Secrets of the Very Small

A medical pathologist, Kerschmann encountered this problem firsthand about 15 years ago, when he was working at the Wellman Laboratories of Photomedicine at Massachusetts General Hospital in Boston. The lab was developing a laser treatment for the inflated blood vessels that cause unsightly skin imperfections such as port wine stains and spider veins. Kerschmann's task was to determine the specific laser energy and pulse length that would collapse the blood vessels without damaging collagen, the protein that gives skin its elasticity.




Doctors would take skin samples from volunteers, then Kerschmann would embed them in
wax and slice them into paper-thin sections. He then examined each section under a microscope to see where in the branching network of blood vessels the laser hit first. But Kerschmann was soon frustrated. The 2-D world he was seeing through existing scopes could not help him answer what was essentially a 3-D problem.




"I couldn't get a good look at them," he recounts. "You see the round cross sections through each of the the vessels, but they don't tell you much about how the vessels interconnect and branch."

It was possible, Kerschmann knew, to generate a composite 3-D computer image that would reflect all the individual sections of his sample. But he also knew that the cutting process so distorts and damages the slices that piecing them back together afterward would create a grossly inaccurate picture. It would be like slicing up a soft loaf of bread, then gluing the squashed slices back together and assuming you'd reconstructed the shape of the original loaf.




Kerschmann eventually figured out a way to get around the cutting problem. Instead of dicing up an object at the outset and placing it on hundreds of glass slides -- the traditional method -- he decided to alternate imaging and cutting the object in a sequential process that is ultimately more precise.




Kerschmann usually begins his process with a sample about the size of a pencil eraser. It is stained with fluorescent dyes and embedded in hard black plastic. He then clamps it to his microscope. The scope shoots laser light through the sample, which excites the fluorescent dyes. A digital camera captures the image, and then a blade slices off the sample's outer layer. That layer, which has been damaged by the cutting, is discarded; the rest of the sample, however, remains intact. Next, the sample's freshly exposed layer is imaged; then it too is removed. As this two-part process is repeated over and over, the sample slowly shrinks until there is nothing left of it. What is left, however, are the 1,000 or so images of each of its layers, stored on a computer. A software program compiles them into a single, luxuriously detailed, three-dimensional view of the original sample.




Adding to the precision of Kerschmann's 3-D images is the fact that the plastic in which he encases his samples is black. That opacity means that when light from the microscope hits the sample, it penetrates only the outermost layer -- the one that will be discarded on the next go-round. As a result, there is no visual overlap from one image to the next.




Once the process is complete, Resolution sends the client a disc containing the composite image and its many components. The company also provides a computer workstation equipped with special software that enables clients to view, analyze, and manipulate the data. The cost of the package is upward of $24,000, but clients say it's well worthwhile.