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Related Concept Videos

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences01:17

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

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.
NMR Spectrometers: Overview01:20

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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...
Standing Waves in a Cavity01:28

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A household microwave and lasers are examples of standing electromagnetic waves in a cavity. When two conducting metal plates are placed parallel at the nodal planes, it creates a cavity where standing waves are formed. The cavity between the two planes is analogous to a stretched string held at the points x = 0 and x = L. Here, the distance 'L' between the two planes must be an integer multiple of half of the wavelength. The wavelengths that satisfy this condition are given by:
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
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Generating Electromagnetic Radiations01:10

Generating Electromagnetic Radiations

The German physicist Heinrich Hertz (1857–1894) was the first to generate and detect certain types of electromagnetic waves in the laboratory. Starting in 1887, he performed a series of experiments that confirmed the existence of electromagnetic waves and verified that they travel at the speed of light. Hertz used an alternating-current RLC (resistor-inductor-capacitor) circuit that resonated at a known frequency and connected it to a loop of wire. High voltages induced across the gap in the...
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.
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Hyperpolarized Xenon for NMR and MRI Applications
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Using time-reversal to generate generalized transversely localized transient waves (X-waves).

S C Walker1

  • 1Marine Physical Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0238, USA. shane@mpl.ucsd.edu

The Journal of the Acoustical Society of America
|March 12, 2009
PubMed
Summary
This summary is machine-generated.

This study demonstrates that causal X-waves can be generated using time-reversal methods with a simple line transducer array. This approach offers a simpler way to create these unique wave phenomena, even in complex environments.

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Area of Science:

  • Acoustics
  • Wave Physics
  • Signal Processing

Background:

  • Traditional X-wave synthesis ignores boundary conditions, leading to acausal solutions.
  • Causal X-waves are linked to physical sources and the inhomogeneous wave equation.

Purpose of the Study:

  • To demonstrate a causal X-wave generation method using time-reversal.
  • To show that this method can be experimentally validated with a simple transducer setup.

Main Methods:

  • Solving the inhomogeneous scalar wave equation for acoustic fields from supersonic sources.
  • Utilizing the time-reversal of acoustic fields generated by a physical source.
  • Experimentally validating the method with a line transducer array in a 2D free space.

Main Results:

  • A causal X-wave is defined by the solution to the inhomogeneous scalar wave equation consistent with a Mach front.
  • Time-reversal of a planar aperture's field can generate X-waves.
  • Experimental validation confirms the generation of approximate acoustic X-waves using a line transducer array.

Conclusions:

  • Time-reversal offers a novel and simpler method for generating causal X-waves.
  • This technique is adaptable for use in inhomogeneous media.
  • Experimental results validate the feasibility of this approach using basic equipment.