(TSE TF) The turbo factor is the number of echoes acquired after each excitation. This is a measure of the scan time acceleration, e.g. at turbo factor 3 the scan time is 3 times faster as a SE sequence with comparable parameters. In combination with profile order and effective echo time the TSE turbo factor controls the echo spacing.

See also Scan Time and Echo Spacing.

(FRFSE, FR-FSE) The fast relaxation fast spin echo sequence provides high signal intensity of fluids even with short repetition times, and can be used with parallel imaging techniques for short breath hold imaging or respiratory gating for free-breathing, high isotropic resolution MR imaging. After signal decay at the end of the echo train, a negative 90° pulse align spins with long T2 from the transverse plane to the longitudinal plane, leading to a much faster recovery of tissues with long T2 time to the equilibrium and thus better contrast between tissues with long and short T2.
Fast relaxation FSE has advantages also for volumetric imaging as the TR can be substantially reduced and thus the scan time. The sequence can be post processed with maximum intensity projection, surface or volume rendering algorithms to visualize anatomical details in brain or spine MRI. Cerebro spinal fluid pulsation artifacts, often problematic in the cervical or thoracic spine may be reduced by radial sampling, in particular when combined with acquisitions of the PROPELLER type.

See also Fast spin echo, Driven Equilibrium.

(CASL) In order to take spins in a different magnetic state to that of the static tissue a train of RF pulses repeatedly saturates blood water spins flowing through the neck. The saturated (or 'labeled') spins flow into the brain and, assuming water is a freely diffusible tracer, exchange completely with brain tissue water, thus reducing the overall tissue magnetization. A steady state progresses where the regional magnetization in the brain is directly related to cerebral blood flow.

(DE FSE) A fast spin echo sequence with application of a pulse, which sensitizes the sequence to variations in T2.

Quick Overview

Pulsatile cerebro spinal fluid flow produces ghost artifacts that are superimposed in the image.

Flow compensation should be used to reduce these artifacts. This applies an additional gradient to eliminate phase differences for both stationary and moving spins at the echo time. At TE no phase differences is measured. If flow compensation is applied and there are still flow artifacts, cardiac triggering is an additional option to reduce these artifacts.
See also Motion Artifact.