There are many sequences used in MRI, each one aimed at increasing the tissue contrast of interest. The basic sequences are:
- Spin echo
- Gradient echo
We will first go through the spin echo sequence. A spin echo sequence aims to remove the effects of the static field (T2*) but leave the tissue characteristic T2 effect.
a. Application of 90° RF pulse
1. A 90° RF pulse is applied. All proton vectors precess in phase and the Mxy signal is at its maximum.
2. The Mxy signal decays rapidly due to the T2* or free induction decay. There are some proton vectors that are fast and lead and some that are slow and lag as they dephase.
b. Application of 180° RF rephasing pulse
3. After a time (t) a 180° RF pulse is applied. This is simply a pulse that is applied twice as long as the 90° pulse in the transverse plane. All proton vectors are turned through 180°. The laggers become leaders and vice-versa.
4. After the same amount of time (t) the proton vectors are again in phase, the Mxy signal is at its peak. This is the echo and this is the signal that is measured. The decay in the signal from the original 90° to the echo is due to the tissue characteristic T2 effect with the effect of magnetic field inhomogeneities minimised.
The time at which this echo is produced is the TE (time to echo). It is produced at exactly 2t, t being the time at which the 180° RF pulse is applied (i.e. the 180° RF pulse is applied at TE/2).
The mechanism through which the echo is created is gone into in more detail in the diagram below.
d. Repeat cycle
5. One cycle has now been completed. This cycle is repeated hundreds of times in the sequence. The time to the next cycle is TR (time to repetition).
- Initial 90° RF pulse
- At TE/2, 180° pulse applied
- At TE, echo signal measured
- Cycle repeated after TR
True T2 weighting
Minimise susceptibility effects
|Long scan times|
Uses more RF power than a GE
There is more to a spin echo than what is covered in this page and I will go into more detail in “Spin echo sequence – detailed”. For the moment, however, I will go through how to create weighted images by altering the parameters of sequences.