Transillumination imaging of physiological functions by NIR light

With a near-infrared (NIR) light, one can get a transillumination image of a living body. Further, one can quantify the physiological change in the body as the change in light attenuation. A fundamental study has been conducted to visualize the functional change inside a living biological body using the NIR light. A technique was developed to visualize the attenuation change occurred in a diffuse scattering medium. Transillumination images are obtained before and after the physiological change. By taking the ratio of the transmitted intensities of these two images, one can obtain the spatial distribution of attenuation change while suppressing the effect of scattering. This principle was derived in theoretical analysis and its effectiveness was verified in experiments. To examine the applicability of this principle to a biological body, localized physiological changes were made in the mouse abdomen and the rat brain. The hypoxia in one of the mouse kidneys was visualized selectively from another normal kidney. The local increase in the blood volume was detected in the somatosensory area of a rat brain when its forelimb was electrically stimulated. The blood increase occurred in a symmetrical position with respect to the sagittal plane, when the forelimb of the opposite side was stimulated. Through these experiments, it was found that the changes in the tissue oxygenation and the blood volume could be detected noninvasively and that they are visualized in the transillumination images using the NIR light.