Dual-slope method for near-infrared spectroscopy (NIRS): in vivo applications to skeletal muscles and brain

Near-Infrared Spectroscopy (NIRS) is a non-invasive, portable, and cost-effective technique that is widely used in the biomedical field to investigate the hemodynamics and oxygenation levels of a variety of biological tissues. To achieve a penetration depth of centimeters, NIRS employs wavelengths in the near-infrared spectral region (typically 650-900 nm). However, non-invasive measurements performed with NIRS are affected by two common problems: (1) the strong sensitivity to hemodynamic contributions from superficial layers, which may confound the measurements of targeted hemodynamics in deeper tissue; (2) artifacts originating from instrumental effects and from subject movements, which may limit practical applicability and clinical use. Over time, different instrumental and methodological approaches have been proposed to address these problems and increasingly sophisticated systems have been introduced, moving from single distance (SD) to multi distances (MD) techniques, which are based on data collected at a single or multiple source-detector separations, respectively. A specific MD method, termed Dual Slope (DS) was proposed in the literature and recently further developed in our laboratory. Previous studies have shown the potential enhancement offered by the DS technique, compared to existing SD and MD methods, to the sensitivity to deep versus superficial tissue, and to minimize instrumental and motion artifacts. These works also demonstrated the implementation of DS methods combining frequency-domain (FD-NIRS), at two discrete wavelengths (690 and 830 nm), and broadband continuous-wave (bCW-NIRS) over a wide range of wavelengths (650-1000 nm), to generate quantitative absorption spectra of biological tissues for measuring the chromophores concentration. This thesis is devoted to a further characterization of the DS technique of layered media, and its application to human subjects for measuring the hemodynamics in skeletal muscle and brain. First, a theoretical study based on diffusion theory was performed to characterize the sensitivity region of the DS data in two-layer media, demonstrating the importance and influence of the optical properties, mainly scattering, of the upper (surface) layers. Second, measurements were conducted on three healthy volunteers using two different protocols: one involving venous and arterial occlusions in skeletal muscles to measure muscle blood flow and oxygen consumption, and one involving brain activation with the Stroop Color and Word Test to measure associated pre-frontal cortical changes in the oxy- and deoxyhemoglobin concentrations. In vivo measurements results showed the different information content of SD and DS data, highlighting the higher specificity provided by the novel approach on deeper tissue (i.e., muscle and brain) versus superficial tissue (i.e., adipose layer and scalp). In summary, this research shows the practical applicability and the potential for research and diagnostics of the new DS NIRS method, paving the way for new in vivo applications.