The Effects of Laser Irradiation on the Central Nervous System†‡

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Abstract
This study of the effect of laser radiation on the brain was made to further our understanding of the interaction of light with living matter and to provide a basis for the possible uses of laser energy in biology and medicine. The physics and engineering aspects of lasers are adequately treated elsewhere (1–3). Light produced by lasers differs from that of conventional sources in that it is strongly coherent and emitted in a narrow, collimated monochromatic beam which can be focused to produce high energy and power densities which are destructive to tissues. The ability to produce sharply circumscribed superficial lesions has been demonstrated (4) and suggests the application as an ablative tool in neurosurgery and neurophysiological research. Using equipment similar to ours, Fine et al. (4, 5) found that 75 per cent of mice died within 24 hours when a single 1-millisecond focused or unfocused laser pulse of about 100 joules struck the brain of the mouse through the intact scalp and skull. In earlier experiments we (G) were able to kill nearly every mouse within a few minutes, using only a single 1-millisecond, 20-joule pulse focused within the brain through the scalp and skull. The lethal mechanism was not explained by the small focal lesions. The occurrence of isolated hemorrhages in the base of the brain away from the site of impact had been thought to be an artifact produced during the removal of the brain from the cranium. If laser light had actually penetrated to that depth in sufficient intensity to produce hemorrhage, a continuous lesion would be expected.

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