Optical Characteristics of Tissue and its Application for Optical CT

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In the medical field, optical CT is expected as a safe and portable methos to get functional image of tissue. Therefore in visible and near infrared light region, absorption is caused mostly hemoglobin, and the absorption spectrum of hemoglobin changes due to oxidization. However it is very difficult to distinguish absorption information, because of strong scattering of light in tissue. At first, we estimated optical parameter of tissue sample, second we investigated the photon propagation in scattering medium, and third we estimated CT image.For the estimation of optical parameter, laser beam amplitude-modulated at 100MHz was used as light source, then measured an amplitude ratio and phase between incident beam and transmitted beam through samples of various thickness. From those experiment we evaluated absorption coefficients, scattering coefficients and anisotropic parameters of blood, muscle, fat and the suspension of intralipid. For theoretical analysis computer simulation was car … More ried out by Monte Carlo method based on radiative transport theory. This results agree with results of phantom experiment.To realize optical CT,we set up experimental system using computer controlled CCD camera and light flux from halogen lamp through Fabry-Perot optical filter. We detected scattered light from phantom as back projected data, and reconstructed images. For our research light is considered as perfect scattering in human tissue, so back projected data is regarded as data from near surface of the phantom. From the experiment using the phantom which had absorbing rods in base material of the same scattering coefficient as tissue, the absorption target at less than 3cm below the surface of the phantom could be distinguished. Experiment using hemoglobin solution as absorber shows that the reconstructed image changed with the change of wavelength.We also measured angular distribution of scattered light from blood and hemoglobin solution to estimate optical anisotropic parameter.From our fundamental study, and it is shown that at less than 3cm blow from the surface of tissue it is possible to get CT image by perfect scattering light, however many problems are remained to realize the image of oxygen saturation which utilize absorption characteristics of hemoglobin. Less