PASADENA--Researchers have recently achieved a certain amount of success in using laser light to see through scattering media such as human tissue. The new technology could eventually have medical applications in situations where X rays are ineffective or downright dangerous.
According to Seth Marder of the California Institute of Technology, his team and a group of collaborators from the University of Arizona have developed a photorefractive polymer that is highly sensitive to near infrared light.
Using this polymer, the group is now able to see through about half an inch of 2 percent milk. They have achieved this by custom-designing a polymer for a state-of-the-art laser holography setup.
Marder, a chemist at Caltech's Beckman Institute, says the work capitalizes on the fact that certain low-energy wavelengths can indeed penetrate solid substances to a certain extent. Visible light, radio and TV waves, infrared heat from an oven, and X rays are all manifestations of electromagnetic radiation that differ only in wavelength and energy.
"If you hold your hand up to a bright light source with your fingers closed tightly, you can actually see light coming through the skin itself," he explains. "In your body there are various things that absorb light such as amino acids, DNA, hemoglobin—which means that you can't see through them with ultraviolet or visible light.
"But many of these tissues stop absorbing light at a low enough energy," he says. "It turns out that, in the near infrared, your body is relatively transparent."
The goal, then, is to analyze this light that has penetrated tissue, and this is where Bernard Kippelen and Nasser Peyghambarian and their team at the Optical Sciences Center of the University of Arizona come in. Using extremely fast lasers, the Arizona team is able to look only at photons of light referred to as "ballistic photons," while filtering out scattered photons that arrive an instant later. The scattered photons lead to tremendous distortions of the image, rendering it useless. By filtering out the scattered photons (leaving only the ballistic photons) it is possible to recapture the original image.
The filtering technique, in fact, is holographic time gating and keys on the ability of a femtosecond laser (with light pulses of a millionth of a billionth of a second) to isolate the information from the ballistic photons.
First, a laser pulse is shot at the tissue to be imaged while a reference beam originating from the same laser source is also introduced into the optical system from another angle. The ballistic photons then interact with laser pulses from the reference beam and record a hologram in the photorefractive polymer developed by Caltech and the University of Arizona.
That hologram contains only the image from the ballistic photons. The delayed scattered photons do not form an interference pattern with the reference pulses because they arrive later and consequently do not write a hologram.
The ballistic photon hologram can then be reconstructed by sending a third laser beam from the opposite side (parallel to the original beam hitting the tissue). This beam is altered by the hologram recorded in the polymer. This altered probe beam then is isolated by the optical system to reconstruct a nearly undistorted image of the object.
The result is an image built of light, using infrared energy that has looked inside the object to a slight extent, but an image that has not required the use of high-energy radiation.
An application that Kippelen sees for the future is the imaging of human retinas. The onset of glaucoma can be detected quite early if certain structural changes just beneath the surface of the retina can be imaged.
Marder cautions that the techniques are still rudimentary, and even in their final form may not see through nearly as much tissue as X rays. But the object is not to replace X rays in all applications, he says—just certain ones in which the conditions are appropriate.
A recent technical report on the research can be found in the January 2 issue of the journal Science. In addition to Kippelen, Marder, and Peyghambarian, the other authors of the paper are Eric Hendrickx, Jose-Luis Maldonado, Gregoire Guillemet, Boris L. Volodin, Derek D. Steele, Yasufumi Enami, Sandalphon, Yonj-Jing Yao, Jiafu F. Wang, Harald Roeckel, and Lael Erskine.
