Recent results with a CdTe imaging portal scanner for radiation therapy

A prototype portal imager has been constructed using a linear array of 256 CdTe photovoltaic detectors, each 2×2×2 mm. 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 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×35 cm near-real-time images with an open field signal-to-noise ratio (SNR) of 143 and 2 mm spatial resolution in less than three seconds. This corresponds to a SNR of 1 for 1% contrast objects. Ultimately, a SNR greater than 5 at 1% contrast should be achievable with no loss of spatial resolution or increase in acquisition time