Diagnostic reference levels for adult computed tomography examinations in Uganda

Abstract

Background: Computed tomography (CT) is a critical tool in medicine. Although CT is associated with higher radiation doses compared with planar radiographic imaging, it is often the most appropriate modality and first line for many examinations. Medical exposures to ionizing radiation constitute nearly half of the total radiation exposures from all sources. There is a higher utilization of imaging services world over, Uganda inclusive. The diagnostic potential of CT is unquestionable, however great care should be taken because of the potential risks associated with ionizing radiation. Medical radiation is a controllable source and should be applied on individualized basis to determine whether each patient fits the appropriate criteria for the diagnostic procedure. Appropriate justification of requested CT examinations should ensure that benefits outweigh the risks. CT scan protocols and radiation doses vary greatly across countries and are primarily attributable to local choices regarding technical parameters, rather than the patient, institution or machine characteristics. These variations call for optimization of doses to consistent standards. Diagnostic reference levels (DRLs) are typically set at the 75th percentile of the dose distribution from surveys conducted across a broad user base using a specified dose-measurement protocol. There was thus a need to develop and implement DRLs as a standardisation and optimisation tool for the radiological protection of patients at Computed Tomography (CT) facilities in Uganda. Specific Objectives: Sub-study 1: To determine the relationship between the patients’ anthropometric characteristics and CT scan exposure variables amongst adults in Uganda.

Sub-study 2: To examine the decision of radiographers in the selection of Computed Tomography scanning parameters in selected facilities in Uganda. Sub-study 3: To determine the dose distribution by volume CTDI and DLP for common adult CT scan examinations and to propose NDRLs for Uganda. Methods: Sub-study 1 was a cross-sectional study design whereby hospitals with CT scan machines and automated computation of radiation dose indices were drawn by simple random sampling. Adults who met the recruitment criteria were included in the study by systematic random sampling. The variables of interest were patient anthropometry and CT scan technical variables. The data collected was analyzed using Stata version 11.2 software. Sub-study 2 was a concurrent triangulation mixed-method using the convergence model in which both quantitative and qualitative methods were employed. The radiographers were drawn from both the public and private facilities that perform CT scan examinations in Uganda. In addition to the quantitative component, an in-depth understanding of the factors influencing the selection of CT doses, barriers and facilitators to best practices to CT radiation protection amongst radiographers through qualitative techniques. Qualitative data were collected using key informant interviews. Sub-study 3 was also a cross-sectional study. A list of all the functional CT scan units at the time of the study was established, then stratified sampling was conducted by facility location and ownership. The study participants were recruited by systematic random sampling. IAEA recommends a minimum of 10 patients and a maximum of 20 adult patients per radiation centre in describing CT dose characteristics and determining the CT scan DRLs. Data were manually extracted from the CT scan console and entered into the data collection tool. The study variables

were kVp, mAs, reference mAs, examination mAs, total mAs, slice thickness, scan length, scan time, DLP, volume CTDI and Effective dose. Results: In sub-study 1, the key findings showed that the examination, reference and total mAs showed significantly positive associations with the anthropometric characteristics namely; weight, height and BMI as opposed to age. Males were found to be generally exposed to higher doses for head and cardiac CT scan studies with females receiving higher doses for abdominal examinations only. In sub-study 2, the respondents either agreed or strongly agreed that DRLs were important in radiation dose selection. The key factors influencing the selection of CT scan doses were; CT scan machine, examination time, age and body size. The key barriers to best practices included the type or level of health facility, radiographer and government level related. Facilitators to best practices included type or level health facility, radiographer and regulator related. In sub-study 3, a total of 574 patients were examined with an average age of 47.1 years. For volume CTDI estimates; there was a strong positive significant relationship between the volume CTDI and examination mAs (rs = 0.9017, p-value < 0.001), and reference mAs (rs = 0.0.7708, p-value < 0.001). For DLP estimates; there was a moderate positive significant relationship between DLP and total mAs (rs = 0.6812, p-value < 0.001), reference mAs (rs = 0.5493, p-value < 0.001). The DRLs were as follows; for head CT scan - the average median volume CTDI was 56.02 mGy and the DLP was 1260.3 mGy.cm; for Chest CT, the volume CTDI was 7.82 mGy and the DLP was 377.0 mGy.cm; for the abdomen CT, the CTDI volume 12.54 mGy and DLP 1418.3 mGy.cm and for the lumbar spine 19.48 mGy and the DLP was 843 mGy.cm, respectively. Conclusion: This study found that the examination, reference and total mAs had a significant positive association with the patient characteristics namely; weight, height and BMI across the various facilities. There was, however, no significant relationship between patients’ age with the CT scan characteristics across all facilities. Males were generally associated with higher exposure characteristics for head and cardiac CT scans whereas the female had a significant relationship with the CT parameters for abdominal CT scans. Based on the opinion of the radiographers concerning CT scan radiation doses, DRLs, make, model and year of manufacture of the CT scanners were found to be important in the patient dose selection. The radiographers rarely had trainings on DRLs and the majority were concerned about the lack of DRLs for Uganda. Selection of the CT imaging parameters were influenced by the make of the machine, examination time and patient characteristics like age and weight. The barriers to the best practices in CT radiation protection included; unavailability of protective devices, delayed repairs of faulty equipment, lack of DRLs, the year of manufacture and installation of the CT scan machine, administrative bureaucratic tendencies, low level of knowledge amongst the radiography community and poor supervision by the regulatory authority. Functional facilities, administrative support, radiographer experience, knowledge and adequate supervision by Atomic Energy Council (AEC) were found to be the facilitators of the best practices. This study confirmed the need to optimize the CT scan parameters in order to lower the national DRLs. We recommend training of all the CT scan radiographers on optimizing the CT scan acquisition parameters. The study also recommends continuous dose audits for all new CT scan equipment’s or after every three years to ensure that values out of range are either justified or further investigated.