Decoupling of a double-row 16-element tight-fit transceiver phased array for human whole-brain imaging at 9.4 T.

One of the major challenges in constructing multi-channel and multi-row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight-loop single-row (1 × 8) human head tight-fit TxRx array. In this work, we designed and constructed a 9.4-T (400-MHz) 16-loop double-row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole-brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit-only/receive-only (ToRo) array, which consists of a larger Tx-only array and a nested tight-fit 31-loop receive (Rx)-only array. At the same time, the ToRo array provides greater peripheral signal-to-noise ratio (SNR) and better Rx parallel performance in the head-feet direction. Overall, our work provides a recipe for a simple, robust and very Tx-efficient design suitable for parallel transmission and whole-brain imaging at ultra-high fields.

In vivo estimation of transverse relaxation time constant (T2 ) of 17 human brain metabolites at 3T.

PURPOSE: The transverse relaxation times T2 of 17 metabolites in vivo at 3T is reported and region specific differences are addressed. METHODS: An echo-time series protocol was applied to one, two, or three volumes of interest with different fraction of white and gray matter including a total number of 106 healthy volunteers and acquiring a total number of 128 spectra. The data were fitted with the 2D fitting tool ProFit2, which included individual line shape modeling for all metabolites and allowed the T2 calculation of 28 moieties of 17 metabolites. RESULTS: The T2 of 10 metabolites and their moieties have been reported for the first time. Region specific T2 differences in white and gray matter enriched tissue occur in 16 of 17 metabolites examined including single resonance lines and coupled spin systems. CONCLUSION: The relaxation time T2 is regions specific and has to be considered when applying tissue composition correction for internal water referencing. Magn Reson Med 80:452-461, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Novel splittable N-Tx/2N-Rx transceiver phased array to optimize both signal-to-noise ratio and transmit efficiency at 9.4T.

PURPOSE: The goal of this study was to optimize signal-to-noise ratio (SNR) and parallel receive (Rx) performance of ultrahigh field (UHF) (≥7T) transceiver arrays without compromising their transmit (Tx) efficiency. UHF transceiver head phased arrays with a tight fit improve Tx efficiency in comparison with Tx-only arrays, which are usually larger so that Rx-only arrays can fit inside. However, having ≥16 elements inside a head transceiver array presents decoupling problems. Furthermore, the available number of Tx channels is limited. METHODS: A prototype of a splittable transceiver phased array was constructed. The array consisted of four flat surface Tx loops positioned in two rows. Each loop could be split into two smaller overlapped Rx loops during reception. RESULTS: Experimental data demonstrated that both SNR and parallel reception performance improved substantially by doubling the number of Rx elements from four to eight. CONCLUSION: As a proof of concept, we developed and constructed a novel splittable transceiver phased array that allows doubling of the number of Rx elements while keeping both Tx and Rx elements at the same distance from the subject. Both Tx and Rx performance can be optimized at the same time using this method. Magn Reson Med 76:1621-1628, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Safety testing and operational procedures for self-developed radiofrequency coils.

The development of novel radiofrequency (RF) coils for human ultrahigh-field (≥7 T), non-proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor-suggested test specifications for third party coils and custom-developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh-field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR-related safety and electrical performance at ultrahigh-field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright © 2015 John Wiley & Sons, Ltd.