A radiation sensor 10 includes an array of imaging pixels 13 electrically connectable to a readout port. At least one first peripheral row of trigger pixels 11 is located at a first edge of the array, the trigger pixels also being electrically connectable to the readout port but responding faster to x-rays than the imaging pixels. At least one second peripheral row of trigger pixels 12 may be located at a second edge of the array opposed to the first edge. Trigger pixels 11, 12 from the first and second peripheral rows may be addressable alternately in a predetermined pattern to detect a radiation signal for triggering the sensor. Various methods of clocking the sensor are also described. The sensor has particular applicability to intra-oral x-ray imaging.