Pocock N A, Noakes K A, Majerovic Y, Griffiths M R
Department of Nuclear Medicine and Bone Densitometry, St. Vincent's Hospital, Sydney, NSW 2010, Australia.
Calcif Tissue Int. 1997 Jan;60(1):8-10. doi: 10.1007/s002239900177.
Hip axis length (HAL) has been reported as an independent risk factor for hip fracture. DEXA machines using fan beam techniques have become increasingly available. Errors in calculated hip axis length may be expected because of different degrees of magnification by the fan beam. The magnitude of this error on measurement of hip geometry was studied, using an anthropomorphic femur phantom with both fan beam (Lunar Expert and Hologic QDR-2000) and pencil beam (Lunar DPXL) densitometers. The clinical relevance of these findings was also examined using patient measurements of buttock soft tissue thickness. Femoral neck axis length (FNAL), which correlates well with HAL, was used as a measurement of hip geometry. There was a linear increase or decrease of FNAL with increasing distance of the phantom above the scanning table, when measured with the Lunar Expert or Hologic QDR-2000, respectively. There was no significant difference in FNAL at different heights using the pencil beam densitometer. The maximal difference in buttock soft tissue thickness in 30 women studied was 8.7 cm. From the equations, derived from the phantom studies, this difference would result in an 8.2% (1.4 SD) increase, or an 11.4% (1.9 SD) decrease in FNAL in the largest woman as compared with the smallest woman when measured using the Lunar Expert or Hologic QDR-2000, respectively. We conclude that there may be unpredictable degrees of magnification of FNAL in vivo, caused by differences in buttock thickness, when measured using a fan beam densitometers. Until these problems are resolved. FNAL, or related parameters of femoral geometry, should be measured using pencil beam instruments.
髋轴长度（HAL）已被报道为髋部骨折的一个独立危险因素。采用扇形束技术的双能X线吸收仪（DEXA）已越来越普及。由于扇形束的放大程度不同，预计计算出的髋轴长度可能会有误差。使用带有扇形束（Lunar Expert和Hologic QDR - 2000）和笔形束（Lunar DPXL）骨密度仪的人体股骨模型，研究了这种误差对髋部几何形状测量的影响程度。还通过测量患者臀部软组织厚度来检验这些发现的临床相关性。与HAL相关性良好的股骨颈轴长度（FNAL）被用作髋部几何形状的测量指标。当分别使用Lunar Expert或Hologic QDR - 2000测量时，随着模型在扫描台上方距离的增加，FNAL呈线性增加或减少。使用笔形束骨密度仪时，不同高度的FNAL没有显著差异。在研究的30名女性中，臀部软组织厚度的最大差异为8.7厘米。根据模型研究得出的公式，当分别使用Lunar Expert或Hologic QDR - 2000测量时，这种差异会导致最大女性的FNAL比最小女性增加8.2%（1.4标准差）或减少11.4%（1.9标准差）。我们得出结论，当使用扇形束骨密度仪测量时，体内臀部厚度差异可能会导致FNAL出现不可预测的放大程度。在这些问题得到解决之前，应使用笔形束仪器测量FNAL或股骨几何形状的相关参数。
