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Related Concept Videos

NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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...
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

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...
2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)01:19

2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does...
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

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...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...

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Fat-Water Phantoms for Magnetic Resonance Imaging Validation: A Flexible and Scalable Protocol
07:59

Fat-Water Phantoms for Magnetic Resonance Imaging Validation: A Flexible and Scalable Protocol

Published on: September 7, 2018

Robustness of fat quantification using chemical shift imaging.

Katie H Hansen1, Michael E Schroeder, Gavin Hamilton

  • 1Department of Radiology, University of California San Diego, San Diego, CA 92103–8226, USA.

Magnetic Resonance Imaging
|November 8, 2011
PubMed
Summary
This summary is machine-generated.

Investigating fat quantification accuracy, this study found minimal impact from parameter changes in chemical shift imaging. Significant variations were linked to T1 bias and noise bias, not sequence adjustments.

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Area of Science:

  • Medical Imaging
  • Quantitative MRI

Background:

  • Accurate fat quantification is crucial for diagnosing and monitoring various medical conditions.
  • Chemical shift imaging (CSI) is a common MRI technique for fat measurement.
  • Understanding parameter influences is vital for reliable CSI results.

Purpose of the Study:

  • To assess the impact of systematic parameter variations on fat quantification accuracy using CSI.
  • To identify which imaging parameters most significantly affect fat fraction measurements.
  • To determine the causes of observed discrepancies in fat quantification.

Main Methods:

  • Spoiled gradient echo sequences were employed for chemical shift imaging.
  • Parameters systematically varied included 2D/3D acquisition, echo number, echo time, repetition time, flip angle, and others.
  • Fat fraction was calculated across all tested parameter combinations.

Main Results:

  • Most parameter variations showed no significant effect on fat fraction measurements.
  • A few parameters resulted in significant but small changes in fat fraction.
  • Observed significant changes were primarily attributed to T1 bias and noise bias.

Conclusions:

  • Chemical shift imaging is robust to many parameter variations for fat quantification.
  • T1 bias and noise bias are key factors influencing fat fraction accuracy.
  • Standardized protocols and bias correction are important for reliable MRI-based fat quantification.