Echo rephasing :

The re-establishment of MR signal coherence which is achieved with either a 180° RF pulse or by gradient switching.

   Echo train length (ETL) :

This is the number of echoes individually phase encoded for a fast (turbo) spin-echo sequence. The ETL corresponds to the number of lines of k-space measured per repetition time (T R) interval. ETLs can range typically from 3 to 128 depending on the pulse sequence type.

   Even echo rephasing :

Re-establishment of spin-echo coherence of moving spins on symmetric even echoes in multi-echo sequences as a result of sequential integration of signal phase shifts summing to zero.

   Fat/water suppression :

A method that suppresses the signal with the imaging volume from either fat or water protons by application of frequency-selective saturation RF pulse.

   Ferromagnetic substance :

A substance, such as iron, cobalt or nickel, that is attracted by a magnetic field and retains its magnetism once removed from the magnetic field.

   Flip angle :

The angle through which the magnetization vector moves, relative to the longitudinal axis of the static magnetic field, as a result of the application of an RF pulse. This variation in flip angle is used in gradient echo imaging to obtain the various tissue-weighted images.

   Flow-related enhancement :

A process by which the signal intensity of moving fluids can be increased as compared with signal from stationary tissue; it occurs when in-flowing, unsaturated fully magnetized spins replace saturated spins within the imaging slice between successive RF pulses.

   Fourier transform :

A mathematical process by which the frequency components of a signal are separated from its amplitudes as a function of time and vice versa.

   Free induction decay (FID) :

A transient signal that occurs as the transverse magnetization decays towards zero following application of an RF pulse.