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Related Concept Videos

Atomic Nuclei: Types of Nuclear Relaxation01:28

Atomic Nuclei: Types of Nuclear Relaxation

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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers...
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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

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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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Atomic Nuclei: Magnetic Resonance01:05

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The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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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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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.
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Updated: May 12, 2025

High-resolution Structural Magnetic Resonance Imaging of the Human Subcortex In Vivo and Postmortem
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In Vivo Characterization of Magnetic Inclusions in the Subcortex From Nonexponential Transverse Relaxation Decay.

Rita Oliveira1, Quentin Raynaud1, Ileana Jelescu2

  • 1Laboratory for Research in Neuroimaging, Department of Clinical Neuroscience, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland.

NMR in Biomedicine
|May 6, 2025
PubMed
Summary
This summary is machine-generated.

Nonexponential MRI signal decay reveals magnetic inclusions in human brain tissue. This advanced technique offers new insights into brain microstructure and potential applications for diseases like Parkinson's.

Keywords:
brain irondopaminergic neuronsnonexponential decayquantitative MRIrelaxometrysubstantia nigratransverse relaxation

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Area of Science:

  • Neuroimaging
  • Biophysics
  • Magnetic Resonance Imaging (MRI)

Background:

  • Magnetic Resonance Imaging (MRI) signal decay in brain tissue is influenced by magnetic inclusions like blood vessels and iron-rich cells.
  • Theoretical models predict a transition from Gaussian to exponential decay with increasing echo times.
  • Standard gradient-echo MRI typically captures only the long-time exponential decay, limiting information about inclusions.

Purpose of the Study:

  • To provide experimental evidence of nonexponential transverse relaxation decay in human subcortical grey matter using in vivo MRI.
  • To characterize magnetic inclusions within brain tissue by analyzing MRI signal decay patterns.
  • To explore the potential of nonexponential decay analysis for understanding brain microstructure and disease pathologies.

Main Methods:

  • Acquired in vivo 3 Tesla MRI data with short interecho spacings and minimal echo time (1.25 ms).
  • Employed novel acquisition strategies to minimize motion and cardiac-induced artifacts.
  • Fitted the acquired gradient-echo data using both exponential and nonexponential models to assess signal decay behavior.

Main Results:

  • Experimental data confirmed nonexponential transverse relaxation decay in human subcortical grey matter, deviating from the expected exponential behavior.
  • Nonexponential models provided significantly superior fits to the MRI signal decay compared to exponential models.
  • The substantia nigra and globus pallidus showed the most pronounced deviations from exponential decay, correlating with non-heme iron concentrations.

Conclusions:

  • Nonexponential transverse relaxation decay in MRI provides a more specific characterization of the spatial distribution of magnetic material in subcortical tissues.
  • Analysis of nonexponential decay can estimate properties of magnetic inclusions, such as magnetic susceptibility, volume fraction, and size (~2.4 μm).
  • This technique holds significant potential for applications in diagnosing and monitoring neurodegenerative diseases like Parkinson's disease.