Available on-demand - *F.SF03.02.02
B1+ Homogenization in 7T Brain MRI with Inversely-Designed Metasurfaces
Namkyoo Park1,Hansol Noh1
Seoul National University1
Offering enhanced SNR, contrast, and spatial resolution, ultra-high field (UHF, B0 ≥ 7T) MRI has shown its potential for clinical diagnostics, such as evaluation of strokes, Alzheimer’s, and Parkinson’s disease [1-4]. However, at the Larmor frequency corresponding to B0 above 7 Tesla, the effective wavelength of the B1+ field becomes smaller than the dimension of the head, leading to the spatial phase variation and inhomogeneity of B1+ fields inside. For the mitigation of this detrimental B1+ inhomogeneity creating uneven contrast and the variation of SNR in the UHF MRI image, various approaches have been investigated, such as parallel transmission techniques (PTx), high permittivity material (HPM) structures, and metasurfaces. While PTx improves B1+ homogeneity in the whole brain, hardware complexity, and the need for real-time SAR assessment prevents its clinical 7T practice [5,6]. Structures with metallic inclusions, such as metasurfaces and hybridized meta-atoms (HMAs) have also been reported to manipulate field profiles at subwavelength scales [7,8], but often resulted in deterioration of the global B1+ homogeneity over the ROI.
In this talk, we propose and present an inverse-design of metasurfaces addressing the global B1+ homogeneity over the region of interest (ROI) in the whole axial planes. Approximating MRI environments as a 2D system governed by the Helmholtz equation, we inversely obtain permittivity distributions of cylindrical metasurfaces enclosing the head at proximity. With the metasurface constructed of cylindrical HPMs of different permittivity, stacked along the superior direction, it is numerically confirmed that the proposed metasurface excite the target fields near the surfaces and ROI in both approximated and real MRI environments. When tested with an actual anatomical head model (MIDA), it is found that the CV (coefficient of variation) and Max-to-min ratios of B1+ in the ROI are reduced on average by 40% and 33% respectively, along with SAR reduction in the head by 34% (average) and 44% (peak). Since the B1+ distribution itself is globally homogenized, the proposed structure is expected to be most useful in spin-echo based imaging in UHF MRI, such as T2-weighted imaging and diffusion-weighted imaging in the whole brain.
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