A unifying view on extended phase graphs and Bloch simulations for quantitative MRI

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Computational Physics
• Biophysics

Background:

• Quantitative MRI and machine learning algorithms rely on accurate forward simulations.
• Slice-selective radiofrequency (RF) pulses critically influence magnetization dynamics in MRI.
• Existing simulation methods provide final magnetization states, hindering theoretical insights and understanding of echo formation with slice profiles.

Purpose of the Study:

• To develop a mathematically exact and intuitive simulation framework for MRI.
• To incorporate slice-profile effects naturally within the simulation.
• To enable fundamental understanding of echo formation and signal behavior.

Main Methods:

• Development of an analytical, hybrid Bloch-EPG formalism.
• Utilizing the spatial representation of slice profiles.
• Derivation of the formalism and its connection to existing methods (Bloch, EPG, partitioned EPG).

Main Results:

• The spatially-resolved EPG approach accurately predicts signal dependence on off-resonance, spoiling moment, microscopic dephasing, and echo time.
• The formalism unifies simulation for both gradient-spoiled and balanced SSFP sequences.
• Application of the framework to MR Fingerprinting demonstrates its utility.

Conclusions:

• The proposed hybrid Bloch-EPG framework offers an exact and intuitive method for MRI simulations.
• It provides deeper insights into echo formation influenced by slice profiles.
• This formalism enhances quantitative MRI techniques like MR Fingerprinting.