The aim of this research is to develop and build a low cost portable integrated frequency-domain and continuous wave (CW) system for real-time spectroscopic imaging of human tissue. This system measures four tissue chromophore concentrations (water, lipid, deoxygenated, and oxygenated hemoglobin) using eight near-infrared wavelengths ranging from 660nm to 980nm, in real-time. The frequency domain (FD) module measures the phase and amplitude of photon density waves from 50-500 MHz with an operating speed of 0.5Hz, while the CW module uses frequency multiplexing to achieve sampling rates up to 250Hz. The FD component provides quantitative information about optical scattering and absorption using the acquired phase and amplitude data at four wavelengths (660, 690, 785, and 830nm). The CW component expands spectral bandwidth and improves acquisition speed by measuring only low frequency (11-19 KHz) intensity changes at four wavelengths (880, 904, 915, and 975nm). The CW system has a 50 dB dynamic range, enabling measurements in tissue with a source-detector spacing up to 4 cm and it is immune to background noise from ambient light by utilizing low-frequency modulation and bandpass filtering.
The performance of this hybrid system and its equivalency to previous diffuse optical spectroscopy systems are tested and validated using a tissue-simulating phantom with an embedded inclusion. The system enables continuous scanning of surfaces which can replace discrete measurements on a grid pattern used in previous systems. The instrument outperforms the current system's data acquisition speed by 2 orders of magnitude while reducing the overall cost by $9000. We demonstrate in-vivo applications of this combined instrument by measuring abdomen, muscle and brain tissues. The extremely fast data acquisition enables high resolution characterization of the pulsatile waveform. Finally, vasculature reactivity and hemodynamics are measured and characterized by using vascular occlusion and paced breathing models.