## ABSTRACT

Magnetic resonance imaging (MRI) is based on a rather complex physical phenomenon of nuclear magnetic resonance (Bloch, 1946 [50]; Purcell et al., 1946 [51]), which is basically the exchange of energy between elementary particles placed in a strong magnetic field and the irradiating electromagnetic field of a particular frequency. It is, precisely taken, governed by laws of quantum mechanics and should be described in corresponding terms. Nevertheless, from the viewpoint of medical image data acquisition, processing, and interpretation, the description at the level of individual atoms or their nuclei is, in most respects, not necessary. We will thus present a macroscopic approximation, introduced originally by Bloch (1946), where large sets of nuclei are the subject of observation or measurement. These sets (sc. spin isochromats
^{1}
) can often be considered to encompass a volume of a voxel—a three-dimensional space element of the discrete image data. In some cases, when some of the parameters influencing the behavior of the group are not homogeneous throughout the voxel volume, a subvoxel view is necessary; nevertheless, even such a smaller subvolume contains a huge number of particles, allowing the acceptance of the macroscopic view when analyzing the external magnetic behavior of the subgroup.