Reduced field of view imaging using a static second-order gradient for functional MRI applications.

Area of Science:

• Magnetic Resonance Imaging
• Functional Magnetic Resonance Imaging (fMRI)
• Image Resolution Enhancement

Background:

• Reduced field of view (FOV) excitation in MRI offers higher resolution or signal-to-noise ratio (SNR) but typically requires lengthy radiofrequency pulses.
• Existing methods for reduced FOV excitation often have limitations in the shape and location of the excited regions.

Purpose of the Study:

• To enhance a novel reduced FOV excitation method utilizing a second-order shim gradient to shorten radiofrequency pulse duration.
• To evaluate the efficacy of this improved method for functional MRI (fMRI) applications.

Main Methods:

• The reduced FOV method was extended to excite thicker regions beyond the isocenter, demonstrating accurate excitation profiles on a grid phantom.
• Visual stimulation fMRI scans were conducted comparing full and reduced FOV acquisitions.
• Image resolution was assessed using the full-width half-maximum of autocorrelation functions (FACFs) for time series and activation maps.

Main Results:

• Reduced FOV imaging resulted in significantly higher resolution in both time series images (4.1% ± 3.7% FACF reduction) and functional activation maps (3.1% ± 3.4% FACF reduction).
• A decrease in signal-to-noise ratio (SNR) was observed (26.5% ± 16.9%).
• In some subjects, precise localization to the reduced FOV was challenging due to insufficient Z2 gradient strength.

Conclusions:

• The developed reduced FOV excitation method is feasible for fMRI.
• The method's performance, particularly region localization, could be significantly improved with a stronger gradient coil.