An initial evaluation of digital radiography automatic exposure control system performance at a major public hospital

Abstract

Background: Automatic Exposure Control (AEC:) devices incorporated into radiographic devices provide consistent image quality across variations in tube voltages and patient body habitus provided that their performance is sufficient. Objectives: This study assessed AEC performance of three identical planar digital radiographic (DR) devices equipped with table and chest systems. Methodology: Assessments were conducted across variations in tube voltage and phantom thicknesses using two setups involving copper sheets and Poly-methyl methacrylate (PMMA) phantoms based on the ”scatter-free” and ”scatter” geometries of IPEM (2010). Measurements of the phantom exit doses and post-exposure mAs: were compared with tolerances of the manufacturer, IPEM, IAEA, and European Commission’s RP162 document. Finally, the post-exposure mAs of the table systems as well as the three chest systems were statistically compared separately. Results: Most AECs produced repeatable results across various tube voltages and phantom thicknesses. The usage of the different AEC chambers of each table/chest AEC device produced similar exposures at 81kVp. Exit dose: increased rapidly beyond 81kVp with 1mm copper. Conversely, when using 20cm PMMA, exit doses reduced at higher tube voltages. Exit doses were steady across different PMMA thicknesses and for thicker copper sheets (>1mm) but higher exit doses were measured for thinner copper sheets. Copper exit doses corresponded well with the RP162 10µGy suspension level for the DetectorAir Kerma: (DAK) but PMMA exit doses were incomparable to the suspension level due to the larger contribution of scatter. The post-exposure mAs of the three table systems as well as the three chest systems differed significantly across the different AEC chamber combinations but not across variations in tube voltage and phantom size. Conclusions and Recommendations: Results of most AECs corresponded well with tolerances but one chest AEC performed poorly across different phantom sizes. Standards often express tolerances in terms of the DAK but due to being impractical to measure in DR, the study proposed using exit doses and mAs as AEC performance indicators.