Multi-channel DDS Light Source Modulation And Digital Lock-in Detection System Based On Single Photon Counting

As an emerging medical imaging modality, diffuse optical tomography (DOT)has played an increasingly important role in medical optical diagnosis technology byits characteristics of non-invasive, no radioactive, high sensitive and continuousmeasurement.As a common technology for encoding the light source in continuous wave DOTimaging, Time Division Multiplexing would cause a long measurement time andrelatively poor real-time when the number of light sources was large. In order to solvethis problem, under FPGA platform, a multi-channel light source modulation modulehas been developed to produce four channel analog sine signals with adjustablefrequency based on DDS theory. Meanwhile, the module can produce digitalreference signals for the digital lock-in module that would ensure the reference signaland the modulation signal to be homologous. For the requirement of the high detectordynamic range and the low measurement signal to noise rate when using thetechnology of Frequency Division Multiplexing, a digital lock-in module based onsingle photon counting technique was designed in this paper. The PMT has thecharacteristic of high dynamic range and the digital lock-in technology can extract theweak periodic signal buried in noise. In order to verify the accuracy of the digitallock-in results, the modulation and demodulation experiment were implemented andthe results proved that with the increase of the modulation amplitude, thedemodulation results increased in proportion.To further verify the effectiveness of the system, it was been applied to thecontinuous wave DOT system. Firstly, for the homogeneous solid phantom, themeasurements in the different source detection separation were compared with theresults measured by single photon counting method. The results indicated that therelative error was less than5.36%when using the single wavelength sourcemodulated and was less than8.01%when using the dual wavelength sourcemodulated. Secondly, for the solid phantom with single target, the measurements wereused to reconstruct the absorption coefficient by using the single wavelength sourceand dual wavelength source, respectively. The results show that the system can be used to reconstruct the coefficient and can separate the response of the dualwavelength sources which were modulated by signals of different frequencies.