When it comes to medical imaging, it’s often a trade-off between how deep you can probe and what resolution you have. With light microscopes, you get excellent resolution, down to submicron structures inside cells or tissue — but the vision only penetrates to a about 1 millimeter or so, which renders it unusable in many situations (beyond this depth, light starts scattering).
Magnetic resonance imaging (MRI), meanwhile, uses radio frequencies that can reach deep enough to go through the entire body — but the resolution is much worse (a resolution of about one millimeter, 1,000 times worse). Now, researchers have found a way to make the best of both worlds.
Now, a new study from a University of California-Berkeley researcher reports a new technique that brings new light into the world of medical imaging. With the aid of microscopic diamond tracers, doctors may soon obtain information via MRI and optical fluorescence simultaneously, obtaining high-resolution images from up to one centimeter below the surface of the tissue — 10 times deeper than with light alone.
“This is perhaps the first demonstration that the same object can be imaged in optics and hyperpolarized MRI simultaneously,” said Ashok Ajoy, UC Berkeley assistant professor of chemistry. “There is a lot of information you can get in combination because the two modes are better than the sum of their parts. This opens up many possibilities, where you can accelerate the imaging of these diamond tracers in a medium by several orders of magnitude.”
The technique, which would be useful for studying cells and tissues outside the body when looking for markers of disease, utilizes diamonds as a type of biological tracer. This relatively new approach relies on the fact that microdiamonds can have some of their carbon atoms kicked out and replaced by nitrogen. This process leaves empty spots in the crystal lattice that become fluorescent when hit by a laser.
“It turns out that if you shine light on these particles, you can align their spins to a very, very high degree—about three to four orders of magnitude higher than the alignment of spins in an MRI machine,” Ajoy said.
A somewhat similar approach can also work with magnetic imaging, Ajoy realized. Some atoms in the microdiamonds can be polarized as they become fluorescent. In other words, the diamonds can be simultaneously used to image tissue with optical microscopes and magnetic methods like MRI.
“Optical imaging suffers greatly when you go in deep tissue. Even beyond 1 millimeter, you get a lot of optical scattering. This is a major problem,” Ajoy said. “The advantage here is that the imaging can be done in radio frequencies and optical light using the same diamond tracer. The same version of MRI that you use for imaging inside people can be used for imaging these diamond particles, even when the optical fluorescence signature is completely scattered out.”
Of course, there’s still a long way to go before the approach can actually be used practically. This is, for now, just a proof of concept — getting it to work on medical equipment (let alone equipment that is not very costly) is a whole different ball game.
The good news is that these diamond tracers are inexpensive and easy to design, Ajoy says. This could allow for inexpensive and performant imaging machines. Meanwhile, MRIs go for millions of dollars, which often means that only large, rich hospitals can afford them.
Journal Reference: Xudong Lv el al., “Background-free dual-mode optical and 13C magnetic resonance imaging in diamond particles,” PNAS (2021). www.pnas.org/cgi/doi/10.1073/pnas.2023579118