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Enhancing myocardium and fibrosis segmentation in LGE-SA CMR images using diffusion models

Sofia Marques Ribas Fernandes

Portuguese National Funding Agency for Science, Research and Technology (RCAAP Project by FCT), 2026

Abstract

Myocardial scar segmentation from Late Gadolinium Enhancement (LGE)-Cardiac Magnetic Resonance (CMR) is essential for infarct quantification and clinical decision-making. Accurate measurement of scar tissue traditionally relies on manual delineation, which is time-consuming and observer-dependent. However, the development of automated and reproducible segmentation models remains technically challenging due to pronounced class imbalance, ill-defined tissue interfaces, and heterogeneous image quality. These challenges are further exacerbated by the limited availability of expert-annotated datasets, thereby necessitating data-efficient training paradigms. This dissertation examines the extent to which synthetic image generation using Diffusion Models can mitigate annotation scarcity for myocardium and scar segmentation. We propose a mask-conditioned diffusion model that synthesizes LGE image slices directly from input segmentation masks, thereby enabling anatomically constrained sampling at inference time without the need for accompanying reference images. The generative model is trained in a two-dimensional setting using a DDPM-style noise-prediction objective and is assessed as a source of synthetic training data for nnU-Net. In addition, a baseline learning curve derived exclusively from real data is established using nested, incrementally expanded subsets that preserve the prevalence of fibrosis. We further compare mask-conditioned diffusion models against exemplar-guided…

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