Tube Current Modulated Computed Tomography Effective Dose and Size Specific Organ Dose Estimates with ImPACT: Total Scan versus Slice by Slice Parameters for Urogram Protocols

Purpose: One of the predominant computed tomography (CT) dosimetry estimation programs, ImPACT, was not designed to estimate organ absorbed dose or effective dose for modern tube current modulated-CT (TCM-CT). ImPACT also only estimates organ doses for a standard adult hermaphrodite mathematical phantom, not for a specific patient. Two methods for calculating size specific TCM-CT organ absorbed dose and non-size dependent TCM-CT effective dose were developed and compared with conventional dose estimation methods. Methods: A sample of 48 TCM-CT urogram procedures was obtained. Patient specific dose was calculated for each data set by two methods. The first method was a summation of slice by slice (localized) parameter estimates. Parameters from each slice were input separately in ImPACT the output organ dose and effective dose were recorded. The organ dose was then multiplied by a size dependent conversion factor to acquire a size specific organ dose. Then the size specific organ doses and the slice effective doses were summed over all slices to calculate the total organ doses and effective dose. The second method estimated doses based on global scan parameters. The effective dose was calculated with global average scan parameters in ImPACT. The output organ doses were multiplied by the average size dependent conversion factor to get the size dependent organ doses. The organ doses were then compared with a non-size adjusted method and the effective doses with a conventional k-factor method. Results: The two size dependent organ dose estimation methods fell within acceptable difference criteria when compared directly with each other. The three effective dose estimation methods also fell within the criteria. Conclusion: These results suggested that using the global parameter method is acceptable for calculating effective dose and patient specific organ doses for TCM-CT urogram protocols.