Multiecho segmented EPI with z-shimmed background gradient compensation (MESBAC) pulse sequence for fMRI

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging (MRI)

Background:

• Susceptibility artifacts, image distortion, and signal dropout limit fMRI in specific brain regions like the orbitofrontal cortex (OFC).
• Existing methods often compromise spatial or temporal resolution, or fail to adequately address motion and ghosting artifacts in prolonged acquisitions.

Purpose of the Study:

• To propose and validate a novel MultiEcho Segmented EPI with z-shimmed BAckground gradient Compensation (MESBAC) pulse sequence for fMRI.
• To improve the reliability of blood oxygenation level-dependent (BOLD)-weighted fMRI signals in anatomically challenging brain areas.

Main Methods:

• The MESBAC sequence was developed, incorporating z-shimmed background gradient compensation and segmented EPI acquisition.
• Navigator echoes in readout and phase-encoding directions were used to mitigate physiological motion artifacts.
• Even and odd echoes were strategically placed in k-space to eliminate ghost artifacts inherent in single-shot EPI.

Main Results:

• The MESBAC sequence effectively reduced image distortion and signal dropout in fMRI.
• Reliable BOLD-weighted fMRI signals were obtained in the orbitofrontal cortex (OFC).
• The sequence demonstrated a favorable tradeoff between spatial and temporal resolution, while eliminating ghosting artifacts.

Conclusions:

• The MESBAC sequence offers a robust solution for fMRI in regions prone to susceptibility artifacts.
• It is particularly advantageous for pharmacological and affective fMRI studies targeting deep brain structures like the OFC, nucleus accumbens, and amygdala.