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Selective excitation localized by the Bloch-Siegert shift and a gradient.

Jonathan B Martin^1,2, Sai Abitha Srinivas^1,2, Christopher E Vaughn^1,2

¹Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA.

Magnetic Resonance in Medicine

|April 25, 2022

Summary

Researchers developed a novel radiofrequency pulse technique for gradient-free selective excitation in MRI. This method utilizes the Bloch-Siegert shift for precise spatial localization, improving slice and slab excitation in magnetic resonance imaging.

Keywords:

Bloch-Siegert shift RF pulse design RF-encoded MRI low-field MRI multiphoton selective RF excitation

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Area of Science:

Magnetic Resonance Imaging (MRI)
Radiofrequency (RF) Pulse Design
Spatial Localization Techniques

Background:

Conventional MRI techniques often rely on magnetic field gradients for spatial encoding.
Achieving selective excitation without gradients presents a challenge in MRI pulse sequence development.
The Bloch-Siegert shift, a phenomenon in nuclear magnetic resonance, offers potential for novel excitation strategies.

Purpose of the Study:

To develop and evaluate a method for gradient-free selective excitation in MRI using the Bloch-Siegert shift.
To achieve spatial localization of magnetic resonance signals without relying on magnetic field gradients.
To enable precise excitation of slices or slabs in radiofrequency gradient-encoded MRI.

Main Methods:

A novel radiofrequency (RF) pulse was designed by summing two component pulses: one inducing a Bloch-Siegert shift and another providing frequency selectivity.
The Shinnar-Le Roux algorithm was employed for precise control over the magnetization profile.
Simulations assessed pulse robustness to off-resonance conditions and suitability for multi-echo spin echo sequences.
Experimental validation was performed on a 47.5 mT MRI scanner.

Main Results:

The proposed RF pulses successfully achieved selective excitation with desired profile characteristics, even with corrections for varying field strengths.
The pulses demonstrated robustness against off-resonance effects and radiofrequency amplifier distortions.
Experimental results closely matched simulation predictions, confirming the technique's efficacy.
The pulses proved suitable for multi-echo spin echo pulse sequences.

Conclusions:

The Bloch-Siegert shift can be effectively utilized to perform gradient-free selective excitation in MRI.
This technique enables the excitation of specific slices or slabs without the need for magnetic field gradients.
The developed method offers a promising alternative for spatial localization in radiofrequency gradient-encoded MRI.