A detection scheme is described that allows one to accomplish dual-energy scanned projection digital radiography without switching the x-ray tube voltage. The method employs a high/low atomic number detector sandwich that simultaneously separates the x-ray beam transmitted by the patient into low and high energy components. To test the method, the response of a scanning linear array of energy-sensitive detectors was simulated, and bone and soft tissue images of an anthropomorphic chest phantom were obtained at 140 kVp. These were compared with similar images obtained by switching the x-ray tube voltage from 80 kVp to a heavily filtered 140 kVp. For comparable entrance skin exposures, the dual-energy detector images required a lower tube load and resulted in higher noise levels. The latter is attributable to the fact that the separation in energy between the high and low energy components is smaller with the dual-energy detector than with the voltage switching technique, and to misregistration problems associated with the simulation methodology. A detector design is also discussed that would result in improved energy separation and lower noise levels. In view of this possibility and the tube loading advantage, the method looks promising for digital scanned projection radiography.
Measurements of the physical performance of a prototype digital chest unit (DCU) are presented. The parameters evaluated were entrance skin exposure, system exposure response and dynamic range, system modulation transfer function (MTF), image noise levels, detective quantum efficiency (DQE) of the detector, and scatter suppression efficiency. Compared with conventional chest imaging systems, the unit has markedly greater exposure latitude, limited spatial resolution, a lower detector DQE, and virtually scatter-free images. Routine clinical exposure levels are comparable with the 1982 national average.
The general features of a prototype digital chest unit are described along with the rationale for the choice of design factors employed. It is shown that the scanning-slit, linear-detector-array approach employed can, with available x-ray tube technology, achieve a spatial resolution of 1 cy/mm and detector radiation levels comparable with those obtained with conventional screen-film systems. Also discussed are the unit's exposure latitude and its ability virtually to eliminate scatter.
We are evaluating the performance of a charge‐coupled device (CCD) detector for digital mammography. The detector is an array of four 1 × 5 cm CCD's coupled (without demagnification) through a fiber optic plate to a 180‐micron layer of Cesium Iodide. The detector has time‐delay integration electronics that enables it to be used in slot‐scanning full field digital mammography. With 27‐micron pixel pith, its resolution approaches that of film‐screen mammography (FSM). The detector also acquires 16‐bit data, which provides more contrast levels and facilitates imaging the edges of the breast. We measure the modulation transfer function, noise power spectrum and detective quantum efficiency in the scan and frame (detector stationary) modes. Our results show that the detector is capable of resolving information near its Nyquist frequency and has a DQE(0) approaching 50% in the scan mode. The DQE is higher than that of FSM cassettes and is comparable to other digital mammography detectors.
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