Three-dimensional mapping of the B1 field using an optimized phase-based method: application to hyperpolarized 3He in...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Medical Physics
• Radiologic Technology

Background:

• Accurate three-dimensional (3D) mapping of the B(1) field is crucial for quantitative MRI.
• Existing methods using composite pulses can lead to excessive radio-frequency power deposition and long acquisition times.
• Limitations include increased Specific Absorption Rate (SAR) and extended repetition times (TRs), exceeding single breath-hold capabilities.

Purpose of the Study:

• To develop a novel, highly sensitive 3D B(1) field mapping technique for MR coils.
• To enable rapid B(1) mapping suitable for hyperpolarized (3)He lung imaging within a single breath-hold.
• To improve sensitivity to radio-frequency inhomogeneities compared to existing methods.

Main Methods:

• A modified radio-frequency (RF) pulse sequence scheme was developed, altering RF excitation phases.
• The new method utilizes a 3D gradient recall echo with a modified composite pulse for excitation.
• Validation involved computer simulations, phantom studies using (1)H MRI, and in vivo imaging of hyperpolarized (3)He in human lungs at 1.5T.

Main Results:

• The novel method demonstrated up to a 10-fold increase in sensitivity to RF inhomogeneities.
• The total pulse duration was significantly reduced, allowing for 3D B(1) mapping within a single breath-hold.
• In vivo results with hyperpolarized (3)He in human lungs confirmed the feasibility and effectiveness of the technique.

Conclusions:

• The developed method offers a significant improvement for 3D B(1) field mapping, particularly for lung MRI.
• This technique enhances accuracy and efficiency, enabling faster quantitative MRI protocols.
• The increased sensitivity and reduced scan time make it suitable for clinical applications, especially with hyperpolarized gases.