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1. 1
Physics & Instrumentation in
Positron Emission Tomography
Paul Vaska, Ph.D.
Center for Translational Neuroscience
Brookhaven National Laboratory
July 21, 2006

2. 2
Non-invasive Medical Imaging
Techniques
Anatomical
• X-ray
• CAT
• MRI
• Ultrasound
Functional
• “nuclear medicine” - SPECT, PET
CAT
X-Ray
MRI

3. 3
Positron Emission Tomography
Recent mainstream acceptance
- relatively expensive
- cyclotron for tracer production
- detectors must stop high-energy
gamma-rays
- low resolution (>2 mm), limited
counting statistics
- BUT unique functional capabilities
Applications
- Diagnosis of disease
- cancer (WB), cardiac, …
- Research
- brain function
- animal studies

4. 4
Technical Challenges in PET Imaging
Radiochemistry - better tracers
Imaging Physics - better images by
• Detector design
– Spatial resolution
– Sensitivity
• Image processing
– Corrections for physical effects
– Image reconstruction algorithms
Data Analysis & Biological Modeling - better
interpretation of images

5. 5
PET Imaging Overview
- Synthesize radiotracer
- Inject radiotracer
- Measure gamma-ray
emissions from isotope
(~20-60 min)
- Reconstruct images of
radiotracer distribution
(nCi/cc)

6. 6
+
+ +
NucleusNucleus
NeutronsNeutrons
ProtonsProtons
ElectronsElectrons
Positron (Positron (ββ++
) Decay) Decay
18
F-FDG

7. 7
ββ++
DecayDecay
+
+
+
+
+
+
+
+
+
Neutron-deficient isotopes can decay by emittingNeutron-deficient isotopes can decay by emitting
positronspositrons
+
anti-neutrinoanti-neutrino
positronpositron Net effect: oneNet effect: one
protonproton
replaced byreplaced by
• neutronneutron
• anti-neutrinoanti-neutrino
• positronpositron

8. 8
Positron annihilation
Annihilation gives
• 2x 511 keV gamma rays
• 180 degrees apart
• Line of response
Positron range & gamma
noncollinearity
Scanner is just a photon
counter!
• Counts gamma-ray pairs vs.
single gammas
• Time window ~ 1 ns
511 keV
511 keV
e+
e-

9. 9
Raw Data & Image Reconstruction
0° projection
0°
180°
90°
90°projection
image
reconstruction
“sinogram”

10. 10
Important Detector PropertiesImportant Detector Properties
- Spatial resolution
- Directly controls spatial resolution
in reconstructed image
- Currently ~ 1 - 5 mm
- Depth-of-interaction?
- Reduces “parallax”

11. 11
Important Detector PropertiesImportant Detector Properties
- Detection efficiency (aka sensitivity, stopping power)
- Reduces noise from counting statistics
- Currently > ~ 30% (singles)
55M Events1M Events

12. 12
Important Detector PropertiesImportant Detector Properties
Random (accidental) coincidence
- Time resolution
- Affects acceptance of random
coincidences
- Currently ~ 1 - 10 ns
- Time-of-flight (TOF)?
- c = ~ 1 ft/ns
- Need << 1 ns resolution

13. 13
Important Detector PropertiesImportant Detector Properties
Scatter and Attenuation
511 keV
- Energy resolution
- Scattered gammas change
direction AND lose energy
- Affects acceptance of scattered
coincidences
- Currently ~ 20%
- Deadtime
- Handle MHz count rates!
511 keV 400 keV

14. 14
Scintillation
Crystal
PMT
Pre-Amplifier
+ Electronics
Gamma photon
converts to optical
photons
(proportional to
gamma energy, typ.
1000’s)
photons are
collected at the end
of the crystal
light is converted to an electrical signal
& amplified
Front-end electronics
condition the signal for
further processing
Prototypical PET Detector
Gamma Ray
Optical reflector

15. 15
New Developments
• Detectors
• Multimodality imaging
• Specialized applications

16. 16
Scintillator NaI(Tl) BGO GSO LSO LuAP LPS LaBr
τ (ns) 230 300 60 40 18 30 35
µ (cm-1
) 0.35 0.95 0.70 0.86 0.95 0.70 0.47
∆E/E (%) 6.6 10.2 8.5 10.0 ~15 ~10 2.9
Rel. light output (%) 100 15 25 70 30 73 150175
25
New Developments: Detectors
• Scintillators
• No perfect choice - tradeoffs
• Also practical qualities
• Rugged?
• Hygroscopic?
• Cost?

17. 17
New Developments: Detectors
• Photosensors
• Photomultiplier tubes
• Avalanche photodiodes
• Arrays, position-sensitive
• Compact but noisier
• Silicon photomultipliers
• Very new
• Best of both?
APD array
PMT
SiPM

18. 18
New Developments: Detectors
• Solid-state
detectors
• Direct conversion,
no photodetector
• Great dE/E &
spatial resolution
• Poorer timing &
stopping power
• CZT Z2
Z1
Sa2
Sa1
Sc
⊕
∅
⊕
⊕
∅
∅
⊕
∅

19. 19
New Developments: Detectors
• Pb converters & ionization
HIDAC Pb-walled straws (50 cm long)

20. 20
New Developments: Detectors
• 3D gamma-ray event positioning
• Depth of interaction
• Reduces parallax problem
LSO
slab
γ γ
APD
slab
s
vs.
LSO slab
crystal
holder
APD
decoupling
capacitor
HV filter
capacitor
Current-
limiting
resistor
signal
output
connector
SHV
connector
unused APD
slot

21. 21
New Developments: Detectors
• Time of flight using LaBr3
noTOF300psTOF
1 Mcts 5 Mcts 10 Mcts

22. 22
New Developments
• Multimodality imaging
• PET/CT
• PET/MRI
• Specialized applications
• Brain, breast, prostate
• Small animal - microPET
• Arterial input function
• Humans - wrist scanner
• Animals - microprobe
• Awake rat brain - RatCAP

23. 23
RatCAP: Rat Conscious Animal PET
Eliminate anesthesia in preclinical neuroscience
using PET in order to:
• Remove confounding effects of anesthetic on
neurochemistry
• Enable stimulation in animal PET
• Enable correlations of behavior and neuro-PET

24. 24
Architecture
Detector blocks x12
• LSO 2.2 x 2.2 x 5 mm in 4 x 8 array
• 1:1 coupling to APD
• ASIC - single all digital output
Timestamp & Signal Processing Module
• Programmable real-time logic (FPGA)
• 1 ns bins (debugging, now 10 ns)
Data acquisition
• PCI card in standard PC
• Up to 70 MB/s = ~10 Mcps singles
• Offline software for coincidences, corrections, recon, …
TSPM
TDC
PCI
card
ASIC
opticaldifferential
RatCAP

25. 25
Architecture
RatCAP
TSPM
LSO APD
ASICs
all interconnections
38 mm FOV
72 mm OD optical links to PCI
high voltage
194 g
data, clock, power
18mmaxialFOV

26. 26
Performance
Spatial resolution (FWHM @ CFOV)
• FBP: 2.1 mm
• MLEM: <1.5 mm
Energy resolution: 23% FWHM
Time resolution: 14 ns FWHM
• window = 30 ns
Sensitivity (point @ CFOV): 0.7%
Peak Noise Equivalent Count rate: 14 kcps @ 5 µCi/cc
1st prototype: LLD = 150 keV average, variable

27. 27
Imaging Conditions
Anesthetized 250-350 g rats
Limited DAQ livetime >> long scans for statistics
Artifacts

28. 28
F-18 Fluoride Bone Scan
1.3 mCi fluoride
RatCAP microPET R4

29. 29
C-11 Raclopride
1.8 mCi raclopride
In the RatCAP

30. 30
C-11 Methamphetamine
Time-activity curve for striatum

31. 31
Thanks!
DOE OBER funding