Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI With Metasurfaces

Abstract

Phased antenna arrays of dipoles are widely used in ultra-high field magnetic resonance imaging for creating the controllable radio-frequency magnetic field distributions in a human body. Due to safety and imaging quality reasons each individual channel of the array should be decoupled-electromagnetically isolated from the others. The required number of channels is large and in some techniques the dipole antennas should be located in the close proximity of the human body. Their ultimately dense arrangement leads to a strong mutual coupling and makes the conventional decoupling structures inefficient. This coupling needs to be suppressed without a significant distortion of radio-frequency fields in the imaged area. In this work, we propose and study a novel decoupling technique for two ultra-high field transceiver on-body dipole antennas. The decoupling is performed by a periodic structure of five parallel resonant wires referred to as a metasurface. In contrast to conventional decoupling with a single resonant wire, the metasurface decoupled by means of excitation of a higher-order coupled mode of the wires, which field is highly confined. The main advantage is a low distortion of the RF-field in the central region of the body.

Interelement Decoupling Strategies at UHF MRI

“…The drawback of this technique is the existence of the secondary RF field produced by the passive element/structure which can reduce the radiation efficiency of the active array. In the work in reference [34], it is shown that the higher number of passive elements and their proper spacing can reduce interaction with the transmit RF field. Passive decoupling structures could also interfere with the receive array elements (in transmit-only receive-only (ToRo) configurations) since the passive elements are not detuned.…”

“…[25] Elements are ended with meandered structures High degree of interelement isolation is achieved/meandered structure should be optimized vs. distance between the coils Microstrip, loop, monopole, dipole [26][27][28][29][30] Insertion of a single passive element Maximized isolation between the adjacent elements/the primary RF field's efficiency and homogeneity could be affected Dipole [32][33][34] Insertion of periodic passive structures with multiple unit cells Can be applied also to microstrip, monopole, and loop coils; the interaction with the primary RF field is eliminated/the effect disappears for densely populated unit cells Dipole [37] Folding of the end of the dipole High degree of interelement isolation is achieved/the presence of the RF shield significantly affects performance…”

“…Dipole elements are linear elements, and therefore, decoupling techniques such as geometrical decoupling cannot be applied. There have been several dipole decoupling techniques proposed recently: decoupling with the electromagnetic bandgap (EBG) structures [32], decoupling with stacked resonators [33], and decoupling with metasurfaces [34].…”

“…Decoupling of dipole antennas using metasurfaces without distortion of the transmit field has been proposed in the work in reference [34]. The metasurface is a periodic structure which consists of five parallel resonant wires and is a continuation of the work presented in reference [30].…”

“…The single wire acts as a scatterer and produces a strong parasitic resonance. In the work in reference [34], the transmit field of the coil has been studied in the cases without decoupling elements and with one and five resonant decoupling elements. It is shown that adding more resonant elements improves the isolation between the channels without distorting the transmit RF field.…”