A study of the midpoint dose to CTDI_vol ratio: Implications for CT dose evaluation

In multidetector CT, the volume CT dose index (CTDI_vol) is reported for each scan series and dose conversion factors are used for the size-specific dose estimate (SSDE) and scanner-independent organ dose evaluation. This study aimed at examining the dependencies of conversion factors on scan length, tube voltage, and subject size. The results may be insightful for evaluating the dose from CT examinations with large variations in patient size and scan length.

A previously developed Monte Carlo simulation program was used to simulate single rotation axial scans of two standard CTDI phantoms [material polymethyl methacrylate (PMMA), diameters 16 and 32 cm] and multiple water cylinders at five tube voltages (70, 80, 100, 120, and 140 kV). The resultant longitudinal dose profiles were used to calculate CTDI_L(water)/CTDI_w (PMMA), where L was dose integration length. The ratio was equal to the midpoint dose D_L(0) (water) to CTDI_vol ratio in a CT scan series with a scan length equal to L.

For water phantom diameters from 11 to 50 cm and scan lengths from 15 to 30 cm, the changes of D_L(0)/CTDI_vol from that of 120 kV and 20-cm scan length were between -18.4% and 11.7%. This was consistent with the CTDI_vol to SSDE conversion factors of AAPM Report No. 204. For scan lengths less than 15 cm, D_L(0)/CTDI_vol decreased considerably as L decreased. D_L(0)/CTDI_vol was shown to be 17.3%-31.4% lower for L = 5 cm than for L = 15 cm, when the tube voltage was 120 kV and phantom diameter ranged from 11 to 50 cm. As tube voltage increased from 80 to 140 kV, D_L(0)/CTDI_vol decreased at small diameters while it increased at large diameters. The change was 9.4% with a diameter of 18 cm and a scan length of 20 cm and 17.6% with a diameter of 40 cm and a scan length of 30 cm.

The midpoint dose to CTDI_vol ratio varies widely across the clinical scan lengths from a few millimeters to about 1 m and varies moderately across tube voltages from 70 to 140 kV. The comprehensive data provided in the Appendix can be used for assessing the dose from short-length scans and improving the dose evaluation in CT.