Phase-corrected bipolar gradients in multi-echo gradient-echo sequences for quantitative susceptibility mapping.

Area of Science:

• Medical Imaging
• Biophysics
• Magnetic Resonance Imaging

Background:

• Current multi-echo gradient-echo (GRE) sequences for quantitative susceptibility mapping (QSM) utilize unipolar readout gradients with large echo spacing, limiting acquisition efficiency.
• Quantitative susceptibility mapping (QSM) is crucial for visualizing and quantifying magnetic susceptibility in biological tissues.

Purpose of the Study:

• To investigate the use of bipolar readout gradients in multi-echo GRE sequences to improve acquisition efficiency for QSM.
• To address and correct for phase discrepancies introduced by bipolar gradients.

Main Methods:

• Phase discrepancies between odd and even echoes in bipolar readout gradients were measured and modeled using spatial polynomials.
• A linear phase correction was applied to compensate for non-ideal gradient behaviors.
• The bipolar approach was compared to the conventional unipolar approach for QSM in the human brain.

Main Results:

• Phase discrepancies were found to be approximately constant along the phase encoding direction and linear along the readout and slice-selection directions.
• A simple linear phase correction enabled accurate QSM of the human brain using the bipolar multi-echo GRE sequence.
• Bipolar multi-echo acquisition achieved QSM in good quantitative agreement with unipolar acquisition and demonstrated reduced noise.

Conclusions:

• Bipolar readout gradients can be effectively implemented in multi-echo GRE sequences for QSM.
• Linear phase correction between odd-even echoes is sufficient to overcome challenges associated with bipolar gradients.
• This approach enhances acquisition efficiency and image quality in QSM.