Measurement of temperature distributions after pulsed IR radiation impact in biological tissue models with fluorescent thin films

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Abstract
Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at 2.94 micrometers wavelength. To achieve control over the thermal damage caused to the tissue and over the extent of the coagulation zone, a thorough knowledge of the local temperature distribution arising near the impact zone is necessary. Calculations are possible in some simple cases, whereas in others, where liquified tissue material acts as a secondary heat source long after the pulse, a time resolved direct measurement of the temperature distributions with microscopical spatial resolution would be desirable. We have developed a method for measuring two-dimensional temperature distributions in optically transparent media with a high time resolution (up to 4 ns) and with microscopical spatial resolution by imaging the temperature dependent fluorescence distribution of 2 micrometers thin films positioned inside the target. With this method we have measured the temperature distributions at different times after the impact of single pulses from an Er:YAG laser at various fluences in gelatin targets, which we use as model for biological tissue. The results are compared with the thermal damage inflicted in vitro to different types of animal tissue. A strong dependence of the temperature distributions and their dynamical behavior on pulse fluence and water content of the target is observed, in congruence with the coagulation zones observed biological tissue.

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