Nuclear Medicine Instrumentation

1. Nuclear Medicine Instrumentation

2. Radionuclides Isotopes Half-life Energy (keV) main decay 99mTc 6.03 hrs 140 I.T. 131I 8.05 days 364 ?? 125I 60.2 days 35 E.C. 123I 13.0 hrs 160 E.C. 201Tl 73.0 hrs 135, 167 E.C. 111In 67.2 hrs 247, 173 E.C. 67Ga 78.1 hrs 300, 185, 93 E.C. 127Xe 36.0 days 172, 203, 375 E.C. 133Xe 5.31 days 81 ??

3. Photon-Matter Interaction Photoelectric effect entire energy converted into kinetic energy high Z material, ? ? Z4E-3 Compton scattering part of its energy converted into kinetic energy proportional to electron density, ? ? ZE-1 predominant interaction in tissue, ( Z ? ? )

4. Attenuation Effect Ina = I0 exp { -?????d?} ? : both photoelectric effect & Campton scatter

5. Gamera camera

6. Collimators

7. Collimator Select the direction of photons incident on camera defining the integration paths Types: parallel slanted parallel fan-beam cone-beam varifocal cone-beam pinhole convergent divergent

8. Parallel Collimator Resolution : Rc = S (1+L/H) ? ?L, ? = S/H Distance dependent (DDSR) Sensitivity : g ? Rc2/L2 = ?2 (S(S+T))2 Septa penetration not considered

9. Resolution v.s. Distance Septal thickness T is determined by photon energy low-energy collimator < 150 keV medium-energy collimator < 400 keV

10. Typical Performance Characteristics

11. Scintillator (inorganic) Convert a gamma-ray photon to light photons for subsequent processing by the PMTs A large flat NaI (Tl) crystal (eg., 20�x15�) Issue: sensitivity vs. resolution Thickness: 1/4� ~ 3/8� The thicker the crystal, the better the sensitivity but the worse (larger) the resolution.

12. NaI properties 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

13. Inorganic Scintillators (Crystals)

14. Crystal vs. Light yield

15. Detector response vs. Energy resolution Output signal amplitude proportional to energy deposited in the scintillator Energy resolution = 100% ? ????? Complete electron transfer (ideal condition)

16. Photofraction (real condition) Spreading due to Poisson effect

17. Factors affecting Energy resolution: Counting statistics + Electronic noise Causes uncertainty in measured deposited energy Poisson Statistics

18. Factors affecting Energy resolution: 1. Incomplete energy transfer Detector size Attenuation effect: density, effective Z number 2. Pile-ups & Baseline shifts

19. Pile-up and Baseline shift Problems occurs at high counting rates Both can be reduced by decreasing the pulse width, but this also increases the electronic noises, thus degrading energy resolution. Baseline shift: 2nd pulse occurring during the negative components of the 1st pulse will have depressed amplitude Shift in the energy of the 2nd event Corrected by pole zero cancellation or baseline restoration Pile-up: Two or more pulses fall on top of each other to became one pulse Incorrect energy information Lost events

20. What is measured ? 2D vs 3D

21. Light guide

22. PMTs Convert a light photon to electrical charges

23. Pulse Processing: Pre-Amplifying Preamplifier (preamp): To match impedance levels to subsequent components To shape the signal pulse (integration) RC = 20~200�s To (sometimes) amplify small PMT outputs Should be located as close as possible to the PMT

24. Pulse Processing: Amplifier Amplifier To amplify the still relatively small signal Perform pulse shaping Convert the slow decaying pulse to a narrow one To avoid pulse pile-ups at high counting rates

25. Positioning logic (Anger)

26. Anger Positioning logic Position determination X ? k (X++X-)/Z Y ? k (Y++Y-)/Z A PHA (pulse height analyzer) is to select for counting only those pulses falling within selected amplitude intervals or �channels� A SCA (single channel analyzer) is a PHA having only one channel:

27. Analog System

28. Digital System

29. PSPMT position sensitive PMT essentially light guide is not necessary perform multi-positioning within one PMT

30. SPECT scanner Multi-head systems: 1. Provide higher sensitivity 2. Allow simultaneous emission and transmission scans 3. More expensive

31. Performance Characteristics: Image Non-linearity straight lines are curved X and Y signals do not change linearly with the distance of the detected events variations in PMT collection efficiency acrossing its aperture variations in PMT sensitivity non-uniformities in optical coupling, etc. Image Non-uniformity flood field-image shows variations in brightness non-uniform detection efficiency and nonlinearities differences in pulse-height spectrum of the PMTs

32. Performance Characteristics: Spatial Resolution overall resolution R2 = Ri2 + Rc2 affecting image contrast and visualization of small structures introduce bias intrinsic resolution Ri crystal thickness (light distribution) crystal density, effective Z number (multiple scattering) light yield (statistical variations in pulse heights) degraded with decreasing g-ray energy (light yield) improves with increased light collection and detection efficiency improves with image uniformity and digital positioning expected resolution limit for NaI (Tl) = 2mm collimator resolution Rc collimator design source to collimator distance

33. Performance Characteristics: cont�d Detection Efficiency: Crystal thickness, density, effective Z number almost 100% at up to 100 keV, but drops rapidly with increasing energy to about 10~20% at 500 keV Collimator efficiency affecting image noise introduce variance while quantitative studying 100 ~ 200 keV is the best optimal energy of Anger camera (g-ray) at low energy, deteriorating spatial resolution at high energy, deteriorating detection efficiency

34. Performance Characteristics: Count rate: Mis-positioning baseline shift pile-up simultaneous detection of multiple events at different locations dead time 0.5~5?s behaves as nonparazable model: 2nd event ignored if it occurs during the deadtime of the preceding events

35. 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

36. Newer developments: Coincidence Imaging (PET like) Low cost Poor sensitivity and resolution g ray septa penetration Simultaneous Transmission and Emission Imaging Registered attenuation map Spill-down scatters from the transmission source Truncation error remains unsettled ��������.. Dual Isotope Imaging Increase diagnosis specificity Issues: spill-down scatters from high to low energy window

37. Newer developments: cont�d Small-animal gamma camera Small FOV, higher resolution Depth-of-interaction (DOI) detectors Better spatial resolution Allow use of thicker NaI crystal Semi-conduction imager Converts g ray directly into electrical signals Promising candidate: CdZnTe detector Novel designs Scintimammography Placed closer to the source by odd geometry Optimizing resolution & sensitivity

38. Newer developments: cont�d Novel designs CERESPECT A single fixed annular NaI (Tl) crystal completely surrounding the patient�s head A rotating segmented annular collimator Modular systems: SPRINT II brain SPECT 11 modules in a circular ring around the patient�s head, each module consists of 44 one-dimensional bar NaI (Tl) scintillation camera Rotating split or focused collimators FASTSPECT A hemispherical array of 24 modules for brain imaging Each module views the entire brain through one or more pinholes Stationary system, easy dynamic imaging