A method for calculating the dose length product from CT DICOM images

Abstract

Objective: The dosimetric calculations in CT examinations are currently based on two quantities: the volume weighted CT dose index (CTDI(vol)) and the dose-length product (DLP). The first quantity is dependent on the exposure factors, scan field of view, collimation and pitch factor selections, whereas the second is additionally dependent on the scan length.

Methods: In this study a method for the calculation of these quantities from digital imaging and communication in medicine (DICOM) CT images is presented that allows an objective audit of patient doses. This method was based on software that has been developed to enable the automatic extraction of the DICOM header information of each image (relating to the parameters that affect the aforementioned quantities) into a spreadsheet with embedded functions for calculating the contribution of each image to the CTDI(vol) and DLP values. The applicability and accuracy of this method was investigated using data from actual examinations carried out in three different multislice CT scanners. These examinations have been performed with the automatic exposure control systems activated, and therefore the tube current and tube loading values varied during the scans.

Results: The calculated DLP values were in good agreement (±5%) with the displayed values. The calculated average CDTI(vol) values were in similar agreement with the displayed CTDI(vol) values but only for two of the three scanners. In the other scanner the displayed CTDI(vol) values were found to be overestimated by about 25%. As an additional application of this method the differences among the tube modulation techniques used by the three CT scanners were investigated.

Conclusion: This method is a useful tool for radiation dose surveys.

Figure 1

The results of the dose-length product (DLP) indications (normalised per 100 mAs) vs planned scan length for the three multislice CT (MSCT) scanners are given. Apart from data points the best fit lines (solid and dashed lines) and the respective equations are also given.

Figure 2

The mAs variations for the examination of patient A—1 of Table 3 performed in scanner A are shown with respect to the slice location and superimposed on the scout scan image.

Figure 3

(a) The mAs variations for the examination of patient B—4 of Table 3 performed in scanner B are shown with respect to the slice location and superimposed on the scout scan image. (b) The mAs variations for the examination of patient B—1 of Table 3 performed in scanner B are shown with respect to the slice location and superimposed on the scout scan image.

Figure 4

The mAs variations for the examinations of patient C—5 and C—8 of Table 3 performed in scanner C are shown with respect to the slice location and superimposed on the scout scan image. This was a chest, abdomen and pelvis examination that was performed with two scans, one for the chest examination and one for the pelvis–abdomen examination.