Chang Lyong Kim

Multi-Pixel Photon Counters for TOF PET detector and its challenges

Multi-Pixel Photon Counter (MPPC) is a Geigermode APD developed by Hamamatsu Corp. We proposed that it could be a suitable photo-sensor for next-generation time-of-flight PET detectors due to mainly its high photon detection efficiency. Therefore, we concentrated on the measurement of coincidence timing performance of various MPPCs in conjunction with LYSO crystal scintillators. With 3mm × 3mm devices of 50µm sub-pixels coupled to 3mm × 3mm × 10mm crystals, we have demonstrated a strong dependence of timing performance on over-voltage and temperature, and analyzed how changes in photon detection efficiency and dark counts would explain the measurements. The best coincidence timing resolution we have achieved between two identical LYSO/MPPC detectors was 240ps in FWHM. This was worse than the timing resolution of 220ps obtained with Hamamatsu H6533 fast PMT, and contradicted the expected improvement from higher photon detection efficiency. The contradiction could be explained by slow rise-time of MPPC pulse shape, transit time spread, dark counts and electronics noise from large capacitance of MPPC. In particular, the slow rise-time of MPPC pulse suggested that the need for a very low trigger threshold on the timing circuit. Since it in turn makes the detector system more sensitive to noise, this poses additional challenges for ganging multiple devices together into a commercially viable time-of-flight PET block detector. We will discuss it in detail including other challenge imposed by MPPC characteristics.

Multi-Pixel Photon Counters for TOF PET Detector and Its Challenges

The Multi-Pixel Photon Counter (MPPC) is a Geiger-mode avalanche photo-diode (APD) developed by Hamamatsu Corp. We propose that it could be a suitable photo-sensor for next-generation time-of-flight PET detectors due to its high photon detection efficiency. We concentrate on the measurement of coincidence timing performance of various MPPCs in conjunction with LYSO crystal scintillators. With 3 mm times 3 mm devices of 50 mum sub-pixels coupled to 3 mm times 3 mm times 10 mm LYSO crystals, we have demonstrated a strong dependence of timing performance on over-voltage and temperature, and analyzed how changes in photon detection efficiency and dark counts would explain the measurements. The best coincidence timing resolution we have achieved between two identical LYSO/MPPC detectors was 240 ps in FWHM. This was worse than the timing resolution of 220 ps obtained with a Hamamatsu H6533 fast PMT, and contradicted the expected improvement from higher photon detection efficiency. The contradiction could be explained by the slow rise-time of MPPC pulse shape, transit time spread, dark counts and electronic noise from the large capacitance of the MPPC. In particular, the slow rise-time of the MPPC pulse suggests the need for a very low trigger threshold on the timing circuit. Since this makes the detector system more sensitive to noise, this poses additional challenges for ganging multiple devices together into a commercially viable time-of-flight PET block detector. We will discuss this work in detail including other challenge imposed by MPPC characteristics.

Time-of-Flight PET Detector Based on Multi-Pixel Photon Counter and Its Challenges

Geiger-mode multi-pixel APD is being recognized as the best alternative solid-state photo-sensor to vacuum PMT for various specific applications. Especially, its magnetic field immunity and high gain made it popular in MR/PET detector research. In this paper, we utilized its compactness, high gain and high photon detection efficiency in the design of TOF PET detector. In a typical block detector based on PMT, the full timing capability of both PMT and scintillator could not be achieved due to its light sharing for Anger logic scheme. Since Geiger-mode APD is a solid-state based technology, we can apply one-to-one coupling between a scintillator and the photo-sensor to optimize the signal-to-noise ratio. Also, the high photon detection efficiency of MPPC, Geiger mode APD from Hamamatsu, would help to improve timing resolution. So, we made a block detector based on a 4 × 4 array of 3 × 3 mm2 MPPC coupled to a 4 × 4 array of 3 × 3 × 25 mm3 LYSO crystals to evaluate its performance. We have achieved the average of 9% energy resolution and 314 ps coincidence timing resolution with very good uniformity. This block timing resolution showed no degradation in timing compared to individual single channel timing resolution as expected from one-to-one readout. On top of that, the result proves that the solid-state based photo-sensor can be used for TOF PET detector. During the development and setup of the detector, we recognized that a compact and low power electronics readout scheme is one of the biggest challenges, including its cost, for MPPC or other Geiger-mode APD to be used in products.

Dependence of timing resolution on crystal size for TOF PET

Recently, with the prospect of great improvement in image quality, the development of time of flight technology has become an exciting topic for positron emission tomography. The excitement was further accelerated by the introduction of various fast and high light output scintillators as well as photosensors. However, the development of improved time of flight detectors is not only about the selection of crystals and photosensors, but also about how detectors are assembled to optimize their performance. For example, depending on crystal block structure, photo-sensor layout, and coupling methods, a detectors timing resolution can be drastically different. Since the effect of block structure for timing resolution is complex and less understood it is essential to first dissect the block structure and understand the impact of its basic components on timing resolution. In this paper, we will present the dependence of timing resolution on varying the dimensions of the scintillator crystals that are the main component of a block detector.

A compact SPECT detector based on a quad PMT

Traditionally, the technology of general purpose SPECT cameras has been based on a large panel of NaI:Tl scintillator, optically coupled with a number of 2” or 3” PMTs [1]. With its rather good performance at low cost, the SPECT camera design has not changed essentially since Hal Anger invented it in the late 1950s. For the last decade, however, with progress in new scintillators and photosensors, there have been renewed efforts to improve spatial and energy resolutions. A series of new compact detectors have been mostly designed for small animal or organ specific SPECT cameras. In this paper, we present a concept of a compact SPECT detector design using a NaI:Tl crystal array and a quad anode PMT. By using a crystal array and a light guide, we demonstrate that all individual crystals can be identified without any dead area at the edge of the detector. This proves the possibility of a compact SPECT detector having high spatial resolution using an array of regular types of PMTs, for example, a 2×2 or 3×3 array of PMTs. Also, we show that this block structure design enables very small gaps at the edge of the detectors resulting in a more compact geometry when packed in full size SPECT cameras.

Time-of-flight PET detector based on multi-pixel photon counter

Geiger-mode multi-pixel APD is recognized as the best alternative solid-state photo-sensor for vacuum PMTs in various applications. Its magnetic field immunity and high gain have made it popular in MR/PET and high-energy physics detector research. In this paper, we utilized its compactness and high gain in the design of a TOF PET detector. In typical block detectors based on PMT, the full timing capability of both the PMT and scintillator could not be achieved due to the light sharing used in the Anger logic scheme. Since the Geiger-mode APD is a solid-state based technology, we can utilize one-to-one coupling between a scintillator and the photo-sensor to utilize the best timing capability of both. Also, the high photon detection efficiency of MPPC, Geiger mode APD from Hamamatsu, would help improve timing resolution. We made a block detector based on a 4 × 4 array of 3 × 3 mm2 MPPC coupled to a 4 × 4 array of 3 × 3 × 25 mm3 LYSO crystals to evaluate its performance. We have achieved an average energy resolution of 9% and 314ps coincidence timing resolution with very good uniformity. This shows that the solid-state based photo-sensor can be used for TOF PET detector. During the development of the detector, we recognized that a compact and low power electronic readout scheme is one of the biggest challenges, including its cost, for MPPC or other Geiger-mode APDs to be realized as products.