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NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.

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Multiparametric Optical Mapping of the Langendorff-perfused Rabbit Heart

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Published on: September 13, 2011

Rapid B1 mapping using orthogonal, equal-amplitude radio-frequency pulses.

Yulin V Chang¹

¹Mechanical Engineering and Materials Science, Washington University, St. Louis, Missouri 63130, USA. yc3z@virginia.edu

Magnetic Resonance in Medicine

|June 30, 2011

Summary

This study introduces a novel phase-based method for rapid 3D radio-frequency field (B(1)) mapping. The technique offers a simple, robust, and quick alternative to existing B(1) mapping methods, especially for challenging imaging scenarios.

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Area of Science:

Magnetic Resonance Imaging
Radio-Frequency Physics

Background:

Accurate mapping of the radio-frequency (B(1)) field is crucial for quantitative MRI.
Existing B(1) mapping techniques can be time-consuming or unsuitable for certain applications.

Purpose of the Study:

To develop and validate a new, rapid, and robust phase-based method for 3D B(1) field mapping.
To demonstrate the method's applicability in scenarios where magnitude-based methods are less effective.

Main Methods:

Exploited the noncommutation relation between orthogonal rotations.
Implemented a sequence by adding a specific radio-frequency pulse to a standard 3D gradient-echo sequence.
Focused on flip angles (FA) less than 90°, particularly up to 60°.

Main Results:

Achieved reliable 3D B(1) mapping within 1 minute using flip angles up to 60°.
Demonstrated robustness against T(1) variations, chemical shift, and mild magnetic field inhomogeneity.
Established a simple relationship between final magnetization phase and flip angle.

Conclusions:

The proposed phase-based method provides a rapid and simple approach for B(1) mapping.
This technique is particularly advantageous for applications like long-T(1) imaging and hyperpolarized-gas imaging.
The method offers a valuable alternative for B(1) mapping where conventional techniques are limited.