Characterization of a re-designed clinical fluoroscopic portal imager
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文摘
Several investigators have demonstrated that images produced with current fluoroscopic portal imagers can be improved by reducing optical quantum noise and electronics noise, eliminating fixed pattern noise, and minimizing the light spread in the optical chain. We have completely re-designed the Philips SRI-100 fluoroscopic imager to address these specific concerns. The x-ray detector consists of a fast Gd2O2S screen (165 mg/cm2) bonded to a 1.5 mm thick stainless steel plate. This screen is approximately 35 % faster than the Lanex Fast (Back) screen, demonstrates a minimum of screen non-uniformities, and has an improved modulation transfer function (MTF) as compared to the unconventionally thick screens used in some systems. The light emitted by the screen is collected by an f/0.95 (f=50mm) lens and focused onto a large CCD light sensor (512×512 array, 27μm×27μm pixels). The array is back-thinned, increasing its optical quantum efficiency to 84 % (@ λ=550nm) and is air-cooled to −20°C to reduce dark current (
60 e-/pixel/sec). The charge collected is digitized to 12 bits. The rms read-out noise is 0.93 counts in 4096. The spatial frequency dependent signal (MTF) and noise (NPS) characteristics of the system are presented. The detective quantum efficiency quantifies the improvement over previously described fluoroscopic systems. High portal image quality is achieved with patient doses of 1-2 cGy. Contrast-detail analysis demonstrates that objects with radiographic contrasts of 0.4 % or greater (7mm in diameter) can be detected using a 2 cGy exposure. Patient images from the clinical system will also be presented.
60 e-/pixel/sec). The charge collected is digitized to 12 bits. The rms read-out noise is 0.93 counts in 4096. The spatial frequency dependent signal (MTF) and noise (NPS) characteristics of the system are presented. The detective quantum efficiency quantifies the improvement over previously described fluoroscopic systems. High portal image quality is achieved with patient doses of 1-2 cGy. Contrast-detail analysis demonstrates that objects with radiographic contrasts of 0.4 % or greater (7mm in diameter) can be detected using a 2 cGy exposure. Patient images from the clinical system will also be presented.