Multiscale Photoacoustic Tomography of a Genetically Encoded Near-Infrared FRET Biosensor

Photoacoustic tomography (PAT) with genetically encoded near-infrared probes enables visualization of specific cell populations in vivo at high resolution deeply in biological tissues. However, because of a lack of proper probes, PAT of cellular dynamics remains unexplored. Here, the authors report a near-infrared Forster resonance energy transfer (FRET) biosensor based on a miRFP670-iRFP720 pair of the near-infrared fluorescent proteins, which enables dynamic functional imaging of active biological processes in deep tissues. By photoacoustically detecting the changes in the optical absorption of the miRFP670 FRET-donor, they monitored cell apoptosis in deep tissue at high spatiotemporal resolution using PAT. Specifically, they detected apoptosis in single cells at a resolution of ≈3 µm in a mouse ear tumor, and in deep brain tumors (>3 mm beneath the scalp) of living mice at a spatial resolution of ≈150 µm with a 20 Hz frame rate. These results open the way for high-resolution photoacoustic imaging of dynamic biological processes in deep tissues using NIR biosensors and PAT.