A radiation imaging apparatus comprising: an optical imaging device configured to image real world structures; a position and orientation detector configured to determine an instantaneous position and orientation of the radiation imaging apparatus in six degrees of freedom relative to the real world structures as a fixed frame of reference; a radiation detector configured to detect ionising radiation; a radiation source determination section configured to determine a distribution of radiation sources based on data of the ionising radiation detected by the radiation detector combined with data of the position and orientation of the radiation imaging apparatus relative to the real world structures; and an image generator configured to generate data for a combined image comprising an optical image of the real world structures imaged by the optical imaging device and an overlay image representing the distribution of radiation sources based on data from the radiation source determination section.