T2-enhanced steady-state gradient echo
|Type of sequence||Philips||Siemens||GE||Hitachi||Toshiba|
|T2-enhanced steady-state GE||T2-FFE T2||PSIF||SSFP||FE|
In T2-enhanced steady-state gradient echo sequences:
- residual transverse magnetization is conserved
- the sequence is inverted in time, compared to the preceding sequences
- only the echo corresponding to the Hahn echo, dependent on T2 but weaker tthan spin echo, is recorded
The fact that the sequence is reversed in relation to standard gradient echo sequences creates a Hahn echo after two excitation pulses (separated by TR). The amplitude of this echo will depend on the relaxation time T2. It is weaker than that of a true spin echo. The T2 image weighting is linked to double TR (since the Hahn echo will not occur during the repetition corresponding to the first excitation pulse, but in the following one, after a second excitation pulse).
The weak intensity of the Hahn echo, produces a low signal to noise ratio in T2-enhanced steady state gradient echo sequences. This type of sequence is rarely used in clinical practice.
DESS (Double Echo Steady State)
The DESS sequence is a combination of FISP and PSIF sequences . The FID signal improves spatial resolution and the Hahn echo improves T2-weighting.
Interest: high-resolution T2 -weighted 3D gradient echo imaging.