Pseudo Steady-State Free Precession for MR-Fingerprinting

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Physics

Background:

• Steady-state free precession (SSFP) sequences are fundamental in MRI.
• MR fingerprinting (MRF) utilizes continuously varied flip angles in SSFP.
• Flip angle variation disrupts the steady state, causing instabilities due to magnetic field inhomogeneities and intravoxel dephasing.

Purpose of the Study:

• To investigate signal behavior in SSFP sequences with continuously varied flip angles for MR fingerprinting.
• To demonstrate how a pseudo steady state can be achieved to overcome instabilities.
• To restore the spin echo nature of SSFP signals in the context of MR fingerprinting.

Main Methods:

• Derivation of relationships between flip angle, repetition time, and echo time based on geometrical considerations for pseudo steady state establishment.
• Validation of the theoretical framework using Bloch simulations.
• Experimental verification with phantom and in vivo studies.

Main Results:

• The proposed conditions successfully restore a typical SSFP passband.
• The stability of the pseudo steady state was confirmed by comparing single isochromat evolution to spin ensemble behavior.
• Experiments demonstrated that the pseudo steady state requires fewer degrees of freedom for magnetization description, leading to more reliable parameter maps.

Conclusions:

• The derived conditions re-establish spin-echo-like signal behavior in SSFP fingerprinting sequences.
• This approach enhances robustness against B0 variations (magnetic field inhomogeneities).
• The pseudo steady state method offers improved reliability for MR fingerprinting parameter mapping.