Modern image guided radiation therapy involves the use of an isocentrically mounted on-board imager (OBI) to take kV
images of a patient’s position. Orthogonal OBI images are used with 2D-2D match software to determine the treatment
couch shifts required for ideal alignment based on digitally reconstructed radiographs created in treatment planning. The
lack of an automatic exposure control (AEC) on Varian OBI systems requires x-ray techniques to be selected manually
which may result in over or under exposed images and compromise the accuracy of the image matching. A software
based AEC system is being developed in order to predict the optimal, patient specific exposure factors. This software
requires that each OBI system be uniquely characterized in terms of both the x-ray tube output and the detector response
for a clinical range of energies (kVp). Characteristic curves show that the detector is highly energy dependent at low
energies and increasingly energy independent with increasing energy. The detector response (per unit exposure) was
determined as a function of the beam quality and the level of detector saturation due to scattered radiation was modeled
based on patient thickness, kVp, and field size. Using this model, the optimal exposure can be determined to produce the
highest quality image.
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