Parallel Transmit Methods & Hardware

Wednesday 14 May 2014

Martijn A Cloos¹, Christopher Wiggins², Graham Wiggins¹, and Dan Sodickson^1
1NYU Langone Medical Center, New York, NY, United States, ²Scannexus, Maastricht, Netherlands

Parallel transmission (PTX) is often proposed as a framework for transmit non-uniformity mitigation in ultra high field MRI. However, routine application of PTX has hitherto been hampered by technical challenges. In particular, optimal performance is contingent on subject-specific transmit-sensitivity maps and tailored RF-pulses, which impede the workflow and increase the duration of the exam. In this work we demonstrate a novel approach to transmit non-uniformity mitigation inspired by the recently proposed MR Fingerprinting method that enables simultaneous quantitative mapping of the T1 relaxation and an array of transmit-sensitivity profiles without prior calibration scans or tailored RF-pulses in less than 15seconds.

Xiaoping Wu¹, Jinfeng Tian¹, Sebastian Schmitter¹, Tommy Vaughan¹, Kamil Ugurbil¹, and Pierre-Francois Van de Moortele^1
1CMRR, Radiology, University of Minnesota, Minneapolis, Minnesota, United States

Simultaneous MultiSlice (SMS) MR imaging using MultiBand (MB) RF pulses is becoming increasingly popular in neuroimaging. Recently, there has been an interest in utilizing multielement RF arrays combined with multichannel (pTx) MB pulse design to reduce transmit B1 inhomogeneity and SAR for SMS/MB imaging. Meanwhile, it has been shown that the use of transmit coil elements that approximately align with the slice direction, such as Z-stacked arrays with azimuthally distributed elements in two rings displaced from each other along the Z-direction versus axial slices, can provide improved RF performance for pTx non-MB pulses at 3T and 7T, as compared to conventional single ring arrays. In this study, we evaluate the performance of such transmit coil element/slice geometries for achieving whole brain SMS/MB imaging at 7T by designing pTx MB RF pulses based on electromagnetic simulations.

Samaneh Shooshtary¹, Marcel Gratz², Mark E. Ladd^2,3, and Klaus Solbach^1
1High Frequency Technology, Duisburg-Essen University, Duisburg, Germany, ²Erwin L. Hahn Institute for Magnetic Resonance Imaging, Essen, Germany, ³Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany

High-field 7T MR systems increasingly apply multichannel transmit systems to overcome artifacts resulting from comparably short wavelengths inside the human tissue. Individual phase and amplitude modulation for each channel is required to gain the desired image homogeneity. A high-speed digitally controlled modulator system is presented for independent phase and magnitude control of 32 transmit channels in a 7T MR-scanner, thus allowing RF shimming and dynamic trajectories as well as TX Sense. Experimental results show stability and reproducibility of the RF signal while calibration of imperfect amplitude- and phase tracking of I- and Q components for 32 individual channels is demonstrated.

Natalia Gudino¹, Qi Duan¹, Joe Murphy-Boesch¹, Jacco A de Zwart¹, Hellmut Merkle¹, Peter Van Gelderen¹, and Jeff H Duyn^1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States

An approach to perform parallel RF transmission with on-coil CMCD amplifiers at 7T is evaluated. An optimized 2-channel prototype based on eGaN FET technology was built and tested. Artifact-free images of a gel phantom were obtained by transmitting with the array and receiving signal with a detunable volume receive coil. The results suggest that this technology provides a practical solution for the use of multi-channel B1-shimming and other parallel transmit applications at high field.

Michael Twieg¹ and Mark A Griswold^1,2
1Dept. of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, United States, ²Dept. of Radiology, Case Western Reserve University, Cleveland, OH, United States

We present a fast and powerful method of experimentally quantifying the effective open loop output impedance and power efficiency of coupled power amplifier topologies across their full control space. We demonstrate the use of the method for characterizing a Current Mode Class D (CMCD) amplifier at 10 MHz. The results demonstrate some of the unique qualities associated with switch mode power amplifiers, such as the complex nature of their effective output impedance, and the ability to recover, rather than dissipate, power coupled from other transmit elements. This methodology greatly facilitates the evaluation of amplifiers for parallel transmit applications.

Neal Hollingsworth¹, Katherine Moody¹, Jon-Fredrik Nielsen¹, Douglas Noll¹, Mary Preston McDougall^1,2, and Steven Wright^1,2
1Electrical Engineering, Texas A&M University, College Station, Texas, United States, ²Biomedical Engineering, Texas A&M University, College Station, Texas, United States

Transmit systems have historically been single channel and used to excite a simple slice or slab, however the desire to use spatially selective RF pulses has driven an interest in using the Transmit SENSE to accelerate excitation. This requires the array to have known and stable patterns, but coupled arrays are sensitive to varying loads. Current source and low output impedance amplifiers have been suggested for decoupling transmit arrays. We have constructed current source, ultra-low output impedance, and standard power amplifiers to investigate the strengths of different architectures by comparing the peak-power and isolation of the different amplifier architectures.

Zohaib Mahmood¹, Bastien Guérin², Boris Keil², Elfar Adalsteinsson^1,3, Lawrence L. Wald^2,3, and Luca Daniel^1
1Dept of Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²A. A. Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ³Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States

We present a methodology to design a robust decoupling matrix for high field parallel transmit arrays. The matrix is placed between the power amplifiers and the coupled coil and yields almost perfect decoupling of the array and hence improves the power efficiency. We demonstrate the methodology by designing a decoupling matrix for a coupled 7T 4-channel head transmit-array. Our design is robust with respect to component value variations. A sensitivity analysis revealed that only two lumped elements need to be implemented as variable in order to guarantee good performance of the matrix even in the presence of component values variations.

Pei-Shan Wei^1,2, Michael J. Smith², Christopher P. Bidinosti³, Jarod Matwiy², and Scott B. King^1,2
1Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada, ²National Research Council of Canada, Winnipeg, Manitoba, Canada, ³Department of Physics, University of Winnipeg, Winnipeg, Manitoba, Canada

For determining an optimal Tx-SENSE Tx-array design, we modeled Tx-arrays with and without proper mutual inductance coupling and coil termination to compare accuracy of excitation and SAR. We found that with incorrect coupling models, average SAR was over-estimated by as much as 23%, peak SAR under-estimated by as much as 26%, and peak SAR/Average SAR underestimated by as much as 40%. Therefore, proper coupling in simulations is absolutely necessary for RF coil designers to optimize the Tx-array design for Tx-SENSE applications and for accurate prediction of peak local SAR distribution.

Francesco Padormo^1,2, Arian Beqiri¹, Shaihan J. Malik¹, and Joseph V. Hajnal^1,2
1Division of Imaging Sciences and Biomedical Engineering, Kings College London, London, London, United Kingdom, ²Centre for the Developing Brain, Kings College London, London, London, United Kingdom

In this work we demonstrate an improved model for global Q matrix calculation using power monitoring hardware. This model accounts for RF cable attenuation so that the reconstructed global Q matrix is more accurate than previous approaches.

Andre Kuehne^1,2, Sigrun Goluch^1,2, Ewald Moser^1,2, and Elmar Laistler^1,2
1Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Vienna, Austria, ²MR Center of Excellence, Medical University of Vienna, Vienna, Vienna, Austria

The bower balance of an RF coil gives valuable insight into its performance. For multi-channel arrays, the amount of power deposited in the load, the coil itself or radiated depends on the superposition of the single-channel fields. In this work, the power correlation matrix formalism is extended to encompass all terms of the power balance, thus allowing straight-forward evaluation of the power balance for any excitation, worst-case estimates, and providing means to verify EM simulation integrity.