(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.

Shots are split phase encoding steps (sub-acquisitions).

(SNR or S/N) The signal to noise ratio is used in MRI to describe the relative contributions to a detected signal of the true signal and random superimposed signals ('background noise') - a criterion for image quality.
One common method to increase the SNR is to average several measurements of the signal, on the expectation that random contributions will tend to cancel out. The SNR can also be improved by sampling larger volumes (increasing the field of view and slice thickness with a corresponding loss of spatial resolution) or, within limits, by increasing the strength of the magnetic field used. Surface coils can also be used to improve local signal intensity. The SNR will depend, in part, on the electrical properties of the sample or patient being studied. The SNR increases in proportion to voxel volume (1/resolution), the square root of the number of acquisitions (NEX), and the square root of the number of scans (phase encodings). SNR decreases with the field of view squared (FOV2) and wider bandwidths. See also Signal Intensity and Spin Density.

Measuring SNR:
Record the mean value of a small ROI placed in the most homogeneous area of tissue with high signal intensity (e.g. white matter in thalamus). Calculate the standard deviation for the largest possible ROI placed outside the object in the image background (avoid ghosting/aliasing or eye movement artifact regions).
The SNR is then:
Mean Signal/Standard Deviation of Background Noise

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.

Relates to the addition of phase encoding steps for 3D volumetric imaging.