Digitally Reconstructed Radiographs (Drr) in Radiotherapy: Natural Contrast Enhancement For Virtual Simulation.

Abstract

As a tool for image reconstruction treatment planning systems offer the possibility to generate sagittal, coronar, and arbitraryly orientated slices on the basis of primary transversal slices or spiral scans. These additional slices can be used for calculation of dose distribution in radiotherapy. For better review of dose distribution and beam directions according to patient's anatomy and divergent beam geometry new views are reconstructed by ray tracing from the source through the patient: DRR (digitally reconstructed radiography). This technique is named "vitual simulation" with reference to the conventional technique by using a specific X-ray-equipment. Compared to conventional X-ray-recordings the contrast is reduced by reason of the different image content: absorption- or transmission values vs. density values (Hounsfield Units HU). This reduction of contrast depends on the X-ray-energy used for CT-scanning, and can be restituted by special filter functions, the so called LQO-filter '. According to HU units and X-ray energy an attenuation coefficient is added. The visual impression of reconstructed CT-scans then is comparable to conventional planar X-ray-images, especially for the bone-soft tissue contrast. This demonstrated by some examples, showing encellent contrat and ressognizabiliity detalsi. If the X-ray-energy for simulation is increased up to 5 MV (comparable to a 15 MeV-X treatment beam) the contrast in images is reduced progressively, as it is well known from therapy verification films with poor contrast.