1. High Resolution and High Sensitivity PET Scanner with Novel Readout Concept
Erlend Bolle1,Michael Rissi1, Michelle Böck1, Jo Inge Buskenes1, Kim-Eigard Hines1, Ole Dorholt1, Ole Røhne1, Steinar Stapnes1,2, Jan G. Bjaalie3, Arne Skretting4
1 Universitet i Oslo, Norway
2 CERN, Geneva, Switzerland
3Department of Anatomy & CMBN, Oslo, Norway
4Rikshospitalet-Radiumhospitalet Medical Center, Oslo, Norway

2. Why small-animal PET? Biomedical Applications:
detectibility of small tumors in transgenetic mice/rats -> concepts for human cancer
longtime-study at each animal -> more control, disease progression, therapeutic response -> statistical power improved
functional information available -> PET: functional and kinetic data, non-invasive, in-vivo

3. Challenges for improving small-animal PET Systems
PET systems’ performance improved by enhancing important parameters of detector systems:
photon sensitivity:
fraction of coincident annihilation photon pairs emitted from a source
spatial resolution:
determined by: positron range, annihilation photon acollinearity, intrinsic detector resolution
contrast resolution:
ability to differentiate/quantify one region from background or two adjacent regions
Current Trends in Preclinical PET Systems Design – C.S. Levin, H. Zaidi

4. Challenges for Classical PET
measure depth of interaction (DOI)
no information on DOI -> parallax error
δP = L sinα
increases with distance from center of field of view (FOV)
solution: spatial resolution improved by measuring interaction point in 3D

5. Challenges for Classical PET
True
Scatter
Random
Body
scatter events occur in body and scanner
Scatter
Scatter
Scanner

6. COMPET - Detector Arrangement

7. COMPET - Detector Arrangement
LYSO crystal: information on x-, y-coordinates, energy of event
wave length shifter (WLS): z-coordinate high resolution (sub-millimeter)
readout on the end of WLS and LYSO → reflector on opposite end
light below angle of total reflection escapes LYSO → absorbed at one side by WLS
light transport relies on internal reflection → polished and crack-free surface
resolution along the 3 dimensions tuned by varying LYSO/WLS dimensions
based on method proposed by AX-PET collaboration

8. COMPET Flexible Geometry

9. COMPET - Facts 4-8cm bore opening (adjustable) with 8cm axial view
very high sensitivity (16%)
high resolution (sub-millimeter)
3D event reconstruction
no inter module and inter crystal gap
High data throughput FPGA/ethernet readout (10Mevents/sec)
Backend computer farm for data taking and image reconstruction
MRI compability y x z

10. Detector Module 2×3mm2 MPPC 2×3×80mm3 1×3×80mm3 1×3mm2 MPPC
600 LYSO crystals
400 WLS
2×3×80mm3 LYSO
1×3×80mm3 WLS
2×3mm2 MPPC (LYSO)
1×3mm2 MPPC (WLS)
2×3mm2 MPPC
2×3×80mm3
1×3×80mm3
1×3mm2 MPPC

11. Detector Module-Main Components
LYSO
3x3mm2 MPPC
WLS –wave length shifter

12. AX-PET Workshop, Valencia
MRI Insert
AX-PET Workshop, Valencia

13. MR Insert - Challenges no ferromagnetic components do not disturb MR
fit inside existing MR
shielding
cooling

14. Ultimate Goal
(a) Dynamically acquired PET images from a C57BL/6 mouse injected with [11C]-d-threo-methylphenidate show a specific dopamine transporter binding in the striatum (S) and nonspecific uptake in the Harderian glands (H). (b) Brain morphology is revealed from the magnetic resonance images simultaneously acquired using a 3D TSE sequence. (c) The fused images show enhanced tracer uptake, matching the morphology of the striatum in the magnetic resonance data. (d) Time-activity curves (TACs), derived from the PET data simultaneously acquired during MRI, allow further analysis such as kinetic modeling to determine the dopamine transporter binding potential. The clear separation of the striatum and cerebellum (Cb) curve indicates more specific tracer binding in the striatum than in the cerebellum. Both TACs include unbound tracer. Scale bars, 1 cm.

15. Simulation Setup software: Gate v5.0.0p01 (based on GEANT4)
list output via ROOT
3 different scenarios:
total deposited energy in 2 modules > 2∙ 450 keV (no inter-module scatter accepted)
total deposited energy in less than 3 crystals > 2 ∙ 450 keV (accepting inter-crystal scatters)
total deposited energy in exactly 2 crystals > 2 ∙450 keV (accepting only photoelectric events)
Simulation of back-to-back gamma rays (511 keV) x y

16. Sensitivity Simulation
source: back-to-back gammas (511 keV)
5 layers, 4 modules
diameter: 50 mm, length: 72 mm
3D event reconstruction: 3 cases assumed
LYSO Crystals
Photoelectric event
Multiple scatter
Single scatter g

17. Detector Sensitivity All events (multiple scatters) Single scatters
Photoelectric events

18. COMPET - High Resolution
Sensitivity vs resolution
Ref.: C. Levin et.al., “Current Trends in Preclinical PET System Design”

19. COMPET - High Sensitivity
COMPET LYSO Box ,
Ref.: C. Levin et.al., “Effects of system geometry and other physical factors on photon sensitivity of high-resolution positron emission tomography”

20. Central Point Source Resolutiong
MPPC
LYSO
WLS
POI resolution
s(POI)
[mm]
FWHMx [mm]
FWHMy [mm]
0.3
0.8
0.4
0.5
0.9
0.6
0.7
1.0
1.2

21. Point Source Resolution
activity: 4x106 Bq
1s run time
assumed POI resolution: s = 0.5mm of back-to-back g-rays
xSource [mm]
ySource[mm]
zSource[mm]
FWHMx[mm]
FWHMy[mm]
0.0
0.86
0.89
5.0
0.80
1.05
10.0
0.87
1.22
15.0
0.92
1.50
20.0
0.96
1.52
-20.0
1.59
1.67

22. AX-PET Workshop, Valencia
Reconstruction
3D Fourier rebinning to transaxial slices
2D filtered back projection (using ramp filter) for each transaxial slice
Derenzo - phantom
AX-PET Workshop, Valencia

23. Resolution vs. Sensitivity
Resolution FWHM [mm]

24. Simulation - Summary according to simulations:
DOI resolution of 0.5 mm is achievable -> CPS resolution of approx x 0.9 mm2
point-source-resolution degrades for off-axis objects to approx. 1.6 x 1.6 mm2 close to the detector
maximal sensitivity of to back to back gammas:
if multiple scatter events reconstructed -> sensitivity: 16%
if only single scatter events accepted -> sensitivity: 14%.

25. Conclusion and Future Work
Detector concept -> demonstrated
New readout scheme -> proven to work
All parts bought -> implement full scanner
Hoping for first results beginning of 2011!
2010
Simulation
Image reconstruction
Prototype testing
System implementation
2011
System verification
Preclinical tests
2012
Preclinical test
Fused MRI/PET images
Thanks a lot to:
Norwegian Research Council
Swiss National Fund