Abstract: An apparatus dedicated to taking diagnostic quality images to confirm the accuracy of radiotherapy treatments. The apparatus is sufficiently compact to be used in the existing treatment heads of linear accelerators capable of delivering both electron and x-ray radiation. A compact electron drift tube is made long enough to penetrate the substantial shielding of a linear accelerator, thereby obviating the need to locate the entire generating device within the head of the machine. The diameter of the drift tube of the present invention is made small enough to penetrate the shielding without causing undue leakage of radiation.