Research On Optical Topography System Based On Digital Lock-in Detection Technique

As an extension to tissue near-infrared spectroscopy (NIRS) technique, optical topography has the ability to realize the mapping of the high-order functions in the brains. Now it has been one of the most beneficial supplements to the other established methods used for the functional imaging of the cerebral cortex, such as functional magnetic resonance imaging, and can play great roles in the brain research, diagnosis of the neurological system, clinical rehabilitation.This paper presents a dual-wavelength optical topography system, based on digital lock-in technique that can effectively compress channel bandwidth, ameliorate the signal-to-noise ratio and the dynamic performance, realize the effective segregation of the weak dual-wavelength signal and extract the amplitude information of every modulation signals. Also, we have optimized the modulation signals and the data acquisition procedure, completed the design of the optode array and the processing program. In the system, a photo switch is used to accomplish the orderly import of coupling signal at the 5 source places, and then the 4-channel data is synchronously collected by a Data Acquisition Card and processed in the computer. We introduce every components of the system, and verify the availability of the system and its measured amplitude information by the proportional relationship between the input amplitude changes and the output ones.Through monitoring the variation of hemoglobin concentration, this optical topography system can apply in the study of the brain functional imaging. In validation experiments of the system, we utilize changes of the absorption coefficient to represent the variation of hemoglobin concentration. A dual-wavelength pilot validation was performed using liquid phantom with targets of different absorption coefficient in an area of 40×40 mm2. Then we use the modified Lambert-Beer's Law to reconstruct the distribution of the absorption coefficient's variation. The experimental results clearly show the variation tendency of the absorption coefficient with a reasonable accuracy, and validate the feasibility and availability of the system and method.