Nuclear medicine Pet/Spect

1. Nuclear medicinePet/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 • AZX • A mass number, total # of protons and neutrons • Z atomic number (z# protons)

7. Definitions: Nuclide • Radionuclide: Nuclide with measurable decay rate • A Radionuclide can be produced in a nuclear reactor by adding neutrons to nucleides 59Co + neurtron -> 60Co

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 (l)

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

10. Radioactive Decay • Father and daughter. • Is Y is not stable will undergo more splitting (more daughters) Daughter Father

11. Radioactive Decay Processes

12. Radioactive Decay Processes

13. Alpha decay • Spontaneous nuclear emission of a particles • a particles identical to helium nucleus -2 protons 2 neutrons • a particles -> 4 times as heavy as proton carries twice the charge of proton

14. Alpha decay • Occurs with heavy nuclides • Followed by g and characteristic X ray emission • Emitted with energies 2-10MeV • NOT USED IN MEDICAL IMAGING

15. Positron emission b+ • 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

16. Positron emission b+ • 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

17. Positron emission b+ Fluorin oxygen

18. Positron emission b+ Fluorin oxygen

19. Positron emission b+ • 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. b- decay • Happens to radionuclide that has excess number of neutron compared to proton • A negatron is identical to an electron • Antineutrino neutral atomic subparticle

22. Electron captive e • Alternative to positron decay for nuclide with few neutrons • Nucleus capture an electron from an orbital (K or L)

23. Electron captive e • 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

24. Electron captive e • As the electron is captured a vacancy is formed • Vacancy filled by higher level electron with Xray emission • Used in studies of myocardial perfusion

25. Isomeric transition • During a radioactive decay a daughter is formed but she is unstable • As the daughter rearrange herself to seek stability a g 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 b Tomography?

29. Positron emission b+ Fluorin oxygen

30. PET Positron emission tomography • 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

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 don’t 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 don’t have a simple line trajectory Detector 1 False coincidence Detector 2

38. Total signal is the sum of the coincidences Ctotal = Ctrue+Cscattered+Crandom

39. PET noise sources • 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 MRI PET resolution • Modern PET ~ 2-3 mm resolution (1.3 mm)

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 • 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 • Enclosure • Shielding • Collimator • NI(Tl) Crystal • 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. d L Collimators • Purpose: Image formation (acts as “optic”) • Parallel collimatorSimplest, most common 1:1 magnification • Resolution • Geometric efficiency • Tradeoff: Resolution  Efficiency Aopen Aunit

50. Converging d L L d d Diverging 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