Influence of reference baselines on size-specific dose estimate (SSDE) in head CT examinations

This study aimed to evaluate the influence of three reference baselines-specifically, supraorbitometal baseline (SOML), orbitometal baseline (OMBL), and Reid's baseline (RBL) on size-specific dose estimate (SSDE) in head computed tomography (CT), addressing limitations of conventional dose metrics that neglect baseline-dependent anatomical variations. Experiments were performed on a GE LightSpeed VCT scanner. The positioning of the head phantom corresponding to the SOML, OMBL, and RBL baselines was achieved by tilting the CT gantry and elevating the phantom. Using automatic tube current modulation, 48 imaging datasets were acquired through both axial and helical scans. Thermoluminescent dosimeters (TLDs) were employed to measure the eye lens dose. The SSDE(z) was calculated for each slice, with the central slice SSDE values denoted as SSDEcen. The volume-averaged SSDE (SSDEgro) was derived by weighted integration of the former, enabling comparative analysis of radiation dose parameters across groups. In both axial and helical scanning modes, the SOML group demonstrated lower CTDI_vol(a), SSDEcen, and SSDEgro compared to the OMBL and RBL group. When the gantry tilted along SOML, SSDEgro values reduced by 20.67% (vs. RBL-phantom elevation) to 32.19% (vs. OMBL-gantry tilt). Within a single scan, SSDEgro values were consistently less than the corresponding CTDI_vol(a), with a maximum reduction of 10.71%. Additionally, TLD measurements revealed that the eye lens radiation dose varied between 47.48 and 87.91 mGy, with the lowest dose (47.48 mGy) achieved in SOML-based axial scans using gantry tilt. This study demonstrates that the choice of reference baseline influences SSDE values in head CT imaging. Furthermore, SSDE effectively corrects the overestimation of the actual radiation dose by CTDI_vol. Based on the findings, we recommend adopting the SOML as the reference baseline in clinical practice, which reduces radiation exposure during head CT examinations and optimises ocular radiation protection.