4. Image-based methods of stiffness computation
Image-based stiffness computation involves three stages: 1) segmentation of the lumen and IMC 2) estimation of lumen diameter (LD) and IMT, and 3) computation of Young’s modulus (or β-stiffness and Peterson’s modulus). Algorithms used for segmentation of lumen and IMC are of two types: boundary-based and region-based. Boundary-based (edge-based) approaches include methods that applied Hough transform [88], snake algorithm [89], active contours [90], and level-sets [91]. However, information about the intensity and curvature of the artery is required before initialisation of contour and energy functions [92]. Region-based methods involve split-and-merge algorithms that include watershed [93], region splitting, region merging [94] and graph-based approaches [95]. Concisely, based on operator intervention splitting, these algorithms are either semi-automatic or automatic. Moreover, a few techniques process multi-frame while others process a single frame. After segmentation of the ROI, estimation of LD and IMT is done using distance measurement techniques such as mean-absolute distance or polyline distance. The final stage of stiffness computation includes the application of Eq. 1 or Eq. 2.