Following work by other researchers with dielectrics, metals and various biological tissues, scientists from the University of California, San Diego, Colorado School of Mines in Golden, and Science Applications International Corp. of Arlington and McLean, Va., have used an ultrafast Ti:sapphire laser to selectively ablate layers of brain tissue in vitro. Moreover, they have demonstrated that the same setup, when producing nonamplified pulses, is a suitable excitation source for two-photon laser scanning microscopy, yielding a single apparatus for all-optical Reporting in the July 3 issue of Neuron, the team describes its investigations into the suitability of the technique for sequentially sectioning and imaging adult and embryonic rodent brains. Layers of the fluorescently labeled tissue were imaged by two-photon microscopy and then removed by raster-scanning the amplified pulses across the sample. Examination of the tissue below an ablated layer indicated that the 100-fs, 300-µJ pulses of 800-nm radiation caused no photodamage.

In application, the images of the individual layers, which have a resolution of approximately 0.5 x 0.5 x 1.0 µm, are combined to produce 3-D renderings of the samples. An optimized system would take 18 hours to image the brain of an adult mouse, but the scientists note that the technique lends itself to automation.