Evaluation of the Ar+ laser thermal effect in rabbit brain tissue by means of optical absorption coefficients. Photoacoustic measurements.

In predicting Ar+ laser-neural tissue interaction a determination of the optical absorption properties of brain parenchyma appears indispensable. In this study a determination of absorption spectra of different areas of rabbit brain at the wavelength of an Ar+ laser beam is made. The areas considered in the study are frontal and occipital gray and white substance, thalamus, basal ganglia, cerebellar cortex and choroid plexus. Specimens of these areas measuring 2 X 3 X 3 mm were obtained from coronal sections 1 cm thick of 20 rabbit brains. The analysis of optical properties was made by means of a photoacoustic spectrometer which appears the only suitable technique for testing the optical absorption properties of biological materials. The absorption coefficient results were similar in magnitude and constant in all the areas examined, measuring approximately 50 cm-1. The prediction of lack of any selective effect for the different areas is confirmed by the sizes of lesions made in cerebral specimens of the same areas of the contralateral hemisphere by means of Ar+ laser: the lesions increase in size in proportion to the increase in output power and exposure time, independently of the areas considered. Also the histological examination of the areas shows the same morphology to be independent of the variations of both output power and exposure time. The only differences were the presence for short exposure time and low output power of a carbonized layer inner to the layer of coagulative necrosis.