S. P. Stoll

A study on the properties of lead tungstate crystals

Abstract This report summarizes the results of a study on the properties of five large and five small size lead tungstate (PbWO 4 ) crystals. Data are presented on the longitudinal optical transmittance and light attenuation length, light yield and response uniformity, emission spectra and decay time. The radiation resistance of large crystals and possible curing with optical bleaching are discussed. The result of an in depth materials study, including trace impurities analysis, are also presented. The general conclusion from this investigation is that further research and development is needed to develop fast, radiation-hard PbWO 4 crystals for the CMS experiment at the CERN LHC.

Small animal simultaneous PET/MRI: initial experiences in a 9.4 T microMRI

We developed a non-magnetic positron-emission tomography (PET) device based on the rat conscious animal PET that operates in a small-animal magnetic resonance imaging (MRI) scanner, thereby enabling us to carry out simultaneous PET/MRI studies. The PET detector comprises 12 detector blocks, each being a 4 × 8 array of lutetium oxyorthosilicate crystals (2.22 × 2.22 × 5 mm(3)) coupled to a matching non-magnetic avalanche photodiode array. The detector blocks, housed in a plastic case, form a 38 mm inner diameter ring with an 18 mm axial extent. Custom-built MRI coils fit inside the positron-emission tomography (PET) device, operating in transceiver mode. The PET insert is integrated with a Bruker 9.4 T 210 mm clear-bore diameter MRI scanner. We acquired simultaneous PET/MR images of phantoms, of in vivo rat brain, and of cardiac-gated mouse heart using [(11)C]raclopride and 2-deoxy-2-[(18)F]fluoro-D-glucose PET radiotracers. There was minor interference between the PET electronics and the MRI during simultaneous operation, and small effects on the signal-to-noise ratio in the MR images in the presence of the PET, but no noticeable visual artifacts. Gradient echo and high-duty-cycle spin echo radio frequency (RF) pulses resulted in a 7% and a 28% loss in PET counts, respectively, due to high PET counts during the RF pulses that had to be gated out. The calibration of the activity concentration of PET data during MR pulsing is reproducible within less than 6%. Our initial results demonstrate the feasibility of performing simultaneous PET and MRI studies in adult rats and mice using the same PET insert in a small-bore 9.4 T MRI.

Design and performance of 0.18-/spl mu/m CMOS charge preamplifiers for APD-based PET scanners

The CMOS 0.18-/spl mu/m technology was investigated for two analog front-end projects: the low-power budget rat-head mounted miniature rat conscious animal PET (RatCAP) scanner, and the high-performance, low-noise, high-rate PET/CT application. The first VLSI prototypes consisted of 1- and 5-mW charge sensitive preamplifiers (CSP) based on a modified cascode telescopic architecture. Characterization of the rise time, linearity, dynamic range, equivalent noise charge (ENC), timing resolution and energy resolution are reported and discussed. When connected to an APD-LSO detector, time resolutions of 2.49 and 1.56 ns full-width half-maximum (FWHM) were achieved by the 1- and 5-mW CSPs, respectively. Both CSPs make it possible to achieve performance characteristics that are adequate for PET imaging. Experimental results indicate that the CMOS 0.18-/spl mu/m technology is suitable for both the low-power and the high-performance PET front-end applications.

Further results on cerium fluoride crystals

Abstract A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the “Crystal Clear” collaboration in view of a p

Digital Coincidence Processing for the RatCAP Conscious Rat Brain PET Scanner

The RatCAP has been designed and constructed to image the awake rat brain. In order to maximize system performance, offline digital coincidence data processing algorithms including offset delay correction and prompt and delayed coincidence detection have been developed and validated. With offset delay correction using a singular value decomposition (SVD) technique, overall time resolution was improved from 32.6 to 17.6 ns FWHM. The experimental results confirm that the ratio of prompts to randoms was improved because a narrower timing window could be used. 18F-fluoride rat bone scan data were reconstructed using our fully 3-D ML-EM algorithm with a highly accurate detector response model created from Monte Carlo simulation.

Radiation damage in undoped CsI and CsI (Tl)

Radiation damage has been studied in undoped CsI and CsI(Tl)-crystals using /sup 60/Co gamma radiation for doses up to approximately 4.2*10/sup 6/. Samples from various manufacturers were measured ranging in size from 2.54-cm-long cylinders to a 30-cm-long block. Measurements were made on the change in optical transmission and scintillation light output as a function of dose. Although some samples showed a small change in transmission, a significant change in light output was observed for all samples. Recovery from damage was also studied as a function of time and exposure to UV light. A short-lived phosphorescence was observed in undoped CsI, similar to the phosphorescence seen in CsI(Tl).<<ETX>>

A Simultaneous PET/MRI scanner based on RatCAP in small animals

The ability to acquire high resolution anatomical data as well as quantitative functional information in vivo is becoming an increasingly important factor in the diagnosis of disease. Simultaneous acquisition of PET and MRI data would provide essentially perfect co-registration between the two images which is particularly important for tissues whose position and shape can change between sequential scans. RatCAP is a complete 3D tomograph that is designed to image the brain of an awake rat. A special MRI coil composed of 2 saddle elements working in quadrature mode was mounted on a Delrin cylinder specifically designed to fit inside the RatCAP but allowing the rats head to be placed inside as well. Simultaneous PET/MRI images of the rat brain have been acquired in a 4 T MRI scanner using the RatCAP detector, with minimal effect on MRI images.

A study on the radiation hardness of lead tungstate crystals

This report presents recent progress of a study on the radiation damage in lead tungstate (PbWO/sub 4/) crystals. The dose rate dependence of radiation damage in PbWO/sub 4/ has been observed, confirming our early prediction based upon a kinetic model of color centers. An optimization of the oxygen compensation through post-growth thermal annealing, carried out in Shanghai Institute of Ceramics, has led to PbWO/sub 4/ crystals with significantly improved radiation hardness. A comparison between front versus uniform irradiations revealed that the later caused a factor of 2 to 6 times more severe damage. A measurement of a preliminary batch of lanthanum doped PbWO/sub 4/ crystals indicates that the La doping seems not a determine factor for PbWO/sub 4/ radiation hardness improvement. Finally, a TEM/EDS analysis confirmed our previous conjecture that the radiation damage in PbWO/sub 4/ crystals is caused by oxygen vacancies.

Results from prototype II of the BNL simultaneous PET-MRI dedicated breast scanner

At Brookhaven National Laboratory, we are developing a simultaneous PET-MRI breast imaging system. A prototype II version of the PET system has been constructed. This device consists of 24 detector blocks where each block consists of a 4 × 8 array of 2.2 × 2.2 × 15 mm3 LYSO crystal directly coupled to a 4 × 8 non-magnetic APD array. The scanner has an inner diameter of 100mm and an axial extent of 18mm. Resolution measurements were carried out for the prototype system to evaluate the depth of interaction effects. Average resolution less than 2mm FWHM was maintained throughout the field of view. The prototype PET system was operated unshielded inside the RF coil of the Aurora 1.5 T dedicated breast MRI machine. Artifact free MRI images with good SNR were obtained.

Development of a simultaneous PET/MRI scanner

A combined magnetic resonance imaging (MRI) and positron emission tomography (PET) scanner would be a great benefit to nuclear medicine. The anatomical detail given by MRI and spectroscopy available with magnetic resonance spectroscopy (MRS) complement the quantitative physiological imaging obtained with PET. Such a device has not become a reality because of the incompatibilities of photomultiplier tubes (PMTs) and their associated electronics with MRIs high magnetic fields, as well as significant constraints on PET camera size due to the limited patient port of MR scanners. Recent advances in solid-state electronics have opened the possibility of replacing photomultiplier tubes with avalanche photodiodes (APDs) that are compact and do not share the vulnerabilities of PMTs to magnetic fields. Currently, we are planning to build a miniature PET tomograph using only solid-state electronics to give a combination MRI/PET scanner for small animals. This technology, once developed, can be extended to human scanner designs.