ABSTRACT
To see how this method works, consider Eq. (1), at different positions in the sample (i.e., different values of r) there will be a phase shift because we are applying a magnetic gradient. Visualised along the direction of the gradient the magnetisation is spatially encoded into a helix [68). We note that it is the vector suni of the magnetisation in the transverse plane that gives the NMR signal and the vector sum of the magnetisation helix when integrated over the sample volume will approach zero. The degree of suppression mainly depends on the gradient strength and the frequency selectivity and flip angle accuracy of the selective rf pulse. However, the suppression is also sensitive to the local static field homogeneity and the sample geometry. For simple sample geometries, the maximum signal attenuation occurs when the gradient is directed along the body diagonal of the detectable sample, since this corresponds to the maximum length along the gradient and results in a gradual increase in sample volume with distance in the gradient direction. Hence, the best results should be obtained for Bo gradients by simultaneous applications of gradients in all three directions [92).
