1. Novel high resolution detectors for Positron Emission Tomography (PET)
Siena
2002
8th Topical Seminar on Innovative Particle and Radiation Detectors October 2002 Siena, Italy
Novel high resolution detectors for
Positron Emission Tomography (PET)
N. Belcari, M. Camarda, A. Del Guerra, A. Motta, S. Righi, A. Vaiano
Department of Physics, University of Pisa and
INFN Sezione di Pisa,
Via F. Buonarroti 2, I Pisa (Italy)
G. Di Domenico, G. Zavattini
Department of Physics, University of Ferrara and
INFN Sezione di Ferrara,
Via Paradiso 12, I Ferrara (Italy)

2. Positron Emission Tomography (PET)
Outline of the talk
Positron Emission Tomography (PET)
Clinical PET
Dedicated PET systems
Novel detectors for High Resolution PET
Requirements
PS-PMT
Multi-Pixel HPD

3. PET principle A b+ emitting radiotracer is injected to a “patient”
The radiotracer marks a specific function
(e.g. glucose metabolism) - Uptake process
The positron annihilates with an electron and one pair of nearly collinear 511keV photons are emitted in opposite direction.
511keV photon
A set of detectors surrounding the “patient” detects the pair of photons
(time coincidence) (Q,Z or X,Y coordinates of the point of interaction)
An algorithm performs the 3-D reconstruction of the activity density within the body.
Ring geometry
Parallel plane geometry

4. Physical problem:
Position sensitive detection of both the 511 keV annihilation photons
Matrices of scintillators
are usually used for the
photon detection
Fast, good energy resolution, high Z
For the position determination we have to determine the crystal
where the interaction occurs  Position Sensitive Photodetector
YAP:Ce matrix
4cm  4cm  3cm
(400 elements,
2  2  30 mm3 each)
PIXEL IDENTIFICATION!

5. Whole body PET scanners
Needs of dedicated scanners
Whole body PET scanners
2% Solid Angle Coverage
4-8 mm spatial resolution
High cost for routine scanning
For specific application such as “small animal functional imaging” or
“Positron Emission Mammography (PEM)” dedicated scanners are needed so as to offer:
Higher spatial resolution ( <3mm FWHM for PEM) to reliably detect small tumors (< 5 mm Ø)
( <2mm FWHM for Small Animal PET) to detect very small structures (hot spots)
Higher sensitivity to reliably detect very low specific activity (1mCi/cc) with a low uptake ratio
(hot spot/Background < 10) in a short time (10-20 min.)
Better handling (easier positioning of the mouse or of the breast within the FoV)
Application specific design (Rat or Mice for Small Animal PET), (Fit the breast for Axillary lymph
nodes scanning or compression mode PEM)
Multimodality scanning (PET-SPECT, RX-PEM)
Use of a reduced number of detectors  reduction of cost

6. Spatial Resolution Requirements
Human body: ~70 kg
Heart mass: ~300 g
Aorthic cannula Ø :
~ 30 mm
Rat body: ~200 g
Heart mass: ~1 g
Aorthic cannula Ø : mm
Mouse body: ~20 g
Heart mass: ~0.1 g Aorthic cannula Ø : mm
Relative heart size
Required spatial resolution
4-8 mm FWHM
( mm3)
2 mm FWHM
(8 mm3)
 1 mm FWHM
( 1 mm3)
How to achieve this ?
Small size detectors (high pixellization)
Individual detectors or “perfect” coding

7. Sensitivity Requirements
Imaging of low activity sources
low uptake processes such as in small cancers
Imaging of small hot spots
discrimination of small regions with a low spot/background ratio
How to achieve this ?
High geometry efficiency (large solid angle covered by detectors)
High detection efficiency (e.g. for crystals: high Z, high density)

8. Detectors for High Resolution PET

9. Detector configuration- Light sharingX
Center of gravity calculation for X and Y B A D C
Identification from a LUT Y
The light distribution is measured by 4 PMTs
A BGO block is saw at different depth. The cuts are filled with a reflective material and provide a light guide to the PMTs.
CTI Exact HR
detector block*
Advantages
Reduction of the number of PMT
Cheap
Easy to pack
Drawbacks
Loss of resolution
- Statistical light sharing
- Distortion
- Pileup at high count rate
*Casey & Nutt, IEEE TNS 33 (1986)

10. Detector configuration- Individual coupling
Early PET detector element
Novel configuration
Single PMT
Array or independent
photodiodes (APD)
Scintillator block
(BGO)
Matrix of
scintillators
(BGO/LSO)
+ Simple coding
(“parallel” operation)
+ High spatial resolution
with no distortion
+ Small size
+ High count rate
- Quite difficult to pack
- Tricky tuning
- Gain dependence on
bias & temperature
- Expensive
+ Simple coding
(“parallel” operation)
- Difficult to pack
- Expensive
Si APD array (8×4)
Hamamatsu S8550
A detector module made of a 8×4 LSO matrix couped to a
Hamamatsu S8550
(Pichler B., IEEE TNS 45 (1998) )

11. Electronic coding - PS-PMT readout (Single tube)
A single position sensitive PMT is used for the identification of the hit crystal in a scintillator matrix
Advantages
Large active area (up to 100 mm Ø)
Good spatial resolution (up to 0.5 mm FWHM)
Good uniformity
Limited number of channels (up to 28 x + 28 y crossed wire anodes)
Easy to use with a resistive chain (4 channels)
Drawbacks
Distortion at the borders
Low packing fraction (limited active area)
Round shape (difficult to pack)
The PS-PMT R2486 by Hamamatsu.
Active area 50 mm Ø
16 x + 16 y anodes
Flood field irradiation(122 keV) of a
20 × 20 crystals(2 mm × 2 mm each)
YAP:Ce matrix read by a
Hamamatsu R2486 (resistive readout)

12. Electronic coding - PS-PMT readout (Multi tube)
An array of densely packed small and square
position sensitive PMT is used to build a large area
photodetector for the readout of a scintillator matrix
Advantages
Higher packing fraction with respect to round tubes (up to 73%)
Good spatial resolution
Good uniformity
Easy to use with a resistive chain (4 channels)
YAP:Ce matrix,
11 × 11 crystals
(2mm × 2mm each)
Drawbacks
Dead area between adjacent PMTs
Small active area for each tube
CsI:Tl matrix, 8 × 8 crystals
(2.8 mm × 2.8 mm each)
The PS-PMT R8520-C12 by Hamamatsu.
Active area mm × 22 mm
6 x + 6 y anodes
Flood field irradiation(122 keV) of a matrix of scintillators read by a Hamamatsu R8520-C (single tube, resistive readout)

13. PS-PMT readout - Multi tube recovery of dead area - light sharing
Matrix of scintillators
Flood field (122 keV) image of a matrix of a NaI(Tl) crystals (2 × 2 × 6 mm3 each) coupled to an array of four R7600-C12 via the transparent window of the NaI matrix*
Transparent
window
+ Simple and cheap - Resolution worsening
- Requires enough light yield
*Courtesy of R. Pani, University of Rome, La Sapienza

14. Effect of: Quartz window - White reflector
Hamamatsu R8520-C12 readout
Effect of: Quartz window - White reflector
YAP:Ce Matrix
25 crystals
2230 mm3 each
Hamamatsu R8520-C12 (resistive readout)
YAP:Ce Matrix
121 crystals
2230 mm3 each
YAP:Ce matrix
White reflector
FWHM  0.8 mm
Direct contact
(optical grease)
511 keV source
Quartz window
Direct contact
(optical grease)
FWHM 2.6 pixel (1.0 mm)
FWHM  0.7 mm
FWHM 1.4 mm
3mm quartz window (optical grease)
FWHM  1.1 mm NO
YES NO
3mm quartz window (optical grease)
White reflector
(about 15% more light with a white reflector)

15. Hamamatsu R8520-C12 readout - Multi tube
recovery of dead area - Quartz window
Two Hamamatsu
R8520-C12
(resistive readout)
YAP matrix 1124 crystals
(2mm  2mm  30mm each)
White reflector
22Na source
BGO crystal + PMT
for selecting 511keV
annihilation photons
Quartz window
3 mm thick
Two rows of crystals
(2 mm +2 mm) outside
the active area
22mm
Flood field irradiation of the YAP matrix 1124 crystals (511 keV).
All crystals can be identified
22mm

16. recovery of dead area - fibers
PS-PMT readout - Multi tube
recovery of dead area - fibers
Matrices of scintillators
Matrix of scintillators
Fiber optic bundle (individual coupling)
FOP
(Fiber Optic
Plate)
Example: the detector element of the MicroPET®
+ Linear demagnification of the image
- Expensive
+ No demagnification
- Only for ring
configuration
PS-PMT Hamamatsu R7600-C8
8 × 8 array of 2 mm square fibres by Kuraray
8×8 LSO matrix

17. Hamamatsu Flat Panel Multi Anode PMT
See next talk at this conference
“Flat Panel PMT: advances in position sensitive photodetection”
R. Pani, University of Rome
Hamamatsu R8500

18. (better than 92% with a quartz window)
Comparison
Application: readout of a 5 cm × 5 cm matrix of scintillating crystals
R2486
Array of 4 R8520
R8500 Flat Panel
See next talk
78%
77%
(better than 92% with a quartz window)
96%
Active area

19. What is an HPD (Hybrid Photo Diode)* ?
*R. de Salvo et al. NIM A315 (1992)
Main characteristics
Good quantum efficiency
Low noise
Single photon counting capabilities
Single or Multi - pixel structure
Very low output signal
(Low noise and high gain readout
electronics is crucial)
HV (up to tens kV required)
Available HPD structures:
Proximity focused: Electrons reach the Si sensor by straight lines (no demagnification)
(multi-pixel: up to 73 pixel, up to ~1” Ø, good spatial resolution)
Fountain focused: Electrons are focused on the Si sensor (linear demagnification)
(multi pixel: up to 73 pixel, up to ~ 3” Ø, poorer spatial resolution, spherical entrance window)
Picture from:

20. Multi Anode HPD - WLS Fibres readout
Large number of crystals and large area crystal read out
Separation of scintillating crystal from detector and read out electronics
No dead zone around the detector
Possibility of Z coordinate measurement
To 61 Pixel HPD
Scintillator
matrix
To 61 Pixel
HPD
WLS fibres
2.5 p.e. for
122keV
(57Co source)*
~10 p.e. for 511keV
(22Na source)
(photopeak event)*
YAP:Ce matrix
Not enough light yield!
61 pixel HPD proximity
focusing by DEP ~1”
WLS fibres by Kuraray
*N. Belcari et al. NIM A461 (2001)

21. Multi Anode HPD - Direct readout
Multi anode HPDs can be used as a PS-PMT for the direct readout of a matrix of scintillating crystals and readout by electronic coding.
Four peaks of the second row: the peak-to-valley ratio is about 4.
3-D profiles of 4  2 crystals of a YAP:Ce matrix (2 mm side each ). Pixels are clearly separated.*
*N. Belcari et al. Presented at the conference “New Developments in Photodetection” Beaune,
June 17-21, 2002 (NIM 2002)
HAMAMATSU
PMT R-5900
4  4 YAP:Ce
MATRIX
2 mm 2 mm
2 mm
12 mm
DEP HPD 61 PIXEL
Larger HPD with higher
pixellization will be soon
available.
A 5” HPD is produced
at CERN*
*A. Braem et al., NIM A (2002)

22. Multi Anode Large area HPD - Direct readout
Near ?? future
Further developments are needed in order to make this technology available. New devices and new front-end electronics (low noise-high gain-fast triggering preamplifiers) are necessary.
Still a dream*
5” Pad HPD CERN
Optimized for medical applications
Spherical window + bundled small fibers
or Fiber Optic Plate (FOS)
K2CsSb photocatode
*C. Joram et al. Presented at the conference “New Developments in
Photodetection” Beaune, June 17-21, 2002 (NIM 2002)

23. Dedicated Scanners for
Small Animal PET
To fulfil the spatial resolution and sensitivity requirements dedicated
instruments are needed.
Many different technologies are actually used in these scanners.
Scintillator crystals (BGO, LSO, YAP)
Position sensitive PMT (electronic coding)
Avalanche Photo Diode (individual coding)
Multi Wire Proportional Chambers (charge sharing)
Review of Small Animal PET Scanners*
*(A. Del Guerra, N. Belcari. Q. J. Nucl. Med. 46(1) (2002) 35-47).

24. microPET®4 - Images FDG in rat myocardium
Courtesy of S. Cherry, UCLA, 2002
FDG in rat myocardium
Imaging dopamine receptors in the rat
FDG in rat brain
Gene expression in mouse liver

25. In vivo dynamic imaging of rat brain
YAP-(S)PET
University of Ferrara
Ex vivo images of a rat
brain injected with
18F-FDG (left) and
18F-FESP (right)
A. Del Guerra et al. IEEE-MIC Seattle, 1999, M10-45
In vivo dynamic imaging of rat brain
with 11C-Flumazenil
The rat was injected with 500 µCi of 11C-Flumazenil.
Sagittal section through the center of the brain.
(Nose towards the left. Upper skull towards top)

26. Dedicated scanners for Positron Emission Mammography (PEM)

27. Pisa YAP-PEM prototype*
YAP:Ce crystal matrix
6cm × 6cm × 3cm
2mm × 2mm × 3cm each
(900 elements)
Monte Carlo Simulation
PS-PMT
R8520-C12
(Hamamatsu)
2.2cm × 2.2 cm
active area
The tumor is embedded in an active breast tissue, with specific activity ratio 10:1 between cancer (1.0 mCi/cc) and tissue (0.1 mCi/cc). The distance between detectors is 5 cm.
Variable distance
for compression
( cm)
Optical diffuser (Quartz)
Reconstructed images of simulated tumors in active breast tissue. Tumors have different size (diameter/volume): (a) 12.4 mm / 1.0 cc; (b) 9.8 mm / 0.5 cc; (c) 5.8 mm / 0.1 cc; (d) 5.0 mm / cc
Profiles of the images a,b,c
SNR values as a function of tumor size.
A. Del Guerra et al. To be presented at IEEE-MIC, Norfolk, November 2002

28. Scintillator: YAP:Ce 30  30 crystals 2  2  30 mm3 each
YAP-PEM simulation
Results with YAP-PET (2 heads)
Tumor diameter 5.0 mm (in breast tissue)
Tumor / background activity:
1 mCi /cc / 0.1 mCi /cc (10:1)
Moderate compression (5 cm)
Events 3×106 (~10 min.) - Threshold 50keV
The simulated breast is a specially designed
68Ge solid phantom.
A tumor is simulated by a cylinder 0.5 cm Ø and
0.5 cm height with an activity of 44 nCi.
The breast tissue is simulated by an active cylinder with diameter of 10.8 cm and height of 6 cm with a distributed 68Ge planar source of 25 mCi . The specific activity tumor/breast ratio is about 10:1 for all sources.
Scintillator: YAP:Ce 30  30 crystals 2  2  30 mm3 each
1 detector
6.0  6.0 cm2
4 detectors
2.2  2.2 cm2
Experimental
Monte Carlo
  2.3%
9.3  106 counts
SNR = 10.7  1.0
  0.8%
3.0  106 counts
SNR = 6.0  0.9
The image of the 5.0 mm source after the subtraction of the backprojection of the planar source.

29. Conclusions Dedicated scanners for small animals
Needs well established
Commercially available
Further developments required
Dedicated scanners for Positron Emission Mammography
Needs under scrutiny
Few research prototypes available
Similarities between these two types of scanners
A high density / medium Z scintillator (such as YAP) coupled to novel
PS-PMTs could be a very successful detector for these apparatus (e.g. YAP-(S)PET and YAP-PEM)