An image is a representation of the spatial distribution of objects, structures, materials, and so forth within a region.  is might seem somewhat obvious, but it is worthwhile to begin by recalling exactly what it is we are trying to accomplish. In Chapter 3 it was shown how di˜erent materials, or more speciŸcally, tissues or ¥uids for MR imaging (MRI) of the body, have di˜erent proton densities and relaxation times and can be distinguished by their di˜erent MR signal intensities as a result. To create an image it is therefore necessary to put (or encode) spatial information into the MR signal as well.  at is, the signal we detect must somehow re¥ect where it originated; otherwise, we will not be able to create an image.