An imager 200 includes: an array of imager elements 206 configured to generate image signals based on radiation received by the imager； and circuit 230 configured to perform readout of image signals, wherein the circuit is configured to be radiation hard. An imager includes: an array of imager elements configured to generate image signals based on the radiation received by the imager； and readout and control circuit coupled to the array of imager elements, wherein the readout and control circuit is configured to perform signal readout in synchronization with an operation of a treatment beam source.
