Purpose: Dual-energy x-ray absorptiometry (DXA) is the clinical standard for bone mineral density (BMD) assessments, with lumbar-spine, femoral-neck, and total hip as preferred scans. Therefore, developing a quality-control (QC) protocol to manage longitudinal equipment performance changes is important. However, the low exposures and unique x-ray beam geometry/scanning in DXA complicate measurement and QC strategies relative to traditional x-ray imaging. Here, dose-area-product (DAP) was measured using different types of dose sensors on different DXA scanner platforms. Measurement variability across both sensors and scanners was analyzed, as well as differences between measured and vendor-reported DAP.
Methods: Four Hologic DXA scanners (two Horizon-A, one Horizon-W, one Discovery-A) at two institutions and all four available scan modes were tested. DAP was derived from sensor-measured entrance dose and CR plate-confirmed scan field sizes. Three types of Radcal dose sensors were used: 60-cc ion chamber (IC), 180-cc IC, and solid-state detector. Linear regression analysis assessed the dependence of DAP on scan length (1 to 8 inches) with coefficients of determination (R²). Variations between dose sensors and measured vs. vendor-reported DAP were evaluated using Bland-Altman analysis (mean±95% prediction interval: 1.96σ, PI).
Results: Dose sensor variations in DAP were minimal, with mean±95% PI of ‒3.5±3.5% between the two sizes of IC’s. The solid-state detector produced highly similar measurements to the 180-cc IC. DAP values were strongly linear vs. scan length for both vendor-reported (R²>0.999) and measured (R²>0.99) values. Differences between measured and reported DAP were higher, with mean±95% PI of ‒6±48%. Likely errors in vendor-reported DAP calculations were identified from the consistency of sensor-measured DAP and linear scan length dependence.
Conclusion: It is feasible to measure DXA DAP with a variety of dose sensors and the scan length used is not critical. Measured and vendor-reported DAP may differ significantly, however, reinforcing the importance of established and effective QC.