Energy Spectrum of Electromagnetic Radiation for a Dual-Energy Cone-Beam Computed Tomography in Radiotherapy

To implement dual-energy cone-beam computed tomography (CBCT), a 1 to 3 mm thick copper filter was applied in front of the kilovolt generator mounted on a medical linear accelerator. The energy calibration was performed from three energy peaks among the electromagnetic radiation, gamma rays, emitted by the Ir-192 source. The energy spectrum was measured using a cadmium zinc telluride (CZT) detector. A figure-of-merit (FOM) value was defined and evaluated in order to consider both the increase in the mean energy and the decrease in the intensity according to the filter. For a 1 mm thick filter, the FOM values were 0.82 and 0.92 for tube voltages of 100 and 120 kVp, respectively. The intensity was maintained reasonably and the effect of increasing the mean energy was evident, thus indicating its suitability for implementing dual-energy CBCT.