Planar imaging at energies below 2.4 MV with carbon and aluminum linear

Background: Standard linear accelerators employ a high-Z target designed for therapeutic treatment, but produce poor image quality. Exchanging the high-Z target for a low-Z one, while reducing the incident electron beam energy recovers a significant component of photons in the diagnostic energy range, providing improved contrast-to-noise (CNR) characteristics. Purpose: To assess the improvement of megavoltage planar image quality with the use of carbon and aluminum linear accelerator targets combined with incident electron beam energies ranging from 1.90 to 2.35 MeV. Methods: The bending magnet shunt current was adjusted to allow selection of mean electron energy from 1.90 to 2.35 MeV. CNR and spatial resolution measurements were performed to quantify the improvement of image quality. The mechanism for improvement is explained with reference to Monte Carlo generated energy spectra. Results: A five-fold increase of CNR compared to a 6 MV therapy beam was observed. Spatial resolution decreased slightly, by 4.8% and 2.7% at 0.20 and 0.40 lp/mm when reducing energy from 2.35 to 1.90 MV. The percentage of diagnostic x-rays for the beams examined here ranges from 46.3 to 54.0%. Conclusion: By lowering the beam energy below 2.4 MeV substantial gains in CNR are possible, with only a slight degradation of spatial resolution.