Pranav Bhagirath

56 Chapter 3 the observers’ segmentations. The final consensus segmentation was then obtained by thresholding this probability above 0.7 or 70%. This is referred to in the rest of the text as the consensus ground truth. Common algorithms: n-SD and FWHM Quantification of scar in LGE CMR images using a fixed model is often desirable and commonly used as it includes fewer image processing steps, with some studies advocating its reproducibility (Flett et al., 2011; Amado et al., 2004). In fixed models, scar is quantified by thresholding intensities at a fixed distance from a reference intensity value. Two types of fixed models were used, namely FWHM and the n-SD method. FWHM is a technique where half of the maximum intensity within a user-selected hyper-enhanced region is selected as the fixed intensity threshold for an ensuing region-growing step (Amado et al., 2004). In the region-growing step, infarcted regions are segmented based on user-selected seed points. These are used to initialize the region-growing step. The n-SD method (where n = 2, 3, 4, 5, 6), uses a fixed number of standard deviations frommean signal within healthy myocardium. A manual region-of- interest (ROI) selection was required in both techniques. In FWHM, a ROI was delineated in hyper-intense myocardium. In n-SD, a ROI was delineated in remote myocardium. Remote myocardium was defined as a region with no enhancement and normal wall motion. Endocardial and epicardial surfaces were avoided in the delineation. Evaluation metrics Segmentations from each algorithm were compared against the reference standard for scar. As no single metric is advocated as the best metric, two different types of metric were chosen for evaluating the segmentations. These were overlap and volumetric measures, and they are briefly described below: 1. Overlap metric: The Dice similarity is a metric for segmentation overlap measuring the proportion of true positives in the segmentation: where X is the segmented region in the ground-truth and Y is the region in the challenger’s algorithm. 2. Volumetric-basedmetric: The total volume error between the algorithms’s output and reference standard was found: δV = | V T −V G |