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Home Search Collections Journals About Contact us My IOPscience The uses of radiotracers in the life sciences This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 Rep. Prog. Phys. 72 016701 (http://iopscience.iop.org/0034-4885/72/1/016701) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 192.38.67.112 The article was downloaded on 18/09/2010 at 20:40 Please note that terms and conditions apply. IOP PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 72 (2009) 016701 (23pp) doi:10.1088/0034-4885/72/1/016701 The uses of radiotracers in the life sciences Thomas J Ruth TRIUMF, Vancouver, Canada Received 1 August 2007, in final form 27 October 2008 Published 16 December 2008 Online at stacks.iop.org/RoPP/72/016701 Abstract Radionuclides have been used to follow physical, chemical and biological processes almost from the time of their discovery. Probably the application with the biggest impact has been in the medical field where radionuclides have been incorporated into biologically active molecules and used to diagnose a wide variety of diseases and to treat many disorders. Other uses in the life sciences, in general, are related to using a radioactive isotope as marker for an existing species such as nitrogen-13 in plant studies or copper-67 to track copper catalysts in phytoplankton. This review describes in general terms these uses as well as providing the reader with the background related to the physical properties of radioactive decay, the concepts associated with the production of radionuclides using reactors or accelerators and the fundamentals of imaging radioactivity. The advances in imaging technology in recent years has had a profound impact on the use of radionuclides in positron emission tomography and the coupling of other imaging modalities to provide very precise insights into human disease. The variety of uses for radiotracers in science is almost boundless dependent only upon ones imagination. (Some figures in this article are in colour only in the electronic version) This article was invited by Professor G Gillies. Contents 1. Introduction 2. Radioisotope/radionuclide production 2.1. Specific activity 2.2. Reactors 2.3. Cyclotrons 2.4. Generators 3. Radioactive tracers 4. Medical applications 4.1. Historical background 4.2. Radioimmunoassay 4.3. Radiotracers in medicine—ex vivo applications 4.4. Imaging 4.5. Radionuclides for therapy 5. Radiopharmaceuticals 6. Environmental/biological applications 6.1. Agricultural applications 6.2. Plant physiology 6.3. Earth and ocean sciences 6.4. Insect control 6.5. Water resources 7. Concluding remarks Acknowledgments References 1 2 3 3 4 6 6 7 7 8 8 8 powering common household items, to producing electricity for one of every five US homes and businesses. The first practical application of a radioisotope was made by George de Hevesy in 1911. At the time, de Hevesy was a young Hungarian student working in Manchester with naturally radioactive materials. Not having much money he lived in a boarding house and took his meals there with his fellow boarders. He began to suspect that some of the meals 1. Introduction Just as early man harnessed fire to improve his life, society in the last century was able to harness radiation. The development of nuclear technology is one of the most significant achievements of the 20th century. Today nuclear technology is used in nearly every field and aspect of our lives—from medicine, to manufacturing and construction, to 0034-4885/09/016701+23$90.00 15 17 17 17 18 18 18 18 18 20 20 1 © 2009 IOP Publishing Ltd Printed in the UK Rep. Prog. Phys. 72 (2009) 016701 T J Ruth might be made from leftovers from the preceding days or even weeks, but he could never be sure. To try and confirm his suspicions de Hevesy put a small amount of radioactive material into the remains of a meal. Several days later when the same dish was served again he used a simple radiation detection instrument—a gold leaf electroscope—to check whether the food was radioactive. It was, and de Hevesy’s suspicions were confirmed. This anecdotal story illustrates the inquisitive approach de Hevesy took in solving a personal dilemma while in fact he was heavily involved in his research using radioactivity to trace lead (Levi 1976). The use of radionuclides in the physical and biological sciences can be considered tracer science with special application to medicine where they are used for imaging and radiotherapy. Imaging can be further subdivided into planar imaging, positron emission tomography (PET) and single photon emission computed tomography (SPECT). All of these uses rely on the fact that the radionuclides are used at very low concentration. In order to be used in this manner the radionuclides and the compounds to which they are attached must obey the three tracer principles. These state that 140 keV). In addition, the ease with which an iodine atom can be inserted into a compound makes 123 I extremely versatile as a radiotracer in SPECT (Lambrecht et al 1972, Kulkarni 1991, Kung et al 2003). Rhenium-186 is a β − emitter with a low abundant γ -ray with an energy of 137 keV. The 1 MeV (maximum energy) β − -rays and its 90 h half-life make it a promising radiotoxic nuclide for therapy. As an analog of technetium, rhenium possesses similar chemical properties and can thus be used to label some of the same compounds that have been previously developed for imaging tumors (Maxon et al 1990, Kolesnikov-Gauthier et al 2000). Most of the radiotracers have relatively short half-lives (from less than a few hours to at most a few days). There are definite advantages in using short-lived radionuclides. For example, there is a low radiation dose associated with each study, serial studies are possible (sometimes on the same day for tracers such as 11 C) and the radioactive waste disposal problems are minimized if not eliminated. The disadvantages include the need for an accelerator or other source nearby or within easy shipping distance for the longer lived species and rapid chemical procedures, especially for more complex compound formation. Throughout the rest of this paper, examples of the application of radioactive tracers will be provided in some detail and the high sensitivity of the techniques will be illustrated. • the tracer behaves or interacts with the system to be probed in a known, reproducible fashion, • the tracer does not alter or perturb the system in any measurable fashion and • the tracer concentration can be measured. In radiotherapy, the second principle is, in a strict sense, broken since the point of delivering the radiotoxic substance is to have the emitted radiation cause damage to the undesirable surrounding tissues. However, in order for the radiotoxic substance to localize in sufficient quantities it must follow the known chemical behavior without perturbing that pathway, and thus behave like a tracer. When radiotracers are used for diagnostic or therapeutic purposes, imaging and radiotherapy, respectively, they are referred to as radiopharmaceuticals since they must be of pharmaceutical quality for human use. Radiopharmaceuticals will be discussed further later. The following are some typical radionuclides used in each of the broad categories. 2. Radioisotope/radionuclide production Radionuclide production is indeed true alchemy, that is, converting the atoms of one element into those of another. This conversion involves altering the number of protons and/or neutrons in the nucleus (target). If a neutron is added without the emission of proton(s) then the resulting nuclide will have the same chemical properties as the target nuclide—differing only in mass. If, however, the target nucleus is bombarded by a charged particle, for example, a proton, the resulting nucleus will usually be that of a different element. The exact type of nuclear reactions that a target undergoes depends on the number of parameters including the type of bombarding particle and the energy of this projectile. The binding energy per nucleon in the nucleus is on the order of 8 MeV. Therefore, if the incoming projectile has more than this amount of energy, the resulting reaction will cause other particles to be ejected from the target nucleus. By carefully selecting the target nucleus, the bombarding particle and its energy, it is possible to produce a specific radionuclide. Figure 1 illustrates the various exit routes from the production of the compound nucleus generated by bombarding nitrogen14 with protons. A more complete description of the process of radionuclide production is given below. Carbon-11 is a positron emitting radionuclide with a halflife of 20.3 min. It is generally produced as 11 CO2 which can be converted into a wide variety of labeling agents such as 11 CH3 I or H11 CN. Since carbon is a constituent of all biological compounds, 11 C finds widespread use as a tracer in PET. In fact, more than 200 compounds have been labeled with C-11 (Iwata 2002). Nitrogen-13 is also a positron emitting radionuclide. However, in addition to its use as a cardiac blood flow agent (in the form of 13 NH+4 ) it is used in applications other than PET imaging. For example, it is widely used in botany studies to determine the kinetics of nitrogen uptake in a variety of plant systems under a variety of conditions (Bingham 2000, Glass 2002). Detailed examples will be discussed in section 6. Iodine-123 emits γ -rays with an energy of 159 keV. This is ideally suited for imaging in SPECT cameras, as they have been optimized for use with 99m Tc (γ -ray energy = 2 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth 99 235 U Mo 236 U Neutron 135 Figure 1. Schematic illustration of the possible nuclear reactions from the bombardment of N-14 with protons. The relative amounts of each of the product nuclei will depend upon the energy of the incoming proton. Figure 2. Schematic of the fission process following neutron capture by U-235. The unstable compound nucleus breaks into 2 fragments with a distribution as shown in figure 3. Between 2 and 3 neutrons accompany the breakup of the compound nucleus. 2.1. Specific activity 9.3 × 109 Ci mol−1 . In SI units we have 340 exabecquerels per mole (3.4 × 1020 Bq mol−1 ). If the substance had instead been 14 C-labeled with its 5715 y half-life, then following the same process, but using the decay constant (λ = 3.84 × 10−12 s−1 ) for 14 C, the resulting specific activity would be 6.2 × 104 mCi mol−1 or 62 CIF mol−1 . If the radiolabeled glucose had been prepared in a growing plant, the naturally occurring glucose would have lowered the SA due to the non-radioactive glucose molecules. Therefore, it is easy to see that short-lived radioisotopes have the potential for much higher specific activity but this also depends upon the chemical purity. Specific activity is a measure of the number of radioactive atoms or molecules as compared with the total number of those atoms or molecules present in the sample. The specific activity is usually expressed in terms of radiation units per mass unit. The traditional units have been Ci mol−1 (Ci g−1 ) or a fraction thereof (now expressed in SI units as GBq mol−1 ). If there are no stable or radioactive contaminants of the same element, then the sample is referred to as carrier free. For example, a compound labeled with 211 At will be carrier free since there are no stable isotopes of astatine (assuming, of course, that there are no other radioisotopes of astatine present). However, in most cases there are small quantities of unlabeled compounds that have a similar chemical behavior and can act as pseudo-carrier. By pseudo-carrier it is meant that while there may not be true isotopic species present, the molecules co-existing with the compound in question possess similar chemical behavior and thus represents a contamination. The specific activity of an isotope or radiopharmaceutical is important in determining the chemical/biological effect the substance may have on the system under investigation. The number of radioactive atoms, N , in a sample can be calculated from the relationship of radioactivity to quantity of material present and expressed as dN/dt = −λN, Sn 2.2. Reactors Following the Second World War reactors began to be used for a number of research areas including radionuclide production. The use of nuclear reactors for the production of radionuclides relies on the fact that during the fission process in a reactor, there are large numbers of neutrons produced with a wide range of energies. These neutrons can be used directly or thermalized (slowed) by the surrounding media. The term thermal neutron means they have kinetic energy associated with room temperature (about 0.025 eV). These thermalized neutrons are ideal for initiating (n,γ ) reactions. In some reactors, higher energy or fast neutrons (>1 MeV) are used to produce radioisotopes via other reactions, for example, (n,p) or (n,α) reactions. Figure 2 illustrates the fission process. The fission process is a source of a number of widely used radionuclides. For example, 90 Sr, 99 Mo, 131 I and 133 Xe are all produced in reactors by fission and can be separated from uranium fuel cells or from targets of enriched 235 U placed in the reactor for radionuclide production directly. The distribution of isotopes (both radioactive and stable) is illustrated in figure 3. The peak of the lower mass is at mass 99 which includes Mo-99. Approximately 6% of all fissions yield Mo-99; thus the fission approach is a very efficient mode of production for this important radionuclide. The major drawbacks from using fission produced materials are the large quantities of radioactive waste material generated and the large amounts of radionuclides produced (1) where dN/dt is the disintegration rate per second, while λ is the decay constant in reciprocal seconds (λ = ln(2)/t1/2 ). As an example of specific activity assume that glucose has been labeled with 10 mCi of C-11 with a half-life of 20.3 min. Its carrier free specific activity would be obtained by first determining the number of 11 C atoms: − dN (10 mCi)(3.7 × 107 dps mCi−1 ) N11 C = dt = ln(2) λ (20.3 min)(60 s min−1 ) = 6.5 × 1011 atoms. Using Avogadro’s number, the number of moles is then 1.08 × 10−12 . Dividing the amount of radioactivity by the number of moles we have 9.3 × 1012 mCi mol−1 or a SA = 3 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Table 1. The most commonly used positron emitters and typical reactions for their production. Radionuclide t1/2 Decay mode 11 13 C N 20.3 min 9.97 min β+ β+ 15 O 2.03 min β+ 18 F 110 min β+ a Reaction 14 N(p,α) O(p,α) 13 C(p,n)a 15 N(p,n)a 14 N(d,2n) 16 O(p,pn) 18 O(p,n)a nat Ne(d,α) 16 Energy (MeV) 11–17 19 11 11 6 >26 11–17 8–14 These reactions required enriched target material. disrupt the neutron flux. Typically only longer lived (>1 d) radionuclides are produced in reactors. In addition, as very few new reactors are being built, the availability of this source of radionuclides for medical and scientific endeavors is diminishing1 . 2.3. Cyclotrons It is ironic that the first artificially produced radionuclides were created on Lawrence’s cyclotrons (Lawrence and Livingston 1932, Lawrence 1940), but it took another 30 years before accelerator produced radionuclides began to play a major role in the production of medically important radiopharmaceuticals. The principal advantage of accelerator produced radionuclides is the high specific activities that can be obtained through the (p,xn) and (p,α) reactions that result in the product being a different element from the target. Another significant advantage is that a smaller amount of radioactive waste is generated from charged particle reactions in comparison with reactor production. Cyclotrons used for producing medical radionuclides were initially designed for physics experiments and used only part time for medical applications. These cyclotrons were capable of accelerating protons, deuterons, 3 He+2 and α-particles (the nucleus of 4 He). As can be seen from table 1 however, the PET radionuclides are produced from either proton or deuteron reactions. In the early 1980s, small compact protononly cyclotrons became available and cyclotrons specifically designed for producing PET radionuclides were installed in a few hospitals. The principle of the cyclotron is based on the application of small accelerating voltages repeatedly. Figure 4 shows the principal components of a cyclotron. Hollow cavities called dees because of their shape serve as the electrodes for the acceleration. A radiofrequency (RF) oscillator is connected to the dees such that the electrical potential on the dees is alternatively positive and negative with respect to each other. By placing the dees between the poles of a strong magnet so that the magnet field is perpendicular to the plane of motion, the charged particle undergoing acceleration will move in a circular path. As the particle gains energy it moves in a Figure 3. The two curves show the asymmetric yield distribution of radioisotopes as a function of atomic mass from the fission of U-235 (solid curve) and from Pu-239 (dashed curve). Note that the yield peaks on the lower mass hump at mass equal to 99. This is why reactor production of Mo-99 is so favorable (C C Lin, Radiochemistry in Nuclear Power Reactors, National Academy Press (1996)). including isotopes of the desired species. The co-produced radionuclides become a radioactive waste issue if other uses cannot be identified. In producing 131 I from fission, the isotopes 127 I and 129 I are also formed, thus reducing the specific activity. Since 131 I is obtained from the decay of 131 Te, neutron capture on enriched 130 Te is utilized to produce the required 131 Te. As such, the isotopic purity of the 131 I is directly related to the level of enrichment of the target material, 130 Te. 131 I is then extracted from the tellurium oxide via dry distillation at around 600–650 ◦ C. This is an analogous approach to that which is used to extract 123,124 I from cyclotron irradiated targets of 124 Te. Because 235 U enrichment above 20% constitutes weapons grade material (typically the enrichment is as high as 93% 235 U), there is growing concern regarding its use for the production of medical radionuclides. While there are processes in place for the use of lower enrichment (which means dealing with larger waste streams) the major producers have not switched over as of 2008. Reactor production offers some advantages in that production is carried out in a passive mode. That is, in the presence of neutrons, the targets are inserted and withdrawn throughout an operational cycle. Insertions and withdrawals are performed under controlled conditions, so as to not greatly 1 For further reading see IAEA TECDOC 1340 ‘Manual for Reactor Produced Radioisotopes’. 4 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth (3) vacuum tank, magnet, ion source, extraction system) there have been some innovations in the last few decades that have had a major impact on the design of the modern cyclotron. The two most significant changes have occurred in getting the ions into the cyclotron (ion source) and out of the cyclotron (extraction system). Nearly all modern cyclotrons now use a negative ion source. Ions are generated by passing the source gas through an electric field that generates negative and positive ions (e.g. in the case of H2 , the resulting ions will be H+ or protons and H− ions, a proton with 2 electrons). The advantage of negative ions resides in the ability to easily have a variable energy cyclotron, to have nearly 100% extraction (see below) and to be able to extract multiple beams, simultaneously. The design of the ion source has also changed in that the ion source can reside inside the cyclotron where the ions are generated at the center of the cyclotron (central region) or from outside of the cyclotron (external ion source) and subsequently injected into the central region for acceleration. There are obviously advantages and disadvantages to each approach. With an external ion source the vacuum can be operated at very low pressures with very little beam loss due to stripping of the negative ion by the residual gas. However, the vacuum system must be of a very clean nature to maintain this high vacuum. With an external ion source, maintenance can be performed without opening the cyclotron or breaking vacuum. In addition the central region is not disturbed as in the case of the internal ion source that is part of the central region. The simplicity of the design for proton-only cyclotrons resulted in cyclotrons which accelerate H− ions capable of two or more simultaneous beams of varying energies and intensities. The modern cyclotron is completely controlled by a computer and is capable of running for many days with minimal attention. The major drawback from these proton cyclotrons lies in the fact that in some cases an enriched target material must be used for a sufficient product to be generated. One of the major drawbacks to the widespread availability of PET is the high capital cost associated with the cyclotrons and scanners. However, the success of the small low energy cyclotron encouraged research into the design of even lower energy accelerators, i.e. linear accelerators and cyclotrons of a few megaelectronvolts extracted energy. To date, there are very few of these machines in routine use. Regardless of the type of accelerator used to produce the radionuclides, the production rates depend on the flux of the bombarding particles, the number of target nuclei and the probability of the reaction occurring. The equation for the rate of production is R = I σ t, (4) where m is the mass of the ion, e is its charge and v its velocity with B equaling the magnetic field and r is the radius of the ion’s orbit. Thus the orbit of the particle is directly proportional to the particle momentum and the particle orbit frequency is constant and independent of energy. This principle breaks down under relativistic effects where the mass is not constant. While the basic components of modern cyclotrons are essentially the same as the original designs (RF cavities, where R is the rate of nuclei formed per second, I is the flux of the bombarding particles per second, σ is the cross section (probability of the reaction occurring) in cm2 and t is the target thickness expressed as the number of nuclei per square centimeter. It is of historical interest to note that the unit for cross section is the barn, which is equivalent to 10−24 cm2 . The expression ‘barn’ comes from the fact that the probability of a neutron interacting with a target is proportional to the area of Figure 4. Photograph of the interior of a cyclotron shows the copper ‘dees’, the accelerating component of a cyclotron and the 4 ‘hills’ of the steel magnet. This cavity is enclosed with a plate so that a chamber capable of sustaining a vacuum is formed. Ions of a light particle such as hydrogen or helium are injected into the center of the cyclotron where they are accelerated by the electrically charged dees. The dees are high voltage cavities that change polarity (electrical charge) at a high frequency (radiofrequency—tens of megahertz). The magnet forces the charged particles to move in a circular path. As the particle gains energy the circular path increases in radius until it reaches the energy desired whereupon it is extracted and directed to a target material where a nuclear reaction forms the radionuclide of choice. (Photo of TRIUMF TR13 cyclotron.) spiral outward from the center. With the source of negative ions at a point in the center of the cyclotron the positive dee will accelerate the ions toward that dee with the magnetic field forcing them to move in a curved path. Once inside the cavity the particles no longer experience an electric force. Continuing in the circular path the particles will exit the dee and enter the gap between the dees where the second dee has changed its potential to be an attracting force, accelerating the particles to that dee. The dees reverse their potential when the particles are inside the dees so that at each crossing of the gap the particles receive an increase in energy of the order of 20–50 keV. Lawrence discovered the equations defining this principle of operation in 1929 and built the first cyclotron in 1931. Bev = mv 2 /r and r = mv /Be. (2) Since angular velocity ω = v /r, then ω = Be/m, 5 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth the nucleus, which, compared with the size of the neutron, is as big as a barn. The rate of production is, of course, affected by the fact that the resulting nuclide is radioactive and thus undergoes radioactive decay. For short-lived nuclides the competing reaction rates, production and decay will achieve equilibrium at sufficiently long bombardment times since the rate of decay is proportional to the number of radionuclei present. The point where equilibrium is reached is called saturation. This means that there is no benefit to longer irradiations, as the production rate equals the rate of decay, and therefore no additional product will be formed. At shorter irradiation times the fraction of product produced is related to the saturation factor given by (1 − e−λt ), where λ is the decay constant of the decaying nuclide and t is the bombardment time. It is evident that an irradiation equivalent to one half-life would result in a saturation factor of 50%. For practical reasons, an irradiation rarely exceeds three half-lives (90% saturation) except for the shortest-lived radionuclides. For long lived species, the quantity produced is usually expressed in terms of the integrated dose or the total beam flux (µA h). For example, with a long lived radionuclide such as 82 Sr (t1/2 = 25 d) the amount produced will be essentially the same whether it is produced from 100 µA in 1 h or 50 µA in 2 h (both represent 100 µA h of the beam)2 . λd , and the number of radioactive nuclei, N , present (λd N ). The first term accounts for the growth of the daughter as a function of the decay of the parent as well as the disappearance of the daughter due to its own decay. The last term accounts for the presence of daughter nuclei at zero time. All generator systems used routinely in nuclear medicine form an equilibrium between parent and daughter radionuclei. In the case of the 99 Mo/99m Tc generator, the parent (99 Mo) decays at a rate relatively similar to that of the daughter (99m Tc). With a half-life of 66 h for 99 Mo versus 6 h for 99m Tc, there is an appreciable decay of the parent before the daughter reaches steady state. This steady state condition is referred to as transient equilibrium. With transient equilibrium, the daughter radioactivity grows in and surpasses that of the parent before equilibrium is reached. The ratio of the daughter radioactivity to that of the parent is given by equation (6), Tp Ad = , Ap Tp − T d (6) where T is the half-life for each species, respectively (see figure 5). The useful lifetime of the 99 Mo/99m Tc generator is determined by two factors: (1) the amount of 99m Tc that can be eluted from the generator in a volume suitable for use in the diagnostic procedure and (2) the amount of 99 Mo that is co-eluted or the amount of breakthrough. The US Pharmacopeia and the US Nuclear Regulatory Commission or equivalent Agreement State regulations specify a limit of 0.00015 MBq molybdenum Mo-99 per MBq of technetium Tc-99m (0.15 µCi Mo-99/mCi Tc-99m) at the time of administration to each patient. For the situation where the parent has a half-life much longer than the daughter, e.g. 68 Ge/68 Ga and 82 Sr/82 Rb, the change in the amount of the parent during the time for steady state to be reached will be negligible; the steady state condition is referred to as secular equilibrium. The quantity of daughter activity at any time is then expressed by equation (7) 2.4. Generators Finally, the other source of radionuclides used in medicine is the generator. The most widely used generator system is the 99 Mo/99m Tc pair, where over 80% of all nuclear medicine procedures performed worldwide use Tc-99m as the imaging radionuclide. There are numerous Tc-99m kits for producing tracers to examine the brain, kidney, heart, bone, liver, lung, red blood cells and TcO− 4 for thyroid. The parent Mo-99 is produced in a reactor, usually as a fission product from U-235. A radioactive generator takes advantage of the cases where one longer lived (parent) radionuclide decays, usually by β − emission, to a shorter lived (daughter) radionuclide. The chemical differences in the two elements are exploited to separate the daughter product from the parent. The parent radionuclide is produced by one of the methods described above and then attached to an inert substance from which the desired product can be eluted or washed off the support. The product can be used directly as in the case of 82 Rb+ from the Sr/Rb generator or after undergoing a chemical reaction in the case of 99m Tc from the Mo/Tc generator (see below). The equilibrium equations that reflect the relative radioactivity of parent and daughter are given by the general equation: (λd )(e−λp t − e−λd t ) Ad (t) = Ap (0) + Ad (0)e−λd t , (5) λd − λ p Ad (t) = Ap (0)(1 − e−λd t ). (7) Thus, in secular equilibrium, when e−λdt ≈ 0, the daughter and parent radioactivity are approximately equal. From table 2, it is easy to see that generators have a wide variety of uses and half-lives of both parent and daughter nuclides. Obviously, from an end user perspective, the long lived parent makes it possible to have a single generator in use for an extended period of time. The utility of the generator is actually based primarily on the daughter’s half-life and the chemistry required to provide the radionuclide in a useful species. The simplest systems make use of the daughter nuclide directly; 82 Rb+ and 81m Kr are used directly as a K+ ion analog and as an inert gas ventilation tracer, respectively. 3. Radioactive tracers where A is the radioactivity of the daughter ‘d’ and parent ‘p’, respectively. Ad is equal to the product of the decay constant, In addition to the use of radionuclides in medicine there are a wide variety of uses for following the behavior of system, both on the large scale such as the environment and a much 2 For further reading see IAEA TecDoc ‘Theory and Practice of Production of Radioisotopes Using Cyclotrons’ 2007 (at press). 6 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Table 2. Examples of generator systems available today. 99 68 Mo/99m Tc 68 Parent t1/2 Daughter t1/2 66 h 6h Ge/ Ga 270 d 68 m Sr/82 Rb 188 W/188 Re 25.5 d 69 d 76.4 s 16.9 h 81 225 Rb/ Kr Ac/213 Bi 4.58 h 10.0 d 13 s 45.6 m 62 Zn/62 Cu 9.26 h 9.7 m 82 81m Uses 9 Tc-99m is the most widely used radionuclide in nuclear medicine, single photon emitter In equilibrium as a long lived positron source; Ga metal chemistry Cardiac blood flow Radionuclide therapy (β-particles) Lung ventilation studies Radionuclide therapy (α-particles) Blood flow, hypoxia 8 Mo-99 Tc-99m 7 Curies Generator Activity versus Time, Mo-99 and Tc-99m 10 6 5 4 3 2 1 0 0 24 48 72 96 120 144 168 192 216 240 Hours Figure 5. Illustration of the decay of Mo-99 with the in-growth of Tc-99m. smaller scale as in the chemical process in the lab. As indicated in the introduction the term radiotracer refers to a radioactive species that is used to follow (trace) the uptake into or function of an organ system in a living plant, animal or physical/chemical process. Initially, the radiotracers used were radioactive isotopes of naturally occurring elements as in the case where de Hevesy used radioactivity to trace food leftovers in his boarding house (see above). While early radiotracer applications used simple, naturally occurring elements, today the use of radiotracers is based on the production of radionuclides by one of the aforementioned methods. Radioactive isotopes of the elements of sodium and potassium (24 Na, 42 K) have been used as chloride salts to measure the sodium and potassium content of the body employing the isotope dilution technique. Analysis by isotope dilution involves the addition of a known mass and specific activity of a particular isotope to a mixture containing an unknown quantity of that element. An aliquot of the mixture is then analyzed to determine the new specific activity of the substance under investigation. It can be shown that the mass, M, of the substance of interest in the unknown mixture may be expressed as S1 −1 , (8) M = M1 S2 4. Medical applications Nuclear medicine makes use of the fact that certain radionuclides emit gamma rays with sufficient energy for detection outside of the body. In attaching such radionuclides to biologically active compounds, the activity will either localize within particular bodily tissues or be free to follow a particular biochemical pathway. The radiotracers that are used to study bodily function are referred to as radiopharmaceuticals. The term has been used because of the similar properties between these tracers and the drugs or pharmaceuticals that have been developed to treat disease. The following discussion will concentrate on the uses of radioactive substances for the diagnosis of human pathology using imaging as well as by taking samples from blood or exhaled air and in their use for therapeutic treatment. 4.1. Historical background Nuclear medicine (NM) has its origins in the pioneering work of the Hungarian doctor, G de Hevesy, who, in 1924, used radioactive isotopes of lead as tracers in bone studies. These studies were in addition to his amateur detective work at his boarding house. Shortly thereafter, Stevens made intravenous injections of radium chloride to study malignant lymphomas (1926). However, it was not until the discovery of artificially produced radioactive isotopes that the number of available species suitable for use as tracers began to increase. The invention of the cyclotron by Ernest Lawrence in 1932 made it possible to produce radioactive isotopes of a number of biologically important elements. The use of these artificially produced radiotracers continued with J G Hamilton and R Stone using radioactive sodium clinically in 1937. S Hertz, A Roberts and R D Evans, in 1938, used radioactive iodine in the study of thyroid physiology, followed, in 1939, by J H Lawrence, K G Scott and L W Tuttle in the study of leukemia with radioactive phosphorus. By 1940 J G Hamilton and M H Soley were performing studies in iodine metabolism where S1 and S2 are the specific activities of the tracer added before and after addition to the system, respectively, and M1 is the mass of the spike added to the system. As an example, suppose the amount of copper in a system was to be determined. A spike of copper containing 0.5 mg with 5 kBq of 64 Cu is added to 20 mL of the unknown sample. After addition and mixing, a portion of the mixture is isolated and the specific activity (Bq mg−1 ) of copper is determined. In this example, say that the analysis yields a S2 value of 0.024 kBq mg−1 . Using the above equation the amount of copper in the unknown sample is M = 0.5 mg × ((10 kBq mg−1 /0.024 kBq mg−1 ) − 1) = 207 mg of copper. This method of isotope dilution has a number of applications, especially in tracer elemental analyses of living systems. 7 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth by the thyroid gland in situ by comparing the use of radioiodine in control subjects to patients with various types of goiters (Harbert and da Roche 1984). The first medical cyclotron was installed in 1941 at Washington University, St Louis, where radioactive isotopes of phosphorus, iron, arsenic and sulfur were produced. With the development of the fission process during the Second World War (WWII), most radionuclides of medical interest began to be produced in nuclear reactors. After WWII the wide use of radioactive materials in medicine established a new field of what was then called atomic medicine and only later became known as nuclear medicine. Radioactive carbon, tritium, iodine, iron and chromium found increasing use in the study of disease processes. Ben Cassen, 1951, developed the concept of the rectilinear scanner which opened the way to obtaining the distribution of radioactivity in a subject within a short time. This was followed by the production of the first gamma camera by Hal Anger in 1958. The original design was modified shortly afterwards to what is now known as the Anger scintillation camera, thus heralding the modern era of gamma cameras whose principles are still in use today. Powell Richards developed the 99 Mo/99m Tc generator system at the Brookhaven National Laboratory in 1957. Technetium-99m produced via this generator system has become the most widely used radionuclide in nuclear medicine today accounting for as much as 80% of all diagnostic procedures (see table 3 for a list of radiopharmaceuticals). The modern era of nuclear medicine has become known as molecular medicine, as the field translates advances in molecular biology and biochemistry into the treatment of human disease and the diagnosis of pathology and anatomical abnormalities. The advent of clinical PET for cancer diagnosis makes use of sophisticated tracers to unravel cancer biology. for the unknown and the standards to be chemically identical or to have identical biological behavior. The original technique has been modified by making use of a variety of radiotracers to measure concentrations of vitamins, enzymes, peptides, serum proteins, hormones, viruses, drugs and tumor antigens (Ruth 1994). Commercially available kits provide the materials for performing RIA. A typical kit would consist of a series of standard samples each containing a specific amount of an unlabeled antigen, a vial of a labeled antigen, a vial of antibody and a substance used to precipitate the antigen– antibody complex. In most cases I-125 is the isotope employed as the radioactive label on the antigen. 4.3. Radiotracers in medicine—ex vivo applications In the development of drugs or even biomarkers the use of radiotracers labeled with 3 H and 14 C have played an important part in determining the biodistribution as a function of time after injection and the determination of the pharmacokinetics of the substances under investigation. These studies involve the administration of the tracer to a rodent (rat or mouse) and the sacrificing of the animal at specific time points with the organ of interest excised for further analysis. The analysis may involve preparing slices of the organ and placing the slices on film or on phosphorimaging devices so that autoradioradiographic images can be prepared. High resolution images are possible due to the short range of the beta rays from the decay of 3 H and 14 C. Phosphorimagers use a film embedded with light sensitive crystal that absorbs the energy of decay of the radioactive substance to form an excited electronic state in the crystals of the film and the image is read by excitation with a laser digitally stored on a computer. With traditional autoradiography, an x-ray film is exposed to the radioactive substance and the film is developed to provide the image. The advantage of the phosphorimager is that the film can be reused by exposing the film to white light to return the crystals to the unexcited state. Small animal imaging (SPECT and PET) is an attempt to duplicate this technique, however performed in vivo as discussed below. 4.2. Radioimmunoassay Based on the radioisotope techniques developed in the early 1950s S Berson and R Yalow published in 1959 the first use of radioimmunoassay (RIA) to measure the concentration of insulin in an unextracted human plasma. The RIA principle is simple and is illustrated by the schematic in figure 6. The concentration of an unknown amount of unlabeled antigen can be determined by comparing its inhibitory effect on the binding of radioactively labeled antigen to a specific antibody with the inhibitory effect of known standards (Yalow 1978). The use of radioactive tracers makes the method sufficiently sensitive for detecting very small quantities. For example, in some species, concentrations of less than 1 picomolar can be determined. The technique involves the separation of the labeled antigen of interest into bound and unbound fractions after interaction with an antibody in the presence of an unknown amount of unlabeled antigen. The ratios of the bound to free fractions of the labeled antigen are compared with the binding of known standards. The method of measurement requires only that the antigen in test samples and the antigen in standard samples have identical immunologic behavior. Therefore, it is not necessary 4.4. Imaging Nuclear medicine imaging differs from other radiological imaging techniques in that the radiotracers used in nuclear medicine relate to the function of an organ system or metabolic pathway and thus the tracing of these agents reveals the integrity of these systems or pathways. This is the basis for the unique information that a nuclear medicine scan provides. Planar imaging. By far the most common imaging device in nuclear medicine is the planar camera or the Anger camera. The basic components of the camera include a thin crystal of NaI scintillator coupled to a cluster of photomultiplier tubes (PMTs), an X, Y positioning circuit and a readout device that may be an oscilloscope or photographic film. The NaI scintillator design minimizes multiple interactions with the incident γ -rays so that the position of interaction can be 8 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Table 3. Technetium-based radiopharmaceuticals. Generic name Product name Use Manufacturer 99m Technetium generator Ultra-TechneKow FM Supply of Tc Technetium generator Supply of 99m Tc TechneLite Supply of 99m Tc Aggregated albumin Macrotec Lung imaging TechneScan MAA Lung imaging Pulmolite Lung imaging Aggregated albumin Lung imaging MPI MAA Lung imaging Albumin colloid Microlite Imaging of RE system Serum albumin HSA Kit Blood pool imaging Disofenin Hepatolite Hepatobiliary imaging Exametazime Ceretec Cerebral perfusion Lidofenin TechneScan HIDA Hepatobiliary imaging Mebrofenin Choletec Hepatobiliary imaging Medronate Osteolite Bone imaging AN-MDP Bone imaging TecheScan MDP Bone imaging MDP-Squibb Bone imaging Medronate Bone imaging TechneScan MDP Bone imaging Mertiatide TechneScan MAG3 Renal imaging Oxidronate OsteoSan HDP Bone imaging Penetate sodium DTPA Kidney and brain imaging AN-DTPA Kidney and brain imaging Techneplex Kidney and brain imaging Pyro- and tri- metaphosphates TechneScan PYP Bone imaging Phosphotec Bone imaging Pyrolite Bone imaging AN-Pyrotec Bone imaging Red blood cell kit Ultratag RBC Bloodpool imaging RB-SCAN Blood pool imaging Sestamibi Cardiolite Myocardial imaging Gluceptate Glucoscan Kidney and brain imaging TechneScan Gluceptate Kidney and brain imaging Succimer DMSA Renal studies Sulfur colloid Sulfur Colloid Gastrointestinal and organ studies Tesuloid Gastrointestinal and organ studies AN-Sulfur Colloid Gastrointestinal and organ studies Teboroxime CardioTec Myocardial imaging Source: R Brown, Mallinckrodt, Inc., personal communication, April 6, 1994. Mallinckrodt Medi-Physics DuPont-Merck Squibb Mallinckrodt DuPont-Merck CIS-US Merck Sharp & Dohme Dupont-Merck Medi-Physics Dupont-Merck Amersham Merck Sharp & Dohme Squibb DuPont-Merck CIS-US Merck Sharp & Dohme Squibb Medi-Physics CIS-US Mallinckrodt Mallinckrodt Medi-Physics CIS-US CIS-US Mallinckrodt Squibb DuPont-Merck CIS-US Mallinckrodt Cadema DuPont-Merck DuPont-Merck Merck Sharp & Dohme Medi-Physics Medi-Physics Squibb CIS-US Squibb Note: The trade names and the names of the producers may have changed in the intervening years. From Adelstein and Manning (1995). Labeled antigen + Labeled antigen-antibody complex ↔ Ab Ag*-Ab + Ag Unlabeled antigen in known standard solutions or unknown samples ↔ Ag* Specific antibody Ag-Ab Unlabeled antigen antibody complex Figure 6. Schematic representation of the reactions used in RIA. 9 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth determined with great accuracy. Typical scintillation cameras have detectors 25–45 cm in diameter and 0.64–1.27 cm in thickness. The X, Y positioning circuit relies on the light output from the many (19–91) PMTs mounted on the back of the scintillator. The PMT located nearest to the γ -ray interaction will receive the maximum amount of light, and the other PMTs will receive light in proportion to the solid angle subtended by the tube at the point of interaction. The positioning circuit sums the output of the PMTs and produces X and Y pulses proportional to X and Y coordinates of the γ -ray interaction. In between the radioactive source and the detector is a collimator constructed of dense metal such as lead or tungsten. The collimator has one or more holes drilled through it to allow the passage of γ -rays. Since γ -rays cannot be bent or focused the collimator’s function is to absorb those γ -rays that do not pass through the openings. In its simplest form the collimator has a single pin hole and acts like a camera lens. Other collimators are constructed with the holes converging, diverging or parallel to the imaginary line connecting the object to be imaged and the camera face. The converging collimator has the effect of magnifying the imaged object while the diverging collimator magnifies the object. The parallel collimator is used for high resolution. Regardless of which collimator is used they all absorb a large fraction of the photons emitted by the radiotracer in the patient. γ ee+ γ 11 B 11 C Figure 7. Illustration of positron decay. One of the protons (red) in the unstable nucleus is converted into a neutron (blue) with the emission of a positive electron (positron) which travels a short distance until it is annihilated with a neighboring atomic electron resulting in their annihilation giving 2 photons (γ -rays), each with an energy of 511 keV. The photons will travel at nearly 180◦ from each other to conserve momentum. from 201 Tl having a scatter fraction of as much as 40–50% depending on the depth of the source. It is for these reasons that attenuation and scatter exert significant and difficult nonlinear effects that are difficult to correct. Whereas scintillation camera images show the distribution of the radiopharmaceutical in defined regions in planar view, they suffer from the superimposition of organs and background contributions to the areas of interest. It is because of these shortcomings in planar imaging that SPECT has a major role to play in diagnostic imaging regardless of whether SPECT can achieve the difficult task of providing quantitative information. The ability to view the distribution in three dimensions greatly affects the interpretation of the images. Single photon emission computed tomography. As with PET, single photon emission computed tomography, SPECT, acquires views of the emitted photons from many different angles and re-projects these views to reconstruct an image of the three-dimensional distribution of radioactivity in the object or patient. In SPECT, the radiopharmaceuticals used contain radionuclides such as 99m Tc that emit single photons (ones that are not in timed coincidence with one another). Directional information is achieved by collimating the photons incident on the detector of the Anger camera. The collimator thus reduces the sensitivity of the camera because all of the photons not parallel to the holes in the collimator are prevented from reaching the detector surface. Since the reconstructed image contains three-dimensional information on the distribution of radioactivity, SPECT also has the potential for quantification. The factors effecting this capability are similar to those in PET, e.g. system sensitivity, dead-time, spatial resolution, sampling interval, reconstruction filters and the size of the object being imaged. Also, as in PET, the photons emerging from the subject are attenuated by the amount of matter between their origin and the detector, and, of course, they have a definite probability of being scattered along their path. Because of the inherently lower energies (100–150 keV) of the photons emitted by radionuclides used in SPECT, the effect of attenuation can be quite dramatic with reductions as great as a factor of 5 or more. Thus, with single photon emitters, it is difficult to determine whether data reflect a weak source near the surface or a stronger source located at some greater depth. In addition, the amount of scatter is strongly dependent on the energy of the photons, with the photons Positron emission tomography. PET imaging makes use of the self-collimating nature of positron decay (see figure 7), as two nearly collinear photons are utilized to define the location of an annihilation event. PET cameras are typically made of a ring of detectors that are in timed coincidence (resolving time of a few nanoseconds), allowing a line of response to define the cord along which the positron was annihilated (the location of the emission is not known because of the short distance the positron travels before annihilation). By mathematically backprojecting the lines of response, a density map can be generated that reflects the distribution of the positron emitter. There are several physical limitations inherent to PET technology. Firstly, as the emitted positron has kinetic energy, varying from a few hundred kiloelectronvolts to several megaelectronvolts depending upon which radionuclide, it will thus travel a few millimeters to centimeters before annihilating with an atomic electron. As such, the site of annihilation is not the site of emission, thus resulting in a limitation when defining the origin of the decay. Another limitation is the fact that the positron–electron pair is not at rest when the annihilation occurs, thus by conservation of momentum, the two photons are not exactly collinear. Although the lack of co-linearity becomes increasingly important with greater 10 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Figure 8. The three panels show a combined FDG PET/CT image in transaxial, saggital and coronal views, from left to right. The colored hot metal image is the PET image and the gray image is from the CT camera. The combined image enables physicians to determine the precious location of abnormal function (high uptake in the mass visible on the chest wall in the CT image in this case). Photo courtesy of British Columbia Cancer Agency. detector separation, this effect is ignored, for the most part, in existing tomographs because the detector ring diameter is less than a meter at which distance the deviation from 180◦ is a fraction of a millimeter. One of the major strengths that PET has over SPECT is the ability to measure, directly, the attenuation effect of the object being viewed. This is the result of requiring that both photons are detected. Thus, if one photon of the pair is not observed then there is no line of response. Along the path to the detectors, one or both photons (511 keV each, the rest mass of the electron) can undergo absorption by the photoelectric effect or Compton scattering when interacting with surrounding material. Thus, in order to be detected as an event, both photons must be detected in temporal coincidence. By using an external source of positron emitter, the attenuating (absorbing) extent of the object to be measured can be determined. However that advantage has been eliminated now that all commercial PET (and many SPECT) cameras are built with a CT scanner (x-ray tomography) so that a merged image of structure and function can be obtained. In addition, as the CT image is a measure of electron density, it is used to calculate the necessary coefficients for attenuation correction. However, the calculated attenuation coefficients are difficult to perform in the thorax. Nevertheless, the use of the CT image is standard for attenuation corrections now although its primary function is to provide a detailed view of the section of the body under investigation. Figure 8 illustrates the power of this approach. Once the attenuation of the object is measured and the radiotracer is injected the temporal and spatial distribution of the tracer may be determined. However, to make a quantitative estimate of the distribution there are other corrections required. First of all, for true quantitative extraction of information the detector system must be normalized to account for the nonuniform response of the detector system. This is achieved by placing a cylindrical flood phantom of known tracer concentration in the field of view and measuring the responses of all detector pairs. Other corrections are needed to account for scattered photons, which for modern systems can be anywhere from 30% to 50% of the events. The amount of scatter can be reduced by selecting a narrow energy window of acceptance so as to eliminate large angle scatter (large angle scatter results in lower energy of the scattered photon). This will however reduce the efficiency. The remaining scatter profile is removed by analytical techniques, a discussion of which is beyond this review. Finally, there are random coincidences that must be subtracted. Because of the finite timing window for defining a coincidence, there is the possibility of unrelated events arriving within the timing window. The number of random events is related to the size of the timing window and the count rate in any one detector. Random events can be reduced by using fast detectors and electronics which enable a short timing window to be employed. Randoms are usually estimated by monitoring the single event rate and subtracting globally from the image. Once all of these corrections are applied the resulting image can be displayed as what is called a parametric image. In its simplest form this will be disintegrations per second for the volume element of the image. If a mathematical model is employed that describes the time course of the tracer the images can be presented as metric describing a biological function such as glucose metabolism when using 18 F-fluorodeoxyglucose (FDG) or the binding potential in measuring receptor concentrations. The binding potential is 11 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth related to the ratio of the receptor concentration (Bmax ) and the affinity of the tracer for the receptor (KD ). The heart of the PET camera is the detection system. The vast majority of modern PET scanners make use of segmented inorganic scintillation crystals coupled to multiple PMTs. The ideal crystal will have a high stopping power for the 511 keV annihilation photons (high photoelectric absorption), a high light output with wavelength matched to the PMT, a fast decay time for the light and be physically robust. For nearly two decades the detector material of choice was bismuth orthogermanate (BGO). More recently lutetium orthosilicate (LSO) has been introduced. Due to its higher light output, the segmentation of the crystals could be finer, thus reducing the crystal element size from approximately (4 mm × 4 mm) to (2 mm × 2 mm). There are proposals to reduce the crystal elements to below 1 mm2 . In order to accomplish such a task, the packing fraction of the crystals must be improved; in other words, the empty space between crystal elements must remain a small fraction of the total area. The typical crystal is segmented into an 8×8 grid (or more) coupled to four PMTs. There is an algorithm to identify the location of the event by comparing the light sharing amongst the PMTs. While this scheme reduces the cost of the scanner there is a loss in resolution due to the approximate nature of the light sharing approach. There are prototype scanners using avalanche photodiodes coupled to individual crystal elements making the finer pixel identification better. Thus far, such systems have been built only for small animal scanners. Functional imaging using PET started as a research tool in neuroscience in the late 1970s and still remains a major research tool for current-day neurosciences. However, its major impact recently has been in the diagnosis of cancer. While simple tracer molecules such as water, carbon monoxide and carbon dioxide had been used for many years the first complex molecule to be used extensively was the glucose analog, 18 F-fluorodeoxyglucose (FDG), developed at the Brookhaven National Laboratory (BNL) in collaboration with researchers at the National Institutes of Health in the US and the University of Pennsylvania around 1975. Since the human brain uses glucose as its primary energy source, the availability of the tracer led to ground-breaking work for studying the human brain in health and disease. This effort was driven by the successful use of 14 C labeled deoxyglucose at the NIH by Louis Sokolov in the 1960s. Since 14 C is not detectable from outside of the body, the effort went into developing a labeled analog that could be shipped from a cyclotron facility (BNL in this case) and the PET camera (the University of Pennsylvania). Thus F-18 with its nearly 2 h half-life became the radionuclide of choice. Today, many more tracers are used to investigate the various neuronal systems probing both the presynaptic and the postsynaptic pathways. Several hundred tracers have been prepared and tested for the utility in investigating various enzymatic and receptor systems while only a handful are routinely used. There are tracers specifically designed to monitor cell proliferation, the hypoxic nature of cells and cell apoptosis. Because diagnostic imaging is driven by a digital approach (present/absent, yes/no) the desire to have uncluttered images Table 4. Radionuclides used in imaging for SPECT and PET studies. SPECT PET 99 m 11 201 18 Tc Tl 67 Ga 123 I C F 64 Cu 124 I resulting from PET is of great importance. Nevertheless, the true power of PET is its ability to track the distribution of a tracer over time and extract detailed kinetic data as in a physical chemistry experiment where rate constants are determined. So the conflict between using the technology for clinical diagnosis versus using PET as an in vivo biochemistry tool will not be easily resolved, nor should it be. With the advances in the technology enabling increasingly better resolution, it has become possible to build PET scanners capable of imaging small animals. The pharmaceutical industry has recognized the power of using such small animal PET scanners as a screening tool for their pre-clinical research. PET can be used as a surrogate to monitor changes in metabolism or receptor occupation or by labeling the drug directly and determining the distribution and time course of the compound, in vivo. One of the strengths of PET in this regard is that animals can be used many times so that they can serve as their own controls and changes due to interventions monitored. Such an approach reduces the number of animals required and increases the statistical power of the study. See below for more details on small animal scanning. Pharmaceutical companies also recognize that human PET scanning can be used as surrogates for monitoring the therapeutic efficacy of drugs in phase II and III drug trials. By performing baseline scans and scans at intervals following intervention, the PET data can often reveal biochemical changes much sooner than the clinical signs—thus shortening the assessment time. Most often surrogate markers are used to monitor a particular functional change. As the physical limitations of detection are approached, the remaining avenue is to increase the signal to noise by utilizing tracers that are uniquely suited to imaging the function in question and otherwise clear rapidly from surrounding tissue. To this end, the development of more specific tracers is believed to be the most critical component for PET. Radionuclides in imaging. While there is a wide range of radionuclides that are used in imaging, a relatively small number make up the vast majority of all studies in SPECT and PET imaging. Table 4 lists the most widely used radionuclides for imaging along with a couple of potentially useful radionuclides. For the SPECT agents, 99m Tc is the most widely used accounting for approximately 80% of all studies in nuclear medicine. This is primarily due to its availability through the 99 Mo/99m Tc generator as discussed earlier. Tl-201 is widely used in cardiac studies as thallous chloride. The Tl+ ion is an analog of K+ which is used in muscle function. Ga-67 as a citrate is used to detect inflammation and I-123 is used in a variety of radiopharmaceuticals to image brain, 12 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth have high specific activity 123 I available for labeling. However, the production costs are still relatively high in comparison with other radionuclides, which will make its use limited for the foreseeable future. While 123 I can be produced for local use via the 123 Te(p,n) or 124 Te(p,2n) reactions, the co-production of 124,125 I limits the product’s shelf-life. Although 99m Tc can be produced on an accelerator its production in a reactor by extraction of its parent, 99 Mo, from 235 U fission products is much cheaper and more efficient. Thalium-201 has been extensively used for more than 30 years to assess cardiac blood flow as a K+ analog. Over this period there have been numerous reports of its demise, yet the growth in demand for this isotope is still upward. The remaining isotopes listed are used in PET imaging. Carbon-11 is extremely attractive because, in principle, one can replace an existing carbon atom in the molecule of interest with the radioactive isotope. However, because of the short half-life, its availability will be limited to those sites that either possess an accelerator or are close to an accelerator. The demand for 18 F exceeds its availability. To overcome this shortage, a number of central distribution centers have been placed in large metropolitan areas in North America, Europe and Asia. Although several nuclear reactions are available, the (p,n) reaction is the route of choice for producing large quantities of 18 F. If the availability of 18 F continues to grow, 18 F-labeled compounds may begin to compete with other SPECT agents such as 123 I. The other two isotopes, 64 Cu and 124 I, are candidates for both PET imaging and possible use in therapy (see below). The interest in these two is primarily related to the relatively long half-lives. Such properties would enable studies to be performed where the in vivo kinetics are slow and exceed the ability to image with 18 F. The disadvantages include low production rate (124 I) and the need for expensive enriched target material (64 Ni, 124 Te (<1% and <5% natural abundance, respectively)). Results from Washington University in St Louis have shown that even with the high-energy β + -particles associated with 124 I decay and other photons in coincidence with the β + -decay, they can still be imaged at high resolution (64 Cu) (McCarthy et al 1997, Lewis et al 2003). PET imaging has been in use for several decades for human brain and whole body imaging, first only as a research tool, now gaining acceptance as a diagnostic imaging modality in selected applications such as oncology and, very recently, as an aid in the diagnosis of Alzheimer’s disease. All of these advances are made possible through the improvement in resolution and sensitivity of the scanners but more importantly by the development of more specific tracers. Table 5. Nuclear reactions used to produce imaging radionuclides from accelerators. Radionuclide t1/2 Reaction 99m 6.0 h 13.1 h 100 123 Tc I 201 Tl C 73.1 h 20.3 m 18 F 110 m 64 Cu 12.7 h 11 124 I 4.14 d Mo (p,2n) Xe(p,2n)123 Cs 124 Xe(p,pn)123 Xe 124 Xe(p,2pn)123 I 123 Te(p,n)123 I 124 Te(p,2n)123 I 203 Tl(p,3n)201 Pb→201 Tl 14 N(p,α) 11 B(p,n) 18 O(p,n) 20 Ne(d,α) nat Ne(p,X) 64 Ni(p,n) 68 Zn(p,an) nat Zn(d,axn) nat Zn(d,2pxn) 124 Te(p,n) 125 Te(p,2n) 124 Energy (MeV) 30 27 15 25 29 11–19 10 15 14 40 15 30 19 19 13 25 heart and kidney function. The variety of compounds available is based on the ability to chemically insert iodine into complex molecules. Of the PET radionuclides F-18 is by far the most widely used, principally due to its use in FDG. The fluorine atom is about the same physical size as the hydrogen atom in most molecules; thus F-18 is used as a hydrogen substitute. A large number of molecules have been labeled with F-18. C-11 is also widely used because of the obvious isotopic substitution for C-12. The principal disadvantage of C-11 is its short half-life (20 min) which limits its availability to sites with an accelerator. Cu-64 has a half-life of just over 12 h and is thus of interest to probe systems which have a long biological halflife. I-124 has the advantage of being easily inserted in a wide range of molecules but its 4 d half-life limits its utility due to high radiation exposure. Table 5 provides various low energy production routes along with the half-life of the radionuclides. Technetium99m is included since this isotope alone accounts for nearly 80% of all nuclear medicine imaging studies. There have been a number of proposals suggesting that 99m Tc could be produced at an accelerator. However, the economics of accelerator production cannot compare with the extremely low costs of producing it at a reactor. While there is concern about the ability to build new reactors and the availability of this important isotope may be jeopardized, the recent construction of reactors in Canada dedicated to 99 Mo production and the upgrade of other facilities around the world will remove this concern for the present. Iodine-123 has been of interest for nearly three decades because of its unique chemistry that makes it possible to attach this isotope to a wide variety of molecules and the γ -ray energy (159 keV) that is matched well to SPECT cameras. The ability to produce this isotope in high purity from enriched 124 Xe targets made it possible to ship 123 I over long distances and still Small animal scanning (Sossi and Ruth 2005). Compared with human PET scanning, small animal PET presents new challenges, both of instrumentation and biological nature. However, it also offers the possibility of performing in vivo testing of new pharmaceuticals while at the same time allowing for the possibility of direct correlation between in vivo and in vitro measurements thus indirectly providing a deeper understanding of the human PET measurements. For the most part the use of small animal scanning has been dominated by 13 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth metabolism via the use of the labeled drug or to measure the efficacy of action through the use of other PET tracers as surrogate markers of the drug role in altering function (Langstrom et al 1995). While labeling the drug directly may present some challenges, the labeled drug is seen as an important tool for those compounds directed at brain function since an estimate of the degree that (and even whether) a drug penetrates the blood–brain-barrier is required before further drug assessment. In addition, the concentration at which a drug has an effective action is often associated with plasma concentrations when in fact this relationship may not really be measuring the effect of the drug in the brain. The true effect can be measured via PET, either with labeled drug or with surrogate molecules. In drug design a particular neuronal system is to be altered through blocking enzymes, intercepting transmitters or occupying receptors. Using tracers that are sensitive to these changes can provide the needed information in a time frame measured in minutes to hours as opposed to waiting for a pharmacological effect which may take days if not weeks. As mentioned in the introduction, the ability to assess the effects of an intervention longitudinally, on the same animals, significantly reduces the variability of the final results and makes better and more efficient use of the animals themselves. d. Comparison with post-mortem measures. A unique advantage of small animal imaging is the ability to use the same animal as its own control and to perform longitudinal studies. In more traditional animal studies multiple animals were required at various time points so that the animals could be sacrificed and studied to determine the time course of the function under investigation. With small animal scanning the time course can be measured directly, even over days if necessary. The challenge with longitudinal studies is to reposition the animal so that the regional data are correlated. e. Radiotracer and chemistry development. Future advances in functional imaging using nuclear techniques, especially PET, are dependent on tracer development. The PET scanner only measures radioactive decays and cannot by itself identify a biological process of interest. To understand the time course of the tracer, the careful design and development of the radiotracer to make it as specific as possible for the relevant biological sites and processes, while minimizing nonspecific binding to other tissue types, is required (Okarvi 2001, Kawamura 2003). As the imaging instrumentation becomes more powerful, there is an increasing demand for new tracers as more sites and processes become potentially observable in vivo. In addition to undergoing in vitro validation however, the new tracers must undergo a rigorous validation of their in vivo behavior and, where necessary, new imaging protocols and analysis methods must be developed. Presently there are a number of small molecules that have been used in human PET scanning for years as well as in small animal autoradiographic studies using the 3 H- and 14 C-labeled versions. In order to have sufficient signal for the PET scanner the tracers have to be of sufficiently high specific activity (radioactivity units per mass) to provide a high-count rate while not violating the tracer principle. The specific activity required to maintain this principle is on the order of 37 GBq µmol−1 (Hume et al 1998). Thus when research in oncology because of the existing animal models of tumor biology and the relative ease of placement of the tumor in a location with low background. With the increased availability of animal models of disease especially in cancer biology where a wide variety of tumor models are not only being developed but mice are being genetically modified to spontaneously produce tumors, small animal imaging is being used to test new therapies as well as developing more specific diagnostic tests. In addition, small animal imaging has been steadily expanding into the areas of brain and neuroreceptor imaging with a variety of different tracers. The challenges associated with small animal scanning can be divided into the physical and biological issues as discussed below. a. Instrumentation related challenges. The biggest instrumentation challenge that needed to be overcome to successfully apply PET imaging to small animals was to increase spatial resolution, while still maintaining high detection sensitivity. For example, the spatial resolution of traditional human PET scanners ranges typically from (4 mm)3 to (9 mm)3 , while the size of a rat or mouse organ is orders of magnitude smaller compared with the size of the corresponding human organ. b. Biology related challenges. The small size of the animals limits the amount of the tracer that can be administered in a scanning session: PET is based on the tracer principle, that is, the administered radiotracer must not influence the process under investigation. In receptor imaging this is satisfied when the tracer does not occupy more than 1% of the available receptors (Hume et al 1998). This requires tracers to be produced at very high specific activities (generally >37 GBq µmol−1 ) and limits the amount of radioactivity that can be injected, thus rendering detection sensitivity even more important. The second complication due to the small physical size is the fact that the size of the animal’s blood pool is very small. This has direct implications on the applicability of biological models that are applied to the PET data to extract biologically relevant parameters such as binding potentials and process rate constants. Many of these models in fact rely on an input function derived from the radiotracer concentration in plasma, measured by the extraction of several blood samples. Such blood sampling is often not possible with these small animals; therefore, analysis methods that utilize tissue input functions must be used. Such methods require a region where there is no specific binding of the tracer and appropriate regions must be accurately identified for each tracer. Conversely, some research groups are looking into the possibility of measuring the plasma input function from the image of an animal organ, such as the heart (van der Weerdt et al 2001). However, this is in practice only feasible when the radiotracer does not undergo significant metabolism: the PET scanner only detects radioactivity and is not able to separate the chemical form of the radioactively labeled substance. c. Testing of new drugs and their efficacy. Small animal PET imaging is an ideal tool in the process of new drug development and evaluation of treatment efficacy (Campbell 1995). PET imaging can be used to either follow a drug distribution and 14 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Figure 9. Placement of the PET insert inside a 7-T MRI scanner: photograph (a) and drawing (b) show magnified views of insert and the RF coil in place inside the MR scanner and drawing (c) shows axial placement (Catana et al 2006). Figure 10. Black and white MRI images and merged FDG PET images with the MRI scans (Judenhofer et al 2007). injecting a few MBq of a tracer with such specific activities the resulting mass of the injected tracer would be on the order of tens of picomoles. In addition to the need for high specific activity there is a need for high radioactivity concentration (radioactivity units per volume of solution). This requirement stems from the fact that the volume that can be injected into rodents is on the order of 0.5 mL, maximum because of the small blood volume of the animal (typically 20 mL for rats and 2 mL for mice). While there are no requirements to produce the tracers under regulatory conditions, it is obvious that the tracer must be of the highest purity in order to preserve the integrity of the study (Sossi and Ruth 2005). Just as with human scanning, small animal PET has been combined with CT scanners. However, there is a complication associated with the high radiation dose from the CT adversely affecting the animal under investigation. Thus the power of the x-ray beam must be monitored, especially if the animals are to have serial scans. Nevertheless the images are exquisite. To overcome this difficulty and to provide even more information in a single setting investigators are developing combined PET-MRI devices. Unlike the PET/CT systems which are two scanners built back-to-back, the PET/MRI systems are integral as shown in figure 9. The development of this technology is moving forward rapidly and is being applied to human studies as well as illustrated in figure 10 which shows early images from an F-18 FDG scan. 4.5. Radionuclides for therapy The idea of a radionuclide used in therapy is based on the desire to link a radionuclide which has a high linear energy transfer associated with its decay products such as Auger electrons, β-particles or α-particles to a biologically active molecule that can be directed to a tumor site. Since the β − -emitting radionuclides are neutron rich they have, in general, been produced in reactors although a few are best produced via charged particle reactions. Astatine-211 is one such radionuclide [209 Bi(α,2n)211 At]. Table 6 provides a list of radionuclides considered suitable for therapy along with their physical characteristics while table 7 contains the nuclear reactions that can be used for selected radiotoxic nuclides. The attractive feature of 77 Br is its chemical versatility in addition to its half-life. Production rates are relatively low and purity may be an issue since 76 Br is often co-produced. The demand for 103 Pd, which is used in treating prostate cancer, is continuing to grow. A large number of low energy (19 MeV) cyclotrons are dedicated solely to the production of this isotope. Yttrium-90 is an attractive radionuclide for therapy because it is a pure β − emitter and is the product of 90 Sr decay. Strontium-90 has a long half-life (28.8 years) and is readily available as a fission product from nuclear reactors. Because Y-90 does not have an imageable γ -ray, another isotope of yttrium or one of similar chemical properties must be used. In most instances 111 In has been used as the surrogate. 15 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Table 6. Radionuclides that have been proposed for use as possible radiotoxic isotopes for treating cancer. Isotope Half-life (h) Bromine-77 Iodine-131 (131 I) Yttrium-90 (90 Y) Lutetium-177 (177 Lu) Copper-67 (67 Cu) Rhenium-186 (186 Re) Rhenium-188 (188 Re) Bismuth-212 (212 Bi) Bismuth-213 (213 Bi) Astatine-211 (211 At) Actinium-225 (225 Ac) 58 192 64 161 62 90.5 16.9 1 0.77 7.2 240 Receptor Emission (for therapy) Auger electrons β β β β β β α α α α Antibody Cancer Cell Figure 11. Schematic illustrating the antibody labeled with a radiotoxic isotope and how the antibody targets specific receptors on the cancer cell. As the radioisotope decays there is a high probability that some of the beta particles will break the DNA strands in the cell nucleus initiating cell death (drawing courtesy of S Lapi, Simon Fraser University, Burnaby, Canada). Table 7. Production routes for selected therapy radionuclides. Radionuclide t1/2 77 90 Br Y 103 Pd 186 Re 211 At 2.4 d Decay mode Auger electrons Reaction Energy (MeV) α-particle associated with it. Because of its short half-life multiple production sites would be required. Thus the interest in producing its parent radionuclide (211 Rn, t1/2 = 14.6 h) has been suggested as a way of producing and shipping 211 At to remote sites. In spite of its long half-life there is growing interest in the use of Ac-225. The concern with the long half-life is related to the redistribution that may occur during its residence in the body following a therapeutic injection. 75 As(a,2n) Se(p,n) 78 Se(p,2n) 79,81 Br(p,xn)77 Kr nat Mo(p,spall.) 90 Sr decay 27 13 24 45 >200 − 2.7 d β Sr-fission product 103 17.5 d Auger Rh(p,n) 19 electrons nat Ag(p,xn) >70 185 90.6 h β¯ Re(n,γ ) Thermal 186 W(p,n) 18 186 W(d,2n) 20 197 Au(p,spall.) >200 nat Au(p,spall.) >200 nat Ir(p,spall.) >200 209 7.2 h α Bi(α,2n) 28 209 Bi(7Li,5n)211 Rn 60 232 Th(p,spall.)211 Rn >200 77 4.5.1. Targeted radionuclide therapy. In order for these radionuclides to be effective at cancer cell killing, they must either be located at or near the cancer cells (high LET particles) or near the DNA in the nucleus (Auger emitters). Major research efforts have gone into finding the magic bullet, that is achieving site directed delivery of the radionuclide. Most efforts have centered on the use of monoclonal antibodies that are substrates for specific receptors associated with specific cancer cells. Receptors are proteins usually on the cell surface that are used by the cell that act like signal transducers, receive chemical signals from other cells, thus acting as receptors of chemical information. Each of these receptors has unique chemical and physical (spatial) properties so that only certain molecules can be bound or received by the receptor. A close analogy is a lock and key concept. Recently two commercial products have appeared on the market that are based on this concept, antibodies labeled with radionuclides (Zevalin® , labeled with 90 Y (t1/2 = 2.7 d) and Bexxar® labeled with 131 I(t1/2 = 8 d)) (Health Canada (Bexxar) 2005; Health Canada (Zevalin) 2005). Figure 11 illustrates the steps in targeting cancer cells with labeled antibodies. The radionuclide must be attached to the antibody in such a way as not to impact the recognition properties of the antibody for the specific receptor on the cancer cell. With β-particle radiation the damage does not have to occur on the cell on which the antibody is attached. With the range of particles representing a few cell diameters there is the possibility of damage to neighboring cells. This concept is referred to as the crossfire effect. Rhenium-186 is attractive for a number of reasons. It has the desirable physical characteristics of being a β − -emitter with a useful half-life (90 h) and a γ -ray (137 keV) that can be imaged with standard SPECT cameras. This ability to be imaged provides a strong case for its use since radionuclide therapy agents are often pure β¯ emitters and require a surrogate radionuclide for distribution information. In addition, rhenium is in the same chemical family as is technetium; thus much of the chemistry developed for technetium can be applied to rhenium. The production from the neutron capture reaction leads to a low specific activity product which limits its shelf-life and may also limit its utility as a radiotoxic species when attached to a chemical vehicle such as an antibody. Production rates from all of the reactions from charged particles for this radionuclide listed in table 7 are very low. Thus the only practical route to this potentially important radionuclide is via neutron capture in a reactor. And finally, α-emitting isotopes have been of interest for use in therapy because of the high LET associated with the α-decay. Astatine is of interest because it possesses many properties of halogens and each decay of 211 At has an 16 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth neurotransmission, receptor density and occupancy have all been measured via appropriately designed radiotracers. It should be pointed out that the development of radiotracers for PET fundamentally violates rule number 2 for the ideal tracer because PET radionuclides, by nature, emit β + particles. However, the resulting coincident γ -rays from β + annihilation form the basis for the technique. In addition to consideration of the above principles, the radiochemist must plan how to insert the radionuclide into the molecule at a point in the synthetic process where there is minimal handling, yet late enough in the synthesis to minimize loss due to chemical yield and radioactive decay. For these reasons the preparation of radiopharmaceuticals requires planning and techniques not encountered by traditional synthetic chemistry. The development and use of PET tracers can be viewed as covering two major areas: (1) tracers that can be used as surrogate markers for biological processes and (2) those tracers that are specific for a particular process, whether it is intended to measure enzyme activity or receptor concentration or the expression protein synthesis. A major hindrance in tracer development is the complex nature of the synthesis process itself. While major steps have been made to simplify the synthetic steps there are still areas in need of improvement such as miniaturization of the synthesis instrumentation. Miniaturization provides the opportunity to use small amounts of starting materials and radioactivity that would make the purification simpler and easier. Simple solid phase columns could be used instead of cumbersome high performance liquid chromatography. In addition, if the miniaturization can be realized it is conceivable that multiple compounds could be prepared in parallel for testing with a single supply of radionuclide. This can be viewed as the radiochemist’s attempt at screening compounds. 5. Radiopharmaceuticals The term radiopharmaceutical is applied to a biologically active compound that either has a radionuclide attached or in the elemental form behaving as a radiotracer that can be safely administered to humans. The safety of these molecules is determined by their radionuclidic and radiochemical purity and that they are sterile and free from micro-organisms that can cause fevers (pyrogens). Radiopharmaceuticals differ in one major aspect from regular pharmaceuticals in that they are given in such small concentrations that they do not elicit any pharmacological response. Because of this there have been a number of attempts to change the name used to describe these substances to, for example, radiotracers. Present-day radiopharmaceuticals are used for diagnostic purposes in about 95% of the cases and the remainders are used in therapy. However the use of radiopharmaceuticals in therapy is seen as the next major area for growth in the use of radionuclides. In order for a radiotracer (radiopharmaceutical) to be used in humans safely it must meet the quality standards that include chemical and radiochemical purity and it must be sterile and free from pyrogenic material. The ideal radiopharmaceutical for imaging should (1) be readily available at a low cost, (2) be a pure gamma emitter, that is no α and β (such particles contribute radiation dose to the patient while not providing any diagnostic information, (see the section on dosimetry); this is, of course, not followed with PET), (3) have a short effective half-life so that it is eliminated from the body as quickly as possible, (4) have a high target to non-target ratio so that the resulting image has a high contrast, that is the background does not blur the image, (5) possess proper metabolic activity in that it follows or is trapped in the metabolic process of interest. 6. Environmental/biological applications Radioisotopes can be used to help understand chemical and biological processes in the environment and in plants. There are two reasons for this usefulness. Radioisotopes are chemically identical to other isotopes of the same element and will react in the same way in chemical reactions and for many elements some radioactive isotopes of the element have appropriate half-lives and can be easily detected. In other situations elements or simple molecules can be constructed to have similar chemical or physical properties of the chemical systems to be probed. In using surrogate markers their use needs to be validated through experimentation. The ability to measure regional biochemical function requires a careful design process with these principles in mind. However, in reality it is not possible to meet all of these criteria. For example, all decay processes involve the emission of particles as in the case of pure γ -emitters which have Auger electrons emitted during some fraction of the decays. Thus, it is necessary to address the following steps (Eckelman and Gibson 1993) in the development of a biochemical probe: (1) develop a radiotracer that binds preferentially to a specific site; (2) determine the sensitivity of the radiotracer to a change in biochemistry; (3) find a biochemical change as a function of a specific disease that matches that sensitivity. 6.1. Agricultural applications There are many applications of radioisotopes in agriculture. Radiation has been used to breed new seed varieties with higher yields, such as the ‘miracle’ rice that has greatly expanded rice production in Asia. The ionizing radiation from radionuclides increases the number of variations in plants and, with careful selection, can produce crops that are more drought and disease A large number of radiotracers have been synthesized to probe metabolic turnover such as oxygen consumption, glucose utilization and amino acid synthesis. Enzymatic activity, 17 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth resistant, as well as crops with increased yield or shorter growing time. This practice has been in place for several decades and has helped feed some third-world countries. Radioisotopes are ideally suited as tools for the investigations of fertilizers. Important plant nutrients, such as calcium, phosphorus, iron, potassium, copper, sodium, sulfur, and zinc, have radioisotopes with appropriate halflives and decay characteristics to be used as tracers. These elements can be incorporated in fertilizers and applied to the soil to determine the effect on plant utilization of fertilizer composition or the method of application. Plant uptake of the activated fertilizer can be readily measured and can be distinguished from the uptake of the same compound already present in the soil. See the section below on plant physiology which explores some fundamental mechanisms that affect plant interactions with the environment, both natural as well as artificial. at the same time reducing harmful environmental effects of excessive nitrogen use. All of these studies involved using N-13 labeled nitrate and or ammonia in the lab under controlled conditions. The use of C-11 (t1/2 = 20.3 min) provides the opportunity of producing more complex molecules as is seen in the medical applications discussed earlier. This example illustrates how the use of radiotracers has the potential to impact what is understood about plant physiology and the effect of nutrients in the environment. Details of these studies and more are provided in the readings listed. 6.3. Earth and ocean sciences Radiotracers are used studying the biological production in aquatic environments. For example, Si-32 is used to estimate the rates of silicon uptake by diatoms. Diatoms, a group of aquatic algae, are one of the largest contributors to carbon fixation accounting for up to 75% of marine primary production. They have absolute requirements for silicon, which is precipitated as amorphous hydrated silica in their cell walls. Hence, diatoms control the cycling of silicon and contribute significantly to the downward flux of biologically produced silica, nitrogen and carbon in most oceanic regions. Accurate determination of diatom growth is essential for understanding global nutrient cycling and biogeochemical modeling. Another radiotracer that is being used to further our understanding of the carbon cycle and the oceans is Cu-67. Iron (Fe) is an essential micronutrient for phytoplankton growth and has been shown to control primary productivity in large oceanic regions. However the role of copper in this process is poorly understood. With the Cu-67 62 h half-life it has become possible to use this isotope as a tracer in deep ocean studies without having to store and transport the samples back to the lab. 6.2. Plant physiology Probably the most widely used tracer for studies in tracer kinetics in plants is N-13. In spite of its relatively short half-life (<10 min) a wide variety of studies have been undertaken to understand the incorporation of nitrogen into plant systems (Britto 2004). These studies have had a wide impact on understanding the adaptive abilities of plant systems associated with changing environmental conditions to monitor the nitrogen content of genetically modified rice in attempts to increase the protein content of rice species as the primary protein food around the world. By examining the roots of rice plants and the manner in which cellular pools of carbohydrates and various nitrogen compounds regulate the expression of three ammonium transporter genes by measuring the ammonium influx using 13 NH+4 , the researchers found that N and C interact at the cellular level so that the supply of N provided by the root ammonium transporters matches the availability of carbon compounds provided by leaf photosynthesis. This research team has investigated the effect of different transport systems in the root system by genetically modifying Arabidopsis (a small flowering plants related to cabbage and mustard) to express one of the two transporter genes. They have demonstrated that one mutant is unable to grow normally when the nitrate is the sole source of N and that the 13 NO− 3 uptake is dramatically reduced. Thus high-affinity nitrate uptake requires participation of genes encoding both the type of transporter proteins. A large portion (perhaps >50%) of applied nitrogen fertilizer is lost from soils. One significant proportion of this loss is attributed to ammonium blocking nitrate uptake. Using the fungus Aspergillus as a model system this team studied the mechanism of this effect. They found that the effect is rapid and due to ammonium per se not to its metabolic product, e.g. glutamine. They are investigating whether the protein can be modified so it can be eliminated in transgenic plants to reduce nitrate losses from soil and improve fertilizer utilization while 6.4. Insect control About 10% of the world’s crops are destroyed by insects. These pests can sometimes be controlled by releasing sterile laboratory-raised insects into the wild. The male insects are made sterile using ionizing radiation. Female insects that mate with sterile male insects do not reproduce, and the population can be quickly curbed as a consequence. The technique is considered to be safer and better than conventional chemical insecticides since insects can develop resistance against insecticides, and there can be health concerns about chemically treated crops. 6.5. Water resources Adequate water is essential for life. However in many parts of the world water is scarce and in others it is becoming scarcer. Isotope hydrology makes accurate tracing of underground water resources possible. These techniques are important analytical tools in the management and conservation of existing water supplies and in the identification of new, renewable sources of water. The results permit planning and sustainable 18 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Table 8. Common radionuclides and their uses. Calcium-47 Carbon-11,14 Cesium-137 Chromium-51 Cobalt-57 Copper-64 Fluorine-18 Gallium-68 Germanium-68 Iodine-123 Iodine-129 Iodine-131 Nitrogen-13 Oxygen-15 Phosphorus-32 Rubidium-82 Selenium-75 Sodium-24 Strontium-85 Technetium-99m Thallium-204 Tritium (H-3) Uranium-235 Uranium-238 Xenon-133 Yttrium-90 Aid to biomedical researchers studying the cell function and bone formation of mammals. Used in research to ensure that potential new drugs are metabolized without forming harmful by-products. Used to treat cancers; to calibrate the equipment used to measure correct patient dosages of radioactive pharmaceuticals; to measure and control the liquid flow in oil pipelines; to tell researchers whether oil wells are plugged by sand and to ensure the right fill level for packages of food, drugs and other products. (The products in these packages do not become radioactive.) More recently, Cs-137 has become the radioactive source by which attenuation is measured in PET scanners. Used in research in red blood cell survival studies. Used in nuclear medicine to help physicians interpret diagnostic scans of patients’ organs and to diagnose pernicious anemia. MicroPET scanners make use of Co-57 as the radioactive source by which attenuation is measured in small animals. Used in small animal PET imaging. Considered a potential radiotherapeutic. Primary radionuclide used in PET imaging, generally substituted for hydrogen in biologically active molecules. Generator produced PET radionuclide. In equilibrium with its positron emitting daughter has been used as an attenuation source for PET scanners as well as a source for Ga-68. Widely used to diagnose thyroid disorders. Used to check some radioactivity counters in in vitro diagnostic testing laboratories. Used to diagnose and treat thyroid disorders such as Graves’ disease. In the chemical form of ammonia N-13 is used as a blood flow marker in cardiac studies. Also used in plant − + physiology studies (NO− 2 , NO3 and NH4 ). Used in brain studies in various chemical forms to monitor blood flow (H2 O), Oxygen metabolism (O2 ), and blood volume (CO) Used in molecular biology and genetics research. Used as a potassium analog to measure cardiac blood flow with PET Used in protein studies in life science research. Used to locate leaks in industrial pipelines and in oil well studies. Used to study bone formation and metabolism. The most widely used radioactive isotope for diagnostic studies in nuclear medicine. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies. Measures the dust and pollutant levels on filter paper and gages the thickness of plastics, sheet metal, rubber, textiles and paper. Used for life science and drug metabolism studies to ensure the safety of potential new drugs; for self-luminous aircraft and commercial exit signs; for luminous dials, gauges and wristwatches and to produce luminous paint. The source of Tc-99m and other important medical radionuclides when the U-235 undergoes fission. Used in dental fixtures such as crowns and dentures to provide natural color and brightness and in fuel for nuclear power plants and naval nuclear propulsion. Used in nuclear medicine for lung ventilation and blood flow studies. Radiotherapeutic nuclide used in combination with antibodies to treat cancer. management of these water resources. Neutron probes can measure soil moisture very accurately, enabling better management of land affected by salinity, particularly with respect to irrigation. For surface waters they can give information about leakage through dams and irrigation channels, the dynamics of lakes and reservoirs, flow rates and river discharge rate measurements and silt sedimentation rates. Many countries, developed and developing, have used isotope techniques to investigate their water resources in collaboration with the IAEA. imaging, while the big challenge in the next few years will be for the chemists to develop tracers that are more specific and reflective of the functional condition under investigation, while miniaturizing the chemical synthesis and related instrumentation. Two major areas related to tracer development will include the miniaturization of the chemistry for preparing tracers. With the advent of microfluidics and lab-on-a-chip technology, the automated syntheses of tracers on a wafer that can be discarded are not far away. Such developments will speed the availability of tracers for widespread human use because of the possibilities of mass production of the miniature chemistry sets under sterile conditions much like other medical devices such as syringes. The other area ripe for exploitation is in achieving higher specificity of the tracer. This will most likely occur in the use of peptides (protein fragments) and oligonucleotides or short strands of DNA, which will be specific for a particular gene expression for protein syntheses related to pathological conditions. Being able to clearly identify a phenotypic disease in a population could overcome some of the shortcomings related to PET’s lack of sensitivity. Having a tracer that has a very high signal relative to the background enhances the ability 7. Concluding remarks Table 8 illustrates the vast variety of uses for radioactive substances, some of which are obscure while others are essential in modern life. The list is not comprehensive and only represents those associated with life sciences. An equal list can be generated for the physical sciences. The future in imaging now lies in the development of multi-modality imaging approaches such as PET/CT, SPECT/CT and PET/MRI, as well as the use of optical 19 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth to detect small quantities, thus increasing apparent sensitivity. In addition, this approach would truly introduce personalized medicine since the compounds used would be unique to the individual being examined. While the use of radioactive species has become widespread in the health field their use in other fields is still relatively rare. This review paper tried to illustrate the power associated with using radiotracers in a variety of disciplines, both in basic research and in practical applications. While most of the non-medically related applications of radiotracers have used reactor produced species because of their availability, the use of accelerator produced tracers has the potential for a much wider use because of the introduction of PET as a routine diagnostic modality around the world. This advance has placed a very large number of cyclotrons around the world capable of producing a wide variety of short-lived radionuclides for use in the variety of disciplines described here. The only limitation is the imagination of the investigator. Health Canada 2005 Zevalin Available from: http://www.hc-sc.gc.ca/dhp-mps/prodpharma/activit/proj/ sbd-smd/nd ad 2005 zevalin 076192 e.html accessed January 15th 2007 Hume S P, Gunn R N and Jones T 1998 Pharmacological constraints associated with positron emission tomographic scanning of small laboratory animals Eur. J. Nucl. 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Med. 47 1968–76 Eckelman W C and Gibson R E 1993 The design of site-directed radiopharmaceuticals for use in drug discovery Nuclear Imaging in Drug Discovery, Development and Approval ed H D Burns et al (Boston: Birkhäuser) pp 113–34 Glass A D M, Britto D J, Kaiser B N, Kronzucker H J, Kumar A, Okamoto M, Siddiqi M Y and Vidmar J J 2002 The regulation of nitrate and ammonium transport systems in plants. J. Exp. Bot. 53 855–64 Harbert J and da Roche A F G 1984 Textbook of Nuclear Medicine, Volume I: Basic Science 2nd edn (Philadelphia: Lea and Febiger) Health Canada 2005 Bexxar available from: http://www.hc-sc.gc.ca/dhp-mps/prodpharma/activit/proj/ sbd-smd/nd ad 2005 bexxar 084518 e.html. Accessed January 15th 2007 20 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Langstrom B, Bergstrom M, Hartvig P, Valind S and Watanabe Y 1995 Is PET a tool for drug evaluation? PET for Drug Development and Evaluation ed D Comar (Dordrecht: Kluwer) Lecomte R et al 2004 Design considerations for a combined, APD-based µPET /µCT Scanner Proc. IEEE/MIC Meeting (Rome, Italy) Myers R 2001 The biological application of small animal PET imaging Nucl. Med. Biol. 28 585–93 Okarvi S M 2001 Recent progress in fluorine-18 labeled peptide radiopharmaceuticals Eur. J. Nucl. Med. 28 929–38 Rowland D J, Lewis J S and Welch M J 2002 Molecular imaging: the application of small animal positron emission tomography J. Cell Biochem. Suppl. 39 110–5 Seemann M D 2005 Whole-body PET/MRI: the future in oncological imaging Technol. Cancer Res. Treat. 4 577–82 Seidel J, Vaquero J J and Green M V 2003 Resolution uniformity and sensitivity of the NIH ATLAS small animal PET scanner: comparison to simulated LSO scanners without depth-of-interaction capability IEEE Trans. Nucl. Sci. 50 1347–50 Sossi V and Ruth T J 2005 MicroPET imaging: in vivo biochemistry in small animals J. Neural Transm. 112 319–30 Tai Y C, Chatziioannou A F, Yang Y, Silverman R W, Meadors K, Siegel S, Newport D F, Stickel J R and Cherry S R 2003 MicroPET II: design, development and initial performance of an improved microPET scanner for small-animal imaging Phys. Med. Biol. 48 1519–37 Tsyganov E N, Antich P P, Kulkarni P V, Mason R P, Parkey R W, Seliounine S Y, Shay J W, Soesbe T C and Zinchenko A I 2004 Micro-SPECT combined with 3D optical imaging Proc. IEEE/MIC Meeting (Rome, Italy) Weber S et al 1999 First results from MADPET-II: a novel detector and readout system for high resolution small animal PET IEEE Trans. Nucl. Sci. 46 1177–83 Woody C L et al 2004 The RatCAP Conscious Small Animal PET tomography Proc. IEEE/MIC Meeting (Rome, Italy) Yalow R S 1978 Radioimmunoassay: a probe for the fine structure of biologic system Science 200 1236–45 Further reading Burns H D, Gibson R E, Dannals R F and Siegl P K S 1993 Nuclear Imaging in Drug Discovery, Development and Approval (Boston: Birkhauser) Diksic M and Reba R C (ed) 1990 Radiopharmaceuticals and Brain Pathology Studied with PET and SPECT (Boca Raton, FL: CRC Press) Frost J J and Wagner H N (ed) 1990 Quantitative Imaging, Neuroreceptors, Neurotransmitters and Enzymes (New York: Raven) Harbert J and da Roche A F G 1984 Textbook of Nuclear Medicine, Volume I: Basic Science 2nd edn (Philadelphia: Lea and Febiger) Swanson D P, Chilton H M and Thrall J H 1990 Pharmaceuticals in Medical Imaging (New York: Macmillan) Imaging Eckelman W C and Gibson R E 1993 The design of site-directed radiopharmaceuticals for use in drug discovery Nuclear Imaging in Drug Discovery, Development and Approval ed H D Burns et al (Boston: Birkhäuser) pp 113–34 Lyons S K 2005 Advances in imaging mouse tumour models in vivo J. Pathol. 205 194–205 Saha G B 1979 Fundamentals of Nuclear Pharmacy (New York: Springer) Small animal imaging Campbell B 1995 Drug development and positron emission tomography PET for Drug Development and Evaluation ed D Comar (Dordrecht: Kluwer) Chatziioannou A F 2002 PET scanners dedicated to molecular imaging of small animal models Mol. Imaging Biol. 4 47–63 Cherry S R and Gambhir S S 2001 Use of positron emission tomography in animal research ILAR J. 42 219–32 Cherry S R 2001 Fundamentals of positron emission tomography and applications in preclinical drug development J. Clin. Pharmacol. 41 482–91 del Guerra A and Belcari N 2002 Advances in animal PET scanners Q. J. Nucl. Med. 46 35–47 Frese T, Rouze N C, Bouman C A, Sauer K and Hutchins G D 2003 Quantitative comparison of FBP, EM, and Bayesian reconstruction algorithms for the IndyPET scanner IEEE Trans. Med. Imaging 22 258–76 Herschman H R 2003 Molecular imaging: looking at problems, seeing solutions Science 302 605–8 Herschman H R 2004 PET reporter genes for noninvasive imaging of gene therapy, cell tracking and transgenic analysis Crit. Rev. Oncol./Hematol. 51 191–204 Hume S P, Gunn R N and Jones T 1998 Pharmacological constraints associated with positron emission tomographic scanning of small laboratory animals Eur. J. Nucl. Med. 25 173–6 Hume S P and Myers R 2002 Dedicated small animal scanners: a new tool for drug development? Curr. Pharm. Des. 8 1497–511 Jacobs A H et al 2003 PET-based molecular imaging in neuroscience Eur. J. Nucl. Med. Mol. Imaging 30 1051–65 Jeavons A P, Chandler R A and Car D 1999 A 3D HIDAC-PET Camera with Sub-millimetre resolution for imaging small animals IEEE Trans. Nucl. Sci. 46 468–73 Knoess C et al 2003 Performance evaluation of the microPET R4 PET scanner for rodents Eur. J. Nucl. Med. Mol. Imaging 30 737–47 Radiotracers Qaim S M and Coenen H H (ed.) 2004 Advances in Nuclear and Radiochemistry (Jülich: Forschungzentrum, Jülich GmbH) Environmental Albrecht A, Schultze U, Bello Bugallo P, Wydler H, Frossard E and Flühler H 2003 Behavior of a surface applied radionuclide and a dye tracer in structured and repacked soil monoliths J. Environ. Radioact. 68 47–64 Ban-nai T and Muramatsu Y 2002 Transfer factors of radioactive Cs, Sr, Mn, Co and Zn from Japanese soils to root and leaf of radish J. Environ. Radioact. 63 251–64 Brandtberga P-O, Bengtssonb J and Lundkvist H 2004 Distributions of the capacity to take up nutrients by Betula spp. and Picea abies in mixed stands Forest Ecol. Management 198 193–208 Reide Corbetta D, McKeeb B and Duncan D 2004 An evaluation of mobile mud dynamics in the Mississippi River deltaic region Mar. Geol. 209 91–112 Seebaugh D R, Goto D and Wallace W G 2005 Bioenhancement of cadmium transfer along a multi-level food chain Mar. Environ. Res. 59 473–91 Wolterbeek H Th and van der Meer A J G M 2002 Transport rate of arsenic, cadmium, copper and zinc in Potamogeton pectinatus L.: radiotracer experiments with 76 As, 109,115 Cd, 64 Cu and 65,69m Zn Sci. Total Environ. 287 13–30 21 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth Botany Chen X, Gastaldi C, Siddiqi M Y and Glass A D M 1997 Growth of a lettuce crop at low ambient nutrient concentrations: a strategy designed to limit the potential for eutrophication J. Plant Nutrition 20 1403–17 Crawford N M and Glass A D M 1998 Molecular and physiological aspects of nitrate uptake in plants Trends Plant Sci. 3 381–95 Min X, Siddiqi M Y, Guy R D, Glass A D M and Kronzucker H J 1998 Induction of nitrate uptake and nitrate reductase in trembling aspen and lodgepole pine Plant Cell Environ. 21 1039–46 Kronzucker H J, Guy R D, Kirk K, Siddiqi M Y and Glass A D M 1998 Effects of hypoxia on 13 NH+4 uptake in rice roots: kinetics and compartmental analysis Plant Physiol. 116 581–7 Kronzucker H J, Siddiqi M Y and Glass A D M 1997 Conifer root discrimination against soil nitrate and the ecology of forest succession Nature 385 59–61 Glass A D M, Erner Y, Kronzucker H J, Schjoerring J K, Siddiqi M Y and Wang M Y 1997. Ammonium fluxes into plant roots: energetics, kinetics and regulation Z. Pflanzen. Boden. 160 261–8 Glass A D M et al 1999 Inorganic nitrogen absorption by plant roots: physiology and molecular biology Plant Nutrition Molecular Biology and Genetics ed G Gissel-Nielsen and A Jensen (Wageningen: Kluwer) pp 1–16 Min X, Siddiqi M Y, Guy R D, Glass A D M and Kronzucker H J 1999 A comparative study of fluxes and compartmentation of nitrate and ammonium in early successional tree species Plant Cell Environ. 22 821–30 Johnson R R, Glass A D M, Kronzucker H J, Gelbart Z, Venczel E, Paul M, Berkovits D, Cavan A, Kashiv Y and Ghelberg S 1997 Measurement of aluminum transport in wheat at the cellular level Nucl. Instrum. Methods B 123 283–6 Wang M Y, Glass A, Shaff J E and Kochian L V 1994 Ammonium uptake by rice roots: III. Electrophysiology Plant Physiol. 104 899–906 Wang M Y, Siddiqi M Y, Ruth T J and Glass A 1993 Ammonium uptake by rice roots: II. Kinetics of 13 NH+4 Influx across the plasmalemma Plant Physiol. 103 1259–67 Wang M Y, Siddiqi M Y, Ruth T J and Glass A 1993 Ammonium uptake by rice roots: I. Fluxes and subcellular distribution of 13 NH+4 Plant Physiol. 103 1249–58 King B J, Siddiqi M Y, Ruth T J, Warner R L and Glass A 1993 Feedback regulation of nitrate influx in barley roots by nitrate, nitrite, and ammonium Plant Physiol. 102 1279–86 Kronzucker H J, Siddiqi M Y and Glass A 1995 Analysis of 13 NH+4 efflux in spruce roots (a test case for phase identification in compartmental analysis Plant Physiol. 109 481–90 Kronzucker H J, Glass A D and Yaeesh Siddiqi M 1999 Inhibition of nitrate uptake by ammonium in barley: analysis of component fluxes Plant Physiol. 120 283–92 Kronzucker H J, Siddiqi M Y, Glass A D and Kirk G J 1999 Nitrate–ammonium synergism in rice: a subcellular flux analysis Plant Physiol. 119 1041–6 Kronzucker H J, Kirk G J D, Yaeesh Siddiqi M and Glass A D M 1998 Effects of hypoxia on 13 NH+4 fluxes in rice roots: kinetics and compartmental analysis Plant Physiol. 116 581–7 Siddiqi M Y, Glass A D M, Ruth T J and Rufty T W 1990 Studies of the uptake of nitrate in barley: 1. Kinetics of 13 NO3 -influx Plant Physiol. 93 1426–32 Glass A D M, Siddiqi M Y, Ruth T J and Rufty T W 1990 Studies of the uptake of nitrate in barley: 2. Energetics Plant Physiol. 93 1585–9 Kafkafi A U, Siddiqi M Y, Ritchie R J, Glass A D M and Ruth T J 1992 Reduction of 13 NO3 influx and 13 N translocation by tomato and melon varieties after short exposure to Ca2+ and K+ chloride salts J. Plant Nutr. 15 959–75 Glass has used N-13 as a radiotracer extensively over the last 20 years and although the list below is not exhaustive it is included as evidence of the quality of research that can be achieved through the use of tracers. Britto D T, Ruth T J, Lapi S and Kronzucker H J 2004 Cellular and whole-plant chloride dynamics in barley: insights into chloride-nitrogen interactions and salinity responses Planta 218 615–22 Britto D T and Kronzucker H J 2003 Trans-stimulation of 13 NH+4 efflux provides evidence for the cytosolic origin of tracer in the compartmental analysis of barley roots Funct. Plant Biol. 30 1233–8 Kaiser B N, Rawat S R, Siddiqi M Y, Masle J and Glass A D M 2002 Functional analysis of an Arabidopsis T-DNA ‘knockout’ of the high-affinity NH+4 transporter AtAMT1 Plant Physiol. 130 1263–75 Kronzucker H J, Siddiqi M Y, Glass A D M and Britto D T 2003 Root ammonium transport efficiency as a determinant in forest colonization patterns: a hypothesis Physiol. Plant 117 164–70 Glass A D et al 2002 The regulation of nitrate and ammonium transport systems in plants J. Exp. Bot. 53 855–64 (Review) Britto D T and Kronzucker H J 2001 Constancy of nitrogen turnover kinetics in the plant cell: insights into the integration of subcellular N fluxes Planta 213 175–81 Britto D T, Glass A D M, Kronzucker H J and Siddiqi M Y 2001 Cytosolic concentrations and transmembrane fluxes of NH+4 /NH3 : an analysis of a current controversy Plant Physiol. 125 523–6 Kronzucker H J, Britto D T, Davenport R J and Tester M 2001 Ammonium toxicity and the real cost of transport Trends Plant Sci. 6 335–7 Britto D T, Siddiqi M Y, Glass A D and Kronzucker H J 2001 Futile transmembrane NH(+) 4 cycling: a cellular hypothesis to explain ammonium toxicity in plants Proc. Natl Acad. Sci. USA 98 4255–8 Glass A D M et al 2001 Nitrogen transport in plants, with an emphasis on the regulation of fluxes to match plant demand J. Plant Nutrition Soil Sci. 164 199–207 Vidmar J J, Zhuo D, Siddiqi M Y, Schjoerring J K, Touraine B and Glass A D M 2000 Regulation of HvNRT2 expression and high-affinity nitrate influx in roots of Hordeum vulgare by ammonium and amino acids Plant Physiol. 123 307–18 Vidmar J J, Zhuo D, Siddiqi M Y and Glass A D M 2000 Isolation and characterization of HvNRT2.3 and HvNRT2.4, cDNAs encoding high-affinity nitrate transporters from roots of Hordeum vulgare Plant Physiol. 122 783–92 Kronzucker H J, Glass A D M, Siddiqi M Y and Kirk G J D 2000 Comparative kinetic analysis of ammonium and nitrate acquisition by tropical lowland rice: implications for rice cultivation and yield potential New Phytol. 145 471–6 Min X J, Siddiqi M Y, Guy R D, Glass A D M and Kronzucker H J 2000 A comparative kinetic analysis of nitrate and ammonium influx in two early-successional tree species of temperate and boreal forest ecosystems Plant Cell Environ. 23 321–8 Britto D T, Glass A D, Kronzucker H J and Siddiqi M Y 2001 Cytosolic concentrations and transmembrane fluxes of +/ NH4 NH3 : an evaluation of recent proposals Plant Physiol. 125 523–6 Glass A D M et al 2001 Nitrogen transport in plants, with emphasis on the regulation of fluxes to match plant demand Pflanzen. Boden. 164 199–207 Touraine B and Glass A D M 1997 Nitrate and chlorate fluxes in the chl1-5 mutant of Arabidopsis thaliana: does the CHL1-5 gene encode a low affinity nitrate transporter? Plant Physiol. 114 137–44 22 Rep. Prog. Phys. 72 (2009) 016701 T J Ruth C-11 as radiotracer even after proton transport is decoupled Planta 226 541–51 Ferrieri A P, Thorpe M R and Ferrieri R A 2006 Stimulating natural defenses in poplar clones (OP-367) increases plant metabolism of carbon tetrachloride Int. J. Phytoremediation 8 233–43 Babst B A, Ferrieri R A, Gray D W, Lerdau M, Schlyer D J, Schueller M, Thorpe M R and Orians C M 2005 Jasmonic acid induces rapid changes in carbon transport and partitioning in Populus New Phytol. 167 63–72 Babst B A, Ferrieri R A, Gray D W, Lerdau M, Schlyer D J, Schueller M, Thorpe M R and Orians C M 2005 Jasmonic acid induces rapid changes in carbon transport and partitioning in Populus New Phytol. 167 63–72 Thorpe M R, Ferrieri A P, Herth M M and Ferrieri R A 2007 (11 C)-imaging: methyl jasmonate moves in both phloem and xylem, promotes transport of jasmonate, and of photoassimilate 23