The variation of hounsfield units on scanning parameters and influence on CT to red conversion curves

Abstract

© 2015 RAD Conference Proceedings. Since CT scanning of radiotherapy patients is done by different radiation therapists (RTTs), with different daily routine, the influence of different scanning protocols (tube voltage, tube current, position in relation to the center of the table) were evaluated. The dedicated phantom was used for this purpose. Conversion curves were generated, and differences evaluated. The highest dependence of computerized tomography number-Hounsfield unit (CT number-HU) to relative electron density (RED) conversion curve is found to be on the tube voltage. The difference was observed mostly in the high density material, such as bone and dense bone. The voltage decrease of 20 kV gives the difference in range 10% to 20% in the value of Hounsfield number (HU) for the same dense material (different types of bones). The dependence on tube current was negligible. The dependence on the off axis position of a phantom (outer parts of the CT bore) results in difference of 5% - 10% on reconstructed HU values imported in treatment planning system (TPS) in comparison to the same material when phantom is placed in the central part of the CT bore.