An Investigation On Photon Counting Technique In Tissue Optical Measurement

Biomedical photonics develops upon theoretical base of tissue optics, which features the study of interaction between bio-tissue and light. The research field comprises study of light distribution upon bio-tissue under constraint conditions, as well as development of method for in vivo measurement. Generally, the received optical signal used in research of biomedical photonics is comparatively weak, which makes the faint-optical detection a major concern.A traditional way to realize detection of faint light is to utilize the photon counting techniques. Output of a photon detector is discrete by nature, which makes it possible to extract the weak signal by methods of pulse amplitude discrimination and digital counting techniques.Nowadays, tissue-optical measurement systems can be divided into three categories: the continuous wave system, the time resolved system and the frequency domain system. This paper introduces three kinds of photon detectors designed based upon the programmable logic devices. Among them the 8-channel boxcar photon counter may find application in the continuous wave measurement system, which is used extensively in research of hemodynamics of newborn brains and functional brain imaging. The parallel-gated photon counter and the multiscaler photon counter may be employed in the time resolved system, which is suitable to measure characteristic parameters of fluorescence such as the intensity and the longevity of the long lifetime fluorescence-phosphorescence attenuation signal.Every counting channel of the boxcar photon counter has two counters, which work alternately, and a full record of the input pulse will be saved. Through communication mechanism and cyclic storage, the data can be sent back to computer in real time to realize a dynamic monitor of human body.Both the gated photon counters and the multiscaler photon counters have an adjustable gate width, which make it applicable for the measurement of fluorescence-phosphorescence attenuation signal with different lifetime. The logarithmic method and Prony's method can get the lifetime of the fluorescence, which is a parameter relevant to the biochemical process in the body as well as environmental conditions.