Study of oxygen transport in tissue using reflection type diffuse optical tomography

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Study of transport phenomena in human body is essential to understand the complex physiological mechanisms in living organisms. In recent years, noninvasive monitoring of hemodynamics using near infrared (NIR) light has been gaining the attention. NIR light diffusively propagates through biological tissues, and its absorption spectrum varies with blood oxygenation. These allow us Diffuse Optical Tomography (DOT) which can illustrate the spatial distribution of the hemoglobin concentrations within the tissues. This study intended to unveil the hemodynamics in the brain and muscles with DOT.Simple DOT systems using continuous-light can generate tomographic images when transmitted light through the object is detectable. However, it is difficult to obtain images only from the data of the reflected light when the transmitted light is undetectable due to the large size of the object. Pico-second time resolved measurement makes it possible to obtain tomographic images from reflected light. We attempted to monitor the hemodynamics in an adult brain with reflection-type DOT using a pico-second time-resolved measurement system, and discussed the related issues as well.The major achievements of this study are as the following :(i) It is proven by the numerical simulation and in vivo experiments that reflection-type DOT images can be obtained by a pico-second time resolved measurement system in the case of large objects such as an adult head through which the transmitted light is undetectable. (ii) It is found that the precise measurement of the positions of the source-detector optical fibers attached on the skin of the subject is essential for high-quality imaging. (iii) The DOT images of the oxygenation and blood volume in frontal lobe during calculation tasks were obtained. (iv) Optical mapping using the reflected light data is found to be affected by the structure of the head, especially by the cerebrospinal fluid and the skull layers.