PNNL

Abstract

In this article a review is given of the impact of magnetic susceptibility variations in biological objects on the proton NMR metabolite spectra observed in these objects, and of methods to reduce this impact. Susceptibility variations arise near boundaries of inter- and intra-cellular structures, generating local dipolar field gradients. These gradients broaden the 1H NMR lines significantly, seriously hampering a quantitative analysis of the spectra and, henceforth, the utility of this technique for biochemical and biomedical applications. In this paper newly developed methods are discussed to separate the susceptibility-induced line broadening from the isotropic chemical shift information while maintaining the structural integrity of the object. To this end two methods will be considered, which both utilize a slow spinning of the sample about the so-called magic axis (magic angle spinning or MAS), namely phase-adjusted spinning sidebands (PASS) and phase-corrected magic angle turning (PHORMAT). The advantages and disadvantages of these techniques will be discussed. It will be shown that PASS offers the highest NMR sensitivity and the shortest measuring time, but requires spinning speeds of at least 30 Hz, restricting this methodology to small biological samples. PHORMAT has a reduced sensitivity and a relatively long measuring time, but with this technique spinning speeds as low as 1 Hz can be used. Hence PHORMAT can be used in larger biological objects, including live animals. The utility of PASS and PHORMAT will be illustrated with applications on excised rat liver tissue and, for the latter technique, on a live mouse subjected to 1.5 Hz magic angle spinning. It will be shown that in a 7 Tesla magnet field PASS and PHORMAT reduce the widths of the proton metabolite lines by at least an order of magnitude, and that even in a relatively low 2 Tesla field and in a live mouse these widths are reduced by almost a factor 5. Finally, the impact of molecular diffusion on the isotropic line widths, and the dependence of these line widths upon the spinning frequency in PASS and PHORMAT are discussed, and perspectives of further improving the methods and possible clinical applications of PHORMAT are given.