Prevrhal Sven, Fox Julia C, Shepherd John A, Genant Harry K
Osteoporosis and Arthritis Research Group, Department of Radiology, University of California-San Francisco, San Francisco, California 94117, USA.
Med Phys. 2003 Jan;30(1):1-8. doi: 10.1118/1.1521940.
Measurement of the width of thin structures such as the cortical shell of the vertebral body or femoral neck with computed tomography (CT) is limited by the spatial resolution of the CT system. Limited spatial resolution exists both within the CT image plane and perpendicular to it and can be described by the in-plane point spread function (PSF) and the across-plane slice sensitivity profile (SSP), respectively. The goal of this study was to confirm that errors of thickness measurement of thin structures critically depend on the spatial positioning of the object and the spatial resolution limitations of CT in all three dimensions, and to assess the size of the errors themselves. We compared computer models that incorporated both effects to experimentally assessed cortical thicknesses of the European Spine Phantom. Analysis included varying CT slice width, the orientation of measurement and angle beta of misalignment of longitudinal scanner and phantom axes. Agreement of models with measurements was good in all configurations with an overall error of 0.17 mm. This showed that PSF and SSP are adequate system characteristics to predict deviation of measured values from true widths. Errors between measurements and true cortical thickness values delta(true) averaged to 1.5 mm were strongly positively correlated with slice width d and beta. When the across-plane partial volume effect was eliminated, limited in-plane resolution still accounted for overestimation of delta(true) by 0.68 (137%), 0.27 (27%), and 0.06 mm (4%) for delta(true)=0.5, 1.0, and 1.5 mm, respectively. For delta(true) of 1.0 mm and above, it was shown that although the absolute cortical thickness values might not be accurately measurable, relative differences between two values are reflected in measurement. Implications for cortical thickness measurement are that the spinal cortical shell is too thin, whereas accurate assessment at locations of the femoral neck exhibiting a thicker cortical shell of both difference and absolute values should be possible with CT even for larger misalignment angles, especially when a smaller CT slice width is chosen.
利用计算机断层扫描（CT）测量诸如椎体皮质壳或股骨颈等薄结构的宽度，会受到CT系统空间分辨率的限制。在CT图像平面内以及垂直于该平面的方向上均存在有限的空间分辨率，可分别用平面内点扩散函数（PSF）和跨平面切片灵敏度剖面（SSP）来描述。本研究的目的是确认薄结构厚度测量误差在很大程度上取决于物体的空间定位以及CT在所有三个维度上的空间分辨率限制，并评估误差本身的大小。我们将纳入这两种效应的计算机模型与通过实验评估的欧洲脊柱模型的皮质厚度进行了比较。分析内容包括改变CT切片宽度、测量方向以及纵向扫描仪与模型轴的不对准角度β。在所有配置中，模型与测量值的一致性都很好，总体误差为0.17毫米。这表明PSF和SSP是预测测量值与真实宽度偏差的适当系统特征。测量值与真实皮质厚度值δ（真实值）之间的误差平均为1.5毫米，与切片宽度d和β呈强正相关。当消除跨平面部分容积效应时，对于δ（真实值）=0.5、1.0和1.5毫米，有限的平面内分辨率仍分别导致δ（真实值）高估0.68（137%）、0.27（27%）和0.06毫米（4%）。对于δ（真实值）为1.0毫米及以上的情况，结果表明，虽然绝对皮质厚度值可能无法准确测量，但两个值之间的相对差异在测量中是可以体现出来的。对皮质厚度测量的启示是，脊柱皮质壳太薄，而对于股骨颈处皮质壳较厚的部位，即使在较大的不对准角度下，使用CT进行差异和绝对值的准确评估也应该是可行的，尤其是在选择较小的CT切片宽度时。
