Evaluation of Heel-Effect Impact on AEC Cell Balance Quality Control
Abstract
Purpose
The X-ray Heel Effect influences automatic exposure control (AEC) termination at different AEC cell locations. This study evaluates the Heel Effect’s influence on AEC cell balance testing methodologies in digital radiography.
Methods
Cell balance was measured on seven table and nine wall Buckys across five vendors. Flat-field exposures at RQA-5 were performed with individual AEC cells active. mAs, exposure index (EI), and mean pixel value (MPV) from the active cell region were recorded. Air kerma at each AEC cell location was measured using a solid-state dosimeter and manual technique, then scaled to account for discrepancies between AEC-delivered and manually selected mAs. Additional flat-field images were acquired to relate MPV to detector air kerma. Grids were removed for all exposures. Cell balance, defined as percent difference in response relative to the center cell, was evaluated using system-reported mAs, Heel-Effect-corrected mAs (mAscorr), EI, MPV, MPV converted to air kerma (MPVcorr), and measured air kerma. Measured air kerma served as ground truth. For each group, paired measurements from the six methods were compared using repeated-measures analysis, with Holm-corrected post hoc comparisons against ground truth. Statistical significance was defined as p less than 0.05.
Results
For table Buckys, mAs, EI, and MPV differed significantly from ground truth with mean differences of 7.55%, 6.91%, and 9.21%, respectively. mAscorr and MPVcorr did not differ significantly from ground truth. For wall Buckys, mAs, EI, MPV, and MPVcorr differed significantly from ground truth with mean differences of 2.95%, 3.14%, 3.58%, and 3.68%, respectively, while mAscorr showed no significant difference.
Conclusion
Heel-Effect-induced beam non-uniformity produces systematic differences in AEC cell balance measurements, particularly for table bucky systems operating at shorter source-to-image distances. Without accounting for the Heel Effect, AEC cell balance testing may falsely indicate system imbalance. Incorporating Heel-Effect-aware measurement or correction methods improves AEC performance assessment.