Researchers at Rensselaer Poytechnic Institute (RPI) have developed a new approach to optical imaging that makes it possible to quickly and economically monitor multiple molecular interactions in a large area of living tissue such as an organ or a small animal — technology that could have applications in medical diagnosis, guided surgery, or pre-clinical drug testing. The method is capable of simultaneously tracking 16 colors of spatially linked information over an area spanning several centimeters, and can capture interactions that occur in mere billionths of a second.

New method quickly, economically, and accurately tracks multiple in vivo interactions. (Image courtesy of RPI)

In biomedical applications, optical imaging has many advantages over techniques such as MRI and PET, which use magnetism and positron emissions to acquire images inside of living tissue. The method makes use of advanced optical imaging techniques — fluorescence lifetime imaging paired with Forster resonance energy transfer — to reveal the molecular state of tissues. In fluorescence lifetime imaging (FLIM), molecules of interest are tagged with fluorescent “reporter” molecules, which, when excited by a beam of light, emit a light signal with a certain color over time that is indicative of their immediate environment. Reporter molecules can be tuned to offer information on environmental factors such as viscosity, pH, or the presence of oxygen. FLIM is ideal for the thick tissues of a body because it relies on time information, rather than light intensity, which degrades significantly as it travels through tissue. The researchers used Forster resonance energy transfer (FRET), to determine close proximity between two similarly tagged molecules, such as a drug and its target, based on an energy transfer that occurs only when the tagged molecules are delivered into the diseased cells for maximal therapeutic efficacy.

However, while the FLIM-FRET method generates a signal rich in information, collecting that signal quickly and economically is problematic. Current methods rely on expensive cameras, which can image only one reporter at a time, and scanning the subject can take hours as the camera collects information from its full field of vision. To overcome this obstacle, the researchers dispensed with cameras and instead used a single-pixel detection method combined with a mathematical sampling technique (based on a Hadamard transform) that allowed them to collect sufficient relevant information in 10 minutes to construct a precise image. Three detection devices positioned around the sample provided spatial information used to construct a three-dimensional image. The detection method can collect information on 16 spectral channels simultaneously.

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Photonics & Imaging Technology Magazine

This article first appeared in the March, 2019 issue of Photonics & Imaging Technology Magazine.

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