Griffiths M R, Noakes K A, Pocock N A
Department of Nuclear Medicine and Bone Densitometry St. Vincent's Hospital, Sydney, NSW, Australia.
J Bone Miner Res. 1997 Jan;12(1):119-23. doi: 10.1359/jbmr.1997.12.1.119.
Dual-energy X-ray absorptiometry (DXA), using a narrow pencil-shaped X-ray beam coupled to a single detector, has been used extensively. More recently, DXA using a fan- shaped X-ray beam coupled to an array of detectors has been introduced. This new generation of scanners causes an inherent magnification of scanned structures as the distance from the X-ray source decreases. This magnification, which occurs in the medial-lateral direction but not in the craniocaudal direction, does not affect bone mineral density (BMD). There are, however, significant changes of bone mineral content (BMC), bone area, and parameters of hip geometry, with varying distance of the bone scanned from the X-ray source. Variability of soft tissue thickness in vivo, by altering the distance of the skeleton from the scanning table and X-ray source, may cause clinically significant errors of BMC, bone area, and proximal femur geometry when measured using fan-beam densitometers. We analyzed the geometry of Lunar and Hologic fan beam scanners to derive equations expressing the true width of scanned structures in terms of the apparent width and machine dimensions. We also showed mathematic ally that performing an additional scan, at a different distance from the X-ray source than the first scan, provides simultaneous equations that can be solved to derive the real width of a scanned bone. This hypothesis was tested on the Lunar Expert using aluminium phantoms scanned at different table heights. There was an excellent correlation, r = 0.99 (p < 0.001), between the predicted phantom width and the measured phantom width. In conclusion, this study shows that the magnification error of fan beam DXA can be corrected using a dual scanning technique. This has important implications in the clinical usefulness of BMC and geometrical measurements obtained from these scanners.
双能X线吸收法（DXA），使用与单个探测器耦合的窄铅笔形X线束，已被广泛应用。最近，引入了使用与探测器阵列耦合的扇形X线束的DXA。随着离X射线源的距离减小，这种新一代扫描仪会使扫描结构产生固有的放大。这种放大发生在内外侧方向而非头脚方向，不影响骨矿物质密度（BMD）。然而，随着被扫描骨离X射线源的距离不同，骨矿物质含量（BMC）、骨面积和髋部几何参数会有显著变化。体内软组织厚度的变化，通过改变骨骼离扫描台和X射线源的距离，在使用扇形束骨密度仪测量时，可能会导致BMC、骨面积和近端股骨几何形状出现临床上显著的误差。我们分析了Lunar和Hologic扇形束扫描仪的几何结构，以推导用表观宽度和机器尺寸表示扫描结构真实宽度的方程。我们还从数学上表明，在与第一次扫描不同的离X射线源的距离处进行额外扫描，可提供能求解以得出扫描骨真实宽度的联立方程。使用在不同台面高度扫描的铝制体模在Lunar Expert上对这一假设进行了测试。预测的体模宽度与测量的体模宽度之间存在极好的相关性，r = 0.99（p < 0.001）。总之，本研究表明扇形束DXA的放大误差可通过双扫描技术校正。这对从这些扫描仪获得的BMC和几何测量的临床实用性具有重要意义。
