Stationary table CT dosimetry and anomalous scanner-reported values of CTDI_vol

Purpose: Anomalous, scanner-reported values of CTDI_vol for stationary phantomtable protocols (having elevated values of CTDI_vol over 300 higher than the actual dose to the phantom) have been observed; which are well-beyond the typical accuracy expected of CTDI_vol as a phantom dose. Recognition of these outliers as bad data is important to users of CT dose index tracking systems (e.g., ACR DIR), and a method for recognition and correction is provided. Methods: Rigorous methods and equations are presented which describe the dose distributions for stationary-table CT. A comparison with formulae for scanner-reported values of CTDI_vol clearly identifies the source of these anomalies. Results: For the stationary table, use of the CTDI₁₀₀ formula (applicable to a moving phantom only) overestimates the dose due to extra scatter and also includes an overbeaming correction, both of which are nonexistent when the phantom (or patient) is held stationary. The reported DLP remains robust for the stationary phantom. Conclusions: The CTDI-paradigm does not apply in the case of a stationary phantom and simpler nonintegral equations suffice. A method of correction of the currently reported CTDI_vol using the approach-to-equilibrium formula H(a) and an overbeaming correction factor serves to scale the reported CTDI_vol values to more accurate levels for stationary-table CT, as well as serving as an indicator in the detection of bad data.