1. From the Nuclear Magnetic Resonance Laboratory, Neuromuscular Investigation Center, Institute of Myology, Bâtiment Babinski, Groupe Hospitalier Pitié-Salpêtrière, 47-83 Blvd Vincent Auriol, 75651 Paris Cedex 13, France; and Nuclear Magnetic Resonance Laboratory, CEA, DRF, IBFJ, Molecular Imaging Research Center, Paris, France.