A matrix is an array of numbers in rows and columns. The horizontal lines in matrices are called rows and the vertical lines are called columns. A matrix with m rows and n columns is called an m-by-n matrix (or m * n matrix) and m and n are called its dimensions.
The matrix used in MRI determines the scan resolution. See Acquisition Matrix.

A usual variation of sequential plane imaging technique that can be used with selective excitation technique that does not affect adjacent slices. Since, in SE imaging TR longer than TE, the machine would be idling most of the time, if a single slice would be acquired; multiple slice imaging was introduced early on. Adjacent slices are imaged while waiting for relaxation of the first slice toward equilibrium, resulting in decreased image acquisition time for the set of slices. The maximum number of slices of a pulse sequence depends on the repetition time.

Fractional Nex imaging (GE Healthcare term for imaging with a Nex value less than 1) benefits from the conjugate symmetry of the k-space to reduce the number of phase encoding acquisitions. With fractional Nex imaging (similar to partial Fourier or Half Scan), just over half of the data are acquired and the data from the lower part of k-space are used to fill the upper part, without sampling the upper part. Fractional Nex imaging sequences use a number of excitations values between 0.5 and 1. These values are a bit misleading, because the number of phase encoding steps is reduced, and not the NEX.
Fractional Nex imaging reduces the scan time considerable, by preserving the same contrast between the tissues. The effect by acquiring fewer data points is that the signal to noise ratio decreases.

See also acronyms for 'partial averaging//fractional Nex imaging' from different manufacturers.

In single shot techniques (used for EPI, TSE, FSE, RARE, HASTE), the entire raw data set is acquired with a single excitation pulse. The magnetization of a fully relaxed spin system is used. Each of the subsequent echoes is given a different phase encoding. For improved SNR, spatial resolution or FOV, the needed raw data are acquired over a number of sequence repetitions. Each repetition then collects a fraction of the complete raw data set. Only slightly more than a half of the raw data is acquired. The image is obtained through half Fourier reconstruction.
A single shot sequence is useful in cases where movement is to expect e.g. in abdominal Imaging or fetal MRI.

See also Half Fourier Acquisition Single Shot Turbo Spin Echo.

Partial averaging is a scan time reduction method that takes advantage of the complex conjugate of the k-space. The number of phase encoding steps of the acquisition matrix are reduced in the phase encoding direction.
Since negative values of phase encoded measurements are identical to corresponding positive values, only a little over half (more than 62.5%) of a scan actually needs to be acquired to replicate an entire scan. This results in a reduction in scan time at the expense of signal to noise ratio. The time reduction can be nearly a factor of two, but full resolution is maintained.
Partial Fourier averaging can be used when scan times are long, the signal to noise ratio is not critical and where full spatial resolution is required. Partial averaging is particularly appropriate for scans with a large field of view and relatively thick slices; and in 3D scans with many slices. In some fast scanning techniques the use of partial averaging enables a shorter TE thus improving contrast.
Partial averaging is also called Fractional NEX, Half Scan, Half Fourier, Phase Conjugate Symmetry, Single Side Encoding.