Table 6.
Texture descriptor families utilized in this study together with physiologic rationale and implementation.
| Feature Group | Quantity | Description & Rationale |
|---|---|---|
| Histogram measures | 21 | First-order statistics of the original image signal intensity within local pixel neighborhoods, capturing basic variations in signal intensities due to intermixed tissue types (fibrosis, ulceration, mucosa) after nCRT |
| Gradient operators [49] | 10 | Identification of leading gradients and edges in the local signal within small neighborhoods of pixels, likely occurring due to impact of nCRT within the rectal wall |
| Haralick measures [50] | 65 | Quantify heterogeneity and entropy of local intensity texture as represented by the gray-level co-occurrence matrix pixel neighborhoods, widely shown to be related to underlying tissue heterogeneity as a result of intermixed treatment effects, residual disease, and irradiated tissue |
| Gabor operators [51] | 35 | Responses to Gabor wavelets which are defined at specific unit-length scales (λ = 0.765, 0.128, 1.786, 2.296, and 2.806; corresponding to window sizes 3, 5, 7, 9 or 11 pixels) and orientations (θ = ), thus capturing multi-scale and multi-oriented variations within the rectal wall |
| Laws operators [52] | 34 | Responses to local filters targeting combinations of specific textural patterns in the x- and y-directions. Descriptors include all combinations of 1D filters: level (L), edge (E), spot (S), wave (W), and ripple (R), which have been related to underlying abnormal structures or enhancement patterns |
| CoLlAGe [53] | 26 | Captures and exploits local anisotropic differences in voxel-level gradient orientations by assigning every image voxel an entropy value associated with the co-occurrence matrix of gradient orientations, which have been related to reflecting subtle local differences in tissue microarchitecture |
nCRT = neoadjuvant chemoradiation therapy; CoLlAGe = co-occurrence of local anisotropic gradient orientations.