Boston University researchers explain how advanced MRI coils and textile metamaterials could make high-quality imaging more accessible and efficient.

By Xin Zhang, Ke Wu and Xia Zhu, Boston University

With over 100 million magnetic resonance imaging (MRI) scans performed annually worldwide, including 40 million in the U.S., MRI remains a cornerstone of modern medicine due to its safety and noninvasiveness.

Now, wireless and wearable coils, along with textile metamaterials, are emerging as game-changers. These advanced coils manipulate electromagnetic fields to enhance imaging performance, offering a promising, cost-effective upgrade to traditional MRI systems.

The challenge

MRI technology has traditionally relied on rigid, anatomy-specific RF coils, hardwired to a direct RF feed, to ensure high imaging sensitivity. While effective, these coils are bulky and expensive due to their numerous nonmagnetic electronic components, feed boards, cable traps, and adapters.

These bulky, rigid coils increase patient discomfort and complicate handling during imaging procedures. Additionally, imaging centers often need 5 to 7 different coils for various anatomical regions, driving up costs.

The emergence of metamaterials — artificially constructed materials composed of sub-wavelength unit cells — has opened new possibilities for enhancing the signal-to-noise ratio (SNR) of MRI systems through wireless, cost-effective solutions. By precisely tailoring electromagnetic waves in the near-field region, metamaterials can significantly improve SNR as auxiliary devices working with the body coil of MRI, improving image resolution and acquisition efficiency.

Despite this promise, previous metamaterial designs have faced slow clinical adoption due to lower sensitivity compared to state-of-the-art receive coils and incompatibility with common clinical sequences. Their rigid, bulky structures also limit both sensitivity and patient comfort.

To overcome these limitations, we have developed an innovative solution using off-the-shelf coaxial cables. These wearable coils cost less than $100 per unit and maximize sensitivity by closely conforming to anatomical regions, achieving a SNR comparable to or even surpassing that of advanced MRI coils.

Coaxial cables revolutionize resonator design

Coaxial cables, known for their dual-layered internal conductors, have long been a staple in telecommunications, delivering efficient signal transmission with minimal losses and robust protection against external interference. Now, these cables are transforming MRI technology.

In an innovative leap, we have leveraged the superior properties of coaxial cables to develop coaxially shielded loop resonators. By strategically adding welding connections between the inner and outer conductors, we created substantial structural capacitance, enabling the necessary resonance capacity despite the coil’s compact size relative to the RF wavelength in an MRI system.

Unlike traditional resonators that rely on single conductive wires, these coaxially shielded resonators depend solely on their geometric properties and total length, eliminating the need for lumped elements along the cable. This design reduces losses and enhances the quality factor, significantly improving MRI performance.

One of the major advantages of this innovation is its ability to overcome size constraints. The integration of multiple welding connections allows for flexible resonator sizing while maintaining the desired resonance frequency, enabling customization for specific anatomical sites.

Traditionally, MRI coils have been hardwired to the scanner, lacking wireless functionality. To address this, we introduced intelligent self-adaptivity to the resonators. By leveraging the rectifying effect and bi-stable nonlinear behavior of PN junctions in diodes, these resonators selectively amplify the magnetic field, preventing interference with the RF transmission field and ensuring patient safety.

Our research further explores the operational principles of these resonators. By manipulating the electric field distribution, we aim to achieve frequency tunability, ensuring precise resonance alignment with MRI systems. This investigation into coaxially shielded resonators with self-adaptivity and frequency tunability offers advanced and efficient solutions for wireless and wearable imaging.

Form-fitting coils and textile metamaterials for wearable imaging

The proximity of MRI coils to the target anatomy is crucial for optimal magnetic coupling, signal reception, depth of penetration, and image quality. Leveraging the flexibility of coaxial cables and the cable-free design of coaxially shielded resonators, we have developed form-fitting coils that can be bent, twisted, and shaped to conform to the 3D contours of the body.

Supported by 3D-printed scaffolds, these coils can be easily attached to extremities such as fingers, wrists, or ankles without additional tools or fasteners, simplifying coil positioning during MRI scans. By placing these wireless coils close to the anatomical site, the form-fitting design effectively prevents the signal from dissipating into the surrounding environment, resulting in a significant gain in SNR.

Additionally, the wireless standalone coils weigh only 35 grams, a substantial reduction compared to several kilograms for commercial surface coils. Their lightweight, cable-free design reduces positioning constraints, enhances patient comfort, and requires fewer adjustments, offering a streamlined workflow that increases productivity while maintaining excellent image quality. This groundbreaking work is detailed in the June 12 issues of Science Advances.

While these form-fitting coils enhance imaging capabilities for smaller anatomical regions, clinical MRI often requires visualization of larger areas, such as the knee or spine. To address this, we propose the development of metamaterials featuring an array of coaxially shielded resonators inductively coupled to function collectively for signal acquisition. Using computer-aided embroidery, we integrate these metamaterials onto a textile substrate, creating a wearable configuration that conforms closely to patient anatomies, enhancing sensitivity. Weighing only 50 grams, including the housing fabric, these metamaterials improve patient comfort during MRI procedures.

Compared to previous designs, our metamaterials offer substantial SNR gains due to superior magnetic response and minimized electric dipole moments. MRI images acquired with this metamaterial using various pulse sequences achieve an SNR comparable to or surpassing that of a state-of-the-art 16-channel knee coil. This research is detailed in the Aug. 1 issue of Advanced Materials.

Looking ahead

Our objective is to develop advanced MRI coils that seamlessly integrate into routine healthcare settings and existing clinical workflows. This innovation promises to enhance patient care and streamline MRI procedures, making high-quality imaging more accessible and efficient.

To achieve this, we will be working closely with clinical research partners, gathering valuable insights and understanding the specific needs of targeted diseases to refine the wearable imaging technology.

Looking ahead, we plan to partner with industry collaborators to pave the way for the large-scale commercialization of wearable coils and metamaterials for MRI applications, marking a significant step forward in the evolution of MRI technology. If you’d like to help, get in touch by email here.

Xin Zhang is a distinguished professor of engineering at Boston University. Her recent research focuses on metamaterials for applications in clinical medical imaging, photonics and optics, as well as acoustic silencing and noise reduction.

Ke Wu is a postdoctoral researcher at the Photonics Center at Boston University. He specializes in integrating metamaterials and RF electronics to engineer mechanically adaptive devices and systems, with the aim of advancing magnetic resonance imaging technologies.

Xia Zhu is a PhD researcher at the Department of Mechanical Engineering at Boston University. He specializes in developing metamaterials and RF components to engineer wireless devices and systems, with the aim of improving magnetic resonance imaging technologies.

How to submit a contribution to MDO

The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design & Outsourcing or its employees.