1991 Volume 3 Issue 4 Pages 265-276
A megavoltage CT scanner using a 4 MV and a 6 MV radiotherapy beam has been developed. A detector system composed of 120 pairs of Cadmium. Tungustate scintillators and photo-diodes is attached to the treatment unit at the distance of 160 cm from the beam source. image reconstruction is performed at an engineering work-station with a standard filtered back projection algorithm. Scanning time and reconstruction time for a slice are approximately 35 seconds and 60 seconds, respectively. Although spatial resolution is as much as 4 mm the megavoltage CT has sufficient image quality to be applied for treatment planning and verification. Doses delivered to a patient with 4 MV and 6 MV are about 1.4 cGy and 2.8 cGy, respectively. These doses within or close to radiation fields are thought to be acceptable in the patients receiving radiation therapy. The megavoltage CT provides an accurate and quantitative means for detecting patient set-up errors. In addition when it is used for radiation therapy treatment planning, the positioning errors will be highly reduced since the deviations caused by transferring the patient from a CT couch to a treatment couch are minimized. CT numbers of this megavoltage CT can be directly used to calculate electron density for dose distribution calculation.
