Technique of selecting a plane for sequential plane imaging by using an oscillating magnetic field gradient and filtering out the corresponding time dependent part of the NMR signal. The gradient used, is at right angles to the desired plane and the magnitude of the oscillating magnetic field gradient is equal to zero only in the desired plane.

MR imaging techniques in which the image is built up from successive lines through the object. In various schemes, the lines are isolated by oscillating magnetic field gradients or selective excitation, and then the NMR signals from the selected line are encoded for position by detecting the FID or spin echo in the presence of a magnetic field gradient along the line; the Fourier transformation of the detected signal then yields the distribution of emitted NMR signal along the line.

(Gs)The slice select gradient is a magnetic field gradient applied to select the slice position in the direction of this gradient (x-direction). For orthogonal slices, the magnetic field gradient is applied perpendicular to the desired slice plane. Oblique and double-oblique slices are exited by simultaneously applying 2 or 3 gradient fields.

The selective excitation of spins in only a limited region of space. This can be particularly useful for spectroscopy as well as imaging. Spatial localization of the signal source may be achieved through spatially selective excitation and the resulting signal may be analyzed directly for the spectrum corresponding to the excited region. It is usually achieved with selective excitation.
Typically, a single dimension of localization can be achieved with one selective RF excitation pulse (and a magnetic field gradient along a desired direction), while a localized volume (3D) can be excited with a stimulated echo produced with three selective RF pulses whose selective magnetic field gradients are mutually orthogonal, having a common intersection in the desired region. Similar 'crossed plane' excitation can be used with selective 180° refocusing pulses and conventional spin echoes.
A degree of spatial localization of excitation can alternatively be achieved with depth pulses, e.g. when using surface coils for excitation as well as signal detection. An indirect application of selective excitation for volume-selected spectroscopy is to use appropriate combinations of signals acquired after selective inversion of different regions, in order to subtract away the signal from undesired regions.

This imaging technique is probably the earliest, but rarely used today. Most of today's imaging techniques are based on the Fourier transform, and fill the Cartesian grid of points in k-space line by line by a sequence of applied gradients. Back projection imaging performs a radial filling of the k-space by a one dimensional field gradient, applied at different angles. Back projection imaging is still in use in laser polarized noble gas imaging (see ventilation agents and lung imaging).