Frequency-Offset Cartesian Feedback for MRI Power Amplifier Linearization
Frequency-Offset Cartesian Feedback for MRI Power Amplifier Linearization
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DOI：
10.1109/tmi.2010.2087768
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发表时间：
2011-02-01
影响因子：
10.6
通讯作者：
Scott, Greig C.
中科院分区：
文献类型：
--
作者：
Zanchi, Marta G.;Stang, Pascal;Scott, Greig C.
高质量的磁共振成像（MRI）需要精确控制发射射频（RF）场。在发射SENSE等并行激励应用中，高RF功率线性度对于消除混叠激励至关重要。在广泛使用的AB类功率放大器中，增益压缩、交越失真、记忆效应和热漂移都会使RF场调制失真，并会降低图像质量。如果架构中固有的正交失配和直流失调缺陷可以最小化，则笛卡尔反馈（CF）线性化可以减轻MRI中的这些影响。在本文中，我们提出了一种修改后的笛卡尔反馈技术称为“频率偏移笛卡尔反馈”（FOCF），显着减少这些问题。在FOCF架构中，反馈控制是在低中频而不是直流频率下执行的，因此正交重影和直流误差被移到控制带宽之外。FOCF线性化被证明与各种典型的MRI脉冲。用布洛赫方程得到的磁化强度的模拟表明，即使使用廉价的AB类放大器也可以获得高保真RF再现。最后，在1.5 T MRI系统中获得的实际图像证明了FOCF比CF增强的RF保真度。
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.