Raymond P. DeVito

Scatter reduction with energy-weighted acquisition

The theory of energy-weighted acquisition (EWA) in nuclear medicine imaging is outlined, and a system that implements EWA is described. EWA reduces the effects of scattered radiation by allowing events of all energies to contribute to image formation, processing each energy with its own short-range spatial filter. This approach implements short-range energy-dependent filtering with an image buffer called a weighted acquisition module, providing scatter reduction with controllable noise and resolution properties. The systems response to point sources and planar distributions of radioactivity embedded in radiation-scattering media have been measured. EWA is compared to conventional energy-window acquisition, showing that the EWA approach provides improved image contrast. >

Dual-anode high-pressure xenon cylindrical ionization chamber

A new approach to the design of high-pressure xenon cylindrical ionization chambers is investigated. In the new approach, a dual anode is used to replace the single anode surrounded by a shielding grid, which are employed in a conventional design. Two anode wires are stretched near the axis of the 30 cm long cylindrical chamber with a 9 cm diameter cathode. Both the wires are kept at the same (nearly zero) potential and are dc-coupled to the charge-sensitive preamplifiers. For most cases, only one of the wires (it can be either one) collects the electrons produced by an ionizing event. The difference between the signals read out from the wires is proportional to the total produced charge. The experimental results are compared to Monte Carlo simulations. The optimal design of the dual-anode cylindrical ionization chamber is discussed.

HgI/sub 2/ detector with a virtual Frisch ring

We investigated the performance of the modification of a Frisch-ring device design applied to the bar shaped HgI/sub 2/ detector. In this device, the ring electrode, separated from the detector surface by thin insulating layer, is extended to cove the entire area of side surfaces of the crystal, and connected to the cathode. To fabricate this device the side surfaces of 3 /spl times/ 3 /spl times/ 6 mm/sup 3/ HgI/sub 2/ crystals were wrapped in a Cu foil separated from the crystal by a thin layer of a Teflon tape. These bar-shaped HgI/sub 2/ detectors can be used in large-area high-efficiency detector arrays that are in demand for medical imaging and nuclear materials detection. We obtained an energy resolution of /spl sim/5% FWHM at 511 keV with our tested detectors. This is a very good result for the HgI/sub 2/ detector of such thickness. However, this resolution is far away from the theoretical limit predicted for HgI/sub 2/ material. This paper analyzes several factors affecting the energy resolution of this type of the virtual Frisch-grid devices, of which the fluctuation of charge loss due to electron trapping by local defects with high concentration of traps seems is a dominant factor.

Vibration-proof high-pressure xenon electroluminescence detector

We have developed a high-pressure electroluminescence (EL) detector consisting of a cathode, 5 drift rings, and an EL-generating structure which defines a sensitive volume of O5cm /spl times/ 5cm. The EL-generating structure consists of two parallel-plate, chemically etched grids and a high-pressure optical window which is optically coupled to an external photomultiplier tube. Ionizing radiation that is absorbed in the sensitive volume generates electrons, which drift into the EL region and produce an EL flash. The detector was filled with 20-bar Xe gas that was highly purified (>1 msec electron-life time) using a spark purification technique. In order to evaluate the potential for using an EL detector under adverse conditions, we disturbed the detector using a 10 W electric engraver working at 60 Hz. The action of the engraver had practically no effect on spectra acquired from an /sup 241/Am source.

Mercuric iodide x-ray and gamma-ray detectors for astronomy

The recent technological developments and availability of mercuric iodide detectors have made their application for astronomy a realistic prospect. Mercuric iodide, because of its high resistivity and high density, can be used in a variety of astronomy instrumentation where high spectral resolution, low noise levels, stability of performance, resistance to damage by charged particles and overall ruggedness are of critical importance. X-ray detectors with areas of 12 to 100 mm square and 1 mm thickness have absorption efficiencies approaching 100% up to 60 keV. The spectral resolution of these detectors ranges from 400 eV to 600 eV at 5.9 keV, depending on their area, and the electronic noise threshold is less than 1.0 keV. Gamma ray detectors can be fabricated with dimensions of 25 mm x 25 mm x 3 mm. The spectral resolution of these detectors is less than 4% FWHM at energies of 662 keV. Because of the high atomic numbers of the constituent elements of the mercuric iodide, the full energy peak efficiency is higher than for any other available solid-state detector that makes measurements up to 10 MeV a possibility. The operation of gamma ray detectors has been evaluated over a temperature range of -20 through + 55 degrees Celsius, with only a very small shift in full energy peak observed over this temperature range. In combination with Cesium Iodide scintillators, mercuric iodide detectors with 25 mm diameter dimensions can be used as photodetectors to replace bulky and fragile photomultiplier tubes. The spectral resolution of these detectors is less than 7% FWHM at 662 keV and the quantum efficiency is larger than 80 % over the whole area of the detector.

Variance propagation for SPECT with energy-weighted acquisition

The authors compare the noise characteristic of SPECT (single-photon-emission computed tomography) images which are reconstructed from data acquired by both EWA energy-weighted acquisition and normal windowed acquisition (NWA). The comparisons used are propagation of variance and measurement of the noise power spectrum (NPS). For a selected Tl-201 weighing function, the analytically predicted variance (neglecting covariance terms) was compared with the variance from experimental SPECT scans of a phantom. Reasonable agreement was observed between predicted and measured percent rms noise. The NPS for EWA was skewed slightly with respect to the NWA spectrum. >

Characterization of carrier generation and transport mechanisms in single-crystal and thin-film HgI2

Abstract High-electronic-quality thin films of mercuric iodide have been deposited on SnO 2 and other transparent contacts. The crystallites exhibit some preference for c -axis orientation and have lengths comparable to the overall film thickness of 200–400 μm. With visible photons of 584 nm, quantum efficiencies up to 0.5 are observed at an applied voltage of 50 V. amps

Heart Range Determination for SPECT Myocardial Perfusion Studies

We have developed a new algorithm for determining the cross- section range of the heart from a sequence of SPECT projection images. The new algorithm provides accurate estimation of the heart range for a fully automatic myocardial perfusion SPECT processing system. The limits of the heart range are used for reconstructing transverse images for the subsequent analysis. The basis of the approach is the 1D pseudo motion analysis which has three major components, spatial feature to position mapping, knowledge-driven analysis of heart region, and heart range determination. The main advantage of the algorithm is that the processing is fully automatic regarding no user intervention and is less sensitive to the image intensity distribution comparing to other existing methods.

Gallium-67 Imaging With Low Energy Collimators And Energy Weighted Acquisition

The medium and high energy collimators used in 67Ga imaging have poorer resolution than low-energy collimators, such as the LEAP. The low energy collimators could be used for gallium imaging if the background under the 93 and 185 keV peaks could be reduced without degrading the signal-tonoise ratio unacceptably. Energy weighted acquisition provides a means of accomplishing this background reduction. We have developed weighting functions for gallium imaging through LEAP and high resolution collimators. The resolution of the low energy collimators is realized while the background is comparable to, or better than, the background in normal, energy-window imaging with the medium energy collimator. The pixel noise is somewhat greater than the Poisson noise in normal gallium imaging, and some noise correlations, or noise texture, is introduced. I. INTRODUCTION Gallium-67, a nuclide important in nuclear medicine, emits photons at 93, 185,300, and 394 keV, along with less abundant ones up to 888 keV. Gallium images from collimated gamma cameras are corrupted by a diffuse background that is mainly due to two processes: penetration of photons through the collimators walls, or septa, causing events that are not properly collimated; and scatter of photons in the patient before they pass through the collimator, causing a scatter fraction in clinical gallium imaging of about 50%. For reducing the septum penetration, the collimators of choice in clinical gallium imaging [1,2] are medium-energy WED-ENG) and high-energy ones, whose thick septa are designed to shield against the high-energy photons of 67Ga and 1311, respectively. These collimators have disadvantages, however: the large holes are discernible in images, and the resolution and utilization are inferior to those of the thin-walled collimators used to image low-energy isotopes with photons well below 200 keV, for instance the low-energy all-purpose and highresolution collimators (LEAP and HI-RES). The thick septa do not reduce the scattered radiation component of the background, for many scattered photons pass through the collimator holes along with the primary radiation. Since collimator design cannot reduce the scatter, one uses different approaches to control this component of the background. This paper asks whether one can effectivcly image 67Ga with a low-energy collimator, and remove the background mathematically. Various workers have removed background in WmTc and 1231 imaging by using two cncrgy windows at a time and, in a post-processing operation, subtracting one image from the other.[3,4] Here, we consider energyweighted acquisition [5,6] (EWA) to reduce the background in 67Ga imaging. The effect of EWA is to collect a low-statistics image in each 1-keV energy interval between 0 and 511 keV, filter each image with a spatial filter designed for that energy (a weighting function), and sum the filtered images to form an energy-weighted image; or, the energy-weighted images can be acquired on the fly with the WAM hardware described in references [5,61. In the case of 67Ga imaging with low-energy collimators, EWA allows us to remove much of the background under the well-collimated 93 keV peak, while controlling the scatter background and enhancing the spatial resolution of the 185 keV peak.