SPGR, RF-FAST, FLASH

Expansion:

Spoiled Gradient Recalled Acquisition in Steady State

RF-Spoiled Fourier Acquired Steady State

Technique:

In a steady-state gradient echo sequence, e.g. GRASS, a rewinder gradient restores the original phase state (a necessity to maintain a steady state). In such sequences, image contrast is complex and depends on flip angle, TR and TE. An SPGR sequence spoils the transverse steady state by semi-randomly changing the phase of the RF pulse, resulting primarily in T1 or PD contrast. The related FLASH sequence uses a random spoiler gradient to produce a phase shift and spoil the steady state.

Typical Parameters:

optimal flip angle (Ernst angle) at a given TR

cos angle = exp (-TR/T1)

TR: 20-80 msec (longer = more T1-weighting)

TE: 5-10 msec (longer TE = more T2* weighting)

flip angle: 30-50 degrees (increasing angle increases T1)

Reference:

All You Really Need to Know About MR Physics, 7-8,7-9

SPGR:  SMRM 9, 1308

FLASH: JMR 67, 258, 1986

Contributor:

 

Related Sequences/Terms:

GRE, FISP, FAST, Steady State, FLASH, RF-FAST

Comments:

To maintain steady state: TR < T2 phase shifts must be constant spins must be stationary or motion-compensated

SPGR spoils transverse steady state with random phase of RF pulse

FLASH sequences use a random spoiler gradient to produce a spatially-dependent phase shift and spoil steady state

both result in primarily PD and T1 contrast images

Pulse Sequence Diagram:

Spoiled Steady State Gradient Sequence