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Tissue Welding Gets Helping Hand

Photonics Spectra
Apr 1999
Michael Van Belle

Tissue welding is a laser-based surgical technique that is still in the research stage, and several groups are involved in exploring and improving the method's viability. Researchers at Lawrence Livermore National Laboratory have developed a device that more easily and accurately measures the heat resulting from tissue welding. Their two-color mid-infrared thermometer relies on a single hollow glass optical fiber to provide dynamic noncontact temperature and emissivity measurement that is fast and has a high spatial resolution.
The laboratory was specifically looking for better methods of laser control. The work was led by the US Department of Energy as part of a Cooperative Research and Development Agreement with Conversion Energy Enterprises of Spring Valley, N.Y.
"We started with a specific application for tissue welding, where there is a need for accurate temperature modeling," said Peter Celliers, staff physicist in laser programs at Lawrence Livermore. "We have to detect fairly low room temperature heating in a localized region. The temperature of surfaces has to be elevated [to a precise level]. The main advantage of this technology is that, since we have two-wavelength detection, the emissivity effect is canceled out."
Because of the nature of the fiber used to collect the radiation, the scientists can zero in on a small point-like region of the tissue, he said. This also helps to obtain more accurate temperature measurement.
The radiation signal travels through the single optical fiber and is split by a reflective optical chopper. Two thermoelectrically cooled mid-IR HgCdZnTe photoconductors monitor the split signals.
The single fiber eliminates the problem of aligning two fibers to the same spot on the target. Because of the small numerical aperture of the hollow glass fiber, the calculated temperature and emissivity are effectively independent of the fiber-to-target distance, Celliers noted. This feature allows the distance between the fiber tip and target to vary without affecting the measurement.
A laboratory prototype has been effectively demonstrated in laser-assisted tissue welding experiments, according to Barbara A. Soltz, CEO of Conversion Energy. "It generates the feedback signals necessary to control the laser output to maintain a preselected temperature at the tissue site."
"This means of controlling a laser can have a wide variety of medical and nonmedical applications," said Soltz. "Any technique that uses lasers, in which you need very accurate control of the target temperature, would benefit from this."
An upgraded version of the temperature controller is in progress, she said, with the engineering being done jointly by Conversion Energy and Lawrence Livermore. Conversion Energy also plans to develop a commercial version.


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