Ronald Grazioso

Position-sensitive avalanche photodiodes for gamma-ray imaging

In this paper, we report on the investigation of silicon avalanche photodiodes (APDs) for high-energy photon imaging applications. This includes a new APD design that provides X-ray and /spl gamma/-ray imaging with significant reduction in electronic readout requirements. This new APD design, referred to as position-sensitive avalanche photodiode (PSAPD), involves charge sharing amongst the electrodes that enable determination of position of interaction. PSAPDs with 14 /spl times/ 14 mm/sup 2/ area have been fabricated using planar processing. The performance of these devices has been evaluated for energy resolution, timing resolution (4 ns full-width at half-maximum), and spatial resolution (/spl sim/300 /spl mu/m intrinsic spatial resolution). The potential of these APDs in high-energy physics and medical imaging is addressed.

Design studies of a high resolution PET detector using APD arrays

The authors evaluated a compact, high resolution PET detector module using avalanche photodiode (APD) arrays to replace bulky position sensitive PMTs. The newly developed APD array is a planar processed 4/spl times/4 array which has a 2/spl times/2 mm/sup 2/ pixel size with 0.4 mm gaps between pixels, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (>1000) across all channels. A 4/spl times/4 array of 2/spl times/2/spl times/10 mm/sup 3/ LSO crystals was coupled to an APD array. Different readout electronics and signal multiplexing schemes were explored. All crystals in the detector array were clearly identified in the flood source histogram, with average peak-to-valley ratios of about 12:1 using a charge sharing resistor network. The energy resolution was measured to be /spl sim/14% at 511 keV in the detector array. The measured timing resolution was 2.6 ns in coincidence with a LSO/PMT detector. By optimizing the readout electronics currently being used, it is likely that detector performance can be further improved. The authors have also determined depth-of-interaction (DOI) by reading out two APD arrays connected to the ends of a 2/spl times/2/spl times/22 mm/sup 3/ LSO crystal. Preliminary measurements show good DOI measurement capability with DOI positioning uncertainty between 4 and 6.5 mm.

Dual APD array readout of LSO crystals: optimization of crystal surface treatment

Summary form only received as follows: The authors are developing a compact PET detector module by coupling an LSO scintillator array with two APD arrays to achieve high sensitivity, and high and uniform spatial resolution. They report studies on improving the depth-of-interaction (DOI) resolution by optimizing the crystal surface treatment, and on the effect of crystal geometry on DOI resolution. Three 2/spl times/2/spl times/20 mm LSO crystals were treated with different surface finishes along their length: raw saw cut, polished with 12 mm grade AlO2 paper, and fine mirror polish. The 2/spl times/2 mm ends were fine mirror polished. The ratio of the signals from the APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished with the 12 mm grade paper gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for all interactions with energy above /spl sim/150 keV threshold, and uniform light output for different DOI positions. The energy resolution averaged /spl sim/17%. A 1/spl times/1/spl times/20 mm and a 2/spl times/2/spl times/30 mm LSO crystals finished with saw-cut and 12 /spl mu/m grade paper were also measured, and gave DOI resolutions in the range of 2.7 to 4.4 mm, and 4.7 to 6.6 mm, respectively.

Position sensitive APDs for small animal PET imaging

In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8/spl times/8 mm/sup 2/ and 14/spl times/14 mm/sup 2/ area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV /spl gamma/-rays and coincidence timing resolution of /spl sim/3 ns (FWHM) have been recorded with PSAPD coupled to 1/spl times/1/spl times/20 mm/sup 3/ LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of <4.5 mm (FWHM) has been measured by coupling two PSAPDs on opposite ends of a 20 mm long LSO crystal with a 1/spl times/1 mm/sup 2/ cross section. Based on these results, PSAPDs appear to be promising for high resolution PET. An important advantage of these PSAPDs is significant reduction in electronic readout requirements.

Performance measurements of a depth-encoding PET detector module based on position-sensitive avalanche photodiode read-out

We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 x 7 matrix of LSO scintillator crystals (1 x 1 x 20 mm3 in dimension) coupled to 8 x 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3-4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

SiPM performance in PET applications: An experimental and theoretical analysis

Silicon Photomultipliers (SiPMs) are increasingly being studied for their use in clinical and pre-clinical PET applications. Many groups have evaluated the performance of Multi-Pixel Photon Counters (MPPCs) from Hamamatsu Photonics. When coupled to PET scintillator crystals, these devices have shown promising results in terms of energy and timing resolution. The purpose of this paper is to analyze the main factors that determine the performance of SiPM based PET detectors and to provide guidelines for further optimization towards the performance levels of state-of-the-art PMT detectors. We present a statistical signal analysis that links the energy and time resolution to fundamental device characteristics, such as photon detection efficiency, cell density, secondary avalanche probability and dark rate. The trade-offs and impact of these device parameters on the overall detector performance is analyzed and discussed.

Large area APDs and monolithic APD arrays

In this paper, development of large area planar APDs and monolithic APD arrays for scintillation detection is discussed. Single APDs with area as large as 10 cm2 have been fabricated and tested with CsI(Tl) scintillator (3.8 cm diameter, 2.5 cm high). The resolution of the 662 keV photopeak has been measured to be 9% (FWHM). Multi-element APD arrays have also been fabricated in various formats such as 4/spl times/4 to 14/spl times/14 elements (2 mm pixels) and their gain, noise and dark current performance has been characterized. Scintillation and timing studies have also been performed by coupling these arrays to LSO and CsI(Tl) scintillators. Packaging, and electronic readout issues related to these APD devices are discussed.

APD performance in light sharing PET applications

An APD-based, light-sharing detector has been evaluated for use in PET. The detector configuration is a 2 /spl times/ 2 array of 5 mm /spl times/ 5 mm Hamamatsu S8664-55 APDs used to readout multi-crystal LSO blocks. Initially, a basic performance study was undertaken with a single APD coupled to a chemically etched 4 mm /spl times/ 4 mm /spl times/ 10 mm LSO crystal (teflon wrapped) using a custom, single-channel fast ASIC preamplifier. Timing and energy resolution measurements with a /sup 22/Na source were performed using the monolithic LSO crystal coupled to the API). The timing resolution of the APD channel in coincidence with a plastic scintillator coupled to a PMT was 870 ps FWHM. The energy resolution of the 511 keV photopeak was 12.1% FWHM. Based on these initial results, a 9 /spl times/ 9 array of 2 mm /spl times/ 2 mm /spl times/ 20 mm LSO crystals was assembled and evaluated with a 2 /spl times/ 2 APD array. The LSO block had an average energy resolution of 20.9% and a timing resolution of 2.47 ns FWHM. These results show that APDs are promising photodetectors for high-resolution and cost-effective PET systems utilizing light-sharing block detectors.

RbGd/sub 2/Br/sub 7/:Ce scintillators for gamma-ray and thermal neutron detection

In this paper, we report on gamma ray and thermal neutron detection with RbGd2Br7:Ce scintillators. RbGd2Br7:Ce (RGB) is a new scintillator material, which shows high light output (56,000 photons/MeV) and has a fast principal decay constant (45 ns) when doped with 10 percent Ce. These properties make RGB an attractive scintillator for g-ray detection. Also, due to the presence of Gd as a constituent, RGB has a high cross section for thermal neutron absorption and can achieve close to 100 percent stopping efficiency with 0.5 mm thick RGB crystals. Crystals of RGB with three different Ce concentrations (0.1, 5, and 10 percent) have been grown and their basic scintillation properties such as light output, decay time, and emission spectrum have been measured. In addition, high efficiency thermal neutron detection has been confirmed in our studies.

Evaluation of an MR-compatible blood sampler for PET

The integration of magnetic resonance imaging (MRI) and positron emission tomography (PET) is an upcoming hybrid imaging technique. Prototype scanners for pre-clinical and clinical research have been built and tested. However, the potential of the PET part can be better exploited if the arterial input function (AIF) of the administered tracer is known. This work presents a dedicated MR-compatible blood sampling system for precise measurement of the AIF in an MR-PET study. The device basically consists of an LSO/APD-detector assembly which performs a coincidence measurement of the annihilation photons resulting from positron decays. During the measurement, arterial blood is drawn continuously from an artery and lead through the detector unit. Besides successful tests of the MR compatibility and the detector performance, measurements of the AIF of rats have been carried out. The results show that the developed blood sampling system is a practical and reliable tool for measuring the AIF in MR-PET studies.