Deep Tissue Hemodynamics Monitoring by Layered Model-Based Diffuse Optical Spectroscopy.

Abstract

In this study, we developed two spectroscopy techniques, ‘Diffuse optical metabolic spectroscopy’ and ‘Layered model-based diffuse optical spectroscopy’. A diffuse optical metabolic spectroscopy is developed to provide tissue metabolic rate of oxygen consumption in a simple and convenient manner. And A layered model-based diffuse optical spectroscopy is developed to monitor deep tissue hemodynamics in a short source-detector separation channel. Several optical systems have been proposed to measure tissue metabolic rate of oxygen consumption (tMRO2) by the simultaneous measuring the blood flow and oxygenation level. however, current optical systems are costly and require complex optical alignments with precisely fixed optical components, which are inconvenient for clinical applications. Therefore, in this study, we proposed a development of diffuse optical metabolic spectroscopy based on diffuse optical spectroscopy (DOS) and diffuse speckle contrast analysis (DSCA) combined system. The diffuse optical metabolic spectroscopy consists of a broadband light source, a near infrared laser and a spectrometer. To calculate tissue metabolic rate of oxygen consumption (tMRO2), the diffuse optical metabolic spectroscopy simultaneously measures the blood flow, blood volume, and the oxygenation level by sharing a spectrometer as a detector for both DOS and DSCA. To validate the system, the blood phantom, flow phantom, and arm occlusion experiments were conducted. The results of the blood phantom experiment show that the blood volume fraction increased linearly with blood accumulation in the blood phantom. And, the oxygenation was changed with the modulation of oxygen level in gas supply. During the flow experiment, the system shows linear responses to the controlled flow on range 0~0.9 ml/min. Finally, we monitored tMRO2 changes during the in vivo arm occlusion test. The results show us decrease in tMRO2 during the occlusion and recovery following the arm occlusion. The optical metabolic spectroscopy system shows possibility as a tool which can be effortlessly applied in medical domain, by providing simple and convenient application. In the field of neuroscience, DOS has become a favorite tool because DOS provides functional hemodynamic responses to brain activity in a non-invasive and non-ionizing manner. To monitor functional hemodynamic responses to brain activity, DOS have to measure signals deep tissue. However, typical DOS systems are not sensitive to depth, and it is difficult to separate deep tissue hemodynamic responses on measured signals. Therefore, in this study, we developed layered model-based DOS, to separately monitor superficial and deep tissue hemodynamic, by applying a double-layer diffuse reflectance model. To validate the capability of our layered model-based DOS system and to compare our methods and currently used method (corrected DOS), The double layer blood phantom experiments were designed. The experiments consisted of scalp layer blood volume changes, brain layer blood volume changes, brain layer oxygenation changes, and scalp layer oxygenation changes. The experiment results show that the corrected DOS methods are not appropriate when the hemodynamic responses in the scalp layer do not change or the brain hemodynamic responses in short S-D channel are dominant. While the layered model-based methods monitored the blood volume fraction and oxygenation changes in both scalp and brain layer using 3 mm source-detector separation. Although the accuracy of the layered model-based DOS should be improved for the future studies, the results show us the potential of the layered model-based DOS in deep tissue hemodynamic monitoring. We believe that the layered model-based DOS system provides better solution in monitoring deep tissue hemodynamics.