(
FISP) A fast
imaging sequence, which attempts to combine the signals observed separately in the FADE sequence, generally sensitive about
magnetic susceptibility artifacts and imperfections in the
gradient waveforms. Confusingly now often used to refer to a refocused
FLASH type sequence.
This sequence is very similar to
FLASH, except that the spoiler pulse is eliminated. As a result, any
transverse magnetization still present at the time of the next
RF pulse is incorporated into the steady state.
FISP uses a
RF pulse that alternates in sign.
Because there is still some remaining
transverse magnetization at the time of the
RF pulse, a
RF pulse of a degree flips the spins less than a degree from the longitudinal axis.
With small flip angles, very little
longitudinal magnetization is lost and the image
contrast becomes almost independent of T1. Using a very short TE (with TR 20-50 ms,
flip angle 30-45°) eliminates
T2* effects, so that the images become
proton density weighted. As the
flip angle is increased, the
contrast becomes increasingly dependent on T1 and
T2*. It is in the domain of large flip angles and short TR that
FISP exhibits vastly different
contrast to
FLASH type
sequences.
Used for T1 orthopedic imaging, 3D MPR, cardiography and
angiography.