Reconstruction of an image from a MR data set comprising an asymmetric sampling of k-space. For example, it can be used either to shorten image acquisition time, by reducing the number of phase encoding steps required, or to shorten the echo time, TE, by moving the echo off-center in the acquisition window. In either case the signal to noise ratio is reduced and the resolution can be improved to correspond to the maximum available resolution in the data.

The phase conjugate symmetry benefits from the symmetry (see also Hermitian symmetry) of the raw data in k-space and is used to reduce the data acquisition time by acquiring only a part of k-space data.

See also Partial Fourier Technique, Partial Averaging and acronyms for 'phase conjugate symmetry' from different manufacturers.

The relaxation effect is the transition of an atom or molecule from a higher energy level to a lower one. The return of the excited proton from the high energy to the low energy level is associated with the loss of energy to the surrounding tissue. The T1 and T2 relaxation times define the way that the protons return to their resting levels after the initial radio frequency (RF) pulse. The T1 and T2 relaxation rates have an effect of the signal to noise ratio (SNR) of MR images.
The relaxation process is a result of both T1 and T2, and can be controlled by the dependency of one of the two biological parameters T1 and T2 in the recorded signal. A T1 weighted spin echo sequence is based on a short repetition time (TR) and a change of it will affect the acquisition time and the T1 weighting of the image. Increased TR results in improved SNR caused by longer recovering time for the longitudinal magnetization. Increased TE improves the T2 weighting, combined with a long TR (of several T1 times) to minimize the T1 effect.

(SCT) The total scan time is the time required to collect all data needed to generate the programmed images. The scan time is related to the used pulse sequence and dependent on the assemble of parameters like e.g., repetition time (TR), Matrix, number of signal averages (NSA), TSE- or EPI factor and flip angle.
For example, the total scan time for a standard spin echo or gradient echo sequence is number of repetitions x the scan time per repetition (means the product of repetition time (TR), number of phase encoding steps, and NSA).

See also Number of Excitations, Turbo Spin Echo Turbo Factor, Echo Planar Imaging Factor, Flip Angle and Image Acquisition Time.

See also acronyms for 'scan time parameters' from different manufacturers.

(SMASH) Several lines of data are acquired for each phase encoding step, which is also referred to as a k-space trajectory.
SMASH imaging with a four-element array coil is four times faster and can be used to achieve almost real-time imaging. The maximum reduction in acquisition time is determined by the number of array coil elements. Thus, the heart can be scanned with higher temporal resolution and increased spatial resolution.
SMASH and SENSE differ from other techniques in which only one line of k-space data is acquired for each phase encoding gradient step.

See Sensitivity encoding.