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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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Magnetic Resonance Imaging01:24

Magnetic Resonance Imaging

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Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device in clinical use by the early 1980s. The early MRI...
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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

1.2K
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...
1.2K
Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

3.1K
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...
3.1K
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

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Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral or enantiotopic and are related by a mirror plane. Enantiotopic protons are chemically equivalent in an achiral environment. Because most proton NMR spectra are recorded using achiral solvents, enantiotopic hydrogens yield a single signal.
In chiral compounds such as 2-butanol, replacing the methylene hydrogens at C3 produces a pair of...
3.1K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

4.0K
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
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Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease
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T1 -corrected quantitative chemical shift-encoded MRI.

Xiaoke Wang1,2, Timothy J Colgan1, Louis A Hinshaw1,2

  • 1Department of Radiology, University of Wisconsin, Madison, Wisconsin.

Magnetic Resonance in Medicine
|November 15, 2019
PubMed
Summary
This summary is machine-generated.

A new MRI method improves liver fat quantification by correcting for T1 effects, reducing bias and enhancing accuracy. This variable flip angle-chemical-shift encoded MRI (VFA-CSE-MRI) shows promise for precise proton density fat-fraction (PDFF) measurements.

Keywords:
T1 biasT1 correctionchemical-shift encoded imagingfat quantificationhepatic steatosisliver fatmagnetic resonance imagingproton density fat-fraction

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Area of Science:

  • Medical Imaging
  • Magnetic Resonance Imaging
  • Quantitative MRI

Background:

  • Proton density fat-fraction (PDFF) quantification is crucial for assessing liver conditions.
  • Traditional chemical-shift encoded MRI (CSE-MRI) methods can be susceptible to noise and bias, particularly T1-related inaccuracies.
  • Improving the accuracy and reliability of PDFF measurements is essential for clinical diagnosis and research.

Purpose of the Study:

  • To develop and validate a T1-corrected CSE-MRI technique for improved PDFF quantification.
  • To enhance noise performance and reduce bias in liver fat fraction measurements.
  • To assess the feasibility of the new method in a clinical setting.

Main Methods:

  • Development of a variable flip angle (VFA)-CSE-MRI method with joint-fit reconstruction.
  • Evaluation using computer simulations and phantom experiments to investigate bias sources.
  • Comparison of noise performance between VFA-CSE-MRI and low flip angle (LFA)-CSE-MRI.
  • Prospective pilot study in patients undergoing liver MRI to quantify PDFF before and after contrast administration.

Main Results:

  • VFA-CSE-MRI demonstrated accuracy and insensitivity to B1 inhomogeneities in simulations and phantoms.
  • Joint-fit reconstruction eliminated T1-related bias and offered marginal noise improvement compared to LFA-CSE-MRI.
  • Pilot study in 25 patients showed strong correlation and agreement between VFA-CSE-MRI and LFA-CSE-MRI PDFF measurements pre- and post-contrast.
  • High R-squared values (0.97 and 0.93) and good agreement were observed in patient data.

Conclusions:

  • Joint-fit VFA-CSE-MRI is a feasible method for T1-corrected PDFF quantification in the liver.
  • The method is robust against B1 inhomogeneities and effectively eliminates T1 bias.
  • While offering marginal SNR advantages for typical liver T1 values, it significantly improves quantitative accuracy.