Characterization of Varian on-board imaging systems for use in automatic exposure control software

摘要

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.