K.S. Shah

Depth of interaction resolution measurements for a high resolution PET detector using position sensitive avalanche photodiodes

We explore dual-ended read out of LSO arrays with two position sensitive avalanche photodiodes (PSAPDs) as a high resolution, high efficiency depth-encoding detector for PET applications. Flood histograms, energy resolution and depth of interaction (DOI) resolution were measured for unpolished LSO arrays with individual crystal sizes of 1.0, 1.3 and 1.5 mm, and for a polished LSO array with 1.3 mm pixels. The thickness of the crystal arrays was 20 mm. Good flood histograms were obtained for all four arrays, and crystals in all four arrays can be clearly resolved. Although the amplitude of each PSAPD signal decreases as the interaction depth moves further from the PSAPD, the sum of the two PSAPD signals is essentially constant with irradiation depth for all four arrays. The energy resolutions were similar for all four arrays, ranging from 14.7% to 15.4%. A DOI resolution of 3-4 mm (including the width of the irradiation band which is approximately 2 mm) was obtained for all the unpolished arrays. The best DOI resolution was achieved with the unpolished 1 mm array (average 3.5 mm). The DOI resolution for the 1.3 mm and 1.5 mm unpolished arrays was 3.7 and 4.0 mm respectively. For the polished array, the DOI resolution was only 16.5 mm. Summing the DOI profiles across all crystals for the 1 mm array only degraded the DOI resolution from 3.5 mm to 3.9 mm, indicating that it may not be necessary to calibrate the DOI response separately for each crystal within an array. The DOI response of individual crystals in the array confirms this finding. These results provide a detailed characterization of the DOI response of these PSAPD-based PET detectors which will be important in the design and calibration of a PET scanner making use of this detector approach.

Thallium bromide radiation detectors

Crystals of highly purified thallium bromide were used to fabricate radiation detectors. These detectors were rested with isotopic photon sources and successfully detected gamma and X-rays in the 0.01- to 1 MeV energy range. The detectors were tested at and below room temperature, and their performance approaches that of other semiconductor detector materials which have been under development. The performance of TlBr detectors is due to the similarity in the transport characteristics of electrons and holes in TlBr and the relatively large number of charge carriers produced per unit of energy of detected particle. The performance of these detectors is currently limited by shot noise induced by the leakage current in the TlBr. >

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.

A Prototype PET Scanner with DOI-Encoding Detectors

Detectors with depth-encoding allow a PET scanner to simultaneously achieve high sensitivity and high spatial resolution. Methods: A prototype PET scanner, consisting of depth-encoding detectors constructed by dual-ended readout of lutetium oxyorthosilicate (LSO) arrays with 2 position-sensitive avalanche photodiodes (PSAPDs), was developed. The scanner comprised 2 detector plates, each with 4 detector modules, and the LSO arrays consisted of 7 × 7 elements, with a crystal size of 0.9225 × 0.9225 × 20 mm and a pitch of 1.0 mm. The active area of the PSAPDs was 8 × 8 mm. The performance of individual detector modules was characterized. A line-source phantom and a hot-rod phantom were imaged on the prototype scanner in 2 different scanner configurations. The images were reconstructed using 20, 10, 5, 2, and 1 depth-of-interaction (DOI) bins to demonstrate the effects of DOI resolution on reconstructed image resolution and visual image quality. Results: The flood histograms measured from the sum of both PSAPD signals were only weakly depth-dependent, and excellent crystal identification was obtained at all depths. The flood histograms improved as the detector temperature decreased. DOI resolution and energy resolution improved significantly as the temperature decreased from 20°C to 10°C but improved only slightly with a subsequent temperature decrease to 0°C. A full width at half maximum (FWHM) DOI resolution of 2 mm and an FWHM energy resolution of 15% were obtained at a temperature of 10°C. Phantom studies showed that DOI measurements significantly improved the reconstructed image resolution. In the first scanner configuration (parallel detector planes), the image resolution at the center of the field of view was 0.9-mm FWHM with 20 DOI bins and 1.6-mm FWHM with 1 DOI bin. In the second scanner configuration (detector planes at a 40° angle), the image resolution at the center of the field of view was 1.0-mm FWHM with 20 DOI bins and was not measurable when using only 1 bin. Conclusion: PET scanners based on this detector design offer the prospect of high and uniform spatial resolution (crystal size, ∼1 mm; DOI resolution, ∼2 mm), high sensitivity (20-mm-thick detectors), and compact size (DOI encoding permits detectors to be tightly packed around the subject and minimizes number of detectors needed).

LaBr/sub 3/:Ce scintillators for gamma ray spectroscopy

In this paper, we report on a relatively new scintillator - LaBr/sub 3/ for gamma ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. This material when doped with cerium has high light output (/spl sim/60,000 photons/MeV) and fast principal decay constant (/spl les/25 ns). Furthermore, it shows excellent energy resolution for /spl gamma/-ray detection. Energy resolution of 3.2% (FWHM) has been achieved for 662 keV photons (/sup 137/Cs source) at room temperature. High timing resolution (260 ps FWHM) has been recorded with LaBr/sub 3/-PMT and BaF/sub 2/-PMT detectors operating in coincidence mode using 511 keV positron annihilation /spl gamma/-ray pairs. Details of its scintillation properties, and variation of these properties with changing cerium concentration are reported. Potential applications of this material are also addressed.

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.

Properties of lead iodide semiconductor radiation detectors

Abstract Semiconductor radiation detectors have been fabricated from melt grown crystals of lead iodide (PbI 2 ) and the performance of these detectors as room temperature X-ray spectrometers has been measured. These detectors exhibit good energy resolution (915 eV FWHM for the 5.9 keV peak of 55 Fe at 20°C). Preliminary results indicate they are more stable than HgI 2 detectors and capable of operating at temperatures over 100°C.

LaCl3:Ce scintillator for γ-ray detection

Abstract In this paper, we report on a relatively new cerium-doped scintillator—LaCl 3 for γ-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman method. This material when doped with 10% cerium has high light output (∼50,000 photons/MeV) and fast principal decay time constant (∼20xa0ns). Furthermore, it shows excellent energy resolution for γ-ray detection. For example, energy resolution as low as 3.2% (FWHM) has been achieved with 662xa0keV photons ( 137 Cs source) at room temperature. Also, high timing resolution (264xa0ps—FWHM) has been recorded with LaCl 3 -PMT and BaF 2 -PMT detectors operating in coincidence using 511xa0keV positron annihilation γ-ray pairs. Details of crystal growth, scintillation properties, and variation of these properties with cerium concentration are also reported.

Lead iodide X-ray detection systems

Recent progress in the development of room-temperature lead iodide (Phi,) X-ray detectors is reported. Progress has been made in the areas of detector fabrication and preamplifier electronics, and this has resulted in improved detection performance. An energy resolution of 415 eV (FWHM) has been reported for 5.9 keV X-rays (“Fe source) with 1 mm’ detector at room temperature. A better estimation of the Fano factor in PbIz has been carried out and the upper limit of the Fano factor is calculated to be 0.19. Larger lead iodide detectors (up to 2.5 mm*) have been fabricated and their spectroscopic performance has been evaluated. The timing characteristics of lead iodide detectors have been investigated. A compact, portable lead iodide probe assembly has been designed and built for X-ray spectroscopic applications. Finally, optical and charge particle detection properties of lead iodide detectors have also been characterized.