Digital Lock-in Detection For Multi-channel Diffuse Optical Tomography System

Relying on deeper penetration of light in the tissue,DOT(Diffuse Optical Tomography)achieves organ-level tomography diagnosis, which can provide information on anatomical and physiological features. DOT has been widely used in imaging of breast, neonatal cerebral oxygen status and blood oxygen kinetics observed by its non-invasive, security and other advantages.Continuous wave DOT image reconstruction algorithms need the measurement of the surface distribution of the output photon flow inspired by more than one driving source, which means that source coding is necessary. Most currently used source coding in DOT is time-division multiplexing technology, which utilizes the optical switch to switch light into optical fiber of different locations. In case of large amounts of the source locations or using the multi-wavelength, the measurement time and the measurement interval between different locations within the same measurement period will therefore become too long to capture the dynamic changes in real-time imaging applications, such as brain imaging. Frequency division multiplexing source coding technology can solve this problem. It uses several light sources modulated by different frequency sine wave incident to the imaging chamber simultaneously. Signal corresponding to the various sources are obtained from the mixed output light using digital phase-locked detection technology in the detection end. This method greatly accelerates the DOT system measurement, and can also obtain the multiple measurements in different source-detector locations. The imaging results reflect the objective phenomenon more accurately.This paper studies the digital lock-in detection technology in continuous wave DOT imaging system based on photon counting. A digital lock in detection circuit for photon counting measurement system is implemented in a FPGA development platform. A dual-channel DOT photon counting experimental system is preliminary established, including the two continuous lasers, photon counting detectors, digital lock in detection, control circuit, and computer programs to control and display. A series of experimental measurements is taken to validate the feasibility of the system. Finally, the paper summarizes the results and shortcomings, and prospects for future research.