Understanding entrance-air-kerma as a quality-control metric for dual-energy x-ray absorptiometry

Abstract

Purpose: The low exposures, unique x-ray beam geometry, and scanning design in dual-energy x-ray absorptiometry (DXA) make measurement and quality-control strategies different from traditional x-ray equipment. This study examines the dependence of measured entrance-air-kerma (EAK) on both dose sensor type and scan length. The feasibility of using EAK to compare scanner output between different scan modes, individual scanners, and scanner platforms was also established. Finally, the congruence between measured and vendor-reported EAK was analyzed.

Methods: Four Hologic DXA scanners at two institutions and all four available scan modes were tested. EAK was measured directly by three types of Radcal dose sensors: 60-cc pancake ion-chamber (IC), 180-cc pancake IC, and solid-state detector. The coefficient of variation (COV) was used to assess the dependence of EAK on scan length. Variations in EAK between the types of dose sensors as well as measured versus vendor-reported values were evaluated using Bland-Altman analysis: mean ±95% prediction interval (PI): 1.96σ.

Results: Dose sensor variations in EAK were minimal, with a -3.5 ± 3.5% (mean ±95% PI) percent difference between the two sizes of IC's. The solid-state detector produced highly similar measurements to the 180-cc IC. These small differences were consistent across all scanners and all scan modes tested. Neither measured nor vendor-reported EAK values were found to show relevant dependence on scan length, with all COV values ≤4%. Differences between measured and reported EAK were higher at -6 ± 48%. Likely errors in vendor-reported EAK calculations were also identified.

Conclusion: It is feasible to quantify DXA scanner stability using EAK as a quality-control metric with a variety of solid-state and IC dose sensors, and the scan length used is not critical. Although vendor-reported EAK was consistent among scanners of the same platform, measured EAK varied significantly from scanner to scanner. As a result, measured and reported EAK may not always be comparable.

Keywords: bone mineral density; dual-energy x-ray absorptiometry; entrance-air-kerma; x-ray absorptiometry.

FIGURE 1

Bland–Altman plot comparing entrance‐air‐kerma measured with different sensors: 180‐cc ion chamber relative to the 60‐cc ion chamber (blue circles). The mean percent difference and 95% prediction interval range between measurements on the two sensors are included as dashed lines in the plots, along with their values. Data for the solid‐state detector (relative to those for the 60‐cc ion chamber (red diamonds) are also included for comparison

FIGURE 2

Boxplots of measured entrance‐air‐kerma ratios comparing the 60‐ and 180‐cc ion chamber measurements for all four scan modes on three different dual‐energy x‐ray absorptiometry scanners. The ratios are calculated relative to the 60‐cc measurement. Median values are shown with a horizontal line within each box, while the box represents the interquartile range (25th to 75th percentiles). Mean values are shown with a solid dot. The whiskers extend to 95% CI limits. Ratios for the solid‐state detector (relative to the 60‐cc ion chamber are also included for comparison (diamonds)

FIGURE 3

Bland–Altman plot comparing measured (60‐cc ion chamber) and reported entrance‐air‐kerma (EAK). The mean percent difference and 95% PI range between the EAK values are included as dashed lines in the plots, along with their values

FIGURE 4

Boxplots of measured versus reported entrance‐air‐kerma (EAK) ratios for all four scan modes on four different dual‐energy x‐ray absorptiometry scanners: (a) unadjusted ratios, (b) ratios calculated using adjusted reported EAK values for HD and express scan modes. The ratios are calculated with the reported EAK value in the denominator. The boxplots are set up the same as in Figure 2