Screening for asbestos-related pleural disease with digital storage phosphor radiography.

Digital storage phosphor radiography (SR) has a wide dynamic range and unique postprocessing capabilities that may improve the performance of screening studies for asbestos-related pleural disease compared with conventional film radiography (FR). In a group of 32 asbestos-exposed and nine control subjects with established pleural data, we compared the screening performance of FR and SR obtained with a single isoexposure, dual-energy technique (system resolution 0.2 mm, 10 bits). Performance was evaluated for 7320 observations by eight readers using a paired t test (P less than .02 with Bonferroni correction) of averaged receiver operating characteristic curve (ROC) areas (Az +/- standard error). We found that SR alone and SR supplemented by dual-energy soft-tissue and calcium images (SRde) were superior to FR in the overall detection of pleural abnormalities (Az = 0.90 +/- 0.01, 0.90 +/- 0.01, and 0.88 +/- 0.01, respectively). In the specific detection of pleural calcification, SRde was superior to FR (Az = 0.91 +/- 0.01 and 0.87 +/- 0.01, respectively; P less than 0.01). Analysis of variance indicated that SRde most closely reproduced an established pleural score based on the International Labor Organization (ILO) classification of the pneumoconioses (P less than 0.05, Scheffé's multiple comparison test). We conclude that isodose SR performs at least as well as FR in screening for asbestos-related pleural disease. SR supplemented by dual-energy images might improve the specific detection of pleural calcifications compared with FR.

Energy resolution in a high-pressure gas scintillation proportional chamber.

A high-pressure gas scintillation proportional chamber has been designed and constructed to image x and gamma rays for medical applications. The chamber contains 4 atm of pure xenon. Ultraviolet light emitted from excited xenon atoms within the detector is collected by a hexagonal array of seven UV-sensitive photomultiplier tubes, which in turn are separated from the pressurized gas by 1-cm-thick fused-silica windows. A model was used to predict the energy resolution of the device as a function of fill-gas pressure, voltage within the detector, and light-collection efficiency. The energy resolution improved with increasing scintillation region voltage from 17% full width at half maximum (FWHM) at 1.9 kV to 10% FWHM at 3.0 kV for 59.5-keV photons; once above 1.5 kV, there was no improvement with increasing drift voltage. The addition of the signals from the peripheral phototubes to that of the center phototube did not substantially improve the energy resolution of the device. This was because the noise that was present yielded a high correlation between the phototubes; when this noise was incorporated into the model, the energy resolution of the multiphototube system was accurately estimated. The energy resolution of the gas scintillation proportional chamber was found to be superior to the sodium iodide Anger camera at 59.5 keV by a factor of 2. Further improvement can be obtained by increasing the scintillation region voltage and by increasing the light-collection efficiency by moving the scintillation region closer to the phototubes.

Detection efficiency of a high-pressure gas scintillation proportional chamber.

The detection efficiency of a high-pressure, gas scintillation proportional chamber (GSPC), designed for medical imaging in the 30-150 keV energy range, has been investigated through measurement and Monte Carlo simulation. Measurements were conducted on a GSPC containing 4 atm of pure xenon separated from a hexagonal array of seven ultraviolet-sensitive photomultiplier tubes by 1.27-cm-thick fused-silica windows. Experimental measurements of the photopeak efficiency, fluorescence escape efficiency, and the energy collection efficiency were obtained. Results were also obtained for different photon energies and different values of temporal resolution. The measurements were compared with the results obtained from a Monte Carlo simulation designed specifically for investigating the imaging of low-energy photons (below 150 keV) with a gas-filled detector. The simulation was used to estimate photopeak efficiency, fluorescence escape efficiency, photopeak-to-fluorescence escape peak ratio, quantum interaction efficiency, energy collection efficiency, and local energy collection efficiency. The photopeak efficiency of the GSPC relative to that of a 3-in. (7.62-cm)-thick sodium iodide crystal was measured to be 0.284 +/- 0.001 at 60 keV and 0.057 +/- 0.001 at 140 keV. Of the 60-keV photons incident upon the detector, 70% +/- 4% interacted in the detector, with 28% +/- 1% being in the photopeak, as estimated both by experimentation and through the simulation. The maximum energy collection efficiency was found to be 65% at 60 keV, with 46% being deposited within 0.2 cm of the initial photon interaction. The information gained from this study is being used to design an optimized detector for use in specialized nuclear medicine studies.

Development and validation of a Monte Carlo simulation of photon transport in an Anger camera.

The geometric component of the point spread function (PSF) of a gamma camera collimator can be determined analytically, and the penetration component can be calculated readily by numerical ray-tracing. A Monte Carlo simulation of photon transport which includes collimator scatter is developed. The simulation was implemented with an array processor which propagates up to 1024 photons in parallel, allowing accurate estimates of the total radial PSF in less than a day. The simulation was tested by imaging monoenergetic point sources of Tc-99m, Cr-51, and Sr-85 (140, 320, and 514 keV, respectively) on a General Electric Star Cam with low-energy, general-purpose, and medium-energy collimators. Comparisons of measured and simulated PSFs demonstrate the validity of the model and the significance of collimator scatter in the degradation of image quality.

Prospects for efficient detectors for fast neutron imaging.

A physical model describing in detail the process of fast neutron imaging in luminescent screens is presented. The detection quantum efficiency, luminosity and inherent spatial resolution of the screen were calculated using this model. Properties of transparent and disperse screens were compared. Two imaging systems were suggested to improve the detection efficiency and spatial resolution. A stack consisting of alternating neutron converters and image plates can help in obtaining both high spatial resolution and efficiency. A system containing a screen of special form and a diaphragm can be of use especially in the case of the fan beam.

Imaging gas detectors for medicine and biology.

A brief overview is given of the operating characteristics of gas filled detectors especially multi-wire proportional chambers and gas scintillators as applied to imaging of ionizing radiation for medicine and biology. Both intrinsic and practical limitations on these devices and their implications on their areas of applicability are discussed. Comparisons are made with other imaging technologies.

Near-field artifact reduction in planar coded aperture imaging.

Coded apertures for imaging problems are typically based on arrays having perfect cross-correlation properties. These arrays, however, guarantee a perfect point-spread function in far-field applications only. When these arrays are used in the near-field, artifacts arise. We present a mathematical derivation capable of predicting the shape of such artifacts. The theory shows that methods used in the past to compensate for the effects of background nonuniformities in far-field problems are also effective in reducing near-field artifacts. The case study of a nuclear medicine problem is presented to show good agreement of simulation and experimental results with mathematical predictions.