Table 2.
List of image features used in this study
| Histogram descriptors | Definition* |
|---|---|
| F1. Average region gray value [HU] | μ (Gray value in V) |
| F2. Region contrast [HU] | F1 - μ (Gray value outside of V) |
| F3. Region gray value variation [HU] | σ (Gray value in V) |
| F4. Margin gray value variation [HU] | σ (Gray value in M) |
| Shape descriptors | |
| F5. Irregularity | 2.2 × V1/3 / M1/2 |
| F6. Compactness | % of volume of V included in SP |
| F7. Ellipsoid axes min-to-max ratio | Min to max ratio of semi-axes of the ellipsoid fitted to V |
| F8. Margin distance variation [mm] | σ (distances from the center of V to the margin of V) |
| F9. Relative margin distance variation | F8 / μ(distances from the center of V to the margin of V) |
| F10. Average gradient direction | μ (gradient direction of each voxel in M) |
| F11. Margin volume [mm3] | Σ (voxels in M) |
| Margin descriptors | |
| F12. Average radial gradient [HU] | μ (radial gradient of each voxel in M) |
| F13. Radial gradient index (RGI) | F12 / μ (magnitude of image gradient of each voxel in M) |
| F14. Margin strength 1 | μ (magnitude of image gradient of each voxel in M) / F2 |
| F15. Margin strength 2 | σ (magnitude of image gradient of each voxel in M) / F2 |
| F16. Radial gradient variation | σ (radial gradient of each voxel in M) |
| Texture descriptors | |
| F17. GLCM|Energy | 3D version of 2D gray-level co-occurrence | Energy |
| F18. GLCM|Contrast | 3D version of 2D gray-level co-occurrence | Contrast |
| F19. GLCM|Correlation | 3D version of 2D gray-level co-occurrence | Correlation |
| F20. GLCM|Homogeneity | 3D version of 2D gray-level co-occurrence | Homogeneity |
| Surface Curvature descriptors | |
| F21. Total Curvature | μ (|p1| + |p2| over S) / σ (|p1| + |p2| over S) |
| F22. Mean Curvature | μ ( 0.5 × (p1 + p2) over S) / σ (0.5 × (p1 + p2) over S) |
| F23. Gaussian Curvature | μ ( p1 × p2 over S ) / σ ( p1 × p2 over S ) |
*
V refers to the segmented lesion volume. M refers to the margin of the lesion volume. SP refers to the minimum sphere including V. S refers to the surface of V. p1 and p2 refer to the first and second principal component of S. μ and σ indicate mean and standard deviation.