Spoiled gradient echo sequences
|Type of sequence||Philips||Siemens||GE||Hitachi||Toshiba|
In certain cases, the steady state can be detrimental, namely for obtaining T1 weighted sequences. To resolve this problem, gradients and/or RF pulses (spoilers) are used to eliminate residual transverse magnetization.
In this type of echo gradient sequence, image weighting will depend on:
- the flip angle for T1 weighting (the greater the angle, the more T1 weighting)
- the TE for T2* weighting (the shorter the TE, the more T2* deweighting)
These sequences allows for fast 3D imaging during short apnea (10 to 20 seconds). They are used in angiography after gadolinium injection, possibly completed by a prior acquisition to allow subtraction. This type of sequence is also used in intra-voxel water-fat mixture imaging, by choosing in phase and out of phase TEs (cf. Chemical shift artifact of the second type). Like all other gradient echo sequences, these are also sensitive to magnetic susceptibility artifacts.
Spoiling gradients and RF pulses
The spoiing RF pulse consists in randomly varying the excitation pulse phase at each repetition.
A spoiling gradient has 2 components:
- A constant component, consisting of prolonging the readout gradient
- A variable component, changing randomly at each repetition, in the direction of the slice selection gradient
Ernst angle and TR in gradient echo sequences
For a given flip angle and relaxation time T1, there is an optimal TR to obtain the maximum signal after a series of excitation pulse . The relationship between flip angle and optimal TR follows the Ernst angle formula: