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.

Ultra-high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B /√P) in comparison with Tx-only arrays, which are usually larger to fit receive (Rx)-only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx-only/Rx-only RF system, a single-row eight-element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx-only array, which could also be used in combination with an 18-channel Rx-only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B /√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.

As a proof of concept we developed and constructed a 9.4T (400 MHz) overlapped transceiver head array based on results of the analytical modeling. We demonstrated that at UHF overlapping loops not only provides excellent decoupling but also improves both Tx- and Rx-performance. Magn Reson Med 79:1200-1211, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Ultra-high-field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low-field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight-fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx-only arrays, which are larger in order to fit multi-channel receive (Rx)-only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high-power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16-channel tight-fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx-only vertical loops positioned along the central axis (parallel to the magnetic field B ) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16-element array provided 30% improvement of central signal-to-noise ratio (SNR) in vivo relative to a surface loop eight-element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.

The combination of loop and dipole elements could enhance the SNR performance in the human head at ultra-high field strength.