Background.Analysis of hepatic microvascular perfusion in humans by direct imaging has been impossible so far. Orthogonal polarization spectral (OPS) imaging represents a new technology that combines simultaneous epi-illumination of the subject with linearly polarized light and noninvasive imaging of the microcirculation by reflectance spectrophotometry. The aim of this study was to evaluate the feasibility of studying the human hepatic microcirculation by OPS imaging in vivo and to define microcirculatory parameters for physiologic conditions.Methods.The hepatic microcirculation was analyzed in four different regions of both liver lobes in 11 healthy individuals undergoing partial liver resection for living-donor liver transplantation. The optical probe was gently positioned on the liver surface and sequences of at least 20 sec per measurement were recorded by a charge-coupled device camera on videotape. Microhemodynamic parameters were quantified off-line by single-frame and frame-to-frame analysis using a computer-assisted image analysis system.Results.OPS images of the hepatic microcirculation showed an acceptable quality with good resolution. Quantitative analysis revealed a sinusoidal red blood cell velocity of 0.97±0.43 mm/sec, a sinusoidal diameter of 8.8±0.9 &mgr;m, a sinusoidal volumetric blood flow of 58.2±9.6 pL/sec, an intersinusoidal distance of 22.6±2.5 &mgr;m, and a mean functional sinusoidal density of 391±30 cm−1. Apart from the sinusoidal red blood cell velocity, all data of the parameters studied matched the pattern of normal distribution.Conclusions.OPS imaging enabled for the first time direct in vivo visualization and quantification of the human hepatic microcirculation, providing significant insight into microvascular physiology of the human liver, to the extent that these data can be considered to represent physiologic values for human hepatic microcirculation.