ABSTRACT
This chapter describes the evolution of the nuclear spin magnetization in the presence of an external magnetic field and under the influence of magnetic field gradients. Generating magnetic resonance images involves the application of magnetic field gradients in addition to the static and the RF fields. These magnetic field gradients will render the Larmor frequency position dependent. The signal-to-noise-ratio (SNR), defined as the mean signal in a region of interest divided by the standard deviation of the background noise, is a measure of the quality of the image. The SNR depends on the hardware and sample characteristics and on the acquisition parameters. In magnetic resonance microscopy, acquiring very thin slices may not be possible due to gradient strength limitations as thinner slices require stronger gradients. An alternative solution, very common in MRM, is to employ 3D imaging. The chapter explains analytical SNR expressions which can be used to understand the interplay between these factors and to optimize MR protocols.
