Characterizing sources and targets of illumination in living tissue is challenging. MIT researchers have demonstrated that they can detect light deep into tissues like the brain using a customized MRI sensor.
Because so much light is absorbed or scattered as it passes through tissue, imaging light in deep tissues is particularly challenging. The MIT team overcome that difficulty by creating a sensor that transforms light into an MRI-detectable magnetic signal. This type of sensor could map light emitted by 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 image the distribution of light in tissue, and that’s important because people who use light to stimulate tissue or to measure from tissue often don’t quite know where the light is going, where they’re stimulating, or where the light is coming from. Our tool can be used to address those unknowns.”
“We wanted to create a magnetic sensor that responds to light locally and therefore is not subject to absorbance or scattering. Then this light detector can be imaged using MRI.”
Scientists have previously developed MRI probes that can interact with several molecules in the brain, including dopamine and calcium. The magnetic interactions between the sensors and the surrounding tissue are altered when these probes connect to their targets, which results in the dimming or brightening of the MRI signal.
The scientists decided to encapsulate magnetic particles in a liposome 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 investigation. The liposomes’ lipids become more permeable to water, or “leaky,” when exposed to a specific wavelength of light. This makes it possible for the magnetic particles within 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 named liposomal nanoparticle reporters (LisNR), can change from being permeable to impermeable. In this study, the researcher designed particles that, when exposed to ultraviolet light, become leaky and, when exposed to blue light, become impermeable once more. The particles may react to various light wavelengths, the researchers also demonstrated.
Xin Yu, assistant professor of radiology at Harvard Medical School, said, “This paper shows a novel sensor to enable 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 at the striatum, a region of the rat brain that is important in movement planning and rewarding behavior. The scientists could map the spread of light from an optical wire inserted nearby after injecting the particles all over the striatum.
Researchers who conduct optogenetic experiments in the brain may find this sensing valuable because the fiber they utilise is comparable to those used for optogenetic stimulation.
Jasanoff said, “We don’t expect that everybody doing optogenetics will use this for every experiment — it’s more something that you would do once in a while to see whether a paradigm that you’re using is producing the profile of light that you think it should be.”
The sensor is expected to be useful for monitoring patients receiving treatments that involve light, such as photodynamic therapy, which uses light from a laser or LED to kill cancer cells.
Journal Reference:
- Simon, J., Schwalm, M., Morstein, J. et al. Mapping light distribution in tissue by using MRI-detectable photosensitive liposomes. Nat. Biomed. Eng (2022). DOI: 10.1038/s41551-022-00982-3