Dec 7th PET/SPECT

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Transcript of Dec 7th PET/SPECT

  • 1.Nuclear medicine Pet/Spect Chapters 18 to 22

2. Activity Number of radioactive atoms undergoing nuclear transformation per unit time. Change in radioactive atoms N in time dt Number of radioactive atoms decreases with time (- minus sign) = 3. Activity Expressed in Curie 3.7x1010 disintegrations per second dps Becquerel discovers natural radioactive materials in 1896 the SI unit for radioactivity is the Becquerel. 1 becquerel = 1dps = 4. Nuclear medicine Therapeutic and diagnostic use of radioactive substances First artificial radioactive material produced by the Curies 1934 Radioactivity, Radioactive 5. Definitions: Nuclide Nuclide: Specie of atoms characterized by its number of neutron and protons Isotopes Isotones Isobars () 6. Definitions: Nuclide Isotopes are families of nucleide with same proton number but different neutron number. Nuclides of same atomic number Z but different A same element A ZX A mass number, total # of protons and neutrons Z atomic number (z# protons) 6 12 6 13 7. Definitions: Nuclide Radionuclide: Nuclide with measurable decay rate A Radionuclide can be produced in a nuclear reactor by adding neutrons to nucleides 59 Co + neurtron -> 60 Co 8. Radioactive Decay Disintegration of unstable atomic nucleus Number of atoms decaying per unit time is related to the number of unstable atoms N through the decay constant () = 9. Radioactive Decay Radioactive decay is a random process. When an atom undergoes radioactive decay -> radiation is emitted Fundamental decay equation (Number of radioactive atoms at time t -> Nt = 0 10. Radioactive Decay Father and daughter. Is Y is not stable will undergo more splitting (more daughters) 2 3 Father Daughter 11. Radioactive Decay Processes 12. Radioactive Decay Processes 13. Alpha decay Spontaneous nuclear emission of particles particles identical to helium nucleus -2 protons 2 neutrons particles -> 4 times as heavy as proton carries twice the charge of proton 2 4 + 2 4 +2 + 14. Alpha decay Occurs with heavy nuclides Followed by and characteristic X ray emission Emitted with energies 2-10MeV NOT USED IN MEDICAL IMAGING 2 4 + 2 4 +2 + 15. Positron emission + Decay caused by nuclear instability caused by too few neutrons Low N/Z ratio neutrons/protons A proton is converted into a neutron with ejection of a positron and a neutrino Z A X Z 1 A Y + + + +energy positron neutrino 16. Positron emission + Decrease of protons by 1 atom is transformed into a new element with atomic # Z-1 The N/Z ratio is increased so daughter is more stable than parent Z A X Z 1 A Y + + + +energy positron neutrino 17. Positron emission + 9 18 8 18 + + + + Fluorin oxygen 18. Positron emission + 9 18 8 18 + + + + Fluorin oxygen 19. Positron emission + Positron travels through materials loosing some kinetic energy When they come to rest react violently with their antiparticle -> Electron The entire rest mass of both is converted into energy and emitted in opposite direction Annihilation radiation used in PET 20. Annihilation radiation Positron interacts with electron->annihilation Entire mass of e and + is converted into two 511keV photons 511keV energy equivalent of rest mass of electron 21. - decay Happens to radionuclide that has excess number of neutron compared to proton A negatron is identical to an electron Antineutrino neutral atomic subparticle +1 + + + 22. Electron captive Alternative to positron decay for nuclide with few neutrons Nucleus capture an electron from an orbital (K or L) + 1 + + 23. Electron captive Nucleus capture an electron from an orbital (K or L) Converts protons into a neutron ->eject neutrino Atomic number is decreased by one new element + 1 + + 24. Electron captive As the electron is captured a vacancy is formed Vacancy filled by higher level electron with Xray emission Used in studies of myocardial perfusion 81 201 89 201 + + 25. Isomeric transition During a radioactive decay a daughter is formed but she is unstable As the daughter rearrange herself to seek stability a ray is emitted + 26. Principle of radionuclide imaging Introduce radioactive substance into body Allow for distribution and uptake/metabolism of compound Functional Imaging! Detect regional variations of radioactivity as indication of presence or absence of specific physiologic function Detection by gamma camera or detector array (Image reconstruction) 27. Radioactive nuclide Produced into a cyclotron Tagged to a neutral body (glucose/water/ammonia) Administered through injection Scan time 30-40 min 28. Positron Emission Tomography Tomography? 29. Positron emission + 9 18 8 18 + + + + Fluorin oxygen 30. Cancer detection Examine changes due to cancer therapy Biochemical changes Heart scarring & heart muscle malfunction Brain scan for memory loss Brain tumors, seizures Lymphoma melanoma PET Positron emission tomography 31. Principles Uses annihilation coincidence detection (ACD) Simultaneous acquisition of 45 slices over a 16 cm distance Based on Fluorine 18 fluorodexyglucose (FDG) 32. PET Ring of detectors surrounds the patient Obtains two projection at opposite directions Patient is injected with a 18 fluorine fluorodeoxyglucose (FDG) 33. Pet principle Ring of detectors 34. Annihilation radiation Positron travel short distances in solids and liquids before annihilation Annihilation COINCIDENCE -> photons reach detectors, we collect the photons that happen almost at the same time coincidence? I dont think so! Detector 1 Detector 2 35. True coincidence Detector 1 Detector 2 36. Random coincidence Emission from different nuclear transformation interact with same detector Detector 1 Detector 2 37. Scatter coincidence One or both photons are scattered and dont have a simple line trajectory Detector 1 Detector 2 False coincidence 38. Total signal is the sum of the coincidences Ctotal = Ctrue +Cscattered+ Crandom 39. PET noise sources O T S A ij ij ij ijC C C C= + + Noise sources: Accidental (random) coincidences Scattered coincidences Signal-to-noise ratio given by ratio of true coincidences to noise events Overall count rate for detector pair (i,j): 40. Pet detectors NAI (TI) Sodium iodide doped with thallium BGO bismuth germanate LSO lutetium oxyorthosilicate 41. PET resolution Modern PET ~ 2-3 mm resolution (1.3 mm) MRI PET 42. PET evolution 43. SPECT Single photon emission computed tomography rays and x-ray emitting nuclides in patient 44. SPECT cnt One or more camera heads rotating about the patient In cardiac -180o rotations In brain - 360o rotations It is cheaper than MRI and PET 45. SPECT cnt 60-130 projections Technetium is the isothope Decays with ray emission Filtered back projection to reconstruct an image of a solid 46. Typical studies Bone scan Myocardial perfusion Brain Tumor 47. Scintillation (Anger) camera 1. Enclosure 2. Shielding 3. Collimator 4. NI(Tl) Crystal 5. PMT Imaging of radionuclide distribution in 2D Replaced Rectilinear Scanner, faster, increased efficiency, dynamic imaging (uptake/washout) Application in SPECT and PET One large crystal (38-50 cm-dia.) coupled to array of PMT 48. Anger logic Position encoding example: PMTs 6,11,12 each register 1/3 of total Photocurrent, i.e.: I6 = I11 = I12 = 1/3 Ip Total induced photo current (Ip) is obtained through summing all current outputs Intrinsic resolution ~ 4 mm 49. L d Collimators Purpose: Image formation (acts as optic) Parallel collimator Simplest, most common 1:1 magnification Resolution Geometric efficiency Tradeoff: Resolution Efficiency ( )2a L z b R L + + = 2 4 open open unit A A G d A = Aopen Aunit 50. Collimator types Tradeoff between resolution and field-of view (FOV) for different types: Converging: resolution, FOV Diverging: resolution, FOV Pinhole (~ mm): High resolution of small organs at close distances Diverging L d d Converging L d 51. SPECT applications Brain: Perfusion (stroke, epilepsy, schizophrenia, dementia [Alzheimer]) Tumors Heart: Coronary artery disease Myocardial infarcts Respiratory Liver Kidney