Characterizing sources and targets of illumination in living tissue is challenging. MIT researchers have shown they can detect light deep in tissues such as the brain using a custom MRI sensor.
Because so much light is absorbed or scattered as it passes through tissue, it is extremely difficult to image light in deep tissues. The MIT team overcame that difficulty by creating a sensor that converts light into an MRI-detectable magnetic signal. This type of sensor can map light emitted from optical fibers implanted in the brain, such as the fibers used to stimulate neurons during optogenetic experiments.
Alan Jasanoff, an MIT professor of biological engineering, brain and cognitive sciences, and nuclear science and engineering, said: “We can visualize the distribution of light in tissue, and that’s important because people who use light to stimulate tissue or to measure tissue often don’t know exactly where the light is going, where they’re stimulating or where the light is . originating from. Our tool can be used to tackle those unknowns.”
“We wanted to make a magnetic sensor that reacts locally to light and is therefore not subject to absorption or scattering. Then this light detector can be imaged with MRI.”
Scientists have previously developed MRI probes that can interact with various molecules in the brain, including dopamine and calcium. The magnetic interactions between the sensors and surrounding tissue change when these probes connect to their targets, resulting in the MRI signal being dimmed or brightened.
The scientists decided to encapsulate magnetic particles in a liposomal nanoparticle to create a light-sensitive MRI probe. The specific light-sensitive lipids that Trauner had previously created are used to develop the liposomes in this study. The lipids of the liposomes become more permeable to water, or “leak,” when exposed to a specific wavelength of light. This allows the magnetic particles inside to interact with the water and produce a signal that an MRI can pick up.
Depending on the light they are exposed to, the particles, which the researchers called liposomal nanoparticle reporters (LisNR), can change from permeable to impermeable. In this study, the researcher designed particles that become leaky when exposed to ultraviolet light and become impermeable again when exposed to blue light. The particles can respond to different wavelengths of light, the researchers also showed.
Xin Yu, assistant professor of radiology at Harvard Medical School, said: “This paper shows a new sensor that enables photon detection with MRI through the brain. This illuminating work introduces a new avenue to bridge photon- and proton-driven neuroimaging studies.”
The researchers tested the sensors in the striatum, an area of the rat’s brain that is important in planning movements and rewarding behavior. The scientists were able to map the spread of light from an optical wire inserted nearby after injecting the particles all over the striatum.
Researchers conducting optogenetic experiments in the brain may find this sensation valuable because the fibers they use are similar to those used for optogenetic stimulation.
Jasonoff said, “We don’t expect everyone who does optogenetics to use this for every experiment — it’s more something you’d do occasionally to see if a paradigm you’re using produces the profile of light you think it should be. .”
The sensor is expected to be useful for monitoring patients undergoing light treatments such as photodynamic therapy, which uses light from a laser or LED to kill cancer cells.
- Simon, J., Schwalm, M., Morstein, J. et al. Mapping light distribution in tissue using MRI detectable photosensitive liposomes. Wet. Biomed. English (2022). DOI: 10.1038/s41551-022-00982-3