Some studies for a development of a small animal PET based on LYSO crystals and Geigermode-APDs

Some studies for a development of a small animal PET based on LYSO crystals and Geigermode-APDs

E. Lorenz

, I. Britvich

, D. Ferenc

, N. Otte

, D. Renker

, Z. Sadygov

, A. Stoykov

a)

Max Planck Inst. for Physics, Foehringer Ring 6, D 80805 Munich, Germany

b)

ETH Zurich, Hoengger Berg, Schafmattenstr., CH 8093. Zurich, Switzerland

c)

Paul Scherrer Institute, CH 5233 Villigen Switzerland

d)

Physics Dep. UC-DAVIS, One Shield Av, Davis, CA 95616-8677 USA

Joint Institute for Nuclear research, 141980 Dubna, Russia

Abstract

The use of Geiger Mode APDs (G-APD)/micro-cell APDs (MAPD) opens new simplifications in PET designs. We report on some test studies for a Small Animal PET based on LYSO crystals and G-APDs/MAPDs for readout. Emphasis is put on time and energy resolution.

Keywords: PET detector; Geigermode Avalanche Photodiodes

____________________________________________________________________________________________________

1

Corresponding author. Tel.: +49-89-32354241; fax:+49-89-3226704; e-mail: e.lorenz@mac.com.

1. Introduction

Positron Emission Tomography (PET) has become an indispensable tool in biomedical research. The main reason why this technique has not reached its theoretical performance limits, nor achieved more widespread application in medical diagnostics, has been the limitations and high cost/complexity of current PET detectors.

Commercial PET detectors use photomultipliers (PMT) as photosensors for the readout of high Z scintillation crystals.

Some recent developments for small animal PET detectors aim for using semiconductor photosensors like linear mode avalanche photodiodes. Geiger mode avalanche photodiodes (G-APDs, also known by the name microcell/micropixel avalanche photodiode or silicon photomultiplier) are a class of new photosensors, refs [1], [2], [3] that have several very attractive features for use in PET detectors. The main advantages are:

• Very compact

• Sensitive to single photons

• Low bias (20-100 V) and high gain (10

-10

)

• Low excess noise factor

• Insensitive to magnetic fields

• Robust, very low pickup

• Insensitive to passing ionizing particles

• Very fast signals

• Potential for high quantum efficiency (QE), resp. photon detection efficiency (PDE).

• Potential for very low production costs

G-APDs are still in the developmental phase but should soon become commercially available. An overview about G-APDs can be found in [4]. G-APDs could be used either for the readout of small individual crystals with 1:1 coupling but when larger area devices will be available, also for the commonly used Block detector arrangement. A first test of principle for a possible use in small animal PET detectors has been reported, [5], using a two-crystal configuration and Na

as positron source. The results were very promising but limitations of the used G-APDs (low PDE, high noise rate, large optical crosstalk, low number of cells) were obvious. Here we report about a test using a new generation of Microcell Photodiodes (MCPD). These were developed by Z. Sadygov and produced by the company MICRON, Russia.

2. The test setup and some results

For the test we used basically the same set-up (Fig 1) as

for the original study. As crystal we used 2x2x12 mm

LYSO crystals from Saint Gobain. The crystals were

wrapped into one layer of the V2000 foil from 3M and

optically coupled with the 2x2 mm area to a MCPD model

MW-3 diode biased to 132 V (gain ≈ 5.10

). The far crystal

end was either painted black (to simulate losses for a later

depth of interaction study) or covered with white Teflon

plumbing tape. The MCPD has an area of 3x3 mm,10

cells

per mm

and a PDE of 12.5% at 450 nm. Fig 2 shows some

spectra (NA

, the expanded spectrum around the 1275

KeV line, CO

) triggered in the self-trigger mode.

Submitted to Elsevier Science 2

Fig. 1.: Setup for the test

Fig 2: Top: pulse height spectra for Na

. Middle: expanded spectrum for the 1275 KeV line. Bottom: CO

spectrum.

The following resolutions were measured:

Line Energy Resolution Comments 511 KeV 17 % Far end black (40% less light)

511 KeV 12.5 % Far end white reflector 1275 KeV 7.2% <5% deviation in linearity

511 KeV 22% study [5]

Also some timing resolutions for 511 KeV gammas were measured using different discriminator configurations such as simple leading edge discrimination with ≈ 350 KeV threshold, high-low discrimination (Low ≈ 2 photoelectrons; High ≈ 350 KeV) and constant fraction discrimination with 350 KeV threshold for the arming discriminator. No shaping network was used.

Type of Discriminator Threshold Resolution

Leading edge ≈350 KeV 3.1 nsec

High/Low leading edge ≈350 KeV/2 pe 0.85 nsec Const. Fraction ≈ 350 KeV 0.56 nsec Low leading edge, ref [5] 1.5 nsec (The resolution values refer to a single element. The timing distribution for the simple leading edge discrimination had pronounced tails)

3. Conclusions

The current study highlights the progress both in energy resolution, dynamic range and the timing resolution achieved within 1 1/2 year because of better G-APDs.

Nearly a factor two improvement was achieved in comparison to the first study [5]. The finer cell structure, manifesting itself in the larger dynamic range, the larger area and a higher PDE were the main contribution to this improvement. Compared to another very similar test, [6], a slight degradation in performance has been measured. We attribute it to the use of a very thin reflector foil and the absence of any shielding of the set-up, both with the intention to simulate as close as possible conditions for realistic arrangements. In summary, this test has shown that the readout matches the performance of the photomultiplier readout. It can be concluded that a small animal PET can be build already with G-APDs of the current performance.

Optical crosstalk and the high dark count rate were found to be of no problem. Also, a very fine cell structure is more important than an optimal PDE. Nevertheless, the potential to improve significantly the PDE would make a PET construction easier and also allows one to use the block detector read-out when large G-APDs become available.

References

[1] Bisello, D. et al. (1995). Nucl. Inst. Meth. A 367, 212 [2] Golovin, V., V. Saveliev, (2004) Nuc. Inst. Meth. A 518, 560.

[3] Buzhan, P., (2003). Nucl. Inst. Meth. A 504, 48-52

[4] Renker D. Procs. Photon detector conf. Beaune 2005, to be published in Nuc. Inst. Meth.

[5] Otte, N., et al.,(2005) Nuc. Inst. Meth., A545 705 [6[ Stoykov A. et al., Procs, Euromed 2006, to be published G-

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