Implantable SUB-cm wireless resonators For MRI: from circuit theory to medical imaging
Demir, Hilmi Volkan
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Making implantable wireless resonators having small footprints is fundamentally challenging when using conventional designs that are subject to the inherent tradeo between their size and the achievable range of quality-factors (Q-factors). For clinical magnetic resonance imaging (MRI) frequencies (e.g., about 127 MHz for 3 T), conventional resonators either require a diameter of about 20 cm in chip size or o -the-chip lumped elements for successful operation, both of which practically prevent their use as implantable devices. At least two orders-of-magnitude reduction in footprint area is necessary to make on-chip resonators suitable for invivo applications. However, decreasing the size of such a conventional resonator chip comes at the expense of substantially decreased Q-factor. Thus, achieving high Q-factors with reduced footprints simultaneously entails a novel approach in implantable electronics. In this thesis work, to address this problem, we proposed, designed and demonstrated a new class of sub-wavelength, thin- lm loaded helical metamaterial structures for in-vivo applications including eld localization and signal-to-noise ratio (SNR) improvement in MRI. This implantable wireless architecture, implemented fully on chip with partially overlaid helicals on both sides of the chip interconnected by a through-chip-via, enables a wide range of resonant radio frequencies tunable on chip by design while achieving an extraordinarily small footprint area (<< 1 cm2) and ultra-thin geometry (< 30 m). The miniaturization of such microwave circuits to sub-cm range, together with their high Q-factors exceeding 30 in lossy soft tissues, allows for their use in vivo. The fabricated devices correspond to 1/1500th of their operating wavelength in size, rendering them deep sub-wavelength.For the proposed wireless resonant devices, equivalent circuit models were developed to understand their miniaturization property and the resulting high Q-factors are well explained by using these models. Additionally, full-wave numerical solutions of the proposed geometries were systematically carried out to verify the ndings of the developed equivalent circuit models. All of these theoretical and numerical studies were found in excellent agreement with the experimental RF characterization of the microfabricated devices. Retrieval analyses of the proposed architectures showed that these geometries lead to both negative relative permittivity and permeability simultaneously at their operating frequencies, which do not naturally exist together in nature, making these structures true metamaterials. These fabricated wireless devices were further shown to be promising for the in-vivo application of subdural electrode marking, along with SNR improvement and eld localization without causing excessive heating in MRI. MR images support that the proposed circuitry is also suitable for MRI marking of implants, high-resolution MR imaging and electric eld con nement for lossy medium. Although our demonstrations were for the purpose of marking subdural electrodes, RF characterization results suggest that the proposed device is not limited to MRI applications. Utilizing the same class of structures enabling strong eld localization, numerous wireless applications seem feasible, especially where miniaturization of the wireless devices is required and/or improving the performance of conventional structures is necessary. The ndings of this thesis indicate that the proposed implantable sub-cm wireless resonators will open up new possibilities for the next-generation implants and wireless sensing systems.