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Nuclear Medicine Instrumentation

Nuclear Medicine Instrumentation. Radionuclides. Isotopes Half-life Energy (keV) main decay 99m Tc 6.03 hrs 140 I.T. 131 I 8.05 days 364   125 I 60.2 days 35 E.C.

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Nuclear Medicine Instrumentation

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  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 Gamma rays Scattered (Campton) Absorbed (Photoelectric) Non-attenuated

  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 Rc/(H+L) = S/H    Rc  L L S Septa thickness = T H Rc

  9. Resolution v.s. Distance Collimators • Septal thickness T is determined by photon energy • low-energy collimator < 150 keV • medium-energy collimator < 400 keV High sensitivity resolution General purpose High resolution Rc Source to collimator distance

  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. Conduction band Ionization energy Activator excited states Activator ground state Valence band

  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 NaI (Tl) Light yield CsI (Tl) CsI (Na) 420 565 410 Wavelength (nm)

  15. Detector response vs. Energy resolution • Output signal amplitude proportional to energy deposited in the scintillator • Energy resolution = 100%   • Complete electron transfer (ideal condition) Non-scatter photon Scatter photon Count Eo/(1+2Eo) Eo Photon energy

  16. Photofraction (real condition) • Spreading due to Poisson effect Non-scatter photon Scatter photon Count FWHM Eo / (1 + 2Eo) Eo Photon energy

  17. Factors affecting Energy resolution: • Counting statistics + Electronic noise • Causes uncertainty in measured deposited energy • Poisson Statistics g(x) = Poisson ((x)) Prob (g(x)) = [(x) g(x)/g(x)!] exp(-(x)) f (n/) = n exp (- )/n! SNR {n/} =  E {n/} =  Var {n/} =  E {g(x)} = (x) Var {g(x)} = (x)

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

  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 vs3D attenuation distribution radioactivity distribution y –∫ (x, y’)dy’ (x) = ∫{a(x, y, z) * h (x, y, z) } e  dy + s (x , z) = Ε { (x , z) } DDSR attenuation factor scatter 期望值 y x Gamma Camera

  21. Light guide Scintillator Light Guide Light photon PMT photocathod

  22. PMTs • Convert a light photon to electrical charges light guide scintillator dynodes anode e– Output signal  106 e–’s photocathode light photon 10 ~ 12 dynodes 一般約 30% photons 可經 light guide 到 PMTs

  23. Pulse Processing: Pre-Amplifying • Preamplifier (preamp): • To match impedance levels to subsequent components • To shape the signal pulse (integration) • RC = 20~200μs • To (sometimes) amplify small PMT outputs • Should be located as close as possible to the PMT PMT C R 50μs Preamplifier 250 ns 500μs

  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 PreAmp Amplifier

  25. Positioning logic (Anger) Y+ …… X+ X+ X- X- Y+ Y- Position determination PMT array X = X+ + X- Y = Y+ + Y- Z = X+ + X- + Y+ + Y- Y-

  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: NaI (Tl) Detectors Positioning logic circuit SCA Z X Y ULD: upper level discriminator LLD: lower level discriminator PHA Gating signal A/D

  27. Analog System X/Z X SUM Collimator Crystal Y/Z Y SUM Z energy SUM Anger Registor matrix Summed analog outputs PMT array PreAMPs

  28. Digital System ADCPMT ADC PMT BUS SUM ADC SUM ADC SUM ADC SUM Programmable Digital Event Processor Collimator ADC Crystal SUM ADC SUM ADC SUM ADC SUM ADC SUM Individual PMT data to digital event processor ADC SUM PMT array Analog to Digital Converters PreAMPs

  29. PSPMT • position sensitive PMT • essentially light guide is not necessary • perform multi-positioning within one PMT X X Y Y Z

  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: baseline shift 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 pile-up nonparalyzable ideal Observed count rate, Ro paralyzable real True count rate, Rt

  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 M T P exit window fiber NaI (Tl) entrance window reflectors detector Indium hump bonds readout IC

  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

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