Development of a portal imager and of tools for radiation treatment verification

A prototype electrostatic imager has been developed for megavoltage protal imaging in radiation therapy.\sThe imager utilizes amorphous selenium ( a ‐Se) with a front metal plate as the image receptor and a high resolution voltmeter probe for imager readout.\sImaging characteristics of a ‐Se have been investigated theoretically through Monte Carlo simultions, and experimentally by measuring radiation discharging curves and phantom tests.\sThe results of this study have shown that the prototype imager has high sensitivity, good spatial resolution, and low noise level.\sOur study also reveals the potential of electrostatic imaging with metal/ a ‐ Se in megavoltage imaging.\sTwo computer algorithms have also been developed for automatic segmentation and contrast enhancement of digital portal images, and for radiation field shape verification.\sBased on a priori knowledge of the properties of portal images, the segmentation and contrast‐enhancement algorithm employs multiple criteria and dynamic reasoning to achieve optimal segmentation of individual images, and has been proved to be accurate, robust, and fast.\sThe algorithm for radiation field shape verification is an adapatation of the chamfer matching technique to a specific application:\smatching closed contours.\sBy incorporating geometric features of the radiation field and using a simple minimization method which is more specific to this task, the algorithm appears to be able to improve the matching results of the standard method.