Investigation of spatial dependences of NMR phenomena are most often obtained as maps of a two-dimensional section (slice) of the object under examination. Three-dimensional distributions are frequently obtained by obtaining a set of slices spanning the coordinate normal to such slices.
A particular class of such methods that collect data from many slices in a time comparable to the NMR relaxation times is called multislice imaging.
Selection of a slice is often effectuated by imposing a magnetic field gradient along a selected axis in space called the slice selection axis (denoted herein as the A-axis) such that the component of the magnetic field along the slice selection axis is a precisely known (usually linear) function of A. Thus the resonant frequency for the nuclear spins of interest is a function of A. The resonance phenomenon is constrained to an interval on the slice selection axis thorugh use of selective RF irradiation. The excitation profile of an isolated slice would ideally exhibit a rectangular shape to permit the acquisition of an ideal set of precisely contiguous slices without intervening gaps. In non-ideal situations, the shape of the profile has some finite extension along the A-axis outside of the nominal ideal slice; that is, the edges of a slice are not defined precisely. In such cases, the acquisition of contiguous slices is, in reality, an acquisition of partially overlapping slices.
Spatial and temporal proximity for consecutive NMR excitations are recognized to be likely to lead to a degradation in image quality due to overlapping excitation with adjacent slices. It is apparent that one may arrange to obtain a set of adjacent slices by consecutive excitation of every other slice of the set, e.g., even numbered slices and odd numbered slices are separately acquired, each in consecutive order. The spatial and temporal proximity of consecutively excited slides is a matter of degree depending upon the slice thickness (power spectrum bandwidth and shape) as well as the duration of the data acquisition process associated with each such slice.