Feasibility of size-specific organ-dose estimates based on water equivalent diameter for common head CT examinations: a Monte Carlo study

Computed tomography dose index (CTDI) is an unreliable dose estimate outside of the standard CTDI phantom diameters (16 and 32 cm). Size-specific dose estimate (SSDE) for head computed tomography (CT) examination was studied in the American Association of Physicists in Medicine Report 293 to provide SSDE coefficient factors based on water equivalent diameter as size metrics. However, it is limited to one protocol and for a fully irradiated organ. This study aimed to evaluate the dependency of normalized organ dose (ND) on water equivalent diameter as a size metric in three common protocols: routine head, paranasal sinus, and temporal bone. CTDIw measurements were performed for outlined protocols in the Siemens Emotion 16-slice-configuration scanner. Geant4 Application for Tomographic Emission Monte Carlo simulation platform, coupled with ten GSF patient models, was used to estimate organ doses. CT scanner system was modeled. Helical CT scans were simulated using constructor scan parameters and calculated scan lengths of each patient model. Organ doses provided by simulations were normalized to CTDIvol. The water equivalent diameters (D (w)) of patient models were obtained via relationships between D (w) and both effective diameter for a sample of patients' data.NDs received by fully, partially, and non-directly irradiated organs were then reported as a function of D (w). For fully irradiated organs, brain (R (2) > 0.92), eyes (R (2) > 0.88), and eye lens (R (2) > 0.89) correlate well with D (w). For the rest of the results, a poor correlation was observed. For partially irradiated organs, the exception was scalp (R (2) = 0.93) in temporal bone CT. For non-directly irradiated organs, the exception was thyroid (R (2) > 0.90) and lungs (R (2) > 0.91) in routine head CT. ND correlates well in routine head CT than other protocols. For the most part, no relationship seems to exist between R (2) and scan percentage coverage. The results have revealed additional factors that may influence the ND and D (w) relationship, which explains the need for more studies in the future to investigate the effect of scan conditions and organ anatomy variation.