Light dosimetry in brain tissue: an in vivo model applicable to photodynamic therapy

The intensity of blue-green (488 nm) and red (633 nm) light as a function of increasing distance from an implanted laser fiberoptic source in the brains of anesthetized rats was measured. There was significant (90%) attenuation of the light intensity for both 488 nm and 633 nm at a distance of 2.5 mm from the intracerebral laser light-emitting source. At greater distances from the light-emitting source that corresponded to areas in the cerebral cortex, there was greater attenuation of light intensity for increasing distance with 488 nm than with 633 nm because of the increased vascularity of the cortical mantle and absorption of 488 nm by hemoglobin. When hematoporphyrin derivative was given intravascularly, there was increased attenuation of 633 nm through the cerebral cortex that was similar to that seen for 488 nm in normals. Direct assessment of light intensity in living tissue, which we have shown is possible in this report, will aid in our understanding and interpretation of the histological changes that occur following photodynamic therapy.