RECENT RESULTS WITH A CDTE IMAGING PORTAL SCANNER FOR RADIATION-THERAPY

One of the most promising means for improving the quality of radiation therapy is the use of real-time imaging systems for routine portal po sition verification. We have constructed a prototype portal imager usi ng a linear array of 256 CdTe photovoltaic detectors, each 2 x 2 x 2 m m. The array is attached to a compact linear wanner which is to be mou nted in a cassette shaped package located below the patient table. The array of detectors is moved under the patient during image acquisitio n. The high stopping power of the CdTe allows a high contrast image to be made using a single linac pulse for each array position. In tests conducted with a 4 MV linac, this system produced 50 cm x 35 cm images with an open field signal-to-noise ratio of 143 and 2 mm spatial reso lution in less than 3 seconds. This corresponds to a signal-to-noise r atio of 1 for 1% contrast objects. Ultimately, a signal-to-noise ratio greater than 5 at 1% contrast should be achievable with no loss of sp atial resolution or increase in acquisition time.