The impact of patient table on size-specific dose estimate (SSDE)

The use of parameters water equivalent diameter (D _W ) and size-specific dose estimate (SSDE) are becoming increasingly established as a recognised method to relate patient dose from a CT examination to the dose indicator volume CT dose index (CTDI_VOL). However, the role of the attenuation due to the patient table in these estimations requires careful consideration and is the subject of this study. The aim of this study is to investigate the impact of a minimal part of the patient table when calculating the D _W and SSDE. We investigated 164 patients who had undergone CT examinations for the pelvis, abdomen, thorax and head. We subsequently calculated D _W and SSDE using two methods: one using a small circular region of interest (ROI) including a minimal part of the patient table and the other using a ROI fitted to the patient border alone. The results showed that the water equivalent diameter calculated with the table included in the ROI (D _W,t ) is greater, compared to that without the consideration of the patient table (D _W,nt ), by 1.5-6.2% depending on the anatomy being imaged. On the other hand, the SSDE calculated with inclusion of the patient table (SSDE_t) is smaller than otherwise (SSDE_nt) by 1.0-5.5% again depending on the anatomy being imaged. The effect of the patient table on D _W and SSDE in the thorax CT examination was statistically significant, but its effect on D _W and SSDE in the other examinations of head, pelvis and abdomen was relatively small and not statistically significant.