High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Engineering
• Biomedical Engineering

Background:

• High magnetic field strengths in MRI (e.g., 7 Tesla) exacerbate B(1)(+) field inhomogeneity, leading to reduced signal-to-noise ratio (SNR) and image contrast.
• Existing parallel RF transmission methods using k-space trajectory design are effective for small flip angles but fail at large flip angles.
• Severe B(1)(+) inhomogeneity limits the application of conventional high-flip-angle excitation sequences in advanced MRI.

Purpose of the Study:

• To propose and demonstrate a novel large-flip-angle parallel excitation design for Magnetic Resonance Imaging (MRI).
• To mitigate severe B(1)(+) inhomogeneity during 90-degree and 180-degree spin-echo slice-selective excitations.
• To enable robust high-flip-angle excitations in parallel RF systems at very high B(0) field strengths.

Main Methods:

• Development of a large-flip-angle parallel excitation design specifically for 90-degree and 180-degree spin-echo sequences.
• Validation using an 8-channel transmit array at 7 Tesla.
• Testing on a water phantom simulating in vivo human brain B(1)(+) inhomogeneity.

Main Results:

• Successful demonstration of a large-flip-angle parallel excitation design mitigating B(1)(+) inhomogeneity.
• Achieved uniform in-plane magnetization and good slice selection profiles.
• The method proved effective even with B(1)(+) inhomogeneity levels comparable to those in the human brain.

Conclusions:

• The proposed large-flip-angle parallel excitation design effectively addresses B(1)(+) inhomogeneity in high-field MRI.
• This approach allows the implementation of conventional high-flip-angle sequences without significant pulse duration increases.
• It offers a viable solution for advanced MRI at very high field strengths where B(1)(+) inhomogeneity is a major challenge.