Abstract
The notion of mode shaping based on evanescent coupling has been successfully applied in various fields of optics, such as in the dispersion engineering of optical waveguides. Here, we show that the same concept provides an opportunity for the seemingly different field of ultra-high-field MRI, addressing transmit RF magnetic field (B1+) inhomogeneity. In this work, treating the human phantom as a resonator, we employ an evanescently coupled high-index cladding layer to study the effects of the auxiliary potential on shaping the B1+ field distribution inside the phantom. Controlling the strength and coupling of the auxiliary potential ultimately determining the hybridized mode, we successfully demonstrate the global 2D homogenization of axial B1+ for a simplified cylindrical phantom and for a more realistic phantom of spheroidal geometry. The mode-shaping potentials with a magnetic permeability or material loss are also tested to offer additional degrees of freedom in the selection of materials as well as in the manipulation of the B1+ distribution, opening up the possibility of B1+ homogenization for 3D MRI scanning.
Highlights
The notion of mode shaping based on evanescent coupling has been successfully applied in various fields of optics, such as in the dispersion engineering of optical waveguides
Since the uniformities of the B1+ field distribution in the radial and sagittal directions have a trade-off relation in a cylindrical phantom at a given Larmor frequency[27], we focus here on axial 2D magnetic resonance imaging (MRI) scanning, which is widely used in clinical applications
We first simplify the MRI system to maintain the z-translational symmetry by assuming a cylindrical phantom and a cylindrical pad to minimize the mode mixing between the radial (r), sagittal (z), and azimuthal (θ) directions (Fig. 1a)
Summary
The notion of mode shaping based on evanescent coupling has been successfully applied in various fields of optics, such as in the dispersion engineering of optical waveguides. We treat the MRI system, composed of the body and an RF B 1+ coil, as a waveguide operating at the Larmor wavelength and employ a high permittivity cladding layer (auxiliary electromagnetic potential well) to tailor the mode shape inside the body through controlled evanescent mode coupling.
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