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

Abstract

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.

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

“…The 16‐loop 2 × 8 tight‐fit TxRx array provides a robust initial design of our final new head array. Figure depicts an electromagnetic (EM) simulation model of the new array.…”

“…At the same time, an excessive increase of the number of loops in the TxRx array associated with a decrease in their size may lead to a decline of Tx performance because of insufficient loading and problems with decoupling when having to deal with cross‐talk between non‐adjacent loops. Sixteen‐element double‐row (2 × 8) surface loop Tx arrays have been previously constructed and successfully tested for human whole brain imaging at 7T and 9.4T …”

“…The larger array coil in the outer layer serves for local transmission. The other design, a transceiver (TxRx) array, consists of a single layer of elements used during both transmission and reception . TxRx‐arrays allow higher tissue loading because of tighter fit and, therefore, more efficient transmission .…”

“…The other design, a transceiver (TxRx) array, consists of a single layer of elements used during both transmission and reception . TxRx‐arrays allow higher tissue loading because of tighter fit and, therefore, more efficient transmission . Here, Tx efficiency is defined as /√ P , where is the strength of the transmit RF magnetic field, and P is RF power measured at the coil input.…”

“…For example, decoupling only the adjacent elements of the 2 × 8 human head Tx array may require up to 40 decoupling circuits, which must all be adjusted individually. Previously, we demonstrated that at 400 MHz, a 2 × 8 tight‐fit human head TxRx surface loop array can be well decoupled by optimally overlapping adjacent loops without any additional decoupling methods, which substantially simplifies the entire design. This is based on our previous analytical and experimental findings that at 400 MHz the mutual resistance between adjacent overlapped loops of a tight‐fit 8‐loop single‐row (1 × 8) array is minimal, and both the mutual inductance and mutual resistance can be minimized at the same time by optimally overlapping the loops .…”