System Components & Field Control

Thursday 15 May 2014

David Otto Brunner¹, Markus Weiger¹, Thomas Schmid¹, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

TR switches with very short transients have become crucial for the application of high bandwidth, ultra-short T₂^* imaging techniques such as UTE, SWIFT or ZTE. However, to speed-up the switching of inherently slow PIN diodes comparably high reverse voltages and switching currents have to be applied. This in turn can produce so high spikes on the RF lines that the preamplifier shows significant recovery transients. In this work we show a novel TR switch topology based on a double balanced biasing scheme which allows inherently decoupling the RF lines and the biasing signals and removing all major choke inductances from the bias lines.

Xueming Cao¹, Maxim Zaitsev¹, Jürgen Hennig¹, Jan G Korvink², Oliver Gruschke², and Elmar Fischer^1
1Department of Radiology, University Medical Center Freiburg, Freiburg, Germany, ²Institute of Microsystem Technology, Freiburg, Germany

As the coil becomes increasingly smaller and higher gain is required for preamplifiers. However, the stability of the preamplifier conflict with the high gain and the possibilities of oscillation in the preamplifier improve. The unstable performance may also because of the different body noise resistance of the coils. Here, we present some methods that stabilize an unstable two stage preamplifier, while the high gain and other parameters are kept.

Stephen Dodd¹, Chunqi Qian¹, Joseph Murphy-Boesch¹, and Alan Koretsky^1
1Laboratory of Functional and Molecular Imaging, NINDS, National Institutes of Health, Bethesda, MD, United States

A cascode-type preamplifier was modified by integrating a narrowband SAW filter into its output section to minimize out-of-band gain. Minimal deterioration in the noise figure was observed, along with some loss in gain due to the insertion loss of the filter. A two-element array was tested using the preamplifiers, which demonstrated a reduction in out-of-band gain for the split peaks normally observed from the phased-array circuit. Images acquired at 11.7T show no deterioration in image quality from the modified preamplifiers.

Kelly Byron¹, Chris Ellenor¹, Fraser Robb², Shreyas Vasanawala³, John Pauly¹, and Greig Scott^1
1Electrical Engineering, Stanford University, Stanford, California, United States, ²GE Healthcare, California, United States, ³Radiology, Stanford University, Stanford, California, United States

Wireless power transfer (WPT) uses two inductively coupled coils to transmit and receive power at a particular frequency, creating a local power source that can be used inside a strong magnetic field. By adding cable traps and a brick-wall filter to the system, we minimize the RF interactions between the WPT system and the MR scanner. As a result, our WPT system does not change the noise floor of the MRI preamplifiers and does not degrade the MR image quality.

Michael Twieg¹, Matthew J Riffe², Michael de Rooij³, and Mark A Griswold^1,4
1Dept. of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, United States, ²Dept. of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, ³Efficient Power Conversion Corporation, El Segundo, CA, United States,⁴Dept. of Radiology, Case Western Reserve University, Cleveland, OH, United States

We present the first use of enhancement mode Gallium Nitride on Silicon (eGaN FETs) for active detuning of RF receive coils. Experiments demonstrate that the eGaN FET method was superior to PIN diodes in the detuned state, while image SNR during the receive state suffered a slight penalty. Additionally, we show that the eGaN FETs require orders of magnitude less bias current and power than PIN diodes to operate. Our results show that eGaN FETs present advantages over PIN diodes for patient safety and for B0 and B1 distortion, while preserving the performance of the RF coil.

Josip Marjanovic¹, Jonas Reber¹, David Otto Brunner¹, Markus Weiger¹, Benjamin Emanuel Dietrich¹, Thomas Schmid¹, Urs Moser¹, Christoph Barmet^1,2, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, ²Skope Magnetic Resonance Technologies, Zurich, Switzerland

The receiver dead time due to high power RF transmission and switching limits the bandwidth and SNR of gapped acquisitions in SWept Imaging by Fourier Transform (SWIFT) approaches or Zero Echo Time imaging (ZTE). The settling times of digital filters required for overall data volume reduction contribute majorly to this dead time and relate to the specificity of the filtering. Therefore we propose a variable rate scheme oversampling the acquisition increasingly towards the gaps of the acquisitions. By this the settling times can be reduced almost arbitrarily with overall very minor data memory requirements.

Weinan Tang¹, Weimin Wang², and Jiahong Gao^1
1IDG McGovern Institute for Brain Research, Peking University, Beijing, China, ²Institute of Quantum Electronics, Peking University, Beijing, China

A cost-effective, multichannel MRI console was developed to exploit the potential of coil arrays and high-bandwidth EPI acquisitions such as BOLD-fMRI. With distributed system architecture and high-speed serial connectivity, the console makes a digital optical transmission approach to MRI receiver design available at 1.5 T. The performance of an 8-channel prototype has been demonstrated with substantial imaging experiments. As application demands grow, this proof-of-concept design can scale easily by adding custom-built modules.

Michael Wyss¹, Yolanda Duerst¹, Bertram Wilm¹, David Brunner¹, Benjamin Dietrich¹, Thomas Schmid¹, Christoph Barmet^1,2, and Klaas Pruessmann^1
1Institute for Biomedical Engineering, University of Zurich and ETH, Zurich, Zurich, Switzerland, ²Skope Magnetic Resonance Technologies, Zurich, Switzerland

At high field strength T2* weighted imaging suffers from artifacts due to field fluctuations generated by patient breathing and motion. We propose to employ a real-time higher order field feedback system based on NMR field sensors providing full 3rd order real-time field compensation. In this work the effectiveness of the system is quantified for T2* mapping. Field feedback was demonstrated to improve data accuracy of T2* mapping at 7T, which was evaluated in phantom experiments and in-vivo. The method may as well be beneficial to improve image quality and data accuracy in susceptibility weighted imaging and quantitative susceptibility mapping.

Christoph Juchem¹, Omar M Nahhass², Terence W Nixon¹, and Robin A de Graaf^1
1Diagnostic Radiology, Yale University, New Haven, CT, United States, ²RWTH Aachen University, Aachen, Germany

To date, MRI is based on linear field gradients generated by dedicated X, Y and Z coils. Here, multi-slice Dynamic Multi-Coil Technique (DYNAMITE) MRI is presented in which all field gradients are provided by MC field modeling. The obtained image fidelity is identical to conventional MRI based on dedicated gradient coils. Comparable image quality is a milestone towards the establishment of purely MC-based MRI systems. With the gains of DYNAMITE shimming in performance and efficiency, the MC technology has the potential to replace spherical harmonic coil systems for specific applications.

Suryanarayana Umesh Rudrapatna¹, Terence W Nixon¹, Scott McIntyre¹, Robin A de Graaf¹, and Christoph Juchem^1
1Diagnostic Radiology, Yale University, New Haven, Connecticut, United States

Dynamic Multi-coil technique (DYNAMITE) is inherently capable of generating more complex fields than those possible with higher order spherical-harmonic shims. Thus, it has immense potential for improving EPI quality at high fields. We assessed this by acquiring multiple EPI, T ₂^*- and B0-mapping datasets at 7T under DYNAMITE and 3^rd order spherical harmonic static shims in five volunteers. Results showed significant B0 homogeneity improvement in typical hard-to-shim areas using DYNAMITE. EPI image distortions and signal drop-outs were drastically reduced. Histogram analysis of T₂^* data showed a shift in distribution towards higher values. These results endorse the advantages of DYNAMITE for EPI.