Dose Optimization in Digital Radiography
This chapter explored the ICRP’s principle of optimization, optimization of the exposure technique factors [kilovolts (kV) and milliampere-seconds (mAs)] used in a digital radiography examination, including the exposure indicator (EI). Secondly, dose optimization tools for image quality assessment were outlined, specifically the method of visual grading of normal anatomy. Finally the chapter provides an example of a research study examining the optimization of the EI as a dose management strategy in digital radiography. The ICRP principle of optimization is intended to protect the patient from unnecessary radiation by using a dose that is as low as reasonably achievable (ALARA). The ultimate goal of optimization is to minimize stochastic effects and to prevent deterministic effects. Three dose optimization approaches were introduced and reviewed. These include optimization of the exposure techniques factor (kV and mAs) and the EI. The literature review on the use of these approaches in computed radiography (CR) found the EI as a dose control mechanism and as a “surrogate for dose management,” and the use of the EI has been viewed as an opportunity for dose optimization. Furthermore optimization research has focused mainly on optimizing the kV in CR imaging as a means of implementing the ALARA philosophy.
Generally, these studies have produced “conflicting results.” The fourth topic addressed in this chapter is image quality assessment of medical images. These include objective physical measures and observer performance methods such as the use of visual grading of normal anatomy and the various receiver operating characteristic (ROC) methods. One tool that is well established and makes use of the European Guidelines on Quality Criteria for Diagnostic Radiographic Images is visual grading analysis (VGA) procedure. This procedure is used to evaluate image quality based on the visualization and reproduction of defined anatomical structures in images and not on lesion detection. The final section of this chapter examined an empirical research study as an illustration of the use of the EI in the optimization of the radiation dose and image quality for a computed radiography system. Entrance skin doses were measured for phantom models of the pelvis and lumbar spine imaged using the vendor’s recommended exposure settings (i.e., the reference doses) as well as doses above and below the vendor’s recommended settings for both body parts. Images were assessed using visual grading analysis (VGA). While the dose measurement results revealed strong positive linear relationships between dose and milliampere-seconds (mAs), mAs and inverse exposure indicator (EI), and dose and inverse EI for both body parts, the VGA showed that optimized values of 16 mAs/EI = 136 for the anteroposterior (AP) pelvis and 32 mAs/EI = 139 for the AP lumbar spine did not compromise image quality. The conclusions drawn of this study suggest selecting that optimized mAs reduced dose by 36% compared with the vendor’s recommended mAs (dose) values, and optimizing the mAs and associated EIs can be an effective dose management strategy.
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