Steady-state free precession MRI
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Steady-state free precession MRI (SSFP) is a type of gradient echo MRI pulse sequence in which a steady, residual transverse magnetization (Mxy) is maintained between successive cycles. The sequence is noted for its superiority in dynamic/cine assessment of cardiac function.
To understand SSFP (and all MR imaging for that matter), the first thing to bear in mind is that the magnetization vector has two components: Mz and Mxy. Both are at 90° to each other and excitation pulses which flip the vector result in conversion of one component into the other. Added to this interconversion is the spontaneous regrowth and decay of Mz and Mxy governed by T1 and T2 intervals, respectively. Over several sequences, a steady-state of equilibrium is achieved, with constant magnitudes of Mz and Mxy at the beginning of the cycle.
A steady-state is achieved in tissues with a sufficiently long T2 interval, by keeping the TR shorter than T2.
Types of image formed
At steady state, two signal types are present:
- T1/T2* weighted: the mixed steady-state signal - subjected to refocusing gradient; this generates the FID (typical of a GRE sequence)
- T2 weighted: the residual Mxy at the beginning of the next pulse; the α flip (50°-80° in typical GRE) results in spin-echo (analogous to the 180° refocusing pulse used in spin-echo)
- cardiac imaging
- fetal imaging
- abdominal imaging
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