9.4 T double‐tuned ¹³C/¹H human head array using a combination of surface loops and dipole antennas

Abstract

MRI at ultra-high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X-nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal-to-noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double-tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X-nucleus surface loop array with 1 H dipoles can substantially simplify the design of a double-tuned UHF human head array coil. Recently, we developed a novel bent folded-end dipole transceiver array for human head imaging at 9.4 T. The new eight-element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16-element surface loop array. In this work, we developed, constructed and evaluated a double-tuned 13 C/ 1 H human head 9.4 T array consisting of eight 13 C transceiver surface loops and eight 1 H transceiver bent folded-end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four 1 H traps into each 13 C loop. The presented double-tuned RF array coil substantially simplifies the design as compared with the common double-tuned surface loop arrays. At the same time, the coil demonstrated an improved 1 H longitudinal coverage and good transmit efficiency. K E Y W O R D S13 C imaging, double-tuned coil, folded-end dipole, RF head array, transceiver array, ultra-high field MRI, whole-brain coverage

A 32‐element loop/dipole hybrid array for human head imaging at 7 T

“…The major difference with the Lagore's design is that in our array 24 Rx‐loops are arranged in 3 rows (3 × 8) on a tight‐fit helmet (Figure 1), and all 32 elements including TxRx‐dipoles are placed in a single layer close to the head. It is noteworthy that RF field of the dipoles can be strongly altered by interaction with loops even in the case when loops resonate at a substantially different frequency, that is, detuned 51,52 . Therefore, in our design the dipoles are placed in the center of loops.…”

“…Therefore, we numerically investigated ways to minimize interaction between the 2 complementary resonant structures. The method consists of introducing resonant band‐stop filters (traps) into each loop 51 . We investigated the number (1 or 2) and positions (transverse or longitudinal) of the traps inserted into each loop.…”

“…The method consists of introducing resonant band-stop filters (traps) into each loop. 51 We investigated the number (1 or 2) and positions (transverse or longitudinal) of the traps inserted into each loop. Our simulations showed (Figure 3 and Table 1) that the placement of 48 transverse traps (2 in each loop) substantially reduces alteration of the RF magnetic field both globally and locally, such that Tx-and SAR-efficiencies decreased by only 1% and 3%, respectively.…”

“…Previously, studies of our own and others have shown that the presence of loops, even when tuned to resonate at a very different frequency, that is, detuned, compromise the dipoles' performance. 51,52 A common way to minimize interaction between these 2 structures is to introduce resonant band-stop filters (traps) into each loop. 51 However, as shown previously, 51 1 trap may be insufficient to preserve the dipole array Tx-performance.…”

Interleaved and simultaneous multi‐nuclear magnetic resonance in vivo. Review of principles, applications and potential

“…To improve the overall dual-tuned coil performance and to allow for acquisition of high-quality 1 H data, innovative and organ-specific coil designs have been developed. 37,60,[112][113][114][115][116][117][118][119][120][121][122] An in-depth discussion on the trade-offs for single-and multi-structure dual-tuned RF coils designs (focused on the brain but applicable to other anatomical targets) can be found elsewhere. 123…”