Research On Multi-wavelength Digital Lock-in Photoelectric Detection In Dynamic Diffuse Optical Tomography

Diffuse optical tomography (DOT) is a new biomedical imaging modality, based on biomedical photonics theory and modern optoelectronics technique. This modality aims to reconstruct spatial distributions of the optical properties in highly diffusive medium. A dynamic imaging performance with a high temporal resolution and acceptable measurement accuracy has become one of the major issues in the development of DOT technology.Multi-wavelength detection in DOT allows the quantification of oxy-hemoglobin, deoxy-hemoglobin and additional tissue compounds or extrinsic contrast agents, and also helps improve diagnostic specificity and sensitivity. A combination of the multi-wavelength detection and the digital lock-in scheme can achieve simultaneous measurement and effective separation of the signals at different wavelengths and source incident positions, by using digital data acquisition, digital lock-in algorithm, averaging filters, particular modulation and sampling constraints. The temporal resolution of multi-wavelength dynamic DOT detection will therefore be considerably improved, and the noise in the system effectively suppressed.As an important task of digital lock-in detection in dynamic DOT system, a multi-channel frequency generator was designed based on direct-digital-synthesis (DDS) technique for the requirement of the sinusoidal modulation signals of multiple frequencies in the digital lock-in photoelectric detection. DDS uses digital technique to generate a frequency- and phase-tunable output signal, with its merits of micro-hertz tuning resolution of the output frequency, sub-degree phase tuning capability, extremely fast―hopping speed‖in tuning output frequency (or phase) and phase-continuous frequency hops - all the performances under complete digital control. The multi-channel DDS-based signal generator designed in this paper is comprised of DDS devices (AD9831), current-to-voltage convert circuits, programmable amplifiers, power module, and a microcontroller (AT89C51). Under the control of a LabVIEW-programmed graphical user interface on a PC host, the DDS generator can generate frequency- and amplitude-tunable output signals at the different channels.As the major concern of multi-wavelength digital lock-in photoelectric detection, , an lock-in optical detection platform, and correspondingly an digital lock-in algorithm are developed. The platform is comprised of a diode laser system, a DDS generator, a polypropylene phantom, a Si-photodiode detector, and a DAQ card etc. The algorithm is validated by both numerical simulations and experiments. A series of experiments are performed on investigation on the corss-talks among multiple modulation frequencies, validation on the effectiveness of the particular modulation and sampling constraints, and comparison between the normalized experimental data and theoretical one, etc., The results demonstrate that the proposed digital lock-in photoelectric detection scheme can be potentially used in dynamic DOT for parallel detection of the signals at multiple wavelengths and source sites with reasonable accuracy, suppressed noise-level and low cross-talk, and also, the designed DDS is feasible.