Download 06_Medical equipment based on ionizing radiation principle

Interaction ionizing radiation with
biological tissue. Basic dosimetry.
Radioactivity
• The word radioactivity was
first used by Marie Curie in
1898.
• She used the word
radioactivity to describe the
property of certain
substances to give off
invisible “radiations” that
could be detected by films.
Radioactivity
• Scientists quickly learned that
there were three different kinds
of radiation given off by
radioactive materials.
– Alpha rays
– Beta rays
– Gamma rays
• The scientists called them
“rays” because the radiation
carried energy and moved in
straight lines, like light rays.
Radioactivity
• We now know that
radioactivity comes from
the nucleus of the atom.
• If the nucleus has too
many neutrons, or is
unstable for any other
reason, the atom
undergoes radioactive
decay.
• The word decay means to
"break down."
Radioactivity
• In alpha decay, the nucleus ejects two protons and two
neutrons.
• Beta decay occurs when a neutron in the nucleus splits
into a proton and an electron.
• Gamma decay is not truly a decay reaction in the sense
that the nucleus becomes something different.
Radiation
Radiation: The process of emitting
energy in the form of waves or
particles.
Where does radiation come from?
Radiation is generally produced
when particles interact or decay.
A large contribution of the radiation
on earth is from the sun (solar) or
from radioactive isotopes of the
elements (terrestrial).
Radiation is going through you at
this very moment!
http://www.atral.com/U238.html
Isotopes
What’s an isotope?
Two or more varieties of an element
having the same number of protons but
different number of neutrons. Certain
isotopes are “unstable” and decay to
lighter isotopes or elements.
Deuterium and tritium are isotopes of
hydrogen. In addition to the 1 proton,
they have 1 and 2 additional neutrons in
the nucleus respectively*.
Another prime example is Uranium
238, or just 238U.
• Introduction
– Henri Becquerel discovered radioactivity in 1896
• Becquerel named the emission of invisible radiation
from uranium ore radioactivity.
• Radioactive materials was the name given to materials
that gave off this invisible radiation.
• Radioactivity was discovered by Henri Becquerel when he
exposed a light-tight photographic plate to a radioactive
mineral, then developed the plate. (A) A photographic film
is exposed to an uranite ore sample. (B) The film, developed
normally after a four-day exposure to uranite. Becquerel
found an image like this one and deduced that the mineral
gave off invisible radiation that he called radioactivity.
– Ernest Rutherford later discovered that there were three
kinds of radioactivity.
• Alpha particles () is a helium nucleus (2 protons
and 2 neutrons)
• A beta particle () is a high energy electron
• A gamma ray () is electromagnetic radiation with a
very short wavelength.
• Radiation passing through a magnetic field shows that
massive, positively charged alpha particles are deflected one
way, and less massive beta particles with their negative
charge are greatly deflected in the opposite direction.
Gamma rays, like light, are not deflected.
– Radioactivity is the spontaneous emission of particles or
energy from an atomic nucleus as it disintegrates.
– Radioactive decay is the spontaneous disintegration of
decomposition of a nucleus.
• Nuclear Equations
– The two subatomic particles that occur in the nucleus, the
proton and the neutron, are called nucleons.
• The number of protons is the atomic number which
determines the identity of the element.
• The number of protons and neutrons determines the
atomic mass of the element.
• Different isotopes of an element have the same atomic
number (same number of protons) but different atomic
masses (different number of neutrons)
Radioactivity
By the end of the 1800s, it was known that certain
isotopes emit penetrating rays. Three types of radiation
were known:
1) Alpha particles ()
2) Beta particles
()
3) Gamma-rays
()
Where do these particles come
from ?
These particles generally come
from the nuclei of atomic isotopes
which are not stable.
 The decay chain of Uranium
produces all three of these forms
of radiation.
 Let’s look at them in more detail…
Discovery of Radioactivity
• Antoine Henri Becquerel (1852-1908)
– Noticed the fogging of photographic plate by uranium
crystals
• Pierre Curie (1859-1906), Marie Curie (18671934)
– Further studies of uranium and discovery of polonium
and radium. Marie received two Nobel prizes. She died
from the effects of radiation doses received during her
experiments
• Ernest Rutherford (1871-1937)
– His understanding of atomic structure helped us
understand the role of the nucleus. He defined many of
the terms used to discuss radioactivity today
Note: This is the
atomic weight, which
is the number of
protons plus neutrons
Alpha Particles ()
Radium
Radon
R226
Rn222
88 protons
138 neutrons
n p
p n
+
86 protons
136 neutrons
 (4He)
2 protons
2 neutrons
The alpha-particle () is a Helium nucleus.
It’s the same as the element Helium, with the
electrons stripped off !
Net effect is loss of 4 in mass number and loss of 2 in atomic
number.
Beta Particles ()
Carbon
C14
Nitrogen
N14
6 protons
8 neutrons
7 protons
7 neutrons
+
eelectron
(beta-particle)
We see that one of the neutrons from the C14 nucleus
“converted” into a proton, and an electron was ejected.
The remaining nucleus contains 7p and 7n, which is a nitrogen
nucleus. In symbolic notation, the following process occurred:
np+e (+n)
Yes, the same
neutrino we saw
previously
Gamma particles ()
In much the same way that electrons in atoms can be in an
excited state, so can a nucleus.
Neon
Ne20
10 protons
10 neutrons
(in excited state)
Neon
Ne20
+
10 protons
10 neutrons
(lowest energy state)
gamma
A gamma is a high energy light particle.
It is NOT visible by your naked eye because it is not in
the visible part of the EM spectrum.
Gamma Rays
Neon
Ne20
Neon
Ne20
+
The gamma from nuclear decay
is in the X-ray/ Gamma ray
part of the EM spectrum
(very energetic!)
A. Radioactive Decay
Types of Radioactive Decay
• Positron production
• Positron – particle with same mass as an electron but with
a positive charge (antimatter version of an electron)
– Examples
• Net effect is to change a proton to a neutron.
A. Radioactive Decay
Types of Radioactive Decay
• Electron capture
• Inner orbital electron is captured. New nucleus
formed. Neutrino and gamma ray produced
201 Hg
80
+ 0-1e → 20179Au + ν + 00γ
• Net effect is to change a proton to a neutron
How do these particles differ ?
Particle
Mass*
(MeV/c2)
Charge
Gamma ()
0
0
Beta ()
~0.5
-1
Alpha ()
~3752
+2
* m = E / c2
Rate of Decay
Beyond knowing the types of particles which are emitted
when an isotope decays, we also are interested in how frequently
one of the atoms emits this radiation.
 A very important point here is that we cannot predict when a
particular entity will decay.
 We do know though, that if we had a large sample of a radioactive
substance, some number will decay after a given amount of time.
 Some radioactive substances have a very high “rate of decay”,
while others have a very low decay rate.
 To differentiate different radioactive substances, we look to
quantify this idea of “decay rate”
Radioactive Decay
Conservation of Mass Number and Charge Number
− both are retained in a nuclear reaction
− sum of both from the “reactants and products” are constant
Band of Stability
Black squares
indicate stable
nuclei. Decay
occurs to move
isotopes towards
the black line
Nuclear Transformations
• Nuclear Transformation – forced change of
one element to another
• Bombard elements with particles
• Transuranium elements – elements with
atomic numbers greater than 92 which have
been synthesized
UUO
Detection of Radioactivity and the
Concept of Half-life
• Geiger-Muller counter – instrument which measures
radioactive decay by registering the ions and electrons
produced as a radioactive particle passes through a gasfilled chamber
Detection of Radioactivity and the
Concept of Half-life
• Scintillation counter
instrument which
measures the rate of
radioactive decay by
sensing flashes of
light that the radiation
produces in the
detector
X-ray machines
• X-rays are photons,
like visible light
photons only with
much more energy.
• Diagnostic x-rays are
used to produce
images of bones and
teeth on x-ray film.
• Xray film turns black
when exposed to xrays.
X-ray machines
• Therapeutic x-rays are
used to destroy diseased
tissue, such as cancer
cells.
• Low levels of x-rays do
not destroy cells, but
high levels do.
• The beams are made to
overlap at the place
where the doctor wants
to destroy diseased cells.
CAT scan
• The advent of powerful
computers has made it
possible to produce threedimensional images of bones
and other structures within
the body.
• To produce a CAT scan,
computerized axial
tomography, a computer
controls an x-ray machine as
it takes pictures of the body
from different angles.
CAT scan
• People who work with
radiation use radiation
detectors to tell when
radiation is present and to
measure its intensity.
• The Geiger counter is a type
of radiation detector
invented to measure x-rays
and other ionizing radiation,
since they are invisible to
the naked eye.
Detection of Radioactivity and the
Concept of Half-life
• Half-life – time required for
half of the original sample
of radioactive nuclides to
decay
Decay of a Radioactive Element
Half of the
radioactive parent
atoms decay after
one half-life. Half
of the remainder
decay after
another half-life
and so on……..
Half-life activity
Fundamental law of radioactive decay
• Each nucleus has a fixed probability of decaying per unit
time. Nothing affects this probability (e.g., temperature,
pressure, bonding environment, etc.)
[exception: very high pressure promotes electron capture slightly]
• This is equivalent to saying that averaged over a large
enough number of atoms the number of decays per unit time
is proportional to the number of atoms present.
• Therefore in a closed system:
dN
 N
dt
(Equation 3.1)
– N = number of parent nuclei at time t
–  = decay constant = probability of decay per unit time (units: s–1)
• To get time history of number of parent nuclei, integrate 3.1:
N (t )  Noe t
– No = initial number of parent nuclei at time t = 0.
(3.2)
37
Definitions
• The mean life t of a parent nuclide is given by the number
present divided by the removal rate (recall this later when we talk
about residence time):
N 1
t

N 
– This is also the “e-folding” time of the decay:
N(t )  No e
 t
 Noe
1
No

e
• The half life t1/2 of a nucleus is the time after which half the
parent remains:
No
ln 2 .693
 t1/2


t

ln2
(3.3)
N(t1/ 2 ) 
 Noe
 t1/2 

1/ 2
2


• The activity is decays per unit time, denoted by parentheses:
( N )  N
(3.4)
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Decay of parent
0
ln(N)–ln(No)
Activity
No
No
2
No
e
0
t 1/2 t
3t
2t
4t
-1
-2
slope = -1
-3
-4
-5
5t
0
time
t 1/2 t
2t
3t
4t
5t
time
Some dating schemes only consider measurement of parent nuclei
because initial abundance is somehow known.
•
14C-14N:
cosmic rays create a roughly constant atmospheric 14C inventory,
so that living matter has a roughly constant 14C/C ratio while it exchanges
CO2 with the environment through photosynthesis or diet. After death
this 14C decays with half life 5730 years. Hence even through the
daughter 14N is not retained or measured, age is calculated using:
14
t
1
( C) / C
ln 14
14 ( C) / C

o
39
Half-life is the time required for the quantity of a radioactive material to be
reduced to one-half its original value.
All radionuclides have a particular half-life, some of which a very long, while other are
extremely short.
For example, uranium-238 has such a long half life, 4.5x109 years, that only a small
fraction has decayed since the earth was formed. In contrast, carbon-11 has a half-life of
only 20 minutes. Since this nuclide has medical applications, it has to be created where
it is being used so that enough will be present to conduct medical studies.
When given a certain amount of radioactive material, it is customary to refer to the
quantity based on its activity rather than its mass. The activity is simply the
number of disintegrations or transformations the quantity of material undergoes in a
given period of time.
The two most common units of activity are the Curie and the Becquerel.
The Curie is named after Pierre Curie for his and his wife Marie's discovery of
radium. One Curie is equal to 3.7x1010 disintegrations per second.
A newer unit of activity if the Becquerel named for Henry Becquerel who is credited
with the discovery of radioactivity. One Becquerel is equal to one disintegration per
second.
It is obvious that the Curie is a very large amount of activity and the Becquerel is a very
small amount. To make discussion of common amounts of radioactivity more
convenient, we often talk in terms of milli and microCuries or kilo and
MegaBecquerels.
Common Radiation Units – SI
Gray (Gy) - to measure absorbed dose ... the amount of energy actually absorbed in some
material, and is used for any type of radiation and any material (does not't describe the
biological effects of the different radiations)
Gy = J / kg (one joule of energy deposited in one kg of a material)
Sievert (Sv) - to derive equivalent dose ... the absorbed dose in human tissue to the effective
biological damage of the radiation
Sv = Gy x Q (Q = quality factor unique to the type of incident radiation)
Becquerel (Bq) - to measure a radioactivity … the quantity of a radioactive material that have 1
transformations /1s
Bq = one transformation per second, there are 3.7 x 1010 Bq in one curie.
__________________________________________________________________________________
Roentgen (R) - to measure exposure but only to describe for gamma and X-rays, and only in air.
R = depositing in dry air enough energy to cause 2.58E-4 coulombs per kg
Rad (radiation absorbed dose) - to measure absorbed dose
Rem (roentgen equivalent man) - to derive equivalent dose related the absorbed dose in
human tissue to the effective biological damage of the radiation.
Curie (Ci) - to measure radioactivity. One curie is that quantity of a radioactive material that
will have 37,000,000,000 transformations in one second. 3.7 x 1010 Bq
•
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Since we cannot see, smell or taste radiation, we are dependent on instruments to indicate the presence
of ionizing radiation.
The most common type of instrument is a gas filled radiation detector. This instrument works on the
principle that as radiation passes through air or a specific gas, ionization of the molecules in the air
occur. When a high voltage is placed between two areas of the gas filled space, the positive ions will
be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the
positive side (the anode). These charges are collected by the anode and cathode which then form a
very small current in the wires going to the detector. By placing a very sensitive current measuring
device between the wires from the cathode and anode, the small current measured and displayed as a
signal. The more radiation which enters the chamber, the more current displayed by the instrument.
Many types of gas-filled detectors exist, but the two most common are the ion chamber used for
measuring large amounts of radiation and the Geiger-Muller or GM detector used to measure very
small amounts of radiation.
The second most common type of radiation detecting instrument is the scintillation detector. The basic
principle behind this instrument is the use of a special material which glows or “scintillates” when
radiation interacts with it. The most common type of material is a type of salt called sodium-iodide.
The light produced from the scintillation process is reflected through a clear window where it interacts
with device called a photomultiplier tube.
The first part of the photomultiplier tube is made of another special material called a photocathode.
The photocathode has the unique characteristic of producing electrons when light strikes its surface.
These electrons are then pulled towards a series of plates called dynodes through the application of a
positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are
produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards
the next dynode, where more electron “multiplication” occurs. The sequence continues until the last
dynode is reached, where the electron pulse is now millions of times larger then it was at the
beginning of the tube. At this point the electrons are collected by an anode at the end of the tube
forming an electronic pulse. The pulse is then detected and displayed by a special instrument.
Scintillation detectors are very sensitive radiation instruments and are used for special environmental
surveys and as laboratory instruments.
Terms Related to Radiation Dose
Chronic dose … means a person received a radiation dose over a long period of time.
Acute dose … means a person received a radiation dose over a short period of time.
Somatic effects … are effects from some agent, like radiation that are seen in the individual who receives
the agent.
Genetic effects … are effects from some agent, that are seen in the offspring of the individual who
received the agent. The agent must be encountered pre-conception.
Teratogenic effects … are effects from some agent, that are seen in the offspring of the individual who
received the agent. The agent must be encountered during the gestation period.
Stochastic effects … are effects that occur on a random basis with its effect being independent of the size
of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the
effect increasing with dose. Cancer is a stochastic effect.
Non-stochastic effect … are effects that can be related directly to the dose received. The effect is more
severe with a higher dose, i.e., the burn gets worse as dose increases. It typically has a threshold, below
which the effect will not occur. A skin burn from radiation is a non-stochastic effect.
PET
In clinical applications, a very small amount of labelled compound (called
radiopharmaceutical or radiotracer) is introduced into the patient usually by
intravenous injection and after an appropriate uptake period, the concentration of
tracer in tissue is measured by the scanner. During its decay process, the radionuclide
emits a positron which, after travelling a short distance (3-5 mm), encounters an
electron from the surrounding environment. The two particles combine and
"annihilate" each other resulting in the emission in opposite directions of two
gamma rays of 511 keV each.
The image acquisition is based on the external detection in coincidence of the emitted
gamma-rays, and a valid annihilation event requires a coincidence within 12
nanoseconds between two detectors on opposite sides of the scanner. For accepted
coincidences, lines of response connecting the coincidence detectors are drawn
through the object and used in the image reconstruction. Any scanner requires that
the radioisotope, in the field of view, does not redistribute during the scan. A tissue
attenuation correction is performed by recording a short transmission scan using gammarays from three radioactive (Germanium-68/Gallium-68) rotating rod sources.
NMR (MRI)
Nuclear Magnetic Resonance (NMR) Spectroscopy
In NMR, EM radiation is used to "flip" the alignment of nuclear spins from the low energy spin aligned state to the
higher energy spin opposed state. The energy required for this transition depends on the strength of the applied
magnetic field (see below) but in is small and corresponds to the radio frequency range of the EM spectrum.
Nuclei with an odd mass or odd atomic number have "nuclear spin" (in a similar fashion to the spin of
electrons). This includes 1H and 13C (but not 12C). The spins of nuclei are sufficiently different that NMR
experiments can be sensitive for only one particular isotope of one particular element. The NMR behaviour
of 1H and 13C nuclei has been exploited by organic chemist since they provide valuable information that can
be used to deduce the structure of organic compounds. These will be the focus of our attention.
Since a nucleus is a charged particle in motion, it will develop a magnetic field. 1H and 13C have nuclear
spins of 1/2 and so they behave in a similar fashion to a simple, tiny bar magnet. In the absence of a
magnetic field, these are randomly oriented but when a field is applied they line up parallel to the applied
field, either spin aligned or spin opposed. The more highly populated state is the lower energy spin state
spin aligned situation. Two schematic representations of these arrangements are shown below:
Computed Tomography Imaging (CT Scan, CAT Scan)
Computed Tomography is based on the x-ray principal: as x-rays pass through the
body they are absorbed or attenuated (weakened) at differing levels creating a matrix
or profile of x-ray beams of different strength. This x-ray profile is registered on film,
thus creating an image. In the case of CT, the film is replaced by a banana shaped
detector which measures the x-ray profile.
Computed Tomography (CT) imaging, also known as "CAT scanning" (Computed Axial
Tomography), combines the use of a digital computer together with a rotating x-ray
device to create detailed cross sectional images or "slices" of the different organs and
body parts such as the lungs, liver, kidneys, pancreas, pelvis, extremities, brain, spine,
and blood vessels. For many patients, CT can be performed on an outpatient basis
High resolution axial
CT image of the inner
ears and sinuses. A
large polyp in the right
sinus (arrow) can be
seen
Among the various imaging techniques such as MR and x-ray, CT has the unique
ability to image a combination of soft tissue, bone, and blood vessels. For
example, conventional x-ray imaging of the head can only show the dense bone
structures of the skull. X-ray angiography of the head only depicts the blood
vessels of the head and neck and not the soft brain-tissue. Magnetic resonance
(MR) imaging does an excellent job of showing soft tissue and blood vessels, but
MR does not give as much detail of bony structures such as the skull. CT images
of the head allow physicians to see soft-tissue anatomic structures like the brain's
ventricles or gray and white matter. Physician then can selectively "window" the
digital CT images on the computer monitor to look at the soft tissue, then the
bone and then the blood vessels, as needed.
NUCLEAR MEDICINE
- highly specialized on detection and diagnosis
of functional disturbances, the morphology is
mostly secondary
Main advantages ot this method are :
Radioactive isotopes introduced into an organism
are distinguishable by their radiation from the
atoms already present. This permits the relatively
simple acquisition of information about the
dynamics of processes of uptake, incorporation,
exchange, secretion, etc.
The tracer method is extremely sensitive. In
principle even the presence of only one atom can
be detected.
The high sensitivity allows the study of various
processes with amounts of substances so small
that they have no influence on the life processes.
“In vivo methods”
1)
labeled molecules and compounds, which behave virtually
identically to the unlabelled ones in the various chemical,
biochemical and biological processes
2)
radioactive isotopes form compounds in the same way like as
the stable isotopes
3)
isotopes disclose their presence by their radiation, and thus
their movement and fate can be traced
For these purposes are used radionuclides that emit
electromagnetic waves ( rays) but don’t emit any particle
(,  or neutron).
4)
Radiopharmaceuticals

the most widely used radioisotope is Tc, with
a half-life of six hours.

activity in the organ can then be studied
either as a two dimensional picture or, with a
special technique called tomography, as a
three dimensional picture (SPECT, PET)
Thank you for your attention