Scientists from the Netherlands and Russia have developed a new technology for enhancing the local sensitivity of magnetic resonance imaging (MRI) scanners. The metasurface-based MRI has been tested on human test subjects for the first time. The metasurface consists of thin resonant strips arranged periodically. Placed under a patient’s head, it provides much higher image quality from the local brain region.

A new metasurface-based technology enhances the local sensitivity of MRI. (Credit: ITMO University)
Comparison of MRI scans with and without the use of a metasurface. (Credit: ITMO University)

The results, published in Scientific Reports, show that the use of metasurfaces may reduce image acquisition time, make the procedure more comfortable for patients, and acquire higher resolution images, all of which together enable diseases to be diagnosed at an earlier stage.

MRI is a widely used medical technology for examination of internal organs that can provide, for example, information on structural and functional damage in neurological, cardiovascular, and musculoskeletal conditions, as well as playing a major role in oncology. However, due to its intrinsically lower signal-to-noise ratio, an MRI scan takes much longer to acquire than a computed tomography or ultrasound scan. This means that a patient must lie motionless within a confined apparatus for up to an hour, resulting in significant patient discomfort, and relatively long wait times in hospitals.

Specialists from Leiden University Medical Center in the Netherlands and ITMO University in Russia have acquired human MR images with enhanced local sensitivity provided by a thin metasurface — a periodic structure of conducting copper strips. The researchers attached these elements to a thin flexible substrate and integrated them into the close-fitting receiving coil arrays inside the MRI scanner.

“We placed such a metasurface under the patient’s head, which increased local sensitivity by 50 percent. This allowed us to obtain detailed scans of the occipital cortex in half the usual time. Such devices could potentially reduce the duration of MRI studies and improve its comfort for subjects,” says Rita Schmidt, the lead author of the paper and researcher in the department of radiology at Leiden University Medical Center.

The metasurface enhances the signal-to-noise ratio in the region of interest.

“This ratio limits the MRI sensitivity and duration of the procedure,” notes Alexey Slobozhanyuk, a research fellow at ITMO University’s International Laboratory of Applied Radioengineering. “Often the scans must be repeated many times and the signals added together to separate actual data from random noise.”

According to the scientists, the metasurface can also increase the definition of the resulting scans.

“The size of voxels, or 3D pixels, is also limited by the signal-to-noise ratio. Instead of accelerating the procedure, we can adopt an alternative approach and acquire more detailed images in the same amount of time as before,” says Andrew Webb, leader of the project and professor of radiology at Leiden University Medical Center.

Until now, no one has succeeded at integration of metamaterials into close-fitting receiver arrays because the metasurfaces’ dimensions were too wide. The novel 8-mm-wide design of the metasurface helped solve this issue.

“Our technology can be applied for production of metamaterial-based ultrathin devices for many different types of MRI scans, but in each case, one should first carry out a series of computer simulations as we have done in this project. One needs to make sure that the MRI device’s properties do not affect the metasurface’s performance,” concludes Schmidt.

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