Parallel transmit (pTx) with online pulse design for task-based fMRI at 7 T

Area of Science:

• Magnetic Resonance Imaging
• Neuroimaging

Background:

• Ultra-high field (UHF) MRI, particularly at 7 Tesla (7T), offers enhanced sensitivity but faces challenges with image uniformity.
• Parallel transmission (pTx) is a technique designed to mitigate these uniformity issues in MRI.
• Echo planar imaging (EPI) is a fast imaging sequence commonly used in functional MRI (fMRI), but can be susceptible to signal dropout at UHF.

Purpose of the Study:

• To develop and evaluate a subject-specific parallel transmission (pTx) echo planar imaging (EPI) sequence for online use in ultra-high field MRI.
• To compare the performance of the novel pTx-EPI sequence against conventional circularly polarized (CP) EPI sequences.
• To assess the impact of pTx-EPI on image quality and functional activation detection in challenging fMRI paradigms at 7T.

Main Methods:

• A modified EPI sequence incorporating online, subject-specific pTx pulse design was implemented.
• The pTx-EPI sequence was compared with standard CP-EPI in six healthy volunteers using a short acquisition protocol.
• Two fMRI paradigms, a visual localizer and a semantic processing task, known to be affected by signal dropout at 7T, were employed.

Main Results:

• pTx-EPI demonstrated an 11.0% improvement in whole-brain mean temporal signal-to-noise ratio (tSNR) compared to CP-EPI across all subjects.
• Functional activation analysis revealed that pTx-EPI yielded higher median z-scores and detected more activated voxels for all tested contrasts.
• These improvements were observed in brain regions typically affected by signal dropout, such as those involved in face/scene selectivity and semantic processing.

Conclusions:

• The study successfully demonstrated a workflow for EPI acquisitions utilizing online, per-subject pTx pulse calculations.
• pTx-EPI offers significant improvements in both tSNR and functional activation detection compared to conventional CP-EPI at 7T.
• The robustness of online calculated pTx-EPI makes it a promising technique for future fMRI studies, especially in anatomically challenging areas prone to signal dropout.