By allowing clinicians to look noninvasively inside the human body, magnetic resonance imaging (MRI) has become a mainstay of injury and disease detection and treatment planning and monitoring. But not everyone has benefited equally: the most powerful modern MRI tech is typically bulky, rigid, and expensive, limiting its use and impact in low-resource and remote areas.
At Boston University, engineer Xin Zhang is leading a team that's working to democratize access to MRI, developing innovation-infused devices that can make scans faster, cheaper, and more accurate. To do it, they've turned to metamaterials—precisely engineered structures that use surprisingly ordinary building blocks, such as copper, fabric, and plastic, to manipulate electromagnetic waves and radio frequencies.
Their work has led to a string of breakthrough devices that can sharpen and speed up MRI imaging of knees, ankles, spines, and more. Each new metamaterials tool and method—from resonators that manipulate magnetic fields to wearable, jewelry-like bracelets that cut background noise—is capable of dramatically boosting the power of MRI. The researchers have reported their findings in a series of recent journal articles.
"How can we improve MRI technology to enable clear imaging that's also affordable, accessible, and tolerable for patients?" says Zhang, a BU College of Engineering distinguished professor of engineering. "This is a practical problem I've been interested in for a long time."
Manipulating Magnetic Fields
Zhang, who has studied the use of metamaterials in a diverse range of fields, from optical applications to noise reduction, began focusing on their potential to improve medical imaging in 2016. Within a few years, she and her team had developed what she calls an "intelligent metamaterial" to speed up scans, as well as a tunable helmet that could channel an MRI machine's magnetic field to deliver clearer images of the brain and drastically cut scanning time.
In one of the latest papers, published in Advanced Science, they build on that work with computationally designed wearable metamaterials that can be fitted to any part of the body—even irregularly shaped areas like the elbow or knee. In the article, the researchers show examples of how the metamaterials could be used to improve scans of the ankle (picture a brace of connected discs surrounding the joint). Because they "readily conform to a patient's knee, ankle, head, or any part of the body in need of imaging…while ensuring an optimal resonance frequency," the researchers write, the new tech could facilitate "the widespread adoption of metamaterials in clinical MRI applications."
In their earlier work, the team was able to manually design the helmet to fit over the human head. But in the latest study, says Ke Wu (ENG'23), first author of the paper and a postdoctoral fellow in Zhang's lab, "we recognized that free-form deployable metamaterials fitted to other parts of the body would require computational aid."
Wu developed algorithms and programs capable of analyzing a 3D scan of a part of the body and, within less than a second, calculating the geometry and arrangement of helical resonators—structures made of plastic and thin copper coils—that can manipulate the magnetic field of MRI. Critically, these arrays of coils help to improve the signal-to-noise ratio (SNR) of MRI of the target area, reducing the fuzziness of imaging that's caused when background electromagnetic signals seep into view.
Wu's computational programs use the principles of circle packing—a geometric approach to squeezing circles together without any of them overlapping—to determine the best array and architecture for arranging the magnetic coils. They can also be tuned to resonate with a particular radio frequency, while the free-form shapes can be integrated into comfortable, wearable cuffs.
Boosting MRI with Low-Cost Materials
In related work published in an Advanced Materials paper, Zhang's team demonstrated an alternative wearable metamaterial design for MRI that replaces copper and plastic coils with loops made from coaxial cables—the same cables used to bring you the internet. Coaxial cables are designed to transmit and shield high-frequency electrical signals from their surroundings, preventing unintended loss of signal. "This material has inherent advantages because it is lightweight, flexible, and restricts the electrical field to exactly where you want it," says Xia Zhu (ENG'26), first author of the paper and a graduate student in Zhang's lab.
Zhu created fabric-based wearable metamaterials—each using only about $50 of supplies—designed to bring loops of coaxial cables as close as possible to the part of the body undergoing a scan. In the paper, the team illustrates a potential knee scan: a pad of lightweight fabric, covered with a handful of coils, bending to the curve of the patient's leg as they lie in the MRI machine. The researchers found it achieved "substantial electric field attenuation in its proximity, thereby minimizing electric field exposure to the imaging subject."
Pushing even further, the team sought to develop an entirely wireless, formfitting wearable metamaterial that could boost SNR and passively tune and amplify the MRI signal. "To create a design this simple and elegant, we had to solve several problems first," says Zhang, who's affiliated with the BU Photonics Center, which provided technical assistance for much of the latest research.
In a paper published in Science Advances with their longtime collaborator Stephan W. Anderson, a BU Chobanian & Avedisian School of Medicine professor of radiology, Zhang's team demonstrated that the coaxial cables can be arranged into freestanding cuffs without additional support materials—no fabric needed. They prototyped rings and cuffs sized to enhance MRI scans of the spine, the wrist, and a single finger—and in every experiment proved their seemingly simple design could amplify SNR and enable crisp MRI. The looped and ringed cables look like modern art or custom jewelry.
"Our recent designs demonstrate several strategies for using metamaterials to boost MRI using low-cost materials," Zhang says, "which we hope will be translated into technologies that allow more patients around the world to benefit from MRI."
The first paper was supported by the National Institute of Biomedical Imaging and Bioengineering and BU's Rajen Kilachand Fund for Integrated Life Sciences and Engineering; the second and third papers were supported by the Kilachand Fund.
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