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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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In integrated circuit technology, a capacitance multiplier is often utilized to produce a larger capacitance value when a small physical capacitance falls short. This is achieved by a circuit that multiplies capacitance values by a factor of up to 1000, such that a 10-pF capacitor can replicate the performance of a 100-nF capacitor.
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In signal processing, bandpass sampling is an effective technique for sampling signals that have most of their energy concentrated within a narrow frequency band. This type of signal is known as a bandpass signal. The key principle of bandpass sampling involves sampling the signal at a rate that is greater than twice the signal's bandwidth to prevent aliasing.
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The parallel RLC circuit is an arrangement where the resistor (R), inductor (L), and capacitor (C) are all connected to the same nodes and, as a result, share the same voltage across them. The parallel RLC circuit is analyzed in terms of admittance (Y), which reflects the ease with which current can flow. The admittance is given by:
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Series resonance occurs in a circuit containing inductive (L), capacitive (C), and resistive (R) elements connected sequentially. At the resonance frequency, the inductive and capacitive reactances are equal in magnitude but opposite in sign, effectively canceling each other. This causes the circuit's impedance is minimal, primarily determined by the resistance R. The resonant frequency of an RLC circuit is defined as:
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Generation and Coherent Control of Pulsed Quantum Frequency Combs
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Optimized amplitude modulated multiband RF pulse design.

Samy Abo Seada1, Anthony N Price1, Joseph V Hajnal1

  • 1Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom.

Magnetic Resonance in Medicine
|January 19, 2017
PubMed
Summary
This summary is machine-generated.

Amplitude modulation (AM) multiband pulses can be designed with minimal duration increases. This technique avoids errors from rapid radiofrequency waveform modulation, improving MRI precision.

Keywords:
RF pulse designexcitation stabilityinterslice artifactmultiband pulse designroot flippingsimultaneous multislice

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Area of Science:

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

Background:

  • Multiband pulses feature complex temporal modulations.
  • Rapid phase or frequency modulation in RF pulses can cause errors.
  • Amplitude modulation (AM) offers a potential solution by simplifying waveforms.

Purpose of the Study:

  • To modify existing multiband pulse design techniques for AM waveform generation.
  • To evaluate the performance and characteristics of AM multiband pulses.

Main Methods:

  • Adapted phase-optimization, time-shifting, and root-flipping techniques for AM pulse design.
  • Constrained designs by exploiting conjugate symmetry in the frequency domain.
  • Compared AM and unconstrained pulses across various parameters and validated with phantom/in vivo experiments.

Main Results:

  • AM pulses demonstrated precise results, avoiding artifacts seen with unconstrained designs.
  • AM pulses exhibited slightly longer durations compared to unconstrained versions.
  • Duration increases for AM pulses were 26% (phase-optimizing), 38% (time-shifting), and 20% (root-flipping) on average.

Conclusions:

  • AM multiband pulses are feasible with a modest increase in pulse duration.
  • This approach enhances the reliability of RF pulse generation in MRI.