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Comprehensive Overview of Positron Emission Tomography Annihilation

Understand the science and technology of PET scanning, annihilation process, radionuclides, coincidence detection, system configurations, crystal properties, and image reconstruction techniques.

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Comprehensive Overview of Positron Emission Tomography Annihilation

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  1. Positron Emission Tomography

  2. Annihilation • The ejected positron (e+) annihilates with an electron (e-) of the tissue after traveling a short distance The PSF due to positron range has a very long ditail Positron range Molecule in tissue Positron Emission e- Annihilationevent e+ 18FDG

  3. Radionuclides in PET Isotope Half-life Energy Positron range 15O 2.0 mins 1.72 MeV 0.7 mm 13N 10.0 mins 1.19 MeV 0.5 mm 11C 20.5 mins 0.96 MeV 0.3 mm 18F 1.8 hrs 0.635 MeV 0.2 mm 81Rb 4.58 hrs

  4. Annihilation Photons • On annihilation, two 511 keV annihilation photons are emitted at almost opposite directions Eγ = mec2 = 511 keV 180∘± 0.25∘ Annihilation photon e- Annihilation photon e+ 18FDG

  5. Coincidence Detection 180∘± 0.25∘ detector detector e- e+  Line of response (LOR) 18FDG t0 t0-t1  t1 t1 t0

  6. True Coincidences dp ( r ) = a ( r ) 1 2e -L1( r’) dl e -L2( r’) dl = a ( r ) 1 2e -L1+ L2( r’) dl m = a ( r ) 1 2e -L1+ L2( r’) dla ( r) dl L1+L2 (Summation) L2 detector detector 1 2 L1 : As detector efficiency

  7. Scatter Coincidences • At least one annihilation photon experiences (Compton) scattering before detection True LOR Assigned LOR

  8. Scatter Coincidences (Cont’s) • Contribute to a low-resolution background • Degraded image contrast and spatial resolution • Depend on: • Attenuation distribution • System configuration • energy resolution of the detector • BGO detector 350keV scatter window : 10-15% scatter fraction • Difficult to correct

  9. Random Coincidences • Simutaneous detection, as defined by the coincidence window, of two uncorrelated annihilation photons by change Assigned LOR

  10. Random Coincidences (Cont’s) • Contribute to a smooth background • Correction possible: • estimated from single rates: R = 2 C1 C2 • using delayed lines • Depend on: • Coincidence resolving time  • Detector material Detector 2 Coincidence detection Detector 1 delay

  11. Detector Design Four 1” Square PMT Light guide BGO 8 × 8 PMT BGO crystal block Sawed inot 64 seg., each 6mm square Individual coupling Block detector

  12. Deposited Energy Photo-peak or full-energy peak dN/dE Compton continuum () 511 keV E Compton edge ()

  13. Energy Resolution Photopeak Counts Energy (keV) 350 511

  14. Physical Factors Affecting Resolution • Positron range FWHM = s, related to isotope • Photon noncolinearity FWHM = 0.0022D, D = ring diameter • Detector width, d • Intercrystal scatter • Light sharing + positron logic FWHM  1.25 √(d/2)2 +s2 + (0.0022D) 2 + b2 b  2mm for block detectors, 0mm for individual coupled detectors JNM, 34,101, 1993

  15. Scintillation Crystal • detection efficiency for 511keV photons • effective Z number • density • photoelectric effect preferred • light yield • energy resolution • scatter rejection • scintillation decay constant • timing resolution • reducing randoms • TOF (time-of flight) PET • cost, mechanic properties, refractive index, etc.

  16. Scintillation Crystal • Stopping power: • Effective atomic number (Iodine:53, relatively high) • Density: 3.76 g/cm3 • Light yield: 38 photons/keV (4 eV/per photon) • Good light yield, used as reference = 100 • Energy resolution (Poisson statics) • no. generated proportional to deposited energy • 15% scintillation Efficiency • Light decay constant: 230s after glow • Dead time • Position mis-positioning • Wavelength at max. emission: 415 nm • Reflective index: 1.85 • Hygroscopic, relatively fragile

  17. Common Inorganic Crystals

  18. Crystal vs. Light yield NaI (Tl) Light yield CsI (Tl) CsI (Na) 420 565 410 Wavelength (nm)

  19. Parallax Errors • Point source 下，detector width 小、detector length 長，則 resolution 好。 • Eccentric point source 下，則會有 radial & tangential projections 影響 resolution。radial 投射尤其不利。 Positron source Tangential projection Radial projection

  20. Parallax Errors

  21. Noise Equivalent Count (NEC) 3.0 105 2.5 105 2.0 105 NEC 1.5 105 1.0 105 NEC = T2 / (T+S+R) 5.0 104 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Activity [uCi/ml]

  22. PET Imaging Model Continuous Model: R = Rt + Rs + Ra Rt = α a(x,y,z) h(x,y,z) dx dy dz y = Rt + n Discrete Approximation: y = Ha + n α: attenuation n : Poison noise H : system geometry y : known data a : unknown data

  23. PET Image Reconstruction • 1. Filtered Backprojection (FBP) • 2. Iterative Methods • EM, OSEM • Bayesian • PWLS

  24. System Configurations • Multi-ring systems : usually BGO crystal • PENN PET : NaI crystal in 6 heads • C PET : NaI crystal in 2-3 curved heads • Time-of flight PET : (TOF PET) : BaF2 crystal • ultra-short decay constant • SPECT for coincidence imaging

  25. What Use PET (from LPP) • The basis of all tissue function is chemical. • Diseases result from errors introduced into chemical by viruses, bacteria, genetic abnormalities, drugs, environmental factors, aging and behavior. • The most selective, specific, and appropriate therapy is one chosen from a diagnostic measure of the basic chemical abnormality. • Detection of chemical abnormalities provides the earliest identification of disease, even in a pre-symptomatic stages before the disease process has exhausted the chemical reserves or over-ridden the compensatory mechanisms of the brain. • Assessment of restoration of chemical function provides an objective means for determining the efficacy of therapeutic interventions in the individual patient. • The best way to judge whether tissue is normal is by determining its biochemical function.

  26. SPECT reconstruction: • Issues: attenuation, scatter, noise, DDSR, sampling geometry • Filtered Backprojection (FBP) • ignore attenuation, DDSR • usually no scatter correction • ad hoc smoothing for controlling image noise • Iterative Reconstruction • OSEM • allow attenuation, and DDSR corrections • optimal noise control • usually no scatter correction • needs attenuation map • Analytical approaches uniform attenuation • Simultaneous Emission, Attenuation map Reconstruction • Dynamic SPECT by interpolation vs. timing

  27. Advantage of PET over SPECT • Simple model • attenuation • not DDSR (distance dependent systemic resolution) • Higher sensitivity • electronic collimation • large solid angle of detection • Better tracers availability

  28. New Trends (3D PET) • remove septa to allow coincidence detection between detectors at different rings • advantage: • increase detection sensitivity • performance issue: • increased scatter • increased randoms • parallax errors in axial direction • reconstruction: • FORE (Fourier Rebinning) • 3D iterative methods • hybrid

  29. New Trends (DOI Detectors) Depth-of-Interaction (DOI) Detectors • detectors capable of providing depth information • used for reducing parallax errors • increased sensitivity • reduced ringer diameter • current depth resolution: 5~10mm

  30. New Trends (DOI Detectors) • Images of a point source displaced 10cm from the center with 3mm × 3mm × 30mm crystals 5mm DOI resolution no DOI information

  31. New Trends (DOI Detector 1) 1” Square Photomultiplier Tube Array of 64 photodetectors 1” 64 BGO crystals each 3mm square 30 mm Block detector 1”

  32. New Trends (DOI Detector 2) LSO GSO or LSO PMT pulse shape discrimination circuit decay constant : depends on crystal 中雜質 Phoswich detector ( by CTI System )

  33. New Trends (DOI Detector 3)

  34. New Trends (TOF Detector) • Time-of-Flight PET Block detector employing light sharing 2D Wire Chamber Readout 2” Gamma to Electron Converter BaF2/TMAE or Metal (Pb or W) Foils 8-16mm Block detector 2”

  35. New Trends (DOI PET system) • Siemens/CTI HERRT System 31.2 cm 46.9 cm Crystal length: 7.5mm × 2

  36. New Trends (DOI PET System) LSO (7.5 mm) GSO (7.5 mm) light guide PMT PMT 19.5 mm crystal block Rows Columns

  37. New Trends (Small-Animal Systems) • dedicated for small-animal studies • gene expression, gene transfer • drug effects • basic physiology, etc. • requirements: • high resolution • high sensitivity • configuration: • small diameter • elongated detectors

  38. New Trends (Small-Animal Systems)

  39. New Trends (gene expression)

  40. New Trends (gene expression)

  41. New Trends (gene expression)

  42. New Trends (drug effects)

  43. New Trends (basic physiology)

  44. New Trends (Small-Animal System 1) 2mm × 2mm × 10mm LSO 17.2cm diameter detector ring 110mm transaxial FOV, 18mm axial FOV UCLA

  45. New Trends (Small-Animal System 1) 64 mm3 20 Volume resolution in mm Rx × Ry × Rz 15 Rz Ry 10 Rx 8 mm3 8 mm3 5 0 microPET EXACT HR+ 0 10 20 30 40 50 Offset in mm UCLA

  46. New Trends (Small-Animal System 1) 20 Phantom size vs Scatter 15 cps (×105) NEC Medium (350-650 keV) NEC Large (350-650 keV) NEC Small (350-650 keV) 10 5 0 0 5 10 15 20 25 Activity (uCi/cc)

  47. New Trends (Small-Animal System 2) Germany

  48. New Trends (Small-Animal System 2) Tier PET • YAP scintillator • 2mm × 2mm × 15mm crystal • variable detector-to detector distance • transaxial FOV: 40mm • axial FOV: 40mm • resolution: 2.1mm Germany

  49. New Trends (Compact Systems) • systems capable utilizing the entire space inside the detector ring for imaging • Current systems utilization is about 60-70% • advantages: • improved sensitivity • improved resolution • reduced cost • disadvantages: • increased scatter • increased randoms

  50. New Trends (Compact System 1) 85.9mm FOV FOV 57.3mm 56.3mm 56.3mm RFOV = 56.3mm Compact Conventional

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