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

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

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

6. Positron (+) Decay
Nucleus
Neutrons
18F-FDG +
Protons
Electrons

7. Neutron-deficient isotopes can decay by emitting positrons+
anti-neutrino +
positron
Net effect: one proton replaced by
neutron
anti-neutrino
positron

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 e+ e-
511 keV

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

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

11. Important Detector Properties
Detection efficiency (aka sensitivity, stopping power)
Reduces noise from counting statistics
Currently > ~ 30% (singles)
1M Events
55M Events

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

13. Important Detector Properties
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
511 keV
Scatter and Attenuation

14. Prototypical PET Detector
Optical reflector
light is converted to an electrical signal & amplified
Gamma Ray
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
Front-end electronics condition the signal for further processing

15. New Developments Detectors Multimodality imaging
Specialized applications

16. New Developments: Detectors
Scintillators
No perfect choice - tradeoffs
Also practical qualities
Rugged?
Hygroscopic?
Cost?
175 25

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

18. New Developments: DetectorsZ2 Z1
Sa2
Sa1 Sc  
Solid-state detectors
Direct conversion, no photodetector
Great dE/E & spatial resolution
Poorer timing & stopping power
CZT

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

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

21. New Developments: Detectors
Time of flight using LaBr3
no TOF
300 ps TOF
1 Mcts
5 Mcts
10 Mcts

22. New Developments Multimodality imaging Specialized applications 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. 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. Architecture Detector blocks x12 Timestamp & Signal Processing Module
TSPM TDC
PCI card
ASIC
optical
differential
RatCAP
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, …

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

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

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

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

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

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

31. Thanks!
DOE OBER funding