Fast radiofrequency flip angle calibration by Bloch-Siegert shift.

Area of Science:

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

Background:

• Accurate radiofrequency (RF) transmit calibration is crucial for quantitative MRI.
• Existing methods can be time-consuming or less precise.
• The Bloch-Siegert shift offers a novel physical principle for B1+ field mapping.

Purpose of the Study:

• To apply a novel Bloch-Siegert shift-based method for automated, fast radiofrequency transmit gain calibration in MRI.
• To validate the accuracy of this calibration method for predicting flip angles.
• To demonstrate a robust and rapid implementation for clinical relevance.

Main Methods:

• Integration of two off-resonance RF pulses into a slice-selective spin echo sequence.
• Utilizing the Bloch-Siegert phase shift, proportional to B1 field squared, for signal modulation.
• Spatial localization via readout gradient and calculation of signal-weighted average B1 field.
• Calibration of system transmit gain to average flip angle for precise RF pulse energy prediction.

Main Results:

• Demonstration of a robust implementation with a scan time of only 3 seconds.
• Successful prediction of transmit gain for a 90° RF pulse.
• In vivo validation in 32 volunteers yielded an average flip angle of 88.6 ± 3.42°.

Conclusions:

• The Bloch-Siegert shift-based method provides a fast and accurate approach for radiofrequency transmit gain calibration in MRI.
• This technique enhances the precision of flip angle control, crucial for quantitative imaging.
• The 3-second calibration time significantly improves efficiency for potential clinical applications.