LitePoint Corporation, CA 94085, USA.
IEEE Trans Med Imaging. 2011 Feb;30(2):512-22. doi: 10.1109/TMI.2010.2087768. Epub 2010 Oct 18.
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.
高质量的磁共振成像（MRI）需要精确控制发射射频（RF）场。在并行激发应用中，如发射 SENSE，高 RF 功率线性度对于消除混叠激发至关重要。在广泛应用的 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扭曲 RF 场调制，并降低图像质量。如果能够最小化架构固有的正交失配和直流偏移缺陷，那么笛卡尔反馈（CF）线性化技术可用于减轻这些影响。在本文中，我们提出了一种名为“频率偏移笛卡尔反馈”（FOCF）的改进笛卡尔反馈技术，它可以显著减少这些问题。在 FOCF 架构中，反馈控制在较低的中频而不是直流进行，因此正交鬼影和直流误差被移到控制带宽之外。采用各种典型的 MRI 脉冲对 FOCF 线性化进行了验证。通过 Bloch 方程模拟得到的磁化强度表明，即使使用廉价的 AB 类放大器，也可以获得高保真度的 RF 再现。最后，通过在 1.5T MRI 系统中获得的实际图像，证明了 FOCF 相对于 CF 的增强 RF 保真度。
