Background The value of native myocardial T1 mapping and extracellular volume (ECV) fraction in patients who have hypertrophic cardiomyopathy (HCM) but no late gadolinium enhancement (LGE) and no hemodynamic obstruction are currently unknown. Purpose To evaluate myocardial fibrosis in patients with nonobstructive HCM and no LGE by using native myocardial T1 mapping and ECV fraction and to study their relationships to left ventricular (LV) function and LV hypertrophy. Materials and Methods Patients with HCM who underwent cardiac MRI between 2012 and 2015 were retrospectively evaluated. Patients were included if they had no LGE at MRI, LV ejection fraction greater than or equal to 45%, and no LV outflow tract obstruction. Healthy participants had similar age and sex distribution. Native myocardial T1 and ECV were measured with MRI. Results A total of 258 patients with HCM (mean age ± standard deviation, 49 years ± 15; 74% men) and 122 healthy participants (mean age, 50 years ± 14; 76% men) were evaluated. Native myocardial T1 was longer and ECV fraction was higher in the patients with HCM relative to the healthy participants (mean native T1, 950 msec ± 48 vs 913 msec ± 46; mean ECV, 24.5% ± 2.8 vs 23.0% ± 2.7; both P < .001). Maximum T1 and ECV values correlated strongly with LV mass index for the entire patient cohort with HCM (both r = 0.86; P < .001) and for the subgroups (r = 0.86 and 0.85 for interventricular septal group and r = 0.88 and 0.86 for apical group; all P < .001). Conclusion Prolonged myocardial T1 and elevated extracellular volume in hypertrophic cardiomyopathy suggests diffuse myocardial fibrosis, even in the absence of regionally apparent late gadolinium enhancement and hemodynamic obstruction, and is associated with left ventricular hypertrophy. © RSNA, 2019 See also the editorial by Bluemke and Lima in this issue.

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