Boston University Researchers Design Intelligent Magnetic Metamaterial that Boosts MRI Scans

Boston University researchers have developed a new, “intelligent” metamaterial that could dramatically improve magnetic resonance imaging, making the entire MRI process faster, safer and more accessible to patients around the world. Its cost to build looks very affordable, perhaps less than $10 per unit. 

The researchers estimate that the metamaterial array, developed with the support of the National Institute of Biomedical Imaging and Bioengineering, should cost less than $10 to construct. Even though the current magnetic metamaterial prototype is a flat, thick layer, they expect to adapt it to a flexible, ultra-thin MRI enhancement sheet. Integrated with clinical MRI systems, they say, their newly discovered magnetic metamaterials have the potential to usher in a quantum leap in the performance of MRI. 

“The arrangement of this metamaterial is truly groundbreaking and innovative. Researchers have been trying to make use of metamaterials in MRI for a decade and now have achieved a substantial improvement in image quality for the first time,” said Shumin Wang, director of the NIBIB program in Magnetic Resonance Imaging. Once tested for clinical use, the metamaterial may make MRI technology less costly and more accessible globally, says NIBIB. 

MRI is used by clinicians to diagnose medical problems by spotting abnormalities that could indicate anything from a torn meniscus to muscular dystrophy. But MRIs are expensive, expose patients to radiation and they can take a long time—often the greater part of an hour for a single scan. Finding enough MRI time for waiting patients can be a problem, even in US hospitals, but in hospitals in countries like India, waiting periods of a year or more can put patients’ lives at risk. 

Xin Zhang, a BU College of Engineering professor of mechanical engineering and a Photonics Center faculty member, and a team of researchers that includes Boston Medical Center radiologist Stephan Anderson, a BU School of Medicine professor of radiology, and Xiaoguang Zhao, assistant research professor of radiology, are getting creative with metamaterials to shorten the process while also improving the images obtained. 

A schematic of the magnetic metamaterial is shown above. The metamaterial array composes unit cells featuring metallic helices, which are made of copper wiring with central polymeric scaffolding. Scale bar is 3 cm. The technology, which builds on previous metamaterial work by the team, has been described recently in several scientific journals including Advanced Materials and Communications Physics by Nature, and in BU’s The Brink

MRI works by generating a powerful magnetic field and sending radio waves into a patient’s body. “An MRI’s magnetic field is many thousands of times stronger than the Earth’s magnetic field,” says Zhao. “A precisely orchestrated series of higher-energy radio waves are sent into the human body, and the tissues emit lower-energy radio waves that are received by the MRI to produce an image.”  

Two MRI images of an onion: the image on the right shows enhancement produced by an intelligent metamaterial developed by Xin Zhang and colleagues. Image courtesy of Zhang et al.

The quality of MRI images depends to a great extent on what’s called signal-to-noise ratio (SNR). The higher the SNR, the better the image, and the most direct way to improve the SNR is to turn up the magnetic field. Unfortunately, any increase in the magnetic field also increases the complexity and cost of the MRI, as well as potential risks to patients, whose tissue and any implanted medical devices they might carry are heated up by the radiation. For that reason, radiologists who would like to get a better look inside a body cannot simply turn up the magnetic field strength.  

So Zhang and her collaborators developed a new magnetic metamaterial that when placed beside the body part that is the target of a scan boosts the energy emitted by the patient’s body, increasing SNR and improving MRI imaging. The metamaterial is made of simple copper wiring and plastic. 

Now, Zhang, Anderson, Zhao, and other team members have taken their development a big step further, developing what they call an “intelligent” metamaterial that selectively boosts the low-energy emissions from the patient’s body and literally turns itself off during the millisecond bursts of high-energy transmission from the machine.  

Zhang says that the intelligent metamaterial amplifies SNR by tenfold, which greatly enhances image quality and reduces scan time, resulting in a new way to obtain crisper MRI images at very low cost. “Shortening MRI examinations is paramount to maximizing the capacity,” says Anderson. “Not to mention revenue, as well as the overall patient experience of this powerful imaging technology.”  

“The intelligent metamaterial consists of an array of metallic helical resonators closely packed with a passive sensor,” says Zhao. “When the high-energy radio waves are coming in, the metamaterial detects the high energy level and ‘turns off’ the resonance automatically. With low-energy radio excitation, the metamaterial [turns on] the resonance and enhances the magnetic component of the radio wave.” 

That off-time, while only milliseconds long, allows clinicians to use the intelligent metamaterial to enhance the energy sent back to the MRI. It also diminishes the patient’s overall exposure to radio wave radiation and mitigates potential safety concerns, easing the path toward adoption of this technology in clinical imaging. 

“We can now build smart materials that can interact with radio waves intelligently, enhancing the wanted signal while letting the unwanted signal go,” says Zhang.  

Metamaterials represent a class of artificially structured materials, which may be engineered in order to exhibit properties not found in naturally occurring materials. Their unique properties are derived not only from the inherent properties of their constituent materials but also from the precise arrangement of their internal structure, yielding powerful design flexibility. 

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