Near-Infrared Functional Brain Imaging Studies Based On Multi-Channel Digital Lock-in Detection System

Near-infrared imaging (NIRI) is internationally recognized as one of the mostvaluable biological detection&analysis technology, which has been widely applied inbiomedical field. NIRI measurement system possesses the advantages of portability,non-invasiveness, low power consumption and low price. As an effective supplementfor fMRI, EEG, MEG, etc., NIRI has played a very important role in cognitive scienceresearch, human clinical pathological diagnosis, rehabilitation treatment and otherrelated fields.In this paper, we perfected a multi-channel near-infrared functional brain imagingdetection system based on single photon counting digital lock-in technique. Sinewaves with different frequencies are generated so as to amplitude-modulate lasersources with different wavelengths, which enables parallel detection. The diffuse lightis detected by photomultiplier tubes (PMTs) which can greatly improve themeasurement sensitivity and signal-to-noise ratio of the system. The relevantphysiological information of different light can be extracted respectively by using thelogic resources of FPGA to decode the composite digital signal generated by PMT.The improvements of the existing system have been made, including the matching ofthe devices, the reconstruction of the data acquisition program, the selection ofparameters of sine wave used for laser modulation and the global timing constraintsand optimization of the digital lock-in program, which further enhance the overallperformance of the system.Besides, the source-detector distribution has a big effect on detection and imagingquality, and traditional square distribution can’t meet the needs of in vivo functionalbrain imaging well. To this end, a new type of source-detector distribution has beendesigned and evaluated; through the theoretical and experimental verification, boththe space resolution and the quantitative degree are improved compared with thetraditional square distribution.Finally, combined with the improved system and new source-detector distribution,the double-layer phantom experiments were performed, a simple optical bandage forfunctional brain imaging has been designed, and the in vivo experiments of forehead have been made by applying n-back task paradigm. Results show that this method canreflect the variation of relevant area’s absorption coefficient and distinguish iteffectively to a certain extent.