Defining Orientation of the Foot in WBCT using Hounsfield Units Analysis: Do Soft Tissues Skew the Measurements?

Research Type: Level 4 – Case series Introduction/Purpose: Defining orientation axes of the foot typically requires full segmentation of bones from volumetric data, followed by geometric primitive fitting (GPF) or principal component analysis (PCA). This is either a time or computation heavy task. This study aimed to determine whether soft tissue weight influences the overall foot orientation in DICOM datasets using a simplified approach that involves selecting a region of interest and applying Hounsfield Unit (HU) thresholding. We hypothesized that, given the relatively even distribution and lower density of soft tissues, their inclusion would not significantly impact foot orientation. Methods: In this retrospective cross-sectional study, 14 feet from 7 individuals were analyzed using bilateral cone-beam weight-bearing CT (WBCT) scans. Cases with surgical metalwork were excluded. DICOM volumes were processed in 3D Slicer software. Each foot was isolated by defining a region of interest below the ankle joint line. Two segmentation thresholds were applied: one for bone and soft tissue (-500 to 3000 HU) and one for bone only (500 to 3000 HU). Segments were analyzed for volume, mean HU values, centroid position, Oriented Bounding Box (OBB) dimensions, and first PCA coordinates. Statistical analysis included unpaired Student’s t-tests, with significance set at p< 0.05. Statistical analysis was performed with EasyMedStat. Results: The vertical centroid coordinate was significantly lower in bone and soft tissue segments (-1159.53 ± 5.26) compared to bone only (-1162.12 ± 2.5), p< 0.001. Segment volumes were significantly larger when soft tissues were included (807.46 ± 145.29 vs. 61.62 ± 30.63, p< 0.001). Mean HU values were significantly lower in the bone and soft tissue segments (74.25 ± 59.21) compared to bone only (742.6 ± 45.2), p< 0.001. Significant differences were observed in OBB dimensions, with increased diameters in the bone and soft tissue segments (OBB diamx: 101.57 ± 14.9 vs. 83.18 ± 9.18, p< 0.001; OBB diamy: 116.1 ± 17.2 vs. 95.48 ± 8.79, p< 0.001). Centroid shifts, increased volume, and altered HU values highlight the impact of soft tissue inclusion on morphological assessment. Conclusion: Our main finding was that axial orientation of the foot volume was not affected by different levels of thresholding to include or not the soft tissues. However, the centroid was drawn more plantar with soft tissues. Thus, it would not be possible to reproducibly orientate the volume using HU thresholding other than to fix the sagittal plane. The limitations of this study are a small population without major deformities and the exclusion of metal implants. Future research should investigate deformed feet and explore whether upper HU thresholding can yield similar results in the presence of metal implants.