Abstract: An apparatus and method are provided for imaging an object undergoing high energy irradiation, such as a patient being treated by radiation teletherapy, the apparatus employing an array of ion chambers in a detector, the detector being rotatable with respect to the object, the image being obtained by acquiring projection data from each of the ion chambers at a sequence of predetermined orientations, the projection data for each location being reconstructed into an image by convolution filtering or other reconstruction technique, and subsequently employed in a back-projection of all projection data, which may then be displayed on a video monitor. The detector employs a liquid fluorocarbon as the ionizing medium in the plurality of ion chambers.