Master of Philosophy (Medical Radiation Physics)
School of Physics
The usage of Computed Tomography (CT) for diagnostic and therapeutic evaluations has increased dramatically over the past two decades. The rising concerns about radiation-induced carcinogenesis have resulted in great efforts in investigating, monitoring and reporting radiation dose from CT patient examinations. Currently, common CT dose descriptors of volume CT Dose Index (CTDIvol) and Dose Length Product (DLP) are not able to characterise patient-specific dose from CT examinations. The aims of this project were to investigate the CT dose trends at St. Vincent’s Hospital in the last ten years (2007- 2016) and to evaluate the impacts of patient size and tube current modulation (TCM) to the patient CT dose estimation.
Our investigation of CT dose at St Vincent’s Hospital was followed the 2011 ARPANSA recommendation’s protocol (section 2.2.1) in Australian National Diagnostic Reference Levels (NDRLs). A total of 10, 20, 20 and 20 random patients each year for head, chest, abdomen and chest-abdomen-pelvis scanning have been selected respectively, including 50% male and 50% female adult patients. Further data analysis and investigation in this thesis are focused on three aspects: (1) the local CT facility DRLs (FDRLs) are calculated each year for the CT dose trend analysis and compared to the Australian NDRLs; (2) the patient size effect on the CT patient dose estimation was investigated using the AAPM’s approximate method of Size Specific Dose Estimate (SSDE) and an alternative SSDE calculation method using average CTDIvol has been proposed to account for the TCM effect; (3) the verification of patient size and TCM impacts on patient skin dose was evaluated with an anthropomorphic phantom using the CMRP’s MOSkin dosimeters.
Our results show that both CTDIvols and DLPs from the CT facilities at St Vincent’s Hospital were generally under the Australian National Diagnostic Reference Levels in the past ten years (2007-2016). The year-to-year variations are correlated with either the CT scanner or scanning protocol upgrades. The CTDIvols for individual patient examinations exhibit large variations with patient size. The patient-size dependent variations are significantly reduced by SSDEs instead of CTDIvol. Our proposed SSDE calculation method shows high agreement with the AAPM approach but also high computing efficiency. The results from the phantom experiments show that TCM has significant impact on the skin dose related to patient’s size. The non-TCM scan has higher skin dose than the TCM scan. The dose reduction impact of TCM scan is significant on small size patients.
Liu, Wenbo, Retrospective study of clinical CT dose trends at St. Vincent’s Hospital and evaluation of the impacts of patient size and tube current modulation with an anthropomorphic phantom, Master of Philosophy (Medical Radiation Physics) thesis, School of Physics, University of Wollongong, 2018. https://ro.uow.edu.au/theses1/677
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.