Nuclear magnetic resonance imaging: with or without nuclear?

Historical perspective. Nuclear magnetic resonance (NMR) derives its name from the physical property of cer• tain atomic nuclei related to their magnetic characteristic of resonating when placed within a magnetic field and irra• diated with radio waves at a specific frequency. This prop• erty of the nucleus was first reported in 1946 by two in• dependent laboratories, one at Stanford (Bloch et al. [1]) and the other at Harvard (Purcell et al. [2]). Subsequently, physicists and chemists have developed methods for apply• ing NMR to evaluate chemical composition (NMR spec• troscopy [3]), and most recently to obtain diagnostic images (NMR imaging or "magnetic resonance imaging") (4). Sev• eral nuclei that exhibit NMR have medical relevance. These include hydrogen-I, carbon-l 3 , sodium-23 and phosphorus31. During the past 15 years NMR spectroscopy has been applied in vitro to biologic systems to evaluate biochemical composition, pH and enzyme kinetics, and to monitor changes in these induced by clinically relevant perturbations such as ischemia (5). The most important advance in the application of NMR methods to medicine and clinical cardiology began in 1973 with the descriptions of methods by which images could be generated using NMR (4,6). Subsequently, these descrip• tions were converted into clinically applicable realities through the investigations of a number of groups in both university and industrial laboratories (7). The hydrogen nucleus (pro• ton) has been the most important imaging target owing to its high magnetic sensitivity and high concentration within the body. Using proton NMR methods, images of the brain, heart and other organs have been generated in exquisite detail (8). In addition, sodium images of the brain depicting ischemic stroke have been generated (9). Ultimately, other nuclei with clinical importance may be imaged.