In vivo optical path lengths and path length resolved doppler shifts of multiply scattered light

Abstract

In laser Doppler measurements, perfusion values averaged over different and basically unknown path lengths are recorded. To facilitate quantitative path length resolved perfusion measurements, we developed a phase modulated Mach-Zehnder interferometer with spatially separated fibers for illumination and detection. The goal of this study is to measure in vivo optical path lengths and path length resolved Doppler shifts and to compare these with conventional laser Doppler perfusion measurements. With a phase modulated Mach-Zehnder interferometer, we performed path length resolved perfusion measurements on human skin and its variations to external stimuli and compared these with conventional laser Doppler perfusion measurements. The method was evaluated in three human subjects on the dorsal side of the forearm to establish inter-individual within-site variations. Measurements were performed at three different locations of one individual for observing intra-individual inter-site variations resulting from the heterogeneity of the tissue, both in the static matrix and in the microvascular architecture of the skin. In all measurements, perfusion was simultaneously measured with a conventional laser Doppler perfusion monitor. In this study, we show the first results of path length resolved perfusion measurements in skin and its variations to occlusion and Capsicum cream provocation. From our data, we deduced the Doppler shifted fraction of photons, which is related to the blood volume, and the path length dependent average Doppler shift, which is related to the mean velocities of red blood cells. The Doppler shifted fraction of photons is decreased from 28% to 18% during occlusion and increases to 41% when capsicum cream was applied to the skin. Inter- and intra-individual inter-site measurements demonstrated variations in optical path length distributions and path length resolved Doppler shifts. The Doppler shifted fraction of photons measured on the fingertip is about 38% and that measured on the dorsal and palmar sides of the forearm are 32% and 17%, respectively. The path length distributions depend on the skin site that is being probed and the intra-individual inter-site variability is higher than the inter-individual within-site variability measured on comparable sites between different individuals. In this study, we demonstrated, for the first time to our knowledge, that in vivo path length resolved perfusion measurements are feasible. Optical path length distributions of multiply scattered light, spanning a range of 0-6 mm, and their response to external stimuli such as occlusion and capsicum cream provocation have been measured. This method will enable better interpretation of inter- and intra-individual inter-site variations in the LDF readings that are introduced by the variance in tissue optical properties. The inter- and intra-individual inter-site variations measured with our setup results indicate that that these variations should be taken into account while comparing the perfusion readings from comparable sites between individuals and from different sites of the same individual. Furthermore, the observed inter- and intra-individual inter-site variations in path length resolved Doppler measurements indicate the inherent limitation of conventional LDPM that restrict its clinical usefulness, due to its dependence on the unknown photon path length. Consequently, this method will enable to correctly interpret or counter-act the inter- and intra-individual inter-site variations in the LDF readings introduced by the variance in tissue optical properties. This approach also enables to discriminate between the Doppler-shifted photons resulting from interaction with the moving red blood cells and the non-shifted light scattered only by the surrounding static tissue matrices.