Over the past decade there has been a rapid increase in the number of very high field (7 Tesla and above) human scanners installed worldwide. The major challenges associated with these systems are the intrinsically lower B1 + homogeneity compared to 3 Tesla or 1.5 Tesla systems due to wavelength effects and the higher power deposited in the patient. If one attempts to make a body coil for such a high field system using a conventional quadrature birdcage or TEM design, the EM energy couples very efficiently to waveguide modes in the magnet bore and a significant amount of energy is transported away from the body.
One way to at least partly overcome this situation is to construct transmit arrays: state-of-the-art systems at 7 Tesla and above currently have the capability of driving 8, 16 or 32 element transmit arrays. Based either on measurements or simulations, the signal input (phase and magnitude) to each element of the RF coil can be optimized to maximize the uniformity of the transmit field within the patient, minimize local and global tissue heating, or a combination of the two.
A number of different types of designs have been used to form a transmit array. In our work we have concentrated on two relatively new designs. One consists of dielectric resonators, and the second on dipoles, both of which have been developed in the STW project From Coil to Antenna in collaboration with Dr. Alexander Raaijmakers at UMC Utrecht. Another approach has been using metamaterials and artificial dielectrics. This work was funded by grant ERC Advanced 670629 NOMA-MRI, with most of the work performed by Dr.Rita Schmidt, now a faculty member at the Weizmann Institute.
(top row) Dipole, ceramic and metamaterial-based RF coils designed for 7 Tesla operation.
(bottom row) Images and spectra acquired with the respective coils/antennas.
- T.Ruytenberg and A.G.Webb, Design of a dielectric resonator receive array at 7 Tesla using detunable ceramic resonators, J.Magn.Reson, 284:94-98, 2017.
- R.Schmidt and A.G.Webb, A metamaterial combining electric- and magnetic- dipole-based configurations for unique dual-band signal enhancement in ultra-high field magnetic resonance imaging, ACS Applied Materials & Interfaces, 9(40):34618-34624, 2017.
- T.P.A. O’Reilly, T. Ruytenberg, and A.G. Webb, Modular transmit/receive arrays using very high permittivity dielectric resonator antennas, Magn.Reson.Med. Jun 20. doi: 10.1002/mrm.267842017.
- R.Schmidt, A.Zlobozhanyuk, P.Belov and A.G.Webb, Flexible and compact hybrid metasurfaces for enhanced ultra high field magnetic resonance imaging, Scientific Reports, 7(1):1678. doi: 10.1038/s41598-017-01932-92017.
- S.A.Aussenhofer and A.G.Webb, An eight-channel transmit/receive array of TE01 mode ceramic resonators for cardiac MRI at 7 Tesla, J.Magn.Reson, 43:122-9, 2014.