Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Abstract

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 ultimate intrinsic signal‐to‐noise ratio of loop‐ and dipole‐like current patterns in a realistic human head model

“…These elements have out‐of plane currents, which are linearly independent of surface currents and hence add to the overall SNR. At 9.4 T, Avdievich was able to show an SNR boost of approximately 30% in the brain center when combining eight vertical loops with 8 surface loops . This increase is much higher than what could be expected when using 16 surface loops .…”

Double‐row 18‐loop transceive–32‐loop receive tight‐fit array provides for whole‐brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T

“…To decrease radiation losses and coupling between non‐adjacent loops, the array is shielded with the cylindrical shield placed at a 40‐mm distance from the surface loops located in the second row. The distance to the shield was chosen based on previously published data for UHF human head size arrays . Because the original 2 × 8 TxRx‐array lacks RF elements at the superior locations of a head, both Tx and Rx performances in this area are rather poor .…”

“…The addition of such a large number of Rx‐only elements would substantially complicate the EM model, which in turn greatly increases the amount of the computational time and memory. However, previous EM modeling and experimental evaluation of the simpler 16‐element prototype of the array, which consisted of 8 surface TxRx loops and 8 vertical Rx loops, demonstrated that the addition of Rx‐only vertical elements practically did not alter the Tx performance and maximum local SAR . As a result, we did not include vertical loops in the simulations.…”

“…The next step in our design is an addition of Rx‐only elements to the 2 × 8 TxRx array. Recently, we developed a method of increasing the number of Rx elements in a single layer human head TxRx array without compromising the array’s Tx performance . We designed and constructed an array prototype consisting of a single row of 8 TxRx surface loops circumscribing a head, and 8 Rx‐only “vertical” loops positioned along the central axis of each TxRx loop .…”

Evaluation of short folded dipole antennas as receive elements of ultra‐high‐field human head array

“…This problem is more pronounced for a short dipole given that it does not have as broad a frequency band as a full‐length half‐lambda dipole antenna . An alternative method of producing dipole‐like currents on the cylindrical surface of the head array holder has been described recently . In this design, Rx‐only “vertical” loops were added to a single‐row array of 8 TxRx surface loops circumscribing a head.…”