Decoupling methods for radio frequency antennas in ultra-high-field MRI

Abstract

For further development of many applications, such as multi-input multi-output (MIMO), radio frequency identification (RFID), wireless power transfer (WPT), and ultra-high-field magnetic resonance imaging (UHF MRI), decoupling of closely located antennas without significant changes of radiation pattern is a key task. In magnetic resonance imaging, there is a trend to increase the magnetic field strengths to obtain better spatial resolution of the image and perhaps smaller scan time. Since by increasing dc magnetic field, the frequency of the B1 field (created by dipole and loop antennas) necessary increases, the use of standard radio antennas applied in usual MRI is not allowed. Challenges of B1 field inhomogeneity arising at high frequencies and low efficiency of antennas in the new range are currently addressed by using so-called transmit-arrays driven by multiple independent channels and phase-amplitude steering. However, the mutual coupling results in considerable power losses and up to now poses a constraint on the density of these arrays. This dissertation focuses on developing new techniques for decoupling closely located dipole and loop antennas. In the first part, decoupling of closely located dipole antennas using passive scatterers is analyzed. The possibility of decoupling using passive scatterers and the effect of decoupling on the B1 field is comprehensively studied. In the second part, the decoupling of loop antennas is discussed. For that, shielded loop antennas with modified shields are analyzed. These shielded loop antennas suppress the mutual coupling between antennas without any damage on the B1 field. In general, using a metasurface of resonant dipoles, two closely located dipole antennas are decoupled while the presence of metasurface does not affect the B1 field significantly. Moreover, transceiver shielded loop antennas with self-decoupling feature are designed which does not change the B1 field. Besides ultra-high-field MRI application, the possibility of employing this method for wireless power transfer application has been numerically and experimentally checked.