Heart failure occurs when the heart muscle has become too weak to work properly. It is associated with an increase in the amount of connective tissue (collagen) which replaces dead heart muscle cells (scarring). Currently a biopsy of the muscle is the only way to measure the amount of scarring. This is invasive and rarely done in children. Because of this, it is difficult to measure the amount of scarring in a particular patient or disease process, which is important for improving our understanding and treatment of the disease.