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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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Frequency-offset Cartesian feedback for MRI power amplifier linearization.

Marta G Zanchi1, Pascal Stang, Adam Kerr

  • 1LitePoint Corporation, CA 94085, USA. mgzanchi@stanfordalumni.org

IEEE Transactions on Medical Imaging
|October 21, 2010
PubMed
Summary
This summary is machine-generated.

Frequency-offset Cartesian feedback (FOCF) improves magnetic resonance imaging (MRI) RF field control. This novel technique enhances RF power linearity, leading to higher quality MRI scans even with basic amplifiers.

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Area of Science:

  • Medical Imaging
  • Electrical Engineering
  • Physics

Background:

  • High-quality magnetic resonance imaging (MRI) relies on precise radio-frequency (RF) field control.
  • Parallel excitation techniques like transmit SENSE demand high RF power linearity for accurate aliased excitation cancellation.
  • Conventional class AB power amplifiers introduce distortions (gain compression, crossover distortion, etc.) that degrade MRI image quality.

Purpose of the Study:

  • To introduce and validate a modified Cartesian feedback (CF) technique, termed frequency-offset Cartesian feedback (FOCF), for improved RF field control in MRI.
  • To address the limitations of existing CF linearization methods, specifically quadrature mismatch and DC offset imperfections.
  • To demonstrate the effectiveness of FOCF in enhancing RF power linearity and MRI image quality.

Main Methods:

  • Developed a modified Cartesian feedback architecture (FOCF) where feedback control operates at a low intermediate frequency instead of DC.
  • Simulated RF reproduction using the Bloch equation to assess FOCF performance with various MRI pulses.
  • Implemented and tested FOCF on a 1.5 T MRI system, comparing its performance against standard CF linearization.

Main Results:

  • The FOCF architecture effectively shifts quadrature ghosts and DC errors outside the control bandwidth, mitigating inherent CF imperfections.
  • Bloch equation simulations confirmed that FOCF achieves high-fidelity RF reproduction, even with cost-effective class AB amplifiers.
  • Experimental results on a 1.5 T MRI system demonstrated superior RF fidelity of FOCF compared to standard CF, leading to enhanced image quality.

Conclusions:

  • Frequency-offset Cartesian feedback (FOCF) offers a significant improvement over traditional Cartesian feedback for MRI RF field linearization.
  • FOCF enables high-fidelity RF reproduction, crucial for advanced MRI applications like parallel excitation, using less complex power amplifiers.
  • The demonstrated enhancement in RF fidelity translates to improved image quality in practical MRI systems.