Verification of CT number to density conversion for a simulator-T attachment

The calculation and verification of a CT number to density conversion table for a simulator-CT attachment known as Scanvision, which provides CT images for radiotherapy treatment planning, is presented. While the linear fit for CT-number-to-density is similar to most conventional doughnut gantry CT scanners, an offset of approximately 178 Hounsfield units was found for air using a polyethylene normalisation, and approximately 262 for air using air normalisation. The offset continues for other low-density samples. Results show that the simulator-CT reproducibly measures CT numbers. However a separate calibration line needs to be entered into the radiotherapy planning computer to ensure accurate CT-number-to-density. conversion.