Volume-selective excitation. A novel approach to topical NMR

Topical nuclear magnetic resonance (TNMR) is a noninvasive and nonhazardous new technique to obtain high-resolution NMR spectra from a restricted region within a living system. It allows the collection of detailed information about molecular structure, concentration, kinetics, and metabolism in vivo. For detailed TNMR studies of a complex three-dimensional sample such as that of an animal, a precise method for volume selection is crucial. To give the rationale for the development of a new technique, the methods which are presently available to select a sensitive volume are compared briefly. In principle, TNMR spectra can be obtained indirectly via chemical-shift imaging (l-6), a method to investigate the distribution of various chemical compounds throughout the sample, although at significantly reduced sensitivity. To optimize sensitivity, as well as to simplify the experiment and data handling, it is advantageous to use a one-dimensional method to measure a TNMR spectrum. A number of different techniques have been described to restrict the size of the sensitive volume to a predetermined region of interest. They alI rely either on focusing the static magnetic field B0 or on localizing the rf field Br . The methods with static focusing of B0 (7, 8) make use of the fact that highresolution NMR spectra can be obtained only in a volume with high B0 homogeneity. Outside this region, the spectral lines are very much broadened and therefore do not contribute much signal. At the present time, the position of the focused Bo is restricted to the center of the magnet system, which then requires the object under investigation to be moved for every new volume element of interest. Dynamic focusing of BO has also been proposed (9, 10). In this, a steady-state free-precession experiment is performed under the influence of slowly varying linear BO gradients, which eliminate signal contributions from volumes with time dependent Bo. Although this approach allows the selective volume to be moved easily, it su8ers from ill defined boundaries of the sensitive volume and corresponding lineshape problems. The most common method to localize the rf field is the use of a surface rf coil (II). As the name implies, its application is normally restricted to the surface of samples, although efforts have been taken to push the sensitive volume deeper inside the sample by means of special pulse sequences (12, 13). The inherent drawbacks of the methods mentioned above provided the impetus to develop volume-selective excitation (VSE) to localize the sensitive volume. VSE, for which a possible pulse and gradient sequence is given in Fig. 1, is actually a