Proceedings of the British Institute of Radiology Megavoltage portal imaging. 5 April 1993

SRI-100 is a megavoltage imaging (MVI) system designed as an accessory to the Philips SL series of linear accelerators. Many centres consider MVI to be essential for accurate radiotherapy. X-ray energies used in radiotherapy give poor images, as they present a lower contrast ratio per unit waterdepth than kilovoltage diagnostic X-rays. Furthermore, the finite radiation source size does not allow high spatial resolution. For maximal linearity and sensitivity, SRI-100 uses a non-intensified chargecoupled device (CCD) camera; high sensitivity and low image noise are achieved by the use of long exposure and sensor cooling. Image processing is performed on a 486 based personal computer, equipped with frame grabber and frame processor boards.