A NIR optical imaging system with diffuse photons

A near-infrared (NIR) tissue imaging system with diffuse photons is presented. Hemoglobin in the blood has characteristic absorption in the NIR band. Thus, using NIR optics and scanning device it is easy to construct a 2D image of tissue blood distribution. While the blood image of tissue with tumor, which contains more abundant capillaries, will be different from that of normal tissue, the NIR technique has proved to be a useful tool in detecting the tumor, especially small lesion in tissue. However, the major obstacle in NIR application is that human tissue is a very turbid, scattering medium and thus it is difficult to resolve the absorption signal from the strong scattering background. In this paper we introduce a frequency-domain technique to solve this problem. The light source, a 780 nm laser diode, was modulated in 50.001 MHz. A PIN diode detected the transmitted light and a 50 MHz signal was incorporated to demodulate the detected optical signal. The demodulated signal was filtered and fed to a PC. The data acquisition, signal processing and panel control was written in C++ for Virtual Instrument language and the image reconstruction was written in Matlab. The 2D image scanning was implemented by a 2D scanning platform. The intensity of the transmitted light is influenced by the absorption coefficient of tissue. Preliminary phantom tests showed that it could detect small heterogeneity (less than 8 mm in diameter) and could be applied in clinical diagnosis in the future.