Exam-level dose monitoring in CT: Quality metric consideration for multiple series acquisitions

Abstract

Purpose: Multi-series CT examination is common in the clinic, but no metric is agreed upon to report the overall dose from such an examination. This work proposes a relevant metric for tracking patient dose from multi-series examinations and illustrates the evaluation method through explanatory examples.

Materials and methods: In each acquisition series, a previously reported method was used to evaluate the cross-sectional average dose along the z-axis of a water phantom, with inputs of CTDI_vol , scan length, tube current, and patient water-equivalent diameter. With a multi-series examination, the dose at each z-location was accumulated over all acquisition series. This method was applied to four clinical CT examinations. In three abdominal/pelvic examinations (patient weight, 107, 79, 79 kg), tube current modulation was applied in five acquisition series with scan lengths of 30-41.8 cm, while tube current was fixed in other series with short scan lengths (1.0, 7.9 cm). In another CT-guided liver ablation procedure (patient weight, 114 kg), 22 series were acquired with constant mA and scan lengths of 1-30 cm. The maximum value of the overall dose profile of each examination was compared to five dose quantities, including CTDI_vol,sum and SSDE_sum by the ACR CT Dose Index Registry, scan length-weighted CTDI_vol and SSDE by a CT dose monitoring platform, and "max z location CTDI_vol " by a CT manufacturer.

Results: A simple graphic display of dose as a function of the z-axis location was presented for each acquisition series and for the whole examination. Differences up to 43.4% and 42.8%, or down to -93.5%, -93.5%, and -49.0%, were observed between the maximum value of the overall dose profile and five dose quantities (in the above order), respectively.

Conclusion: The overall dose profile gives a complete description of z-axis dose distribution for the studied CT examinations under a wide range of patient variables and acquisition conditions, including multiple acquisition series. Simple visualization of the doses across and beyond the scan ranges may provide a new tool for CT dose optimization.

Keywords: Computed tomography; multiple series; quality metric; radiation dose monitoring.