Whole-liver flip-angle shimming at 7 T using parallel-transmit k_T -point pulses and Fourier phase-encoded DREAM ...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiology
• Medical Physics

Background:

• Achieving homogeneous signal in 7 Tesla (7T) whole-liver MRI is challenging.
• Conventional methods like circularly polarized (CP) mode and phase shimming often result in inaccurate or imprecise flip angle (FA) distributions.
• This leads to inconsistent signal intensity and T1-weighting in gradient-echo scans.

Purpose of the Study:

• To develop and validate a method for obtaining homogeneous signal throughout the human liver at 7T.
• To assess the efficacy of parallel-transmit k_T-point pulses for FA shimming in 7T whole-liver imaging.

Main Methods:

• Determined optimal Fourier phase-encoding steps for PE-DREAM mapping on a 7T eight-channel parallel-transmission system.
• Performed FA shimming experiments in 7 healthy subjects, comparing k_T-point pulses with CP mode and phase shimming.
• Acquired 3D gradient-echo scans and assessed image homogeneity against estimated FA maps.

Main Results:

• k_T-point pulses yielded significantly lower coefficient of variation for estimated FA (7.4%) compared to phase shimming (18.8%) and CP (43.2%).
• k_T-point pulses delivered whole-liver images with nominal FA and superior homogeneity.
• CP and phase shimming resulted in suboptimal FA distributions and inconsistent signal intensity.

Conclusions:

• Homogeneous whole-liver 3D gradient-echo acquisitions at 7T are achievable using eight-channel k_T-point pulses.
• These pulses should be calculated based on subject-specific multichannel absolute Fourier PE-DREAM maps.
• This technique offers a significant improvement for high-field liver imaging.