Reduced physiology-induced temporal instability achieved with variable-flip-angle fast low-angle excitation...

Area of Science:

• Magnetic Resonance Imaging
• Functional Magnetic Resonance Imaging (fMRI)
• Biomedical Engineering

Background:

• Echo Planar Time-Resolved Imaging (EPTI) offers high-resolution, distortion-free imaging for fMRI, overcoming limitations of conventional Echo Planar Imaging (EPI).
• Multishot sequences in EPTI are susceptible to intershot phase variations caused by physiological processes, leading to temporal instabilities in fMRI data.
• Addressing these instabilities is crucial for reliable high-resolution fMRI studies.

Purpose of the Study:

• To reduce temporal instabilities in multishot Echo Planar Time-Resolved Imaging (EPTI) caused by intershot phase variations.
• To improve the consistency and signal maximization across shots in multishot EPTI.
• To enhance the suitability of EPTI for high-resolution fMRI applications.

Main Methods:

• Combined Fast Low-Angle Excitation Echo-Planar Technique (FLEET) with Variable Flip Angle (VFA) to acquire all shots consecutively with minimal time delays.
• Employed a recursive Shinnar-Le Roux RF pulse design algorithm for consistent slice profiles and signal intensities across shots.
• Integrated Blipped Controlled Aliasing in Parallel Imaging Simultaneous Multislice (CAIPI-SMS) to improve temporal resolution and spatial coverage.

Main Results:

• Variable Flip Angle-Fast Low-Angle Excitation Echo-Planar Technique (VFA-FLEET) demonstrated a spatial-specific increase in temporal stability compared to conventional EPTI at 7 Tesla.
• High-resolution task-fMRI experiments using VFA-FLEET EPTI successfully detected reliable Blood-Oxygen-Level-Dependent (BOLD) responses to visual stimuli.
• The proposed VFA-FLEET EPTI shows significant improvements in temporal stability.

Conclusions:

• Intershot phase variations in multishot EPTI contribute to physiological noise enhancement and reduced temporal stability.
• The VFA-FLEET technique effectively mitigates these physiology-induced instabilities in multishot EPTI.
• The VFA-FLEET method provides adequate stability and sensitivity for advanced high-resolution fMRI studies.