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Gamma-camera SPECT PET 17.04.2012. Gamma radiation • Electromagnetic radiation (f>1019Hz, E>100keV (1.6*10-14J), λ<10-12m) • gamma-decay (the atomic nucleus can get into a more stable state by photon emission) - α- and β-decay frequently accompanied by gamma radiation • Ionizing radiation (the energy is used to remove (detach) the electrons from the atoms) • Discovered by Paul Ulrich Villard in 1900, french chemistphysicis • 1903 - Ernest Rutherford used its name at first • radiation emitted by the atomic nucleus ↔ x-ray: radiation emitted by the atom due to electron transitions 1 γ-ray and matter interaction • photoelectric effect – photon (E<50keV) + electron → ejected electron • Compton effect (scattering) – photon (E1:100 keV-10 MeV) + electron → photon (E2<E1, altered direction) + ejected electron • Pair production – photon (E>1.02MeV=2*0.51MeV) + the electric field of the atomic nucleus → electron-positron pair (higher than 1.02MeV → Ekinetic) How to produce the γ-ray • together with α- and β-decay • relaxation of nuclear isomer (a metastable state of an atomic nucleus caused by the excitation of one or more of its protons or neutrons) – Co → Ni* → Ni – 99mTc → 99Tc 2 Isotopes • Isotope = same place in the periodic table • atoms having the same atomic number (p+) but different mass numbers (p+ + n0) • Similar chemical and biological properties • Different physical properties (radioactive isotopes) • pl. Protium: 1H (H) - 99,985 % 2H Deuterium: (D) Tritium: 3H (T) (1p + Øn + 1e) (1p + 1n + 1e) (1p + 2n + 1e) George Charles de Hevesy (1885–1966) • Hungarian radiochemist • Discovered that the radioactive isotopes can be traced (1913) • Nobel prize in chemistry (1943) for his work with radioactive tracers Radioactive tracing technique – Investigating metabolic processes with radioactive isotopes by replacing part of stable isotopes with small quantities of the radioactive isotopes (same biology different physics). – radiopharmacon: radioactive isotope + biologically relevant (active) molecule 3 Gamma-camera Gamma photon detecting device to image gamma radiation emitting radioisotopes (2D image). Scintigraphy (radioactive tracer + scintillation counter) Detecting gamma-ray emitting isotopes. • 99mTc (metastable Technetium isotope) only emmits gamma ray half-life ~ 6 hours PMT Scintillation crystal (NaI; CsF; BaF2; Bi4Ge3O12) scintillation = a spark, a flash Scintillation crystal Collimator (lateral resolution!) can turn radiation into parallel (non-divergent) beams Gamma-camera • disadvantages: – Low sensitivity (high loss on the collimator). – Bad spatial resolution (1.8 cm / 5 cm (distance from the detector)). • advantage: relatively cheap Spatial resolution: the minimum distance between distinguishable objects in an image 4 SPECT – Single Photon Emisson Computed Tomography Imaging technique that works with gamma camera which is capable to record 2D images from different angles (360°). A computer will reconstruct a 3D image. Result: 3D image. Based on the usage of gamma decaying isotopes (99mTc; 123I; 131I; 133Xe). Acqusition time ~ 15-20 minutes (can be better with a multidetector system) disadvantage: bad spatial resolution (~1cm) Advantages: • relatively cheap • functional information can be gained Usage: - monitoring the the myocardial function (CO; MI) - monitoring cerebral functions Weak points of SPECT • Low sensitivity (high loss on the collimator). • Bad spatial resolution (~1cm) • Long acquisition time (positioning of the detector). • Exposure! – Gamma ray - burnes, tumor, mutations 5 PET Positron Emission Tomography – computertomography History • 1973 – St. Louis (Missouri, USA) Washington University Edward J. Hoffman & Michael Phelps The first PET scanner 6 Definition • 3D imaging technique that can monitor functional changes within the living systems by using positron emitting isotopes. • Qualitative analysis of the metabolic processes! positron • Elementary particle: not to be made up of smaller particles. Subatomic particle. • Antimatter of electron (same weight (9.1 x 10-31kg), same energy (0.51MeV = 8.2 x 10-14 J), same charge with different (opposite) polarity (+1.6 x 10-19C). • The first proven antimatter. • Chung-Yao Chao (student at California Institute of Technology – 1930): the first scientist who captured the positively charged electron (positron) • The egsistence of positrons was suggested at first by Paul Dirac in 1928. • Carl D. Anderson – 1932: the first who called it as positron. • production: - pair production - positron emission (+ β decay) • Application: PET. 7 Origin of positrons • Pair production (↔annihilation) • β-decay Generating positrons I. Pair production (↔annihilation) e- Atomic nucleus photon e+ • Ephoton > 1.022 MeV Annihilation: opposite of pair production - 1e- + 1e+ = 2 gamma photon (180°!) Latin nihil = nothing 8 Generating positrons II. β+ radiaton (nuclear decay) 1 0 n →11p + + 00e − + ν ( antineutrino ) 1 1 p + →01n + 00e + + ν ( neutrino ) 137 55 22 11 − Cs→137 56 Ba + e + ν 22 Na →10 Ne + e + + ν β+ radiation emitting atoms 11 6 13 7 15 8 18 9 C N O F 124 53 82 37 I isotopes Half life (t1/2) carrier What can be followed usage C-11 20 min. Amino acids Amino acid metabolism Metabolic changes, protein synthesis, tumors N-13 2 min. ammonia Blood perfusion Viability of the myocardium O-15 10 min. O-15 labeled gases Oxygen transport Oxygen metabolism F-18 110 min. Deoxiglucose (FDG) Glucose metabolism Metabolic changes, tumors I-124 4.15 day Tumor detection Testing the Thyroid Rb-82 25 day Perfusion of the myocardium Coronary vessel diseases Rb 9 Conditions • PET machine (~3 million Euros) • Cyclotron (particle accelerator) to produce the isotopes (~3 million Euros) • Onkology center (85-90% oncological patient)?!?!? Basic units of the PET laboratory PET camera cyclotron isotopes synthesis of the radiopharmacon injection Data acquisition 10 Basics of PET Detector ring t<20ns photomultiplier Scintillation crystal CsFBaF2Bi4Ge3O12- 180o 0.511MeV annihilation Basics of PET Detector ring t<20ns 180o 0.511MeV annihilation 11 What is it good for? • diagnosis • Checking the efficiency of the therapy Indications • Oncology – Lung cancer, breast cancer, …. • cardiology – Myocardial viability, myocardial perfusion • Neuropsychiatry – epilepsy, dementia, Parkinson-disease, Huntington-disease 12 Advantages and disadvantages of PET • • • • sensitive, specific selective (radiopharmacon) tomography technique information about functional changes within the living systems (metabolic pathways) • early diagnosis (before the structural changes) • good spatial resolution (4-6mm) (better than SPECT) • no structural information → fusion techniques (PET-CT) • high cost (expensive to establish it (3+3 M Euros) and expensive to operate (800-1200€/investigation) • low accessibility • time consuming (preparation+measurement) The end! 13