Single crystal targets may improve soft-tissue contrast in megavoltage imaging by means of coherent bremsstrahlung

Coherent bremsstrahlung denotes the process of bremsstrahlung emission by electrons traversing a single crystal, causing prominent peaks in the resulting photon spectra at low energies. While this phenomenon has been known for decades, little attempt has been made to exploit its potential for megavoltage imaging, where the quality of images is affected by low contrast due to the lack of sufficient photons at the energy range suited for diagnostic purposes. We provide a theoretical foundation of coherent bremsstrahlung in the first-order Born approximation without confinement to high energies. Based on this theory, first evidence is given that diamond crystals are capable of boosting the amount of diagnostic photons by about 10-20%. It is shown that this behaviour is largely conserved for polychromatic electrons hitting thick targets, where multiple scattering dominates the energy distribution of the emitted photons.