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Related Concept Videos

NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
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NMR Spectrometers: Resolution and Error Correction01:14

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When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

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Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
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NMR Spectrometers: Overview01:20

NMR Spectrometers: Overview

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NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field...
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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

936
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
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Pure Shift Nuclear Magnetic Resonance: a New Tool for Plant Metabolomics
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Frequency drift in MR spectroscopy at 3T.

Steve C N Hui1, Mark Mikkelsen1, Helge J Zöllner1

  • 1Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, MD, USA; F. M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, USA.

Neuroimage
|July 27, 2021
PubMed
Summary
This summary is machine-generated.

Magnetic Resonance Imaging (MRI) scanners experience B0 field instability due to heating. This study benchmarked typical field drift during MR spectroscopy (MRS), finding significant drift after functional MRI (fMRI) that impacts spectral quality.

Keywords:
3TFrequency driftMagnetic resonance spectroscopy (MRS)Multi-siteMulti-vendorPress

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Spectroscopy
  • Medical Physics

Background:

  • Gradient coil and passive shim component heating causes B0 field instability in MRI scanners.
  • This instability is exacerbated by gradient-intensive sequences, impacting Magnetic Resonance Spectroscopy (MRS) data quality.

Purpose of the Study:

  • To establish a benchmark for typical B0 field drift during MRS acquisitions.
  • To evaluate the necessity of real-time field-frequency locking systems in MRI scanners.
  • To compare field drift data across multiple sites and vendors.

Main Methods:

  • A standardized protocol was used across 95 3T MRI scanners from three vendors.
  • Phantom water signals were acquired before and after a functional MRI (fMRI) sequence.
  • Frequency drift was analyzed, and simulated spectra visualized the impact on metabolites like NAA and GABA.

Main Results:

  • Median B0 field drift was under 1 Hz for short acquisitions but increased significantly after fMRI.
  • Post-fMRI drift affected spectral intensity, reducing NAA singlet intensity by up to 44% across vendors.
  • Drift rates decreased substantially over time, reaching 0.03 Hz/min after three hours.

Conclusions:

  • While median drift is low, extreme cases and post-fMRI drift can compromise MRS data integrity.
  • The observed drift highlights the potential need for real-time field-frequency stabilization in demanding MRI protocols.
  • Scanner performance regarding drift varies, necessitating further investigation into vendor-specific behaviors.