PhD Defense: “Implantable Sub-cm Wireless Resonators for MRI: from Circuit Theory to Medical Imaging,” Sayım Gökyar (EE), EE-314, 3:30PM December 11 (EN)

Ph.D. Defence in Electrical and Electronics Engineering
Supervisor/s: PROF. DR. H. VOLKAN DEMIR

The seminar will be on Monday, December 11, 2017 at 15:30, @EE-314

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, allow 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, circuit equivalent 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 circuit equivalent 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 in this work, 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.

Keywords: Metamaterials, wireless resonators, magnetic resonance imaging (MRI), MR-compatible implants.