Spatially resolved extended phase graphs: modeling and design of multipulse sequences with parallel transmission

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radio Frequency (RF) Engineering
• Biomedical Engineering

Background:

• Spatially variable radio frequency (RF) fields in MRI can lead to signal inhomogeneity.
• Parallel transmission (PTx) systems offer potential for improved RF field control but require sophisticated methods for optimization.
• Static pseudo-steady state (SPSS) turbo spin echo (TSE) imaging is sensitive to RF field variations.

Purpose of the Study:

• To propose a spatially resolved extended phase graph (SR-EPG) framework for predicting echo amplitudes under spatially variable RF fields.
• To develop and evaluate a dynamic RF-shimming approach for PTx systems to improve signal homogeneity in SPSS-TSE imaging.

Main Methods:

• Development of a spatially resolved extended phase graph (SR-EPG) framework to model MRI signal behavior.
• Implementation of a dynamic RF-shimming strategy that optimizes RF pulse amplitudes and phases per channel and per pulse.
• Application and validation of the dynamic RF-shimming approach in SPSS-TSE imaging using PTx systems.

Main Results:

• The SR-EPG framework accurately predicts echo amplitudes in the presence of spatially variable RF fields.
• Dynamic RF shimming significantly improved signal homogeneity compared to static RF shimming.
• SPSS-TSE imaging with dynamic RF shimming yielded excellent image quality in both phantoms and in vivo studies.

Conclusions:

• The proposed SR-EPG framework provides a valuable tool for designing and optimizing PTx systems.
• Dynamic RF shimming is an effective strategy for enhancing image quality and signal uniformity in MRI.
• The developed method demonstrates potential for widespread application in advanced MRI techniques.