Technical Note: Long-term stability of Hounsfield unit calibration for cone-beam computed tomography.

PURPOSE The goal of this study was to investigate the stability of a phantom based Hounsfield unit (HU) calibration for cone beam CT (CBCT). METHODS Three consecutive scans of a large and a small phantom configuration were acquired and reconstructed with a uniform scatter correction method. The CBCT grey-values of the phantom inserts were measured and the three values of each insert averaged. The linear calibration curve was determined and its slope and intercept were evaluated. This procedure was performed for three CBCT scanners (Elekta Synergy) over a period of ten months with one, two, or four weeks between measurements. A dosimetric estimation of the HU fluctuations was made. RESULTS The CBCT HUs were stable over time with only small variations in slope and intercept resulting in HU differences on the water level, i.e. intercepts, of less than 7 HU (standard deviation). Therefore, the dosimetric influence of these HU differences was limited. The inter-scanner disparities (up to ~16 HU) were larger than the intra-scanner ones (up to ~7 HU). CONCLUSIONS Stable HUs were observed over a period of ten months. Due to the differences between the CBCT scanners, scanner specific calibration curves are necessary.