Assessment of the Effect of Bone Density and Soft Tissue Thickness on Phantom Measurements

摘要

This study examined whether there would he a difference in bone density through a phantom test because of the effect of the thickness of the soft tissue on the bone density measured by using dual energy X-ray absorptiometry (DXA). Three scan modes were used: standard, thin and thick modes correspondly to a normal, an underweight and an overweight patient, respectively. Also, the three phantoms used were the Lunar aluminum spine phantom (ASP), the Hologic spine phantom (HSP), and the European spine phantom (ESP). Three acrylic plates of different thicknesses were used, and a water bath was used as the tissue equivalent material for the soft tissue. The ASP, HSP and ESP were measured 20 times without repositioning. In addition, measurements were carried out three times with the HSP and the ESP covered by an acrylic plate. In regard to controlling the depth of the water, the bone density was measured in the standard mode, where the depth of the water was adjusted using the ASP only and was then determined based on the judgment of the investigators. To confirm the accuracy of the mode selection, we measured the dependence of the bone density of the phantom on the depth of the water. The percentage coefficients of variation (CV%) of the bone mineral density (BMD) in the ASP, HSP and ESP were measured to be 0.17%, 0.45%, and 0.95%, respectively. When the soft tissue thickness was increased, the CV% of the BMD in the ASP, HSP and ESP were 0.65%, 2.24%, and 0.79%, respectively. A correlation analysis also revealed a significant positive correlation between the soft tissue thickness and the bone density (ASP: r~2 = .926, HSP: r~2 = .899). In contrast, a generalized phantom, ESP, showed a significant negative correlation between the tissue thickness and the bone density, which was different from those for the other two phantoms (ESP: r~2 = -.733). After assessing the scan mode and the soft tissue thickness, the mean bone densities in the standard and the thick modes were 1.190 ± 0.004 and 1.179 ± 0.006, respectively, when the depth of the water in the water bath was set to 25 cm. When the depth of water was set to 13 cm, the mean bone densities in the standard and the thin modes were 1.197 ± 0.002 and 1.179 ± 0.006, respectively. After a phantom experiment, a significant correlation was noted between the soft tissue thickness and the bone density. After assessing the correlation between the scan mode and the soft tissue thickness, we found the mean bone density measured using the ASP to be closer to the bone density based on the ASP in the scan mode, which was selected based on the judgment of the investigators, than to that selected automatically.