Evaluation of factors affecting the exposure indicator of computed radiography and how it relates to radiation organ dose at a second level hospital in Zambia

Abstract

Introduction: The main purpose of conducting this research study was to evaluate if there was any significance in the difference between the Exposure Index (EI) of the postero-anterior (PA) chest, antero-posterior (AP) pelvis and lateral lumbar spine views obtained with a Computed Radiography (CR) system for patients referred for conventional diagnostic imaging and the manufacturer’s standard/recommended EI. Secondly the factors affecting EI in the application of CR at a second level hospital in Zambia were also explored, and lastly, to understand the relationship that exists between EIs and radiation dose imparted to the patient. Methods: A sample of 334 patients referred for conventional diagnostic imaging of the PA chest, AP pelvis, and that of the lateral lumbar spine views were enrolled in the study. These patients underwent an X-ray examination on a Fuji CR system. Radiographers were asked to record data for each patient. Data that was collected prospectively included exposure factors such as kilo-voltage peak (kVp) and milli-amperage per second (mAs). The other data collected included parameters such as source-skin distance (SSD), focal spot to skin distance (FSD), filtration, field size, anode angulation and exposure index. In addition, the patient’s height as well as weight, the date and time of the radiographic examination were also recorded. The radiation quantity and the entrance skin exposure (ESE) of the PA chest, AP pelvis and the lateral lumbar spine, using exposure technique factors (mAs and kVp) was calculated using the Personal Computer Program for X-ray Monte Carlo (PCXMC). A total of 334 images were later on recorded (114 for PA chest, 107 AP pelvis and 113 lateral lumbar spine respectively) using the dose values alluded to above and recording the reciprocal EIs. The EI generated by the Fuji CR system is denoted as a sensitivity (“S”) value. The study of dose optimisation was associated with measuring the ESE [and changing it to entrance skin dose (ESD)] of a free-in-air to an anthropomorphic phantom model of the pelvis and lumbar spine, using the vendor’s approved exposure factors (kVp and mAs) and dose values higher and lower than the vendor’s values (reference dose) for each of the selected body parts. Later on, Fifty-four images (27 for each of the AP pelvis and lateral lumbar spine) were recorded using the dose values referred to above and recording the matching EIs.

Results: The results indicated that there was a notable difference in EI between examinations of the PA chest, pelvis and lateral lumbar examinations using a p-value which was < 0.05. There was a notable difference between the average EI generated by radiographers at the research site and the recommended EI value for the lateral lumbar spine. The findings from the study also revealed that gender did not have a significant influence on EI – for all three views namely PA chest, AP pelvis and lateral lumbar spine. Similarly, time of examination did not have a significant influence on EI for all three examinations. None of the following variables (kVp, mAs, height and weight) had a significant correlation with EI – in the lumbar spine area. None of the factors and co-variates (patient sex, age, time of examination, kVp, mAs, height and weight) had a significant influence on the EI. There was no notable relationship between effective dose and the EI in the lateral lumbar spine and PA chest examinations, however, there was a notable relationship between effective dose and the EI in the pelvis examinations. Conclusion: The study revealed that radiation dose to the testicles and ovaries was higher when examining dose of the AP pelvis and lateral lumbar spine views compared to when examining the chest. As regards the uterus, dose was only higher when examining the pelvis compared to when examining the lumbar and chest. The lowest EI generated by the radiographer for the PA chest was 53 and the highest being 540 against the manufacturer approved EI range of 200 – 600. The lowest EI generated for the AP pelvis was 45 and the highest was 417 against the manufacturer recommended range of 200 – 400. For the lateral lumbar spine, the highest EI was 419 and the lowest was 35 against the manufacturer recommended range of 200 – 400. Generally, results of the study indicated that a significant difference existed between the average radiographer EI and the manufacturer approved EI value for the lateral lumbar spine and AP pelvis. Secondly, there was no notable relationship between effective dose and radiographer’s EI in the lateral lumbar and PA chest examinations. However, a notable relationship existed between effective dose and EI in the pelvis examination.