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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.

Double Resonance Techniques: Overview

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...

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Composite pulses for RF phase encoded MRI: A simulation study.

Somaie Salajeghe¹, Paul Babyn², Gordon E Sarty³

¹Division of Biomedical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

Magnetic Resonance Imaging

|October 17, 2016

Summary

This summary is machine-generated.

Composite pulses can reduce magnetic field errors in MRI. An asymmetric composite pulse scheme effectively minimizes artifacts caused by inhomogeneous B1 fields, improving image quality.

Keywords:

Composite pulses Low-field MRI Portable MRI RF encoding

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Area of Science:

Magnetic Resonance Imaging (MRI)
Radiofrequency (RF) Coil Technology
Pulse Sequence Design

Background:

Non-linear RF encoding coils in B1-encoded MRI cause inhomogeneous B1 fields and spatially dependent flip angles.
While B1 phase non-linearity is correctable, B1 inhomogeneity persists as a challenge.
Composite pulses can mitigate B1 inhomogeneity effects in conventional B0-encoded MRI.

Purpose of the Study:

To assess the feasibility of using composite pulses with non-linear RF phase encoding coils.
To identify the optimal composite pulse scheme for mitigating B1 inhomogeneity in MRI.

Main Methods:

Simulated RF-encoded signals using the Bloch equation for various composite pulse types (symmetric, asymmetric, antisymmetric).
Reconstructed simulated signals employing a constrained least squares method.

Main Results:

Root mean square reconstruction errors ranged from 6% for asymmetric composite pulses to 9.7% for antisymmetric ones.
Asymmetric composite pulse schemes demonstrated superior performance in reducing artifacts.

Conclusions:

Asymmetric composite pulse schemes are optimal for reducing spatially varying flip angle effects in inhomogeneous B0 and B1 fields.
This finding contrasts with previous conclusions favoring antisymmetric pulses for conventional B0-encoded MRI spin echo sequences.