B 1 + inhomogeneity mitigation for diffusion weighted MRI at 7T using TR-FOCI pulses

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Pulse Sequence Design
• Quantitative Imaging

Background:

• High-field MRI (7T) offers superior signal but suffers from B0 inhomogeneity.
• Diffusion-weighted imaging (DWI) is sensitive to B0 inhomogeneity, degrading image quality.
• Conventional pulse sequences struggle to fully correct for these artifacts.

Purpose of the Study:

• To improve image quality in 7T DWI using a single RF transmit channel.
• To implement time-resampled frequency-offset corrected inversion (TR-FOCI) pulses within a twice-refocused spin-echo sequence.
• To mitigate B0 inhomogeneity effects on DWI signal intensity.

Main Methods:

• Replaced conventional Shinnar-Le Roux pulses with TR-FOCI pulses in a twice-refocused spin-echo sequence.
• Evaluated slice profiles via simulation and phantom experiments.
• Assessed in vivo image quality comparing TR-FOCI and Shinnar-Le Roux at b-values of 0 and 1000 s/mm².

Main Results:

• The modified sequence demonstrated improved image quality across the entire brain.
• Significant SNR increases observed: 113% for b0 and 66% for b=1000 s/mm² images in temporal lobes.
• Adiabatic TR-FOCI pulses effectively mitigated global B0 inhomogeneity with a single-channel setup.

Conclusions:

• TR-FOCI pulses successfully mitigate B0 inhomogeneity in 7T DWI using a twice-refocused spin-echo sequence.
• This approach enhances signal intensity across the brain.
• Further research is required to address specific absorption rate (SAR) limitations.