Feasibility of NMR imaging by a single radiofrequency field gradient and the separation of two species with different chemical shifts

Abstract

The idea of substituting RF field gradients for static field gradients for localization or imaging purposes goes back to Hoult’s rotating-frame zeugmatography ( 1): further developments of this technique have been recently published (2). Obvious advantages of RF gradients lie in the possibility of rapid switching and in the simplicity of the experimental device, which essentially includes a single-turn coil in addition to a classical NMR probe. Most practical applications rely on two-dimensional methods (3-IO), the first dimension referring to a single spatial dimension, the other to a chemical-shift scale; such experiments therefore provide the distribution of different chemical species along that spatial direction. The present work is part of a project aimed at the use of a linear RF gradient for various NMR applications. These include measurements of self-diffusion coefficients (II), spatial localization by the adaptation of the classical DANTE sequence (12), slice selection, and ID imaging (13) through the recently proposed DANTE-Z procedure ( 14). We demonstrate here the possibility of obtaining a two-dimensional (eventually three-dimensional) image by means of a single coil delivering a RF gradient, in contrast with the applications mentioned, which concern only the examination of a single dimension. The successive exploration of the two dimensions is achieved by a fast rotation of the sample during an interval in an appropriate pulse sequence. Moreover, it is shown that a simple variant of this experiment allows one to perform imaging of two chemically shifted species within the same experiment. The experimental arrangement in use ( I1 -13) can be summarized as follows: (i) surrounding the sample is a normal saddle-shaped coil which is employed for generating hard homogeneous pulses and for detecting the NMR signal, (ii) a single-turn coil, orthogonal to the saddle-shaped coil, delivers a linear RF gradient in the sample region (Fig. 1). and (iii) two different transmitters feed these two coils, at the same RF frequency, but with the possibility of adjusting their relative phases so that they can operate in the same rotating frame. The rotation of the object under investigation could be replaced (and probably will be in the near future) by a rotation of the two-coil system.