Department of Imaging Physics, UT MD Anderson Cancer Center, Houston, Texas, USA.
Section of Medical Physics, Department of Radiology, Mayo Clinic Arizona, Phoenix, Arizona, USA.
J Appl Clin Med Phys. 2022 Oct;23(10):e13779. doi: 10.1002/acm2.13779. Epub 2022 Sep 4.
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.
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.
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.
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.
双能 X 射线吸收法（DXA）中的低照射量、独特的 X 射线束几何形状和扫描设计使得测量和质量控制策略与传统 X 射线设备不同。本研究探讨了测量的入口空气比释动能（EAK）与剂量传感器类型和扫描长度的关系。还确定了使用 EAK 来比较不同扫描模式、单个扫描仪和扫描仪平台之间的扫描仪输出的可行性。最后，分析了测量值与供应商报告的 EAK 之间的一致性。
在两个机构的四台 Hologic DXA 扫描仪上测试了所有四种可用的扫描模式。通过三种 Radcal 剂量传感器直接测量 EAK：60 立方厘米的平面电离室（IC）、180 立方厘米的平面 IC 和固态探测器。使用变异系数（COV）来评估 EAK 对扫描长度的依赖性。使用 Bland-Altman 分析评估了剂量传感器类型之间的 EAK 变化以及测量值与供应商报告值之间的差异：平均值±95%预测区间（PI）：1.96σ。
EAK 中剂量传感器的变化很小，两种尺寸的 IC 之间的差异为-3.5%±3.5%（平均值±95%PI）。固态探测器的测量结果与 180 立方厘米的 IC 非常相似。这些微小差异在所有测试的扫描仪和所有扫描模式中都是一致的。既没有发现测量值也没有发现供应商报告的 EAK 值与扫描长度有相关关系，所有 COV 值均≤4%。在-6±48%处，测量值和报告值的 EAK 之间存在差异。还确定了供应商报告的 EAK 计算中可能存在的错误。
使用 EAK 作为质量控制指标，可以使用各种固态和 IC 剂量传感器来量化 DXA 扫描仪的稳定性，并且使用的扫描长度并不关键。虽然同一平台的扫描仪之间的供应商报告的 EAK 是一致的，但测量的 EAK 却在扫描仪之间有很大差异。因此，测量值和报告值可能并不总是可比的。
