I went in and skimmed, though admittedly this is outside my area of expertise. They're basically using a fluorescence technology (FRET) with a reporter attached

I went in and skimmed, though admittedly this is outside my area of expertise. They're basically using a fluorescence technology (FRET) with a reporter attached to a tagged piece of DNA fused to the cell surface. Data capture is through imaging. As is usally the case with "science by press release", I would characterize this as evolutionary not revolutionary. People have been using fluorescence-labeling successfully for a long time. What seems to be novel here is that they are able to do in-vivo sensing at very high rates. Most of the tech I'm aware of hasn't allowed for that very well. Usually you can get a static image, but it appears they are actually able to capture temporal changes. Plus using the microscope in-vivo is pretty interesting, though I'm sure that bit alone has been done before. In a nutshell, they can use a confocal microscope (either inside or outside an animal) to look at the cells emitting light and where they are located. It looks like a really nice study. It's definitely not like the "sensor" you picture when you read the headline though.