Purpose To histologically validate T1 mapping for quantitative assessment of mild-to-severe myocardial fibrosis in a swine model of chronic myocardial infarction (MI). Materials and Methods In this animal study conducted from June 2021 to July 2022, 18 male miniature swine (16 MI animals; two healthy control animals) underwent cardiac MRI, including cine imaging, late gadolinium enhancement (LGE) imaging, T1 mapping, and extracellular volume fraction (ECV) mapping. Two T1 mapping techniques, modified Look-Locker inversion recovery (MOLLI) and shortened MOLLI (ShMOLLI), were evaluated. Pathologic myocardial slices were categorized as infarcted, peri-infarct, remote, and healthy based on triphenyl tetrazolium chloride staining. Fibrosis was quantified using collagen volume fraction (CVF) and classified as severe (CVF, ≥30%), moderate (CVF, >15%-30%), or mild (CVF, 3%-15%). Associations between cardiac MRI parameters and CVF were assessed, and diagnostic performance in detecting myocardial fibrosis was evaluated using the area under the receiver operating characteristic curve (AUC). Results For detection of severe fibrosis, LGE, T1 mapping, and ECV all demonstrated excellent diagnostic performance (AUC range, 0.88-0.96). ECV using ShMOLLI showed significantly higher performance than ECV using MOLLI for detecting severe fibrosis (AUC, 0.96 vs 0.88; P = .03) and MI (AUC, 0.93 vs 0.87; P = .045), as well as the strongest correlation with histologic CVF (r = 0.90 for ECV with ShMOLLI, 0.84 for ECV with MOLLI, 0.74 for T1 with ShMOLLI, 0.77 for T1 with MOLLI, and 0.74 for LGE extent). In remote myocardium with mild fibrosis (CVF, 8.81%) compared with healthy myocardium (CVF, 2.21%), only T1 with ShMOLLI and ECV with ShMOLLI demonstrated significant differences (P < .05), whereas LGE and MOLLI-based parameters did not. Conclusion Cardiac MRI helped detect mild-to-severe myocardial fibrosis in close agreement with histologic findings. While all techniques helped accurately identify severe fibrosis, T1 mapping-particularly ECV using the ShMOLLI sequence-provided unique sensitivity for detecting low-grade fibrosis, underscoring the importance of sequence selection for precise myocardial tissue characterization and clinical trial design. Keywords: Myocardial Fibrosis, Heart, Histological Techniques, Magnetic Resonance Imaging, Swine Supplemental material is available for this article. © RSNA, 2026.