Carolyn R. Tull

HgI/sub 2/ polycrystalline films for digital X-ray imagers

This paper describes recent results obtained with mercuric iodide (HgI/sub 2/) polycrystalline films that we have produced. The ultimate goal of this effort is to develop a new detector technology for digital X-ray imaging based on HgI/sub 2/ polycrystalline films coupled to large-area flat-panel amorphous silicon thin-film transistor-addressed readout arrays. We have employed two approaches for producing the polycrystalline films: 1) thermal evaporation (sublimation) and 2) deposition of films from various solutions. The 50- to 150-/spl mu/m-thick films were characterized with respect to their electrical properties and in response to ionizing radiation. The leakage current was about 40 pA/cm/sup 2/ at an operating bias voltage of /spl sim/50 V. Signals from the HgI/sub 2/ polycrystalline detectors, in response to ionizing radiation, compare favorably to the best published results for all high Z polycrystalline films grown elsewhere, including TlBr, PbI/sub 2/, and HgI/sub 2/.

Fast-timing silicon photodetectors

We are developing a detector module consisting of an array of Lutetium Orthosilicate (LSO) scintillation crystals coupled to a new type of silicon photodetector array specifically designed to provide the excellent energy and timing resolution needed for positron emission tomography. We have designed, fabricated and evaluated prototypical detector structures including single 2/spl times/2 mm/sup 2/ pixel test structures and eight-by-eight arrays of the 2/spl times/2 mm/sup 2/ pixels. Leakage current of /spl sim/1 pA per pixel and electronic noise of /spl sim/26 e/sup -/ RMS were obtained with thermoelectric cooling. Quantum efficiency was 40% at 460 nm and 70% at 560 nm. Variation in photoresponse (/spl plusmn//spl sigma/%) over the pixel active area was less than 0.3%. The coincidence resolving time with a LSO crystal was 16 ns. Energy resolutions of 6%, 10% and 13% FWHM for Cs-137 (662 keV) were obtained at the pixel level with 2/spl times/2/spl times/4 mm/sup 3/ CsI(Tl), 2/spl times/2/spl times/10 mm/sup 3/ BGO, and 2/spl times/2/spl times/10 mm/sup 3/ LSO crystals, respectively.

A novel silicon array designed for intraoperative charged particle imaging.

A novel Si-PIN imaging array is under investigation for a charged particle (beta, positron, or alpha) sensitive intraoperative camera to be used for (residual) tumor identification during surgery. This class of collimator-less nuclear imaging device has a higher signal response for direct interactions than its scintillator-optical detector-based counterparts. Monte Carlo simulations with 635 keV betas were performed, yielding maximum and projected ranges of 1.64 and 0.55 mm in Si. Up to 90% of these betas were completely absorbed in the first 0.30 mm. Based on these results, 300 microm thick prototype Si detector arrays were designed in a 16 x 16 crossed-grid arrangement with 0.8 mm wide orthogonal strips on 1.0 mm pitch. A NIM- and CAMAC-based high-density data acquisition and processing system was used to collect the list mode data. The system was calibrated by comparisons of measured spectra to energy deposition simulations or by direct measurement of various >100 keV conversion electron or beta emitters. Mean electronic noise per strip was <3.6 keV FWHM at room temperature. When detecting positrons, which have an accompanying 511 keV annihilation background, the flood irradiated beta/gamma ratio was approximately 40, indicating that beta images could be made without the use of background rejection techniques. The intrinsic spatial resolution corresponds to the 1 x 1 mm2 pixel size, and measurements of beta emitting point and line sources yielded FWHM resolutions of 1.5 (lateral) and 2.5 mm (diagonal), respectively, with the larger widths due to particle range blurting effects. Deconvolution of the finite source size yielded intrinsic resolutions that corresponded to the image pixel size. Transmission images of circle and line phantoms with various hole sizes and pitch were resolved with either pure beta or positron irradiation without a background correction. This novel semiconductor imaging device facilitates high charged particle and low gamma sensitivity, high signal/noise ratio, and allows for compact design to potentially aid surgical guidance by providing in situ images of clinical relevance.

Intravascular probe for detection of vulnerable plaque

Coronary angiography is unable to define the status of the atheroma, and only measures the luminal dimensions of the blood vessel, without providing information about plaque content. Up to 70% of heart attacks are caused by minimally obstructive vulnerable plaques, which are too small to be detected adequately by angiography. We have developed an intravascular imaging detector to identify vulnerable coronary artery plaques. The detector works by sensing beta or conversion electron radiotracer emissions from plaque-binding radiotracers. The device overcomes the technical constraints of size, sensitivity and conformance to the intravascular environment. The detector at the distal end of the catheter uses six 7mm long by 0.5mm diameter scintillation fibers coupled to 1.5m long plastic fibers. The fibers are offset from each other longitudinally by 6mm and arranged spirally around a guide wire in the catheter. At the proximal end of the catheter the optical fibers are coupled to an interface box with a snap on connector. The interface box contains a position sensitive photomultiplier tube (PSPMT) to decode the individual fibers. The whole detector assembly fits into an 8-French (2.7 mm in diameter) catheter. The PSPMT image is further decoded with software to give a linear image, the total instantaneous count rate and an audio output whose tone corresponds to the count rate. The device was tested with F-18 and Tl-204 sources. Spectrometric response, spatial resolution, sensitivity and beta to background ratio were measured. System resolution is 6 mm and the sensitivity is >500 cps / micrometers Ci when the source is 1 mm from the detector. The beta to background ratio was 11.2 for F-18 measured on a single fiber. The current device will lead to a system allowing imaging of labeled vulnerable plaque in coronary arteries. This type of signature is expected to enable targeted and cost effective therapies to prevent acute coronary artery diseases such as: unstable angina, acute myocardial infarction, and sudden cardiac death.

Development of new high resolution detectors for small animal SPECT imaging

Summary form only received as follows: New high-resolution gamma-ray detectors are being developed for SPECT imaging of animals. Optimal collimators, reconstruction algorithms and image processing are an important part of the development of animal SPECT. These issues and recent advances in miniature gamma-ray detectors will be presented. Two lines of technical developments are being followed for high-performance gamma-ray imaging systems: the first uses state-of-the-art position-sensitive photomultiplier tubes (PS-PMTs); the second uses silicon-based photodiode detector arrays. Each are coupled to arrays of scintillation crystals. A small system has been assembled with newly developed PS-PMTs that has 1 mm CsI(Na) crystals and was fitted with a hexagonal parallel hole collimator (1.25 mm holes, 0.25 mm septa, 2 cm long). Line spread functions (LSF) were measured with 0.5 mm line sources filled with Tc-99m. The average FWHM of the LSF curves were 2.12/spl plusmn/0.16 mm and 2.63/spl plusmn/0.15 mm for 0 cm and 1 cm source-collimator distance, respectively. A LSF was made with 1.1 mm pitch (0.85 mm CsI(Tl), 0.25 mm spacing) CsI(Tl)-PIN detector collimated with matching geometry 1 cm long collimator. Mean FWHM=1.91/spl plusmn/0.32 mm. Preliminary images from each system are presented.

New LSO based scintillators

Cerium-doped lutetium oxyorthosilicate (LSO) is amongst the most promising new scintillators discovered in almost five decades, with a unique combination of important properties for X and gamma-ray spectroscopy, namely: high density, fast decay, and large light yield. LSO seems to be a prime candidate to replace BGO in PET systems. However, the practical utilization of LSO is hindered by difficulties related to crystal growth (Czochralski method) due to the high temperatures employed. A new approach has been developed using a low-temperature crystal growth technology to produce scintillating LSO crystals. Light transparent polycrystalline LSO samples of a few mm/sup 3/ in volume were grown and characterized by XRD, optical absorption, light decay measurement and gamma-ray spectral response. The properties of the new crystals compared well with high-quality crystals grown by the Czochralski method.

Vortex: a new high performance silicon multicathode detector for XRD and XRF applications

VortexTM, a high performance Silicon Multi-Cathode Detector (SMCD), has been developed and extensively tested for potential X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) applications. As a type of Silicon Drift Detector (SEE), it utilizes our patented structure design and has achieved very low capacitance and very low leakage current with a relatively large active area (~50 mm2). Results will be presented to demonstrate its superior performance over the conventional cryogenic Si(Li) detectors, especially in the resolution and throughput at short peaking times. The detector operates at near room temperature and is thus very compact in size. These features make it idea for XRD and XRF applications.

High throughput high resolution Vortex/spl trade/ detector for X-ray diffraction

Vortex/spl trade/ is a multicathode drift-type X-ray detector produced from high-purity silicon using state-of-the-art CMOS production technology . Based on the Vortex/spl trade/ detector, we have developed a compact detector package for X-ray diffraction applications. The spectrometric package contains a 50 mm/sup 2/ detector cooled using a small Peltier element. The detector package was interfaced with a digital pulse processor, and its performance was tested as a function of the input count rate and pulse peaking time. A pure Cu sample was irradiated with varying flux from an X-ray generator, and the output count rate and energy resolution were measured. The system was able to operate at very high rate (>1 Mcps) with virtually zero loss in resolution and no peak shift. Finally, the performance of the Vortex/spl trade/ system was tested on a Philips (Model PW 1835) powder diffractometer, replacing the sealed tube proportional counter and the graphite monochromator with the silicon detector. The elimination of the graphite monochromator resulted in improvement of the detection efficiency by a factor of 2.5 and improvement in the detection limits due to the low background of the semiconductor detector.

Multi-element linear array of silicon detectors for imaging beta emitting compounds in the coronary arteries

Coronary artery disease (CAD) is the leading cause of death in most developed countries. A large portion of CAD is caused by rupture of unstable plaque, which is not detectable by current diagnostic methods. By labeling the unstable plaque with beta emitting radioisotopes, it is possible to detect these plaques with a very narrow in situ detector system. Our intra-vascular detector system is an imaging device, consisting of several silicon detectors mounted on a flexible PC-board inside of a 1.6 mm diameter catheter. The catheter shields the detectors from outside light and enables the device to he guided to the coronary arteries during an angiography session. Each silicon detector consists of a linear array of 20 square pixels,with pixel dimensions of 0.45 mm /spl times/ 0.45 mm.

Novel x-ray and gamma-ray drift detectors based on silicon and compound semiconductors

Large area silicon drift detectors (SDD) with areas up to approximately 1 cm2 have been fabricated for x-rays. Recent novel designs have produced very low dark current, high electric field, and hence low noise and good charge collection. The developed structures were evaluated with low noise input amplification electronic components on Peltier coolers so that the temperature could be adjusted. Energy resolution of 143 eV FWHM at 5.9 keV was measured with a 50 mm2 SDD whose corresponding noise level was 70 eV FWHM.