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The present invention relates generally to methods and devices for x-ray cone-beam computed tomography (CBCT). More specifically, it relates to techniques for improving image quality in CBCT applications such as radiation therapy.BACKGROUND Radiation therapy is the use of ionizing radiation in the treatment of cancer. External beam radiotherapy uses diagnostic imaging techniques for treatment planning. A conventional preparation procedure for radiation therapy includes a priorcomputed tomography (CT) scan using a multi-detector CT (MDCT) scanner for treatment planning and a three-dimensional cone-beam CT (CBCT) scan for patient setup. A three-dimensional registration is performed on the planning CT and the CBCT data toderive appropriate shifts and rotations for patient positioning. CBCT can also be used to calculate the delivered radiation dose, but this application is not commonly applied mainly due to the inferior image quality of CBCT. The treatment typically involves one or two CBCT scans every day for four to six consecutive weeks, resulting in a large undesired radiation dose to the patient. Reducing the CBCT radiation dose decreases secondary cancer risk, but it generallyresults in degraded image quality and reduced image reconstruction accuracy. In addition, x-ray scattering during CBCT imaging causes severe shading and streak artifacts in reconstructed CBCT images, reducing the accuracy of imaging-based proceduresduring radiation treatment. Since the scatter signal increases as the illuminated volume increases, the scatter problem is especially severe in CBCT systems, which use a large illumination beam. Moreover, known techniques for scatter compensationincrease noise in the image. In view of these problems, there is a need for improved techniques for CBCT imaging in radiation therapy.SUMMARY OF THE INVENTION In one aspect, the present invention provides a technique for CBCT imaging that significantly reduces both scattering effects and undesire

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