Nuclear magnetic resonance spectroscopy in diagnostic and investigative medicine.

Nu uclear magnetic resonance (NMR)' spectroscopy, which has been used by chemists and physicists for more than 30 years, is now becoming a powerful technique in the medical sciences because of its unique ability to monitor intracellular metabolites noninvasively. The fundamental principle underlying NMR spectroscopy is that nuclei of certain atoms, such as hydrogen ('H), phosphorus (31p), carbon (13C), fluorine ('9F), and sodium (23Na), are magnetic dipoles. In the presence of a strong magnetic field, the nuclei of these atoms align with or against the field in much the same way that a bar magnet tends to align itself with the earth's magnetic field. A pulse of perturbing electromagnetic energy at the resonant frequency is then used to tip the nuclei away from their orientation along the magnetic field lines. When the perturbing radiofrequency pulse is stopped, the nuclei return to their original lower energy orientations; in the process, they emit energy which can be picked up by a radiofrequency receiver and transformed, with the aid of a computer, into a spectrum. The different nuclei resonate in the same magnetic field at very different frequencies. For example, at a field of 8.4 T, the 'H-resonance is at 360 MHz while '3C and 3'P are near 90.5 and 145.8 MHz, respectively. Members of the same nuclear species occupying different positions in a molecule resonate at slightly different frequencies. The individual peaks of the spectrum arise from differences in the chemical bondings of individual