Recent results with a CdTe imaging portal scanner for radiation therapy

A prototype portal imager using a linear array of 256 CdTe photovoltaic detectors, each 2 mm×2 mm×2 mm, is constructed. The array is attached to a compact linear scanner which is to be mounted in a cassette shaped package located below the patient table. The array of detectors is moved under the patient during image acquisition. 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 produces 50-cm×35-cm images with an open field signal-to-noise ratio of 143 and 2-mm spatial resolution in less than 3 s. This corresponds to a signal-to-noise ratio of 1 for 1% contrast objects. Ultimately, signal-to-noise ratio greater than 5 and 1% contrast should be achievable with no loss of spatial resolution or increase in acquisition time