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.

(TE) The echo time represents the time in milliseconds between the application of the 90° pulse and the peak of the echo signal in spin echo and inversion recovery pulse sequences.

See also Repetition Time, Spin Echo Sequence and Inversion Recovery Sequence.

Production of spin echo by repeated RF pulses. First observed using equal (90°) RF pulses, now commonly used to describe refocusing of transverse magnetization by a 180° RF pulse. By choosing long echo delay times, the spins in a Hahn echo first dephase for a long time, then rephase, which makes the Hahn pulse sequence more susceptible to diffusion effects.

(SE) The Reappearance of the MR signal after the FID has apparently died away, as a result of the effective reversal (rephasing) of the dephasing spins by techniques such as specific RF pulse sequences or pairs of field gradient pulses, applied in time shorter than or on the order of T2. Proper selection of the TE time of the pulse sequence can help to control the amount of T1 or T2 contrast present in the image. Pulse sequences of the spin echo type, usually employs a 90° pulse, followed by one or more 180° pulses to eliminate field inhomogeneity and chemical shift effects at the echo. Caused by this 180° refocusing pulse, spin echo or fast spin echo (FSE, TSE) sequences are more robust against e.g. susceptibility artifacts than sequences of the gradient echo type.

(TE) Time between middle of exciting (e.g., 90°) RF pulse and middle of spin echo production. For multiple echoes, use TE1, TE2, etc. When the RF spin echo and gradient echo are not coincident in time, TE refers to the time of the gradient spin echo. See also Echo Time.