Neuroimaging Data Processing/Field map correction/Echo times
Echo time (TE) 
The echo time (TE) is defined as the time interval between an excitation pulse and data aquisition. It is usually expressed in milliseconds.
- measured 90° pulse
- determines influence of T2 on image contrast
MR-Echos- Different ways to generate a MR Signal over time
- RF pulse decays exponentially (relaxation time T2*)
- to get the signal maximum not directly after HF excitation (RF-pulse) but during signal aquisition you can use MR-Echos
- after RF pulse (90° pulse) apply 180° pulse - out of phase spins are going back into phase and a new MR signal is generated
- spin echo increasing and reaches it maximum after double of run time (echo time)
- several spin echos are genereated by a multi echo sequence
- procedure can be repeated until transverse magnetization is lost throughout T2 relaxation
- dispersed http://www.youtube.com/watch?v=vMh11VtUA5o
- consolidated http://www.youtube.com/watch?v=dFp2Z3wjrmo
The gradient echo is generated through immediate changes of the magnetic field directly after the RF pulse (gradient pulse -). It means that the magnetic field gets smaller in one direction
and larger in the opposing one. As we all might know is the precession frequency of spins proportional to the field strenght. Therefore spins are now rotating at differents speeds along the field change. Spins dephase!
By reversing the polarity of the gradient from - to +, the spins will be in-phase again, they rephase! We measure an echo during the rephasing of the FID.
The echo time TE has to be considerable shorter for a gradient echo sequence than for a spin echo technique. The underlying mechanism for that is like followed.
The 180 degree pulse is omitted in gradient echo technology. This means that the static T2* dephasing mechanism is not cancelled as in spin echo technology. Instead gradient pulses are used to
quickly destroy the FID and build it back up again, all within the T2* decay. The echo time for a gradient echo has to fit into the T2* time. This is why the gradient echo technique is
faster than the spin echo technique.
- Schneider und Fink: "Funktionelle MRT in Psychiatrie und Neurologie", Hrsg. Gereon R. Fink, Heidelberg, 2 Auflage, 2013 S.69