Clinical CT/SPECT systems acquire CT and SPECT data sequentially using different detectors in close proximity to minimise patient movement and interscan delay. We have developed a prototype simultaneous CT/SPECT imager, using a single CdZnTe detector, with the goal of improving image coregistration and decreasing scan time. A 16-pixel CdZnTe detector was operated in pulse-counting mode with 50ns shaping time. Energy discrimination is used to separate the CT and SPECT data. Simultaneous SPECT and CT images were obtained for a phantom with the X-ray flux limited to reduce pulse pile-up in the radionuclide energy window. At 140keV, the efficiency and energy resolution are 70% and 10%, respectively, and were constant for fluence rates up to 103cps per detector element for 140keV gamma rays, but degrade rapidly at higher fluence rates. In pulse-counting mode, the maximum count rate of 103cps per element from the CdZnTe detector is sufficient for SPECT imaging, but is considerably lower than the fluence rates encountered in CT. The smallest lesion visually detectable in SPECT is 9mm and the CT spatial resolution is smaller than 4.5mm. Image registration is intrinsic because the data can be acquired simultaneously with a single detector with the same reconstruction geometry.

Polycrystalline lead iodide (PbI2) is one of only a few materials commonly mentioned as a potential direct converter for digital X-ray sensors. Previous evaluations have noted higher than desirable leakage currents and commented on imaging characteristics from sensors constructed with PbI2 films deposited onto amorphous Si thin film transistor arrays. Changes in film growth parameters show a significant reduction in leakage current, to 10's of pA/mm2 (or less than 1pA/pixel). Sensitivity remains good but is limited by incomplete X-ray absorption and lag. Image samples are derived from two different a-Si TFT designs, demonstrating high resolution and good contrast.

We have developed a simultaneous CT/SPECT imager using a single Cd0.9Zn0.1Te detector in pulse counting mode. Pulse height energy discrimination is used to separate the CT and SPECT data. In pulse counting mode the x-ray flux into the detector must remain below 3 10(5) cps for a linear count rate response. Our SPECT images compare to current clinical systems but our CT images have poor contrast resolution due to the low x-ray flux used to reduce the total photon flux in the detector and pulse pile-up. We are investigating a current mode operation of the detector to obtain high-resolution CT images. Count rate linearity measurements show three orders of magnitude increase in the maximum count rate of the detector and promises to improve the quality of our CT images from the system. Ultimately, interlaced CT/SPECT data could be obtained with no loss in the quality of the SPECT images, and with a significant improvement in the CT images.

In this paper, we report on gamma-ray and thermal neutron detection with RbGd2Br7:Ce scintillators. RbGd2Br7:Ce (RGB) is a new scintillator material that shows high light output (56 000 photons/MeV) and has a fast principal decay constant (45 ns) when doped with 10% Ce. These properties make RGB an attractive scintillator for gamma-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 % stopping efficiency with 0.5-mm-thick RGB crystals. Crystals of RGB with three different Ce concentrations (0.1, 5, and 10%) have been grown. 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.

In this paper, we report on gamma-ray and thermal neutron detection with RbGd/sub 2/Br/sub 7/:Ce scintillators. RbGd/sub 2/Br/sub 7/:Ce (RGB) is a new scintillator material that shows high light output (56000 photons/MeV) and has a fast principal decay constant (45 ns) when doped with 10% Ce. These properties make RGB an attractive scintillator for /spl gamma/-ray detection. Also, due to the prese...

We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.

A medical imaging system providing both x-ray transmission and radionuclide measurements would allow correlation of structural and functional information. We therefore are evaluating a pixellated CdZnTe detector for combined x-ray CT and SPECT imaging with various readout electronics. Gamma-ray spectra of Co-57 measured using NIM electronics (2-mu s shaping time) and multichannel fast photon-counting electronics (50-ns shaping time) produced energy resolutions of 6.5 keV FWHM and 17 keV FWHM respectively at 122 keV. Fast photon-counting electronics achieved linear x-ray count-rate response up to 4x10(5) cps. Dual-mode digital readout electronics are described, which promise to improve SPECT and x-ray CT performance in comparison to the fast-counting electronics. The leakage current and x-ray response with the dual-mode electronics are studied. The leakage current as small as tens of pA is measured, while detector current over 5 orders of magnitude is measured with linearity over 4 orders of magnitude. Results suggest that the CdZnTe detector is capable of performing both x-ray CT and SPECT with the fast photon-counting electronics, and the digital readout electronics can improve the x-ray CT performance.

This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI3 crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C-6-axis. BiI3 is a layered hexagonal lattice similar to the Pbl(2) lattice, where two-thirds of the metal sublattice sites are occupied and the remaining one-third are vacant. The average atomic number of the bismuth iodide (< Z > = 60.5) is very close to that of lead iodide (< Z > = 62.7). Therefore, BiI3 is similar to the better known PbI2 and can be a promising detector material. Our measurements have shown that bismuth iodide crystals have resistivity on the order of 1G Omega.cm and energy gap E-g = 1.72eV. The photocurrent, as a function of bias and wavelength, as well as detector responses from excitation by blue LED and X-ray tube photons were measured. The value of the mobility-lifetime product of the electron (mu tau similar to 10(-5)cm(2)/V) was estimated by the Hecht technique. The optical characterization also included measuring of quantum efficiency for BiI3 detector. The experimental results demonstrate the potential feasibility of using BiI3 as detector material operating at room temperatures.

Combined emission-transmission imaging systems that perform both X-ray CT and SPECT have the potential to correlate anatomical and physical data, and to improve the accuracy of in vivo radiopharmaceutical quantitation. A detection system suitable for the combined CT/SPECT system must achieve both excellent energy resolution at low count rates for radionuclide imaging and must be compatible with the high flux rates and wide dynamic range encountered in X-ray imaging. We therefore are investigating the use of a pixellated CdZnTe for CT/SPECT imaging. Alternative electrode geometries of CdZnTe detectors for SPECT have been investigated with numerical simulation.

System (PbI 2 ) x - (BiI 3 ) 1-x of two layered semiconductors PbI 2 and BiI 3 with various composition were synthesized and grown both as bulk single crystals (by vertical Bridgman method) and films (by thermal vacuum deposition). Grown composites were tested by different techniques (X-ray difflaction analysis, optical transmission, I - V measurements, response to X-ray pulse excitation). Property versus composition showed nonlinear behavior of measured parameters. The structural model of ordered structures formation in this system, which was based upon local electrical neutrality of unit cells, was worked out. It has been experimentally demonstrated that a formation of ordered structures is possible within (PbI 2 ) x -(BiI 3 ) 1-x system, which is consistent with the suggested model. The possibility of crystal structures formation as ordered sets of BiI 3 and PbI 2 layers was also checked.

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. The instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TIBr are compared to other materials, and possible solutions are discussed.

A small pixellated gamma-ray detector has been constructed from a 5 mm thick CdZnTe substrate and characterized for spectroscopic and spatial properties. Emphasis has been placed on revealing 'pixel-level' results to further understand optimum array design and operation. The anode design is (1.5/spl times/1.5) mm/sup 2/ pixels on a 1.625 mm pitch. Spectroscopic results with a /sup 57/Co source display an average energy resolution of 3.9 keV FWHM. Analysis of the electronic noise for different pixels showed good agreement with a theoretical model and similar performance between edge and interior pixels. A test of spatial response with scanned sources incident on the anode face showed good agreement with the pixel geometry and an undetectable amount of dependence on interaction depth.

Position sensitive detectors constructed from compound semiconductors (CdTe, CdZnTe, HgI2) are being developed for a variety of applications where high sensitivity and improved energy resolution are significant advantages over scintillator or gas based systems. We have investigated the possibility of using a CdTe detector array in a SPECT gamma camera that would require a high efficiency at 140 keV. The problem of worsening photopeak efficiencies in thick detectors (due to incomplete charge collection) makes it difficult to maintain a high efficiency which, ironically, is the primary reason for choosing a thicker detector. Recent research has shown that following a simple geometrical design criterion can greatly reduce this deleterious effect. This paper reports on the results from a small prototype pixellated array fabricated using this design. We verify the ‘small pixel effect’ for a detector thickness and pixel size significantly larger than those used in most other work. A 9-element detector (1×1 mm pixels, 4 mm thick) has been fabricated and characterized in terms of energy resolution, peak-to-valley ratio and detection efficiency. Testing of the detector in a fast pulse mode to obtain its high count rate response has also been performed.

Cadmium telluride has been investigated for potential use in a prototype imaging system capable of acquiring both X-ray CT and SPECT data. The system is being designed to accumulate SPECT images with 140 keV gamma rays, thus requiring 4 mm detector thicknesses to achieve satisfactory stopping power. This study primarily investigates whether an alternative pixel orientation can be used with thick detectors to preserve the higher photopeak efficiencies normally associated with smaller thicknesses. Using /sup 57/Co (122 keV) as a substitute isotope to /sup 99m/Tc, small 2 mm CdTe cubes were investigated along with 'edge irradiated' 2/spl times/4/spl times/2 mm bars of both CdTe and CdZnTe. The photopeak efficiency of the large bars can be increased through the use of cooling but energy resolution remains less than desired. Small cubes of either material meet the resolution requirements, but the efficiency of patient dose would be sacrificed.

Boron nitride phosphide (BNxP1-x) films were grown on single crystal GaAs, using chemical vapor deposition. The films were smooth, well adhered to the substrate and exhibited resistivities on the order of 1011 ohm-cm. Photoconductive detectors fabricated from these films showed quantum efficiencies of 33% and 40% at 254 nm and 365 nm respectively, with a drop of an order of magnitude at wavelengths greater than 400 nm. These measurements demonstrate the potential of BNxP1-x as a material for visible- blind UV detectors.