Low-energy imaging with high-energy bremsstrahlung beams

A method is described for portal imaging with low-energy (approximately less than 150 keV) photons from a radiotherapy accelerator operating in a diagnostic mode. The low-energy photons are produced in the bremsstrahlung process, but are normally filtered out by thick high atomic number (Z) target materials. This absorption can be reduced by choosing a low Z target with the minimum thickness required to stop the electrons in the target. If, in addition, the operating energy is kept low (approximately 5 MeV) and the flattening filter is removed, low-energy photon images can be produced from the broad spectrum of photon energies by using standard diagnostic radiology high-Z fluorescent screen/film systems that strongly absorb at low but not high photon energies. Motion artifacts can then be avoided since only a small dose is required for such a procedure.