Introduction

Opportunistic CT is increasingly used clinically, however, no guidance exists regarding optimal region of interest (ROI) size or placement to obtain Hounsfield Unit (HU) data. As ROI size and placement alter HU, standardization is desirable.

Objectives

This studies purposes were to: 1.) Evaluate various L1 and L4 ROI sizes and locations to obtain reproducible HU values and; 2.) Compare HU values from axial and sagittal images.

Methods

On lumbar spine CT, 2 physicians independently placed varying size circular ROIs (100, 200,300 mm2 and maximum) at L1 and L4 on axial and sagittal images. Subsequent analyses placed a 200 mm2 ROI at L1 and L4 in anterior, mid and posterior aspects and also cranial, central and caudal and left/right locations. Intra and inter-observer reliability was determined using intraclass correlation coefficients. ROI size comparison was performed by ANOVA and between axial and sagittal by t-test. Limits of agreement was assessed by Bland-Altman.

Results

The study included 30 spine clinic patients; 21F/9M mean (SD) age 59.2 (14.4) yrs. Vertebral body centroid identification, i.e., CT slice location used for HU measurement, was nearly identical between readers with ICC >0.99 for both projections. Intra- and inter-observer reliability was excellent for all sizes on both projections with ICC >0.95; Table 1. HU did not differ between the 4 ROI sizes on either projection for L1 and L4; all p >0.95; Table 1. Correlations between ROI sizes were excellent; r >0.95. Axial HU was higher than sagittal; p < 0.001 for all ROI sizes at L1 and L4; Table 1. Anterior, mid or posterior placement did not affect axial or sagittal HU; p >0.5 at L1 or L4; Table 2. However, moving away from the vertebral body centroid, either cranial/caudal or left/right, altered HU; Table 3. Limits of agreement between cranial/caudal and left/right location were large (from 10-90 HU).

Conclusions

Non-radiologists can reliably identify the vertebral body centroid to facilitate ROI placement on axial and sagittal CT images and measure HU. While ROI size did not affect HU, 200 mm2 is recommended as larger ROI sizes may extend to the vertebral cortex in small patients which changes HU. Axial results were ∼6-10 HU higher than sagittal. Anterior, mid or posterior ROI location did not affect HU but image plane (left/right or cranial/caudal) alters HU likely reflecting anisotropic vertebral microarchitecture. For clinical use, we recommend default ROI placement be a 200 mm2 circular ROI placed at the vertebral body centroid.

中文翻译：

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优化机会 CT 的感兴趣区域大小和位置

介绍

机会性 CT 越来越多地在临床上使用，但是，对于获得 Hounsfield 单位 (HU) 数据的最佳感兴趣区域 (ROI) 大小或位置，尚无指导。由于 ROI 大小和位置会改变 HU，因此需要标准化。

目标

这项研究的目的是： 1.) 评估各种 L1 和 L4 ROI 大小和位置以获得可重复的 HU 值；2.) 比较轴向和矢状图像的 HU 值。

方法

在腰椎 CT 上，2 位医生在轴向和矢状图像上的 L1 和 L4 独立放置不同大小的圆形 ROI（100、200,300 mm2 和最大）。随后的分析在 L1 和 L4 的前部、中部和后部以及颅、中央和尾部以及左/右位置放置了 200 mm2 的 ROI。使用组内相关系数确定内部和观察者间的可靠性。通过 ANOVA 进行 ROI 大小比较，并通过 t 检验在轴向和矢状之间进行 ROI 大小比较。协议限制由 Bland-Altman 评估。

结果

该研究包括 30 名脊柱门诊患者；21F/9M 平均 (SD) 年龄 59.2 (14.4) 岁。椎体中心识别，即用于 HU 测量的 CT 切片位置，在两种投影的 ICC > 0.99 的读取器之间几乎相同。在 ICC > 0.95 的两个投影中，所有尺寸的观察者内部和观察者之间的可靠性都非常好；表 1. HU 在 L1 和 L4 的任一投影上的 4 个 ROI 大小之间没有差异；所有 p > 0.95；表 1. ROI 大小之间的相关性非常好；r > 0.95。轴向HU高于矢状；对于 L1 和 L4 的所有 ROI 大小，p < 0.001；表 1. 前、中或后放置不影响轴向或矢状 HU；在 L1 或 L4 处 p >0.5；表 2. 然而，远离椎体中心，无论是颅/尾或左/右，都会改变 HU；表3。

结论

非放射科医生可以可靠地识别椎体中心，以促进在轴向和矢状 CT 图像上放置 ROI 并测量 HU。虽然 ROI 大小不影响 HU，但建议使用 200 mm2，因为较大的 ROI 大小可能会延伸到小患者的椎体皮质，从而改变 HU。轴向结果比矢状面高约 6-10 HU。前、中或后 ROI 位置不影响 HU，但图像平面（左/右或颅/尾）改变 HU，可能反映各向异性椎体微结构。对于临床使用，我们建议默认的 ROI 放置是放置在椎体中心的 200 mm2 圆形 ROI。