Frequency-domain instrument with custom ASIC for dual-slope near-infrared spectroscopy

Real-time and non-invasive measurements of tissue concentrations of oxyhemoglobin (HbO2) and deoxyhemoglobin (HbR) are invaluable for research and clinical use. Frequency-domain near-infrared spectroscopy (FD-NIRS) enables non-invasive measurement of these chromophore concentrations in human tissue. We present a small form factor, dual-wavelength, miniaturized FD-NIRS instrument for absolute optical measurements, built around a custom application-specific integrated circuit and a dual-slope/self-calibrating (DS/SC) probe. The modulation frequency is 55 MHz, and the heterodyning technique was used for intensity and phase readout, with an acquisition rate of 0.7 Hz. The instrument consists of a 14 × 17 cm2 printed circuit board (PCB), a Raspberry Pi 4, an STM32G491 microcontroller, and the DS/SC probe. The DS/SC approach enables this instrument to be selective to deeper tissue and conduct absolute measurements without calibration. The instrument was initially validated using a tissue-mimicking solid phantom, and upon confirming its suitability for in vivo, a vascular occlusion experiment on a human subject was conducted. For the phantom experiments, an average of 0.08° phase noise and 0.10% standard deviation over the mean for the intensities was measured at a source-detector distance of 35 mm. The absorption and reduced scattering coefficients had average precisions (variation of measurement over time) of 0.5% and 0.9%, respectively, on a window of ten frames. Results from the in vivo experiment yielded the expected increase in HbO2 and HbR concentration for all measurement types tested, namely SC, DS intensity, and DS phase.