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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.
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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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Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease
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Spin-echo MRI using pi/2 and pi hyperbolic secant pulses.

Jang-Yeon Park1, Michael Garwood

  • 1The Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA. jypark@cmrr.umn.edu

Magnetic Resonance in Medicine
|December 20, 2008
PubMed
Summary
This summary is machine-generated.

Frequency-modulated (FM) pulses, like hyperbolic secant (HS) pulses, offer advantages for magnetic resonance imaging (MRI). This study details methods to overcome their nonlinear phase issues in spin-echo sequences for improved multislice imaging.

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Pulse Sequence Design
  • Radiofrequency (RF) Engineering

Background:

  • Frequency-modulated (FM) pulses, including hyperbolic secant (HS) pulses, provide broadband pi rotation beneficial for spin-echo experiments, especially under inhomogeneous radiofrequency (RF) fields.
  • The nonlinear phase of transverse magnetization induced by FM pulses limits their application in multislice spin-echo MRI.
  • Conventional spin-echo sequences often avoid FM pulses due to phase complexities.

Purpose of the Study:

  • To develop a general theory and methods for conventional spin-echo imaging using a pi hyperbolic secant (HS) pulse for refocusing.
  • To analytically describe the phase profiles generated by the HS pulse.
  • To establish relationships between pulse parameters and gradients for compensating nonlinear phase variations in spin-echo sequences.

Main Methods:

  • Analytical description of phase profiles produced by a pi HS pulse.
  • Extension of analysis to define relationships between pulse parameters and gradients for phase compensation.
  • Development of a spin-echo sequence using pi/2 and pi HS pulses (pi/2 HS - pi HS sequence).

Main Results:

  • Analytical phase profiles for the HS pulse were derived.
  • Specific relationships between pulse parameters and gradients were identified to compensate for nonlinear phase variations.
  • The pi/2 HS - pi HS sequence demonstrated advantages for multislice spin-echo MRI, including superior slice selection and partial RF inhomogeneity compensation.
  • This sequence offers shorter echo times and lower power deposition compared to previous methods using paired pi HS pulses.

Conclusions:

  • The developed theory and methods enable the effective use of hyperbolic secant (HS) pulses in spin-echo MRI.
  • The pi/2 HS - pi HS sequence provides practical advantages for multislice spin-echo MRI, enhancing performance and efficiency.
  • This approach overcomes limitations associated with nonlinear phase variations in FM pulse applications for MRI.