Theory And Experiment Of In Vivo Microcirculation Imaging Based On The OPS Technique

The assessment of microcirculation has great clinical, physiological andpathological significance. Orthogonal polarization spectral imaging (short forOPS imaging) is a newly developed imaging technique in the recent decade.This technique takes advantage of the fact that the photons reflected andbackscattered from superficial layers would not change its state ofpolarization, but the multiple scattering photons coming from deep layerswould be depolarized, so this method can be used to obtain two-dimensionalmorphology of the tissue from the backscattered signal. When the emissionspectrum of the light source is matched with the absorption spectrum of theerythrocyte, and the weak backscattered signal is protected from thebackground noise, the OPS technique has relatively high image contrast forthe detection of microvessels.In this paper, we built a miniaturized imaging system based on the OPStechnique. After simulating the optical path, and choosing proper components,the system can be used to detect microcirculation in vivo. A number ofexperiments are conducted, and the samples include human nailfold capillaryand the microvessel in a mouse ear. In the comparison experiments, we notonly prove that the imaging performance of the OPS technique is superior tothat of a traditional microscopy, but also find the appropriate emissionwavelength band for microcirculation imaging. Furthermore, we prove thatthis method is able to detect the trajectory of a single erythrocyte.One disadvantage of the OPS technique is that it can only be used todetect image in a specific layer, failing to continuously examine the differentdepth. To solve this problem, we build a polarimetric imaging system fordepth selectivity, especially through the use of elliptically polarized light.The experiments are conducted on a animal fat with a target inside, and themicrovessel in a mouse ear. Then we use contrast analysis to get furtherresults. The results show that the co-elliptical imaging channel has thefunction of depth examination. As the degrees of ellipticity increases from 0°to45°, the image contrast of the target is gradually enhanced, and themaximum depth able to be detected also becomes larger. When the angleincreased by45°, the co-circular light has a comparable imaging performanceto the orthogonal polarized light, but the SNR of the latter one is not as goodas the former one.Finally, the backscattering theory of biological tissue is analyzed. Weuse the Stokes-Mueller method to get the spot pattern of the Muellerscattering matrix, and prove that the shape and symmetry is relevant with theradial distance and azimuthal angle between the detector and tissue. A doublebackscattering model is built by using the Monte Carlo numerical simulation,and it is proved that double scattering is able to determine the basic pattern ofthe Mueller matrix. On the other hand, depolarization is largely dependent onhigher degree of scattering events. This simulation is a theoretical support forthe phenomenon that multiple scattering photons can reach deep layers.