US Patent 9517358 Upstream direct x-ray detection

A radiation beam, for example a therapeutic radiation beam such as an IMRT or VMAT X-ray beam is monitored using a Monolithic Active Pixel Sensor (MAPS) detector. Photons of the radiation beam can interact with the MAPS detector, and the radiation beam configuration can be estimated from the determined positions of the interactions. The detector is made sufficiently thin that it interacts only very weakly with the X-ray photons. For example, less than 1 in 103 of the X-ray photons might interact with the detector. Hence, the disturbance to the X-ray beam is negligible.