The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/Si/sub 0.5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number of scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO, with a sufficient amount of yttrium to modify the decay time, to be a potential candidate for depth-of-interaction determination in multicrystal PET detectors.

Properties of LYSO and recent LSO scintillators for phoswich PET detectors

To fully utilize positron emission tomography (PET) as a non-invasive tool for tissue characterization, dedicated instrumentation is being developed which is specially suited for imaging mice and rats. Semiconductor detectors, such as avalanche photodiodes (APDs), may offer an alternative to photomultiplier tubes for the readout of scintillation crystals. Since the scintillation characteristics of lutetium oxyorthosilicate (LSO) are well matched to APDs, the combination of LSO and APDs seems favourable, and the goal of this study was to build a positron tomograph with LSO-APD modules to prove the feasibility of such an approach. A prototype PET scanner based on APD readout of small, individual LSO crystals was developed for tracer studies in mice and rats. The tomograph consists of two sectors (86 mm distance), each comprising three LSO-APD modules, which can be rotated for the acquisition of complete projections. In each module, small LSO crystals (3.7×3.7×12 mm3) are individually coupled to one channel within matrices containing 2×8 square APDs (2.6×2.6 mm2 sensitive area per channel). The list-mode data are reconstructed with a penalized weighted least squares algorithm which includes the spatially dependent line spread function of the tomograph. Basic performance parameters were measured with phantoms and first experiments with rats and mice were conducted to introduce this methodology for biomedical imaging. The reconstructed field of view covers 68 mm, which is 80% of the total detector diameter. Image resolution was shown to be 2.4 mm within the whole reconstructed field of view. Using a lower energy threshold of 450 keV, the system sensitivity was 350 Hz/MBq for a line source in air in the centre of the field of view. In a water-filled cylinder of 4.6 cm diameter, the scatter fraction at the centre of the field of view was 16% (450 keV threshold). The count rate was linear up to 700 coincidence counts per second. In vivo studies of anaesthetized rats and mice showed the feasibility of in vivo imaging using this PET scanner. The first LSO-APD prototype tomograph has been successfully introduced for in vivo animal imaging. APD arrays in combination with LSO crystals offer new design possibilities for positron tomographs with finely granulated detector channels.

A prototype high-resolution animal positron tomograph with avalanche photodiode arrays and LSO crystals.

We have studied nonproportionality and intrinsic energy resolution of cerium doped YSO, GSO, LSO and LGSO crystals. While LSO and YSO have similar light output, GSO and LGSO have ca. 70% and 20% lower light output than LSO, respectively. YSO, as a compound containing fairly light elements, was expected to be proportional for light output vs. energy scale, like YAP:Ce. Surprisingly it is almost the same nonproportionality as LSO and GSO. Nonproportionality of YSO is followed by large values of intrinsic energy resolution. The comparison of nonproportionality of YSO-LSO and YAP LuAP pairs indicates that the high proportionality of scintillators is connected with the structure of the crystal and not with the presence of light elements. To our knowledge, this is the first study of nonproportionality and intrinsic resolution for LGSO.

YSO, LSO, CSO and LGSO. A study of energy resolution and nonproportionality

The measurement of depth of interaction (DOI) within detectors is necessary to improve resolution uniformity across the FOV of small diameter PET scanners. DOI encoding by pulse shape discrimination (PSD) has definite advantages as it requires only one readout per pixel and it allows DOI measurement of photoelectric and Compton events. The PSD time characteristics of various scintillators were studied with avalanche photodiodes (APD) and the identification capability was tested in multi-crystal assemblies with up to four scintillators. In the PSD time spectrum of an APD-GSO/LSO/BGO/CsI(Tl) assembly, four distinct time peaks at 45, 26, 88 and 150 ns relative to a fast test pulse, having resolution of 10.6, 5.2, 20 and 27 ns, can be easily separated. Whereas the number and position of scintillators in the multi-crystal assemblies affect detector performance, the ability to identify crystals is not compromised. Compton events have a significant effect on PSD accuracy, suggesting that photopeak energy gating should be used for better crystal identification. However, more sophisticated PSD techniques using parametric time-energy histograms can also improve crystal identification in cases where PSD time or energy discrimination alone is inadequate. These results confirm the feasibility of PSD DOI encoding with APD-based detectors for PET.

Investigation of depth-of-interaction by pulse shape discrimination in multicrystal detectors read out by avalanche photodiodes

Positron emission tomography (PET) is the most sensitive of all medical imaging modalities for quantitatively probing biologic processes at the molecular level. However, spatial resolution in PET is significantly inferior to that of other imaging modalities that can provide exquisite images of the anatomy, such as X-ray computed tomography (CT) or magnetic resonance (MR) imaging. It has been one of the outstanding challenges of the last decade to combine PET with these complementary imaging modalities in order to synergistically exploit the benefits of each modality and to enhance the role of PET in pre-clinical research as well as in clinical routine and research. The simple juxtaposition of tomographs around a common axial bed, such as with current PET/CT technology, is very successful in allowing sequential acquisition of PET and anatomical data. However, novel imaging combinations are being considered that would enable simultaneous, or at least concurrent, dual-modality imaging through combined PET/MR or PET/CT. The development of these new integrated instruments creates new bewildering challenges for PET detection systems, which, in addition to the ability to measure annihilation radiation in PET, must satisfy several other critical requirements.

Novel detector technology for clinical PET

The performance characteristics of GSO, LSO and YSO scintillators coupled to new "reverse" avalanche photodiodes (ReAPD) were investigated and compared to those of BGO. The energy and timing resolutions were measured as a function of photodiode bias and amplifier shaping time constant. While the performance of BGO and GSO is critically dependent on these operating parameters because the signal-to-noise ratio is degraded by the dark and excess noise from the ReAPD, that of the high-luminosity LSO and YSO scintillators is not. The energy resolution in these crystals is demonstrated to be limited by the scintillator resolution. Further analysis indicates that excess variance in the number of photons generated in the crystal is mostly responsible for their disappointing energy resolution. This feature appears to be related to the important non-linearity in scintillation response as a function of incident gamma-ray energy observed for these high-luminosity scintillators.

Investigation of GSO, LSO and YSO scintillators using reverse avalanche photodiodes

An improved version of a recently developed “Buried Junction” avalanche photodiode (APD), designed for use with scintillators, is described and characterized. This device, also called the “Reverse APD”, is designed to have a wide depletion layer and thus low capacitance, but to have high gain only for e–h pairs generated within the first few microns of the depletion layer. Thus it has high gain for light from scintillators emitting in the 400–600 nm range, with relatively low dark current noise and it is relatively insensitive to minimum ionizing particles (MIPs). An additional feature is that the metallurgical junction is at the back of the wafer, leaving the front surface free to be coupled to a scintillator without fear of junction contamination. The modifications made in this device, as compared with the earlier diode, have resulted in a lower excess noise factor, lower dark current, and much-reduced trapping. The electrical and optical characteristics of this device are described and measurements of energy and timing resolution of this device with several scintillators (BGO, LSO and GSO) of potential interest in high-energy physics and PET imaging systems are presented.

Radiation detection measurements with a new “Buried Junction” silicon avalanche photodiode

A new reach-through avalanche photodiode, designed for use with sources of short-wavelength light such as scintillators, is described. The device has a double junction p/sup +/-p-n-p/sup -/-n/sup +/ structure in which the central three layers, which comprise about 99% of the device thickness, are fully depleted. The p/sup +/ light-entry surface extends across the whole device and can be placed in contact with a scintillator. The multiplying p-n junction is buried and is located about 4 /spl mu/m below the p/sup +/-layer so that only primary photo-electrons generated by short-wavelength (i.e., strongly absorbed) light are fully multiplied. The p/sup -/-n/sup +/ junction, or array of junctions, is located at the back of the wafer and is surrounded by a guard-ring. Typical characteristics for a device 120 /spl mu/m thick and having a 25 mm/sup 2/ sensitive area, are a quantum efficiency (Q.E.) of 80% at 480 nm, a capacitance of 30 pF, operating voltage of <500 V, a speed of response of /spl sim/3 ns, a noise current of less than 1 pA/Hz/sup 1/2/ at a gain of 100, and an effective k value of .030.

A short-wavelength selective reach-through avalanche photodiode

Summary form only given. The authors discussed the properties of two new avalanche photodiodes (APDs) suited for application with scintillating fiber tracking systems, considered optimal packaging approaches for efficient optical coupling from scintillating fibers to APDs which allow the diode area to be minimized, and predicted their performance when used in either the Geiger mode, together with a fast quench circuit, or in the sub-Geiger mode, in conjunction with a low-noise preamplifier. In either case, detection efficiencies of over 85% for a five-photon pulse were predicted and measured. The use of these devices in the linear mode, with possible application to scintillating fiber calorimeters, was also considered. It was shown both theoretically and experimentally that, for short pulses (a few nanoseconds) of greater than about 10 photons, better signal-to-noise ratios are possible using APDs than using photomultiplier tubes. >

P.P. Webb

Papers

Investigation of GSO, LSO and YSO scintillators using reverse avalanche photodiodes

Radiation detection measurements with a new “Buried Junction” silicon avalanche photodiode

A short-wavelength selective reach-through avalanche photodiode

High photon detection efficiency avalanche photodiodes designed for use with scintillating fibers