Improvement of image quality in megavoltage computed tomography with second generation scanning mode

Megavoltage computed tomographic (CT) scanning is a topic of interest in precision radiation therapy. It is useful in verifying and improving the accuracy of the patient's positioning. For this purpose, we developed a third generation mode megavoltage CT scanner. However, insufficient spatial resolution limits its clinical usefulness. A second generation mode megavoltage scanner using a turntable has been newly developed to investigate whether improvements in spatial sampling could result in image quality high enough for clinical use. Scanning is composed of 11 rotations and 12 translations of the table. The scanning beam is a 3 MV X-ray, and the detector consists of 75 elements of cadmium tungstate crystals combined with photodiodes. A spatial resolution of 0.5 mm and contrast resolution of approximately 5% were obtained. The image quality is inferior to that of conventional diagnostic CT scanners, but is estimated to be adequate for some clinical applications of radiation therapy. Based on the satisfactory results, a new third generation megavoltage CT scanner is under investigation.