A time-resolved optical imaging system using near-infrared light has been developed. The system had three pulsed light sources and total 64 channels of detection, working simultaneously for acquisition of the time-resolved data of the pulsed light transmitted through scattering media like biological tissues. The light sources were provided by high power picosecond pulsed diode lasers, and optical switches directed one of the light sources to the object through an optical fiber. The light signals reemitted from the surface of the object were collected by optical fibers, and transmitted to a time-resolved detecting system. Each of the detecting channels consisted of an optical attenuator, a fast photomultiplier, and a time-correlated single photon counting circuit which contained a miniaturized constant fraction discriminator/time-to-amplitude converter module, and a signal acquisition unit with an A/D converter. The performance and potentiality of the imaging system have been examined by the image reconstruction from the measured data using solid phantoms.
This paper discusses the accuracy of the optical determination of the oxygenated and deoxygenated hemoglobin content of human skin under the influence of a melanin layer for a multi-wavelengths imager. The relation between the nonlinear results by Monte Carlo simulation (MCS) and the modified Lambert Beer's law (MLB) is also clarified, emphasizing the importance of the absolute values of skin pigments and their influence on the mean path-length used in MLB. The fitting procedure of the MCS data to the actual skin spectra is shown to obtain the absolute values. It is also shown that once the proper mean path-lengths have been determined, MLB can be used fairly well within an accuracy of 80% compared with MCS. Images of oxygenated hemoglobin with a newly-developed fourwavelength camera are presented to demonstrate the advantages of a multiwavelength system.
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