GRASS, FISP, FAST

Expansion:

Gradient Recalled Acquisition in Steady State

Technique:

A series of sequential RF pulses is applied with a specific flip angle and interval TR. Three echoes result: an FID, SE, STE. In a GRASS sequence, the readout (frequency encode) gradient is extended to shift the SE and STE from the sampling window (to prevent FLASH bands). A rewinder gradient restores the original phase state (a necessity to maintain a steady state).

Typical Parameters:

optimal flip angle (Ernst angle) at a given TR

cos (angle) = exp (-TR/T1)

Short TR necessary to maintain steady state

Short TE increases signal to noise, decreases T2* weighting

Reference:

All You Really Need to Know About MR Physics, 7-6.

Contributor:

 

Related Sequences/Terms:

GRE, FISP, FAST, Steady State, STE

Comments:

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

optimal flip angle at given TR is the Ernst angle

Pulse Sequence Diagram:

Steady State GRE FID Sampling (GRASS)