Assessment and correction of macroscopic field variations in 2D spoiled gradient-echo sequences

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics

Background:

• Gradient-echo (GRE) imaging is susceptible to dephasing artifacts caused by magnetic field variations.
• Accurate estimation of quantitative parameters like R2* and myelin water fraction (MWF) is crucial for various MRI applications.
• Large flip angles in GRE sequences can exacerbate dephasing effects, complicating quantitative analysis.

Purpose of the Study:

• To develop and validate a model for correcting dephasing effects in GRE signals.
• To account for arbitrary radiofrequency (RF) excitation pulses and large flip angles.
• To incorporate macroscopic field variations and their impact on signal magnitude and phase.

Main Methods:

• Numerical solution of Bloch equations to model dephasing across the slice profile.
• Inclusion of regional transmit RF field variations and slice profile scaling due to field gradients.
• Validation through simulations, phantom studies, and in vivo 3 Tesla MRI.

Main Results:

• The proposed model significantly reduces the influence of macroscopic field gradients on R2* and MWF estimations.
• Dephasing dependency on slice-selection gradient polarity was observed for flip angles ≥ 60°.
• Demonstrated improved accuracy and coverage for R2* and MWF mapping.

Conclusions:

• The developed model enhances the accuracy of R2* and MWF mapping, particularly when large flip angles are used.
• Accurate modeling requires considering phase variations along the slice profile and slice-selection gradient polarity.
• This approach is vital for robust quantitative MRI in the presence of field inhomogeneities.