Mark Schnitzer, who recently became an HHMI Investigtor, has a new paper out on improved optics for his group’s miniature probe microscopes. Mark has been pioneering these tiny probes to do optical imaging in deep brain structures, and is one of the only games in town if you want to look deeper then a millimeter into the brain without sucking off all the ‘irrelevant mush’ in between your microscope and the part of the brain you are interested in. A beer-lubricated former Bell Labs employee (not Karel) years ago confided to me that he wasn’t too impressed with these microprobe systems,because “its just a GRIN lens on a stick”, ignoring the painstaking engineering and minaturization of motor, supports and light paths. The limitations were quite significant though, in this new Nature Methods communication, In vivo fluorescence imaging with high-resolution microlenses, Barretto et al. note that “The best Rayleigh resolutions values achieved by two-photon fluorescence imaging with GRIN lenses are ~1.6 um lateral and ~12um axial, yielding highly elongated point spread functions that impede acquisions of high-quality, three-dimensional image stacks.” That elongated point spread function really blurs the image and dramatically reduces the power and brightness of the two-photon imaging mode. I assume this is why most of the data I’ve seen Mark present was with one-photon illumination. Now, the authors have addressed this shortcoming.
They coupled a plano-convex lens to a custom fabricated GRIN lens whose refractive properties were designed to compensate for the spherical aberration of the plano-convex lens. This system achieved near diffraction-limited resolution in both the lateral and axial dimensions. Rather then the dim blur of previous iterations, the new system clearly resolves synaptic spines on the dendrites of fluorescent neurons buried deeply in the hippocampus of live mice. The example system in the paper has a 1mm diameter, while the previous systems were as narrow as 0.3mm, so there is still plenty of room for further miniaturization, although I’m not sure how that would affect the light gathering capacity of the lens. Importantly, Mark’s systems are finally getting commercialized so that a much larger scientific population can start to benefit from the technology soon.