A new 17" x 17" immediate direct digital flat panel detector has been developed to fit the needs of General Radiography. After reviewing a few key aspects of the General Radiography needs (X-ray energy range and associated measurement conditions, system integration and system operation), we describe the new detector Cesium Iodide I Amorphous Silicon based technology, and give measurement results (MTF, DQE, stability). We compare the new detector performance to existing technologies (film I screen combination, storage phosphor devices) and also to other flat panel solutions (Selenium). We conclude that the CsI I a-Si technology is now the best suited one in order to fit the needs of General Radiography, this means all kinds of examinations (chest, abdomen, bones, extremities. ..) which have been up to now done using films.
To judge the potential benefit of a new x-ray detector technology and to be able to compare different technologies, some standard performance measurements must be defined. In addition to technology-related parameters which may influence weight, shape, image distortions and readout speed, there are fundamental performance parameters which directly influence the achievable image quality and dose efficiency of x-ray detectors. A standardization activity for detective quantum efficiency (DQE) for static detectors is already in progress. In this paper we present a methodology for noise power spectrum (NPS), low frequency drop (LFD) and signal to electronic noise ratio (SENR), and the influence of these parameters on DQE. The individual measurement methods are described in detail with their theoretical background and experimental procedure. Corresponding technical phantoms have been developed. The design of the measurement methods and technical phantoms is tuned so that only minimum requirements are placed on the detector properties. The measurement methods can therefore be applied to both static and dynamic x-ray systems. Measurement results from flat panel imagers and II/TV systems are presented.
Amorphous silicon flat panel x-ray detectors (a-Si FXD) are expected eventually to replace traditional x-ray image intensifier systems (XRII) in medical radiography in the long term. The advantages of FXD's are their large detection area, no distortion, no sensitivity to magnetic fields, low weight and compactness. However, they do not provide the high sensitivity of specific optimized systems based on image intensifiers, which approach the sensitivity of single x-ray photon counting in an appropriate configuration whereas the noise equivalent number ofphotons for an a-Si imager is typically several photons at medical energies. That is, the detective quantum efficiency (DQE) of an XMl at low dose is expected to be higher.The consequences of this difference in sensitivity on the performance of both types of detectors are studied for typical conditions in medical fluoroscopy. Detector performance is analyzed in terms of DQE, noise power spectrum (NPS) and modulation transfer function (MTF). Both detectors show similar performance w.r.t. MTF and DQE down to 5 nGy. The comparison is extended to radiographic imaging at 1 tGy and a pixel size of 155 tm. A fmal comparison shows that the inherent spatial resolution of state of the art XMl's exceeds 10002 pixels. An increase in DQE of a factor 3 at 1.6 lp/mm is measured if a camera having 20002 pixels replaces a 1 0002-pixel camera.
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