|Type of sequence||Philips||Siemens||GE||Hitachi||Toshiba|
|Gradient echo (GE)||FFE||GRE||GRE||GE||FE|
The gradient echo sequence differs from the spin echo sequence in regard to:
- the flip angle usually below 90°
- the absence of a 180° RF rephasing pulse
A flip angle lower than 90° (partial flip angle) decreases the amount of magnetization tipped into the transverse plane. The consequence of a low-flip angle excitation is a faster recovery of longitudinal magnetization that allows shorter TR/TE and decreases scan time.
The advantages of low-flip angle excitations and gradient echo techniques are faster acquisitions, new contrasts between tissues and a stronger MR signal in case of short TR.
The flip angle determines the fraction of magnetization tipped in the transverse plane (which will produce the NMR signal) and the quantity of magnetization left on the longitudinal axis.
If the flip angle decreases, the residual longitudinal magnetization will be higher and the recovery of magnetization for a given T1 and TR will be more complete. On the other hand, the result of a lower flip angle excitation is a lower tipped magnetization.
The actual decay of the transverse magnetization is due to several factors:
- spin-spin tissue-specific relaxation (T2) which is random
- B0 field inhomogeneities and magnetic susceptibility, which are static
As GE techniques use a single RF pulse and no 180° rephasing pulse, the relaxation due to fixed causes is not reversed and the loss of signal results from T2* effects (pure T2 + static field inhomogeneities). The signal obtained is thus T2*-weighted rather than T2-weighted. These sequences are thus more sensitive to magnetic susceptibility artifacts than are spin echo sequences.
As there is no 180° RF pulse, a bipolar readout gradient (which is the same as the frequency-encoding gradient) is required to create an echo. The gradient echo formation results from applying a dephasing gradient before the frequency-encoding or readout gradient.
The goal of this dephasing gradient is to obtain an echo when the readout gradient is applied and the data are acquired. The dephasing stage of the readout gradient is in the inverse sign of the readout gradient during data acquisition. Moreover, its dephasing effect is designed so that it corresponds to half of the dephasing effect of the readout gradient during data acquisition. Consequently, during data acquisition, the readout gradient will rephase the spins in the first half of the readout (by reversing the dephasing effect of the dephasing lobe), and the spins will dephase in the second half (due to the dephasing effect of the readout gradient). The time during which the peak signal is obtained is called Echo Time (TE).
In gradient echo, TR reduction may cause permanent residual transverse magnetization in TR below T2: the transverse magnetization will not have completely disappeared at the onset of the following repetition and will also be submitted to the flip caused by the excitation pulse.
Two main classes of gradient echo sequence can be distinguished, depending on how residual transverse magnetiztion is managed:
- gradient echo sequences with spoiled residual transverse magnetization
- steady state gradient echo sequences that conserve residual transverse magnetization and therefore participate in the signal.