Gradient Echo Sequences
Gradient Echo Sequences
The gradient echo sequences show a wide range of variations compared to the
spin echo and inversion recovery sequences. Not only is the basic sequence
varied by adding dephasing or rephasing gradients at the end of the sequence,
but there is a significant extra variable to specify in addition to things
like the TR and TE. This variable is the flip or tip angle of the spins.
The flip angle is usually at or close to 90 degrees for a spin echo sequence
but commonly varies over a range of about 10 to 80 degrees with gradient
echo sequences. For the basic gradient echo sequence FLASH, illustrated below,
the larger tip angles give more T1 weighting to the image and the smaller
tip angle give more T2 or actually T2* weighting to the images.
Images from other gradient echo sequences such as GRASS and FISP have less
intuitive tissue contrast characteristics than FLASH. The FLASH and SPGR
sequences show better tissue contrast between white matter and grey matter
in the brain and spinal cord than GRASS or FISP and are preferred when the
time of acquisition does not have to be very short. GRASS and FISP maintain
better SNR than FLASH at short TR times and are therefore preferred with
breath-holding techniques, for example.
A vector magnetization diagram of the gradient echo sequence is shown below.
Note that the spins are refocused by reversing the direction of the spins
rather than flipping them over to the other side of the x-y plane as occurs
with the spin echo sequence. Gradient refocusing of the spins takes considerably
less time than 180 degree RF pulse refocusing. One big disadvantage of gradient
echo sequences is the loss of signal from static magnetic field inhomogeneity.
This occurs to a lesser degree with spin echo sequences (and for a different
reason). Magnetic susceptibility artifacts are therefore more pronounced
on gradient echo sequences that on spin echo sequences.
