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Related Concept Videos

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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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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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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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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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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Gradient Echo Quantum Memory in Warm Atomic Vapor
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Radiofrequency pulse design using nonlinear gradient magnetic fields.

Emre Kopanoglu1, R Todd Constable1,2,3

  • 1Department of Diagnostic Radiology, Yale University, New Haven, Connecticut, USA.

Magnetic Resonance in Medicine
|September 10, 2014
PubMed
Summary
This summary is machine-generated.

This study introduces a new iterative method for designing radiofrequency (RF) pulse and k-space trajectories, improving magnetic resonance imaging (MRI) excitation accuracy with nonlinear gradient fields.

Keywords:
RF pulse designhigh order gradient fieldsk-space trajectory designmultidimensional excitationnonlinear gradient fields

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Pulse Sequence Design
  • Gradient Field Engineering

Background:

  • Nonlinear gradient magnetic fields in MRI can cause flip angle variations in excitation profiles.
  • Accurate spatial encoding is crucial for high-fidelity MR imaging.

Purpose of the Study:

  • To propose an iterative method for designing k-space trajectories and RF pulses tailored for nonlinear gradient fields.
  • To enhance excitation fidelity in MRI.

Main Methods:

  • Utilized a matching pursuit algorithm to select spatial encoding functions (SEFs) and a conjugate gradient algorithm to design RF pulses.
  • Developed three variants of the method: full, computationally cheaper, and spoke-based trajectory design.
  • Validated the approach through simulations and phantom experiments.

Main Results:

  • The proposed iterative method demonstrated increased excitation fidelity compared to existing iterative and noniterative techniques.
  • Performance was evaluated against uniform density spiral and EPI trajectories.

Conclusions:

  • An effective iterative method for designing k-space trajectories and RF pulses in the presence of nonlinear gradient fields has been developed.
  • The method offers flexibility in guiding trajectory design or optimizing specific trajectories.