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A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.
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Parallel transmit (pTx) with online pulse design for task-based fMRI at 7 T.

Belinda Ding1, Iulius Dragonu2, Catarina Rua3

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Parallel transmission (pTx) echo planar imaging (EPI) improves ultra-high field MRI by enhancing image uniformity and signal-to-noise ratio. This method shows robust performance for functional MRI (fMRI) studies, particularly in brain regions prone to signal dropout.

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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.