A test of silicon photomultipliers as readout for PET

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Abstract

The silicon photomultiplier (SiPM) is a novel photon detector based on Geiger mode operating avalanche photodiodes. In this paper, we present results from a test, demonstrating the feasibility of SiPM as readout elements in scintillator-based positron emission tomography (PET). As scintillator we use the newly developed LYSO crystals having similar characteristics as LSO. With our setup we measure an energy resolution of about 22% and a time resolution of a single crystal element of (1.51±0.07)ns, both full-width at half-maximum. A significant improvement in time resolution could be achieved by triggering on the first photoelectron in the signal. We also present the coincidence rate of two detector channels vs. the position of a small point-like 22Na positron source.

Introduction

Positron emission tomography (PET) scanners are powerful tools for the study of physiological processes in vivo. Current developments aim to build smaller, more compact and less expensive devices with improved resolution and a simpler mode of operation. In case of higher spatial resolution, respectively, finer structures, one has to take into account the body movements to make the correct correlation with anatomical structures. It might be of advantage to combine a PET scanner with an NMR scanner in order to simultaneously acquire information about the morphological structure and physiological processes. To comply with these requirements, a reliable and cheap photon detector is needed, which is insensitive to magnetic fields and pickup as well as causing minimal interference with the NMR detector's data acquisition system.

The so-called silicon photomultiplier (SiPM) is a novel approach towards an inexpensive high efficient photon detector that is developed by several groups [1], [2], [3]. First suggestions to use SiPMs as photon sensor in PET applications can be found in e.g. [4], but no specific study had been performed at that time. We have carried out such a study and presented first results in Ref. [5]. There we came to the conclusion that SiPMs can in principle be used for scintillating crystal readout in PET detectors. Since then we have improved our experimental setup to demonstrate the full feasibility of using SiPMs in PET.

This paper has the following structure. Firstly, we give a short introduction to the working principle of SiPMs. Then we discuss our setup and present experimental results on energy, time and position resolution. Finally, we conclude on the prospects of SiPMs in PET and discuss some special issues and advantages of the SiPM as readout element.

Section snippets

The silicon photomultiplier

A SiPM is composed of an array of small avalanche photodiodes (APD) combined to form a macroscopic unit (typically 500 to 4000 APDs per mm2); see Fig. 1 for the basic concept. In the following, we refer to the individual array element as ‘cell’, while we name the macroscopic unit a ‘pixel’ or SiPM. Each cell operates in limited Geiger mode, i.e. a few Volts above breakdown voltage. In this mode of operation, an electron initiates an avalanche breakdown confined to a cell (the electron

The SiPM used in the test setup

For our studies we used a (1×1)mm2 prototype SiPM developed at MEPhI and PULSAR enterprises. The used device has an n–on–p structure and is therefore less sensitive to the blue part of the emission spectrum of the LYSO scintillator. In the near future, new devices with a p–on–n structure and thus with enhanced blue sensitivity will be available. The characteristics of the used SiPM are summarized in Table 1. For a more detailed explanation of the sensor itself we point the interested reader to

Mechanics

We constructed a simple mechanical support from Lucite for a good fixation and coupling of the crystal and photon detector combination (see Fig. 3). In between the two crystals we placed a small 22Na-source that faked a point-like positronium source. The distance between the two detectors was 5 cm.

As scintillator crystals we used LYSO, which was provided by the company Saint Gobain. The crystals have polished surfaces and dimensions (2×2×15)mm3. LYSO has a high light yield of 32000 photons/MeV

Energy resolution and calibration

The pulse height distribution in Fig. 6(a) shows the γ-spectrum from the 22Na source when triggering on coincidence events. The spectrum was measured with the source placed half way between the LYSO crystals. The spectrum shows the photopeak of the 511 keV-γ's fully absorbed in the LYSO crystal and a contribution from Compton scattered events (Compton continuum). Due to the coincidence alignment condition the Compton continuum is partially suppressed and the valley between the Compton edge and

Influence of the recovery time

Fig. 6(b) shows besides the prominent 511 keV peak indications of the 1.275 MeV line and the corresponding Compton edge (note that the photo efficiency of LYSO around 1200 keV is already highly suppressed). Using the above calibration, we find a number of fired channels exceeding 576. This can only be explained if fired SiPM cells can recover already within the gating time and are therefore sensitive to late emitted photons from the LYSO. In a simple test, the SiPM was illuminated by an intense

Time resolution

For the measurement of the time resolution, we only accepted events within the one sigma limit around the photopeak in each channel (see vertical lines in Fig. 6(a)). The selection of accepted events was done offline.

One series of tests was performed with a constant fraction (CF) discriminator triggering on the rising edge of the signal. Between the SiPM output and the preamplifier input, we inserted a passive element to shape the signal. We varied the time constant of the shaping element and

Position resolution

The sensitivity of the detector setup on the position of the 22Na γ-source was tested by measuring the coincidence rate versus the displacement of the γ-source. For this test, we set the distance between the two crystals to 5 cm and placed the radioactive sample in between. Then we moved the 22Na sample perpendicular to the long side of the crystals. The result is shown in Fig. 11. One expects a linear rise and fall of the coincidence rate as the source moves through the setup. The expected

Conclusions and discussion of the results

In this study, we have confirmed the good prospects of using SiPMs as new photon detectors in PET.

In detail the following conclusions can be drawn from the studies:

  • (1)

    The 511 keV-γ energy resolution of about 22% FWHM was found to be somewhat worse compared to that achieved with larger classical APDs with a 1:1 coupling to small crystals. The achieved energy resolution is remarkable, as the scintillator was coupled to a SiPM, which is four times smaller than the end-face of the crystal, and the 511 

Acknowledgements

The authors would like to thank L. Weiss for producing the mechanical support for the crystal SiPM combination. The LYSO crystals and optical grease were provided by F. Kniest from Saint Gobain. We are grateful that S. Rodriguez was carefully reading and correcting this manuscript. I. Wacker was so kind to scan and prepare the reflectivity measurements of the 3M foil.

References (12)

  • D. Bisello

    Nucl. Instr. and Meth. A

    (1995)
  • V. Golovin

    Nucl. Instr. and Meth. A

    (2004)
  • A.V. Akindinov

    Nucl. Instr. and Meth. A

    (1997)
  • W.W. Moses

    Nucl. Instr. and Meth. A

    (2002)
  • B. Dolgoshein

    Nucl. Instr. and Meth. A

    (2003)
  • N. Otte, et al., Nucl. Phys. B (Proc. Suppl.) (2004), to be...
There are more references available in the full text version of this article.

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