Real-Time Handheld Frequency-Domain Near Infrared Spectroscopy

Frequency-domain near-infrared spectroscopy (fdNIRS) provides quantitative noninvasive measurements of tissue optical absorption and scattering, as well as a safe and accurate method for characterizing tissue composition and metabolism. However, the poor scalability and high complexity of most fdNIRS systems assembled to date have contributed to its limited clinical impact. To address these shortcomings, we present a scalable, broadband, digital-based handheld fdNIRS platform capable of measuring optical properties and tissue chromophore concentrations in real-time. The system can display single frequency, single wavelength fdNIRS amplitude and phase data at a maximum rate of 36,600 Hz, and provides tissue optical property and chromophore concentrations at a maximum of 17,891 and 10,211 Hz, respectively, all of which are world records 100x to 3,600x faster than previous systems in literature. The entire system is enabled by several innovations including an ultra-high-speed k-nearest neighbor lookup table method, embedded FPGA and CPU high-speed co-processing, and high-speed data transfer (due to on-board processing). Optical property accuracy was compared to a traditional network analyzer-based system and was found to be within 10\% over a range of attenuating tissue-simulating phantoms. We demonstrate real-time optical tracking with mm accuracy on tissue used for real-time 2D spatial optical property maps.

These ultra-high speeds can provide advantages in image resolution, accuracy and scalability. For example, a real-time high-speed system allows for higher spatial density imaging of tissue that lends itself to better assessment of tissue heterogeneity and overall resolution. The system's high-speed capability allows for scaling to hundreds of source-detector channels, while maintaining real-time display of optical properties and chromophores. We show use-case examples enabled by our approach including tumor simulating phantom images, in vivo arterial occlusion and high-resolution pulsatile measurements. The platform is applicable to existing applications such as functional NIRS and breast cancer imaging, but also opens new applications as the technology is disseminated in clinical and laboratory settings.