ABSTRACT
This chapter discusses the basics of magnetic resonance imaging, as well as the related topics of diffusion and flow measurement. One sees a spreading of the nuclear magnetic resonance (NMR) signal over a range of frequencies and, if one knows the magnitude and direction of the gradient, it is simple to replace the frequency axis with a distance-from-centre measurement. In this way, the frequency encoding of the NMR signal can be used to directly obtain a one-dimensional image of the sample. The implicit assumption in frequency encoding is that the inherent chemical shifts of all of the spins used in the imaging experiment are the same. Complete signal recovery after the gradient echo depends upon each spin remaining at the same position throughout the sequence, that is, the absence of diffusion between the encoding and decoding gradients. The ability of magnetic field gradients to divide a sample into spatially resolved sectors is critical to the imaging experiment.
