Optimizing Region of Interest Size and Placement for Opportunistic CT

Abstract

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. 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. 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. 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). 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.