Abstract
Recently, several authors have shown contrast improvements in megavoltage portal imaging and cone‐beam computed tomography(CBCT) using low atomic number (Z) targets. This work compliments previous studies by investigating the effects of varying different beam production parameters including target atomic number, target thickness and incident electron energy on spatial resolution. Target materials of beryllium,aluminum and tungsten were investigated over a range of thicknesses between 10% and 100% of the continuous slowing down approximation (CSDA) range of electrons. Incident electron kinetic energies of 4.5 MeV and 7.0 MeV were used in conjunction with custom targets installed above the carousel of a Varian 2100EX linear accelerator (Varian Medical, Inc.) to produce the experimental beams of interest. Monte Carlo simulated results were compared to measured data and it was shown that thinner targets, as well higher incident electron energies generally produce more favourable modulation transfer function(MTF) curves. Due to an MTF dependence of the detector system on photon energy, the experimental results showed that low‐Z targets produced superior MTF curves. Simulations showed a 14.5% and 21.5% increase in f 50 for the 7.0 MeV and 4.5 MeV targets (Al; 60% R % CSDA ) respectively when moved from the carousel to the location of the clinical target. f 50 values for the custom targets were compared to the clinical 6 MV beam and were found to be between 10.4% lower (4.5 MeV/W) and 15.5% higher (7.0 MeV/Be). This study has been partially funded by a Research Grant from Varian Medical Systems.
Published Version
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