When a multi shot technique is applied, each shot will have its own effect on the prepulse, with a scan time increase. Multiple shots allow a shorter IR delay but at the cost of increased scan time.
In multi shot technique (also called mosaic imaging), a group of samples, which are contiguous in k space are acquired in the same sequence repetition. The phase encoding steps or profiles are split into 'shots' (sub-acquisitions). The shot interval is the time between the shots. Usually kept as short as possible. Because the acquisitions are divided into different shots, each shot will have less T1 variation, thereby increasing T1 contrast. Two excitations, each requiring the data for one half of k-space, are the simplest variation of multi shot techniques (e.g. positive versus negative phase encoding). The alternative to this mosaic strategy for multi shot EPI is interleaving. In interleaved sequences, each repetition acquires every nth (n is the number of shots) line in k-space and for the complete raw data set the various repetition data are interlaced.

See also Single Shot Technique.

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Quick Overview

Fat suppression (SPIR or FatSat) is very critical to the magnetic field homogeneity. Eddy currents in the patient results in B1 disturbance from left to right and from anterior to posterior. The artifact is seen as signal intensity variations with SPIR, like a signal intensity loss diagonal in the image. The short T1 inversion recovery (STIR) sequence is due to another type of fat suppression insensitive to this artifact.

Take short T1 inversion recovery (STIR) instead spectral presaturation inversion recovery (SPIR) for fat suppression.

Longitudinal relaxation rate equal to reciprocal of T1 relaxation time (R1= 1/T1).

Reciprocals of the relaxation times, T1 and T2 (R1 = 1/T1 and R2 = 1/T2). There is often a linear relation between the concentration of MR contrast agents and the resulting change in relaxation rate. The rate of relaxation is influenced by molecules with protons that are tumbling. A slower tumble rate will result in faster relaxation rate (shorter relaxation time). Due to the molecular structure of fat with its larger size than water, fat will tumble slower than water molecules. The slower tumble rate of fat enables a faster relaxation rate.