Frequency-Offset Cartesian Feedback for MRI Power Amplifier Linearization

High-quality magnetic resonance imaging (MRI) requires precise control of the transmit radio-frequency (RF) field. In parallel excitation applications such as transmit SENSE, high RF power linearity is essential to cancel aliased excitations. In widely-employed class AB power amplifiers, gain compre...

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Bibliographic Details
Published in:IEEE transactions on medical imaging 2011-02, Vol.30 (2), p.512-522
Main Authors: Zanchi, Marta G, Stang, Pascal, Kerr, Adam, Pauly, John M, Scott, Greig C
Format: Article
Language:English
Subjects:
Bandwidth
Cartesian feedback
control systems
Couplers
Feedback
Fourier Analysis
Frequency control
Image Processing, Computer-Assisted - methods
linearization
Magnetic resonance imaging
magnetic resonance imaging (MRI)
Magnetic Resonance Imaging - instrumentation
Magnetic Resonance Imaging - methods
Mixers
Phantoms, Imaging
Power amplifiers
Radio frequency
radio-frequency (RF) power amplifiers
Signal Processing, Computer-Assisted
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Summary:High-quality magnetic resonance imaging (MRI) requires precise control of the transmit radio-frequency (RF) field. In parallel excitation applications such as transmit SENSE, high RF power linearity is essential to cancel aliased excitations. In widely-employed class AB power amplifiers, gain compression, cross-over distortion, memory effects, and thermal drift all distort the RF field modulation and can degrade image quality. Cartesian feedback (CF) linearization can mitigate these effects in MRI, if the quadrature mismatch and dc offset imperfections inherent in the architecture can be minimized. In this paper, we present a modified Cartesian feedback technique called "frequency-offset Cartesian feedback" (FOCF) that significantly reduces these problems. In the FOCF architecture, the feedback control is performed at a low intermediate frequency rather than dc, so that quadrature ghosts and dc errors are shifted outside the control bandwidth. FOCF linearization is demonstrated with a variety of typical MRI pulses. Simulation of the magnetization obtained with the Bloch equation demonstrates that high-fidelity RF reproduction can be obtained even with inexpensive class AB amplifiers. Finally, the enhanced RF fidelity of FOCF over CF is demonstrated with actual images obtained in a 1.5 T MRI system.
ISSN:0278-0062
1558-254X
DOI:10.1109/TMI.2010.2087768