Biomedical Engineering Graduate Program, University of Calgary, Alberta, Canada; McCaig Institute for Bone and Joint Health, University of Calgary, Alberta, Canada; Department of Radiology, Cumming School of Medicine, University of Calgary, Alberta, Canada.
McCaig Institute for Bone and Joint Health, University of Calgary, Alberta, Canada; David R. Cheriton School of Computer Science, University of Waterloo, Waterloo, Ontario, Canada.
J Clin Densitom. 2024 Jul-Sep;27(3):101504. doi: 10.1016/j.jocd.2024.101504. Epub 2024 Jun 6.
Weight bearing computed tomography (WBCT) utilizes cone beam CT technology to provide assessments of lower limb joint structures while they are functionally loaded. Grey-scale values indicative of X-ray attenuation that are output from cone beam CT are challenging to calibrate, and their use for bone mineral density (BMD) measurement remains debatable. To determine whether WBCT can be reliably used for cortical and trabecular BMD assessment, we sought to establish the accuracy of BMD measurements at the knee using modern WBCT by comparing them to measurements from conventional CT.
A hydroxyapatite phantom with three inserts of varying densities was used to systematically quantify signal uniformity and BMD accuracy across the acquisition volume. We evaluated BMD in vivo (n = 5, female) using synchronous and asynchronous calibration techniques in WBCT and CT. To account for variation in attenuation along the height (z-axis) of acquisition volumes, we tested a height-dependent calibration approach for both WBCT and CT images.
Phantom BMD measurement error in WBCT was as high as 15.3% and consistently larger than CT (up to 5.6%). Phantom BMD measures made under synchronous conditions in WBCT improved measurement accuracy by up to 3% but introduced more variability in measured BMD. We found strong correlations (R = 0.96) as well as wide limits of agreement (-324 mgHA/cm to 183 mgHA/cm) from Bland-Altman analysis between WBCT and CT measures in vivo that were not improved by height-dependent calibration.
Whilst BMD accuracy from WBCT was found to be dependent on apparent density, accuracy was independent of the calibration technique (synchronous or asynchronous) and the location of the measurement site within the field of view. Overall, we found strong correlations between BMD measures from WBCT and CT and in vivo measures to be more accurate in trabecular bone regions. Importantly, WBCT can be used to distinguish between anatomically relevant differences in BMD, however future work is necessary to determine the repeatability and sensitivity of BMD measures in WBCT.
负重位计算机断层扫描（WBCT）利用锥形束 CT 技术，在下肢关节结构负重时提供其结构的评估。锥形束 CT 输出的指示 X 射线衰减的灰度值难以校准，其用于骨密度（BMD）测量的用途仍存在争议。为了确定 WBCT 是否可用于可靠地评估皮质骨和小梁骨的 BMD，我们通过比较与常规 CT 的测量值，旨在确定现代 WBCT 用于膝关节 BMD 测量的准确性。
使用具有三个不同密度插入物的羟磷灰石体模，以系统地量化采集体积内的信号均匀性和 BMD 准确性。我们使用同步和异步校准技术在 WBCT 和 CT 中对体内 BMD 进行了评估（n=5，女性）。为了考虑到沿采集体积高度（z 轴）的衰减变化，我们测试了针对 WBCT 和 CT 图像的高度依赖校准方法。
WBCT 中的体模 BMD 测量误差高达 15.3%，且始终大于 CT（高达 5.6%）。在 WBCT 中同步条件下进行的体模 BMD 测量可将测量精度提高高达 3%，但会使测量的 BMD 更具变异性。我们发现 WBCT 和 CT 在体内的测量值之间具有很强的相关性（R=0.96）以及广泛的一致性界限（-324 至 183 mgHA/cm），并且不受高度依赖校准的影响。
尽管 WBCT 的 BMD 准确性依赖于表观密度，但准确性与校准技术（同步或异步）以及测量部位在视场中的位置无关。总体而言，我们发现 WBCT 和 CT 的 BMD 测量值之间存在很强的相关性，并且体内测量值在小梁骨区域更准确。重要的是，WBCT 可用于区分 BMD 的解剖学相关差异，但是需要进一步的工作来确定 WBCT 中 BMD 测量的重复性和敏感性。
