Evaluation of CT Acquisition Protocols Effect on Hounsfield Units and Optimization of CT-RED Calibration Curve Selection in Radiotherapy Treatment Planning Systems

The acquisition parameters affect the Hounsfield units (HU) on a CT scanner used for radiotherapy treatment planning. The degree of effect depends on the change in parameters, make of scanner, type of phantom, and the density of substitute materials. This work investigates the changes in HU when acquisition protocols are changed. In the literature, there is a lack of investigation in the optimization of the CT-RED calibration curve protocol. A computerized imaging reference system (CIRS) model 062 electron density phantom was scanned using a 16-slice HITACHI Supria CT scanner. The parameters evaluated are: tube voltage (kVp), tube current (mA s), collimation, slice thickness, field of view (FOV) and image reconstruction filters. All parameters have been fixed by varying a single parameter at each scan to see how that parameter affects the HU. Tube voltage, changing the reconstruction filter from body to head, as well as the use of beam hardening correction significantly affect the HU in dense bone (RED value 1.152). However, for other substitute materials, regardless of the parameter varied, the HU has remained within the tolerances reported in recent reviews, 20 HU for soft tissue and 50 HU for air and bone. In some cases, the acquisition parameters need to be modified according to the morphology of the patient and the region of interest. Therefore, several CT-RED calibration curves may be included in a treatment planning system (TPS); however, this may increase the risk of incorrect selection of these curves. This work provides a methodology to minimize the number of CT-RED calibration curves used within the TPS to avoid the risk of incorrect selection, it was found that three CT-RED calibration curves are in agreement with all CT spectra used for patient imaging.