1. From the Centre for Cardiovascular Imaging, UCL Institute of Cardiovascular Science, London, UK (S.M., B.P., K.H.M., A.M.T., V.M.);and National Pulmonary Hypertension Service, Great Ormond Street Hospital for Children, London, UK (S.M., M.B., M.M., S.Q., S.G.H., I.S.-N.).