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Related Concept Videos

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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NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences01:17

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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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Two-Dimensional (2D) NMR: Overview01:12

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The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse....
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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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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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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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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.
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Spatial or flow velocity phase encoding gradients in NMR imaging.

A Constantinesco, J J Mallet, A Bonmartin

    Magnetic Resonance Imaging
    |January 1, 1984
    PubMed
    Summary
    This summary is machine-generated.

    New Nuclear Magnetic Resonance (NMR) sequences use time-modulated field gradients to distinguish between stationary and flowing protons. This technique offers sensitive, quantifiable flow analysis unaffected by T2 relaxation times.

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    Area of Science:

    • Magnetic Resonance Imaging (MRI)
    • Biophysics
    • Fluid Dynamics

    Background:

    • Accurate characterization of proton behavior is crucial in Nuclear Magnetic Resonance (NMR) studies.
    • Distinguishing between stationary and mobile protons is essential for various applications, including medical imaging and materials science.
    • Existing methods may face limitations in sensitivity, directionality, or susceptibility to relaxation effects.

    Purpose of the Study:

    • To introduce novel time-modulated field gradient sequences for Nuclear Magnetic Resonance (NMR).
    • To demonstrate the capability of these sequences to selectively phase encode spatial location or flow velocity.
    • To differentiate between stationary and mobile protons under various flow conditions.

    Main Methods:

    • Development and application of time-modulated field gradient sequences in one-dimensional NMR.
    • Investigation of two flow conditions: constant velocity flow and simple harmonic flow superimposed on steady flow.
    • Utilizing specific modulated gradients to achieve selective phase encoding of spatial location and flow velocity.

    Main Results:

    • Demonstrated ability to discriminate between stationary and mobile protons using the proposed sequences.
    • Showcased effectiveness under both constant and complex (harmonic + steady) flow conditions.
    • Quantified stationary and flow parameters, highlighting the method's precision.

    Conclusions:

    • The described time-modulated field gradient sequences provide a powerful tool for flow analysis in NMR.
    • Phase modulation offers advantages including sensitivity to flow direction and independence from T2 relaxation.
    • The method allows for accurate quantification of both stationary and mobile proton populations.