B0 mapping with multi-channel RF coils at high field.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Medical Physics
• Image Processing

Background:

• Static magnetic field mapping using gradient echo scans is a standard MRI technique.
• Combining phase data from multi-channel coils and denoising field maps are critical for high-field applications.
• Existing methods for combining multi-channel phase data have limitations.

Purpose of the Study:

• To evaluate three methods for combining phase images from multi-channel MRI coils without a body coil.
• To introduce and validate a novel method for calculating separate field maps per channel.
• To propose a formulation for correcting echo-planar imaging distortions at high fields.

Main Methods:

• Tested Hermitian product, phase-matching, and a new separate channel calculation method for combining phase data.
• Evaluated field map quality, signal-to-noise ratio, and unwrapping errors across 8-32 channel coils at 3T, 4T, and 7T.
• Developed a method to reduce local field map gradients for distortion correction.

Main Results:

• The separate channel method produced field maps with higher signal-to-noise ratio and fewer unwrapping errors compared to other methods.
• Standard deviation over channels effectively identified unreliable voxels, serving as a superior denoising technique.
• The proposed formulation effectively reduced echo-planar imaging distortions.

Conclusions:

• Calculating separate field maps per channel is a superior method for combining multi-channel MRI phase data, especially at high fields.
• This approach enhances the accuracy and reliability of static magnetic field mapping.
• The developed techniques and freely available MATLAB toolbox enable effective correction of echo-planar imaging distortions.