Photon migration in tissue : laser induced fluorescence for cancer diagnostics and influence of optical properties on microvascular Doppler spectroscopy

Sammanfattning: Laser induced flourescence (LIF) is an "optical biopsy" method, based on the selective accumulation of fluorophores in neoplastic tissue. Two recently developed, non-photosensitizing tumor seeking carotenoporphyrins were assessed regarding tumor selectivity, and biodistribution, in experimental animals. A tumor to peritumoral ratio of 5-6:1 was seen in the background free substance related fluorescence in vivo, as well as ex vivo. Cerebral cortex and skeletal muscle displayed a low, and liver a high substance related fluorescence.Laser Doppler flowmetry (LDF) is based on the spectral broadening of monochromatic light, that interacts with moving red blood cells in tissue. The power spectral density of the backscattered light can be processed to yield an estimate of microvascular tissue perlusion. Using a Monte Carlo simulation model of human skin, it is demonstt·ated that for a particular light delivery/detection arrangement, Doppler shifted photons that originate from the central core and peripheral parts of blood vessels of physiological dimensions, both contribute to the detected signal. Further, more than 10 times as many photons will interact with the superficial as with the deep vascular plexus. However, due to greater velocities and concentrations of the moving scatterers, the profound circulation still may yield a greater contribution to the LDF perfusion estimate.A multiple polynomial regression method for prediction of photon pathlength and optical properties in tissue, at surlace source detector separations up to two millimeters, was developed. Using the diffuse, backscattered reflectance profile from an array of optical sensors as predictors in the model resulted in root-meansquare errors of less than three per cent for the estimated pathlength. Caucasian human skin displayed a maximum in vivo variation of ~35 % in the photon pathlength between individuals in similar locations, and within individuals comparing fingertip and forearm skin, as a result of varying optical properties.Assuming a homogenous tissue perlusion, the pathlength variations will induce a corresponding variation in the LDF petfusion signal, which can be compensated for by linearization and pathlength normalization, making intra- and interindividual comparisons of the LDF perfusion estimate feasible.