Optically controlled on-coil amplifier with RF monitoring feedback

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Abstract

<div class="section"> <a class="named-anchor" id="S1">  </a> <h5 class="section-title" id="d3125407e133">Purpose</h5> <p id="P1">To develop a new optically controlled on-coil amplifier that facilitates safe use of multi-channel RF transmission in MRI by real-time monitoring of signal phase and amplitude. </p> </div><div class="section"> <a class="named-anchor" id="S2">  </a> <h5 class="section-title" id="d3125407e138">Methods</h5> <p id="P2">Monitoring was performed with a 4-channel prototype system by sensing, down sampling, digitizing and optically transmitting the RF transmit signal to a remote PC to control the amplifiers. Performance was evaluated with benchtop and 7T MRI experiments. </p> </div><div class="section"> <a class="named-anchor" id="S3">  </a> <h5 class="section-title" id="d3125407e143">Results</h5> <p id="P3">Monitored amplitude and phase were stable across repetitions and had standard deviations of 0.061 μT and 0.0073 rad respectively. The feedback system allowed inter-channel phase and <i>B <sub>1</sub> </i> amplitude to be adjusted within two iterations. MRI experiments demonstrated the feasibility of this approach to perform safe and accurate multi-channel RF transmission and monitoring at high field. </p> </div><div class="section"> <a class="named-anchor" id="S4">  </a> <h5 class="section-title" id="d3125407e154">Conclusion</h5> <p id="P4">We demonstrated a 4-channel transceiver system based on optically controlled on-coil amplifiers with RF signal monitoring and feedback control. The approach allows the safe and precise control of RF transmission fields, required to achieve uniform excitation at high field. </p> </div>

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Most cited references 19

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Single-Sideband Transmission by Envelope Elimination and Restoration

Leonard Kahn (1952)

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Exploring the limits of RF shimming for high-field MRI of the human head.

Michael Smith, Christopher M Collins, Weihua Mao (2006)

Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. The adjustment of these drive magnitudes and phases alone to create uniform RF magnetic (B(1)) fields has been called RF shimming, and has certain limits at every frequency as dictated by possible solutions to Maxwell's equations. Here we use numerical calculations to explore the limits of RF shimming in the human head. We found that a 16-element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz, while an 80-element array can shim the whole brain at up to 600 MHz.

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Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil.

Thomas Witzel, Franz Schmitt, V Alagappan … (2008)

Slice-selective RF waveforms that mitigate severe B1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B1+ on the excitation FA. The optimized parallel RF pulses for human B1+ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in-plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse-duration penalty. (c) 2008 Wiley-Liss, Inc.