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Radionuclides. Isotopes Half-life Energy (keV) main decay99mTc 6.03 hrs 140 I.T.131I 8.05 days 364 ??125I 60.2 days 35 E.C. 123I 13.0 hrs 160
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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
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 = 20~200µs
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