Steady-State Free Precession
What is meant by steady-state free precession (SSFP)?
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In a previous Q&A we showed what happens when a long train of evenly spaced RF‑pulses is applied to a sample without spoiling or gradients. In this scenario, free induction decay (FID) signals will occur after each RF pulse, and SEs will be produced by successive pairs of RF pulses. Each set of three of more RF pulses will in turn produce stimulated echoes (STEs), which coincide with the SEs.
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An unspoiled regularly spaced train of RF-pulses will produce "FIDs" after each RF-pulse and an "Echo"-like signal just before each RF-pulse.
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In the above example, the transverse magnetization dies out in the middle between the RF pulses. If the pulses are applied sufficiently rapidly (i.e. TR<<T2), however, the tails of the "FIDs" and "Echoes" will merge, so that a continuous signal of varying amplitude is produced. This is known as a steady-state free precession (SSFP).
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Steady-State Free Precession (SSFP)
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The fact that the MR signal never completely dies out between RF-pulses is equivalent to saying that the transverse components of magnetization never fully dephase. This unique situation can occur only under special conditions: (1) TR must be significantly shorter than T2, or else natural decay processes would destroy the transverse coherence, (2) phase shifts caused by imaging gradients must remain constant from cycle to cycle, (3) field inhomogeneities must be static, and (4) the spins must be stationary or motion-compensated.
Although we have called the two signals "FID" and "Echoes", in reality they are more complex than this. After a long train of RF-pulses, each contain contributions from magnetizations refocused from multiple prior cycles. Perhaps a better terminology would be "FID-like" and "Echo-like". Some authors refer to them as S+, the post-exitation signal, and S−, the pre-excitation signal.
If care is taken not to completely disrupt (spoil) these transverse coherences through the imaging process, it is possible to use dephase-rephase gradients to refocus and record the FID and Echo signals individually or in various combinations. These methods lead to four basic types of coherent gradient echo sequences in the table below.
These coherent GRE sequences will be discussed more completely in subsequent Q&A's.
References
Carr HY. Steady-state free precession in nuclear magnetic resonance. Phys Rev 1958; 112:1693-1701. (Another paper with heavy math and physics, but one serious MR students should have in their libraries).
Chavhan GB, Babyn PS, Jankharia BG et al. Steady-state MR imaging sequences: physics, classification, and clinical applications. Radiographics 2008;28:1147-1160.
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8. (My older review; still accurate, though some vendors have gone out of business. Gives a good history of the development of GRE sequences).
Hargreaves B. Rapid gradient-echo imaging. J Mag Reson Imaging 2012;36:1300-1313. (A great, but not overly technical, modern review).
Carr HY. Steady-state free precession in nuclear magnetic resonance. Phys Rev 1958; 112:1693-1701. (Another paper with heavy math and physics, but one serious MR students should have in their libraries).
Chavhan GB, Babyn PS, Jankharia BG et al. Steady-state MR imaging sequences: physics, classification, and clinical applications. Radiographics 2008;28:1147-1160.
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8. (My older review; still accurate, though some vendors have gone out of business. Gives a good history of the development of GRE sequences).
Hargreaves B. Rapid gradient-echo imaging. J Mag Reson Imaging 2012;36:1300-1313. (A great, but not overly technical, modern review).
Related Questions
It seems as if every manufacturer has adopted a different name for their gradient echo sequences. Why is this? Can you sort this out for me?
If a spin echo results from 2 pulses, and a stimulated echo from 3 pulses, what do you get from 4 pulses?
It seems as if every manufacturer has adopted a different name for their gradient echo sequences. Why is this? Can you sort this out for me?
If a spin echo results from 2 pulses, and a stimulated echo from 3 pulses, what do you get from 4 pulses?