MechE PhD Final Oral Defense: Xia Zhu

TITLE: METAMATERIAL ENABLED NEAR FIELD ENGINEERING FOR MAGNETIC RESONANCE IMAGING

ABSTRACT: Magnetic resonance imaging (MRI) is a powerful and widely used medical imaging modality whose image quality is fundamentally limited by the signal-to-noise ratio (SNR) of radio-frequency (RF) signal reception. Because MRI signal detection occurs in the nearfield regime, receive sensitivity is governed by the local magnetic field distribution produced by RF coils and resonant structures. Conventional strategies to improve SNR primarily rely on increasing coil channel count or refining coil geometry, which inevitably introduce added system complexity, cost, and practical constraints. This dissertation investigates metamaterial-enabled near-field engineering as an alternative and complementary approach for reshaping magnetic field distributions and enhancing MRI signal reception under practical safety and clinical integration considerations. This work explores a progression of resonant and metamaterial-based architectures that leverage distributed resonance, electromagnetic coupling, and electric-field confinement to achieve safety-compatible magnetic field enhancement. First, a Helmholtz coil–inspired volumetric wireless resonator is introduced to demonstrate volumetric receive sensitivity enhancement using a passive and cable-free resonant structure under body-coil-based reception. Building on this foundation, a flexible, wearable metamaterial composed of coaxially shielded resonant unit cells is developed, enabling robust electric-field confinement, frequency tunability, and passive self-detuning while achieving substantial SNR improvement in phantom and ex vivo MRI experiments. To further improve versatility and anatomical adaptability, a modular coaxially shielded circular resonator architecture is presented as a universal building block that can be geometrically reconfigured and assembled into metamaterial-like arrays for application-specific imaging scenarios. Finally, metamaterial-inspired concepts are integrated directly into receive coil architecture through a hybrid resonant design in which coupling-driven collective modes are exploited to tailor magnetic field profiles and enhance sensitivity beyond that of conventional coils. Together, these studies establish electric-field-constrained near-field resonance as a unifying design principle for metamaterial-enabled MRI receive hardware. The results demonstrate that metamaterial-based and hybrid resonant architectures can achieve meaningful and clinically relevant improvements in MRI signal reception while maintaining safety compatibility and system-level practicality. This dissertation expands the design space of MRI receive technology and provides a framework for next-generation, adaptable, and high-performance RF hardware.

COMMITTEE: ADVISOR Professor Xin Zhang, ME/MSE/BME/ECE; CHAIR Professor Russo, ME/MSE; Professor Bifano, ME/MSE/BME/ECE, Professor Duan, ME/MSE; Professor Stephan W. Anderson, Radiology