(FLASH) A fast sequence producing signals called
gradient echo with low flip angles. FLASH
sequences are modifications, which incorporate or remove the effects of transverse
coherence respectively.
FLASH uses a semi-random spoiler
gradient after each
echo to spoil the steady state (to destroy any remaining
transverse magnetization) by causing a spatially dependent
phase shift. The transverse steady state is spoiled but the longitudinal steady state depends on the T1 values and the
flip angle. Extremely short TR times are possible, as a result the sequence provides a mechanism for gaining extremely high T1
contrast by imaging with TR times as brief as 20 to 30 msec while retaining reasonable signal levels. It is important to keep the TE as short as possible to suppress
susceptibility artifacts.
The T1
contrast depends on the TR as well as on
flip angle, with short TE.
Small flip angles and short TR results in
proton density, and long TR in
T2* weighting.
With large flip angles and short TR result
T1 weighted images.
TR and flip angle adjustment:
TR 3000 ms,
Flip Angle 90°
TR 1500 ms,
Flip Angle 45°
TR 700 ms,
Flip Angle 25°
TR 125 ms,
Flip Angle 10°
The apparent ability to trade TR against
flip angle for purposes of
contrast and the variation in
SNR as the
scan time (TR) is reduced.
See also
Gradient Echo Sequence.