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To characterize the performance of a cone-beam computed tomography (CBCT) imaging system based upon an indirect- detection, amorphous silicon flat-panel imager (FPI). Tomographic images obtained using the FPI are presented, and the signal and noise characteristics of reconstructed images are quantified. Specifically, the spatial uniformity, CT linearity, contrast performance, noise characteristics, spatial resolution, and soft-tissue visualization are examined. Finally, the performance of the FPI-based CT system is discussed in relation to existing clinical technologies. A table-top measurements system was constructed to allow investigation of FPI performance in CBCT within a precisely controlled and reproducible geometry. The FPI incorporates a 512 X 512 active matrix array of a-Si:H thin-film transistors and photodiodes in combination with an overlying (133 mg/cm2 Gd2O2S:Tb) phosphor. The commercially available prototype FPI has a pixel pitch of 400 micrometer, a fill factor of approximately 80%, can be read at a maximum frame rate of 5 fps, and provides 16 bit digitization. Mounted upon an optical bench are the x-ray tube (in a rigid support frame), the object to be imaged (upon a precision rotation/translation table), and the FPI (mounted upon a precision translation table). The entire setup is directed under computer control, and volumetric imaging is accomplished by rotating the object incrementally over 360 degrees, delivering a radiographic x-ray pulse (e.g., 100 - 130 kVp, approximately 0.1 - 10 mAs), and acquiring a projection image at each increment. Prior to reconstruction, dark and flood- field corrections are applied to account for stationary nonuniformities in detector response and dark current. Tomographic images are reconstructed from the projections using the Feldkamp filtered back-projection algorithm for CBCT. The linearity of the CBCT system was compared to that of a commercial scanner (Philips SR-7000) using materials ranging in CT number from approximately 900 to 1100. The contrast sensitivity of the CBCT system and the conventional scanner was compared using these same materials. Images of a uniform water bath were acquired for characterization of the response uniformity and the dependence of noise on exposure. The spatial frequency response characteristics of the system were measured using a steel wire, from which the point spread function and modulation transfer function were determined. Finally, the soft-tissue contrast and spatial resolution of the CBCT system was demonstrated in volumetric images of a euthanized rat. The image quality was compared to images of the same subject acquired with an equivalent technique on the commercial scanner. A table-top CBCT scanner based upon an a- Si:H FPI has been constructed, and a system for CBCT image acquisition, processing, and reconstruction has been implemented. This system is capable of producing high-quality volumetric images. Reconstructions were generated from 300 radiographs (100 kVp; 1 mAs per projection) obtained at 1.2 degree increments through 360 degrees. Image acquisition and reconstruction required approximately 30 min and approximately 2 h 20 min (250 MHz UltraSparc), respectively. The system has demonstrated signal and noise performance comparable to that of commercial CT scanners. The imaging performance of the prototype supports the hypothesis that FPIs can be employed in computed tomography applications.
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