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The concentration of gadopentetate dimeglumine in myocardium and blood can be assessed from T1 measurements and can be used to calculate the extracellular volume (ECV) of the myocardium. We hypothesized that diffuse myocardial fibrosis in a small-animal model could be quantitatively assessed by measuring myocardial ECV using small-animal Look-Locker inversion recovery T1 mapping.Sprague-Dawley rats (n=10) were subjected to continuous angiotensin-2 (AT2) infusion for 2 weeks via a subcutaneously implanted minipump system. Magnetic resonance imaging (MRI) was performed both before and after AT2 infusion. The MRI protocol included multislice cine imaging and before-and-after contrast small-animal Look-Locker inversion recovery T1 mapping and late gadolinium enhancement imaging. Myocardial ECV was calculated from hematocrit and T1 values of blood and myocardium. During the course of AT2 infusion, the mean±SD systolic blood pressure increased from 122±10.9 to 152±27.5 mm Hg (P=0.003). Normalized heart weight was significantly higher in AT2-treated animals than in control littermates (P=0.033). Cine MRI documented concentric left ventricular hypertrophy. Postcontrast myocardial T1 times were shortened after treatment (median [interquartile range], 712  versus 820  ms; P=0.002). Myocardial ECV increased from 17.2% (4.3%) before to 23.0% (6.2%) after AT2 treatment (P=0.031), which was accompanied by perivascular fibrosis and microscarring on myocardial histological analysis. There was a moderate level of correlation between ECV and collagen volume fraction, as assessed by histological analysis (r=0.69, P=0.013).In a small-animal model of left ventricular hypertrophy, contrast-enhanced T1 mapping can be used to detect diffuse myocardial fibrosis by quantification of myocardial ECV.