We plan to use late gadolinium enhancement (T1 mapping) to measure the amount of scarring in patients with heart failure (we have evidence that their heart biopsies show increased amounts of scar tissue) and children having MRI scans for other reasons. We will use measures of function including echocardiography and 6 minute walk test to compare to the amount of scarring. This will help us to know whether the amount of scarring will be clinically useful.