In MRI, an echo is the emission of energy in form of an electromagnetic resonance signal of a nuclei after its excitation. At this point spins are back in phase again and the signal is measured. The desired number of echoes is selectable. Often until eight echoes are permissible for 2D or 3D scans using spin echo, inversion recovery or MIX techniques. Two echoes are permissible for all other techniques. A multi echo imaging sequence is needed for simultaneous measurement of T2 and density weighted images.

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(FA) The flip angle a is used to define the angle of excitation for a field echo pulse sequence. It is the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of a RF excitation pulse at the Larmor frequency. It is also referred to as the tip angle, nutation angle or angle of nutation.
The radio frequency power (which is proportional to the square of the amplitude) of the pulse is proportional to a through which the spins are tilted under its influence. Flip angles between 0° and 90° are typically used in gradient echo sequences, 90° and a series of 180° pulses in spin echo sequences and an initial 180° pulse followed by a 90° and a 180° pulse in inversion recovery sequences.

(T) Times between successive RF pulses used in pulse sequences. Particularly important are the inversion time (TI) in inversion recovery, and the time between 90° pulse and the subsequent 180° pulse to produce a spin echo, which will be approximately one half the spin echo time(TE). The time between repetitions of pulse sequences is the repetition time(TR).

(TI) The time period between the 180° inversion pulse and the 90° excitation pulse in an inversion recovery pulse sequence. The inversion time controls the signal of different tissues and with the change of this parameter also fat and water suppression is attainable.