We present a wearable time-domain near infrared spectroscopy (TD-NIRS) system (two wavelengths, one detection channel), which fits in a backpack and performs real-time hemodynamic measurements on the brain and muscle tissues of freely moving subjects. It can provide concentration values of oxygenated hemoglobin (O₂Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb = O₂Hb + HHb) and tissue oxygen saturation (StO₂). The system is battery-operated and can be wirelessly controlled. By following established characterization protocols for performance assessment of diffuse optics instruments, we achieved results comparable with state-of-the-art research-grade TD-NIRS systems. We also performed in-vivo measurements such as finger tapping (motor cortex monitoring), breath holding (prefrontal cortex monitoring and forearm muscle monitoring), and outdoor bike riding (vastus lateralis muscle monitoring), in order to test the system capabilities in evaluating both muscle and brain hemodynamics.

Diffuse optics (DO) encompasses a range of photonics tools based on the study of random photon migration in highly scattering media—biological tissues in particular. Due to its unique features, DO is emerging as a powerful means for clinical or homecare diagnostics.¹ The basic physics of DO is related to the detection of temporal or spatial alteration in photon distribution re-emitted on the tissue surface.² Due to the low power (typically few mW) of injected near-infrared light (600- to 1100-nm range), DO is inherently noninvasive. The photon temporal (or spatial) distribution carries information on the absorption—ultimately related to tissue chemical composition, such as water, lipid, collagen content, oxy- and deoxy-hemoglobin concentration, cytochrome c-oxidase³—and scattering properties—linked to tissue microstructure. Further, DO is one of the few noninvasive modalities capable of providing functional information, e.g., brain or muscle activation.⁴ It can be operated noncontact through remote light illumination and collection.⁵ It explores the tissue well below the skin at depths of up to 2 to 4 cm. It can provide quantitative operator-independent assessment of the tissue status, such as hemoglobin oxygenation level in the brain. Finally, it is highly scalable, sharing the same technology from large clinical tomographic systems down to wearable devices or homecare appliances.⁶ For all these aspects, DO is attracting more and more interest in many fields, such as monitoring vital signs like brain oxygenation in critical care or during interventions,⁷ tumor diagnostics as for breast cancer,⁸ investigating the impact of lifestyle and nutrition on our body,⁹ or other fields such as neuroscience and psychology,¹⁰ sports, and leisure.¹¹ Even further, the operator independence, the depth sensitivity, and the scalability in addition to noninvasiveness make this option attractive for telemedicine and homecare of patients remotely.

In this work, we present a new multi-distance diffuse correlation spectroscopy (DCS) device integrated with a compact state-of-the-art time domain near infrared spectroscopy (TD-NIRS) device. The hybrid DCS and TD-NIRS system allows to retrieve information on blood flow, tissue oxygenation, and oxygen metabolic rate. The DCS device performances were estimated in terms of stability, repeatability, ability in retrieving variations of diffusion coefficient, influence of the tissue optical properties, effect of varying count rates and depth sensitivity. Crosstalk between DCS and TD-NIRS optical signals was also evaluated. Finally, in vivo experiments (venous and arterial cuff occlusions on the arm) were conducted to test the ability of the hybrid system in measuring blood flow variations.

The possibility to probe human tissue in depth and non-invasively by visible and near-infrared light^1,2 pushed scientists and technologists to develop physics, methodologies, and instrumentation to study photon propagation in highly scattering media. In the 600 to 1100 nm wavelength range, human tissues are diffusive media characterized by a scattering coefficient much higher than the absorption. Furthermore, visible and near-infrared diffuse spectroscopy (NIRS) experienced a huge improvement as a suitable technique for detecting the structural or functional information inside the tissues by studying the light-tissue interaction process.

Gaiting is a natural skill, making humans able to move around in free space in upright position. Almost 65% of people over 70 years old suffer from gait disorder and this issue is often linked to neurological pathologies including Alzheimer’s, Parkinson’s, and multiple sclerosis.^1,2 Falling is a severe consequence of walking dysfunctions, frequent in elderly population.³ Other than provoking injuries and physical debilitation, ambulatory difficulties lead to psychological problems such as depression or fear of falling.⁴ Gaiting disorders and related issues involve more than 1% of the total expenses for the health care system in the USA.⁵ Although this phenomenon has been widely studied,¹ a satisfactory understanding of neuronal and cerebral hemodynamic processes occurring during a gaiting task is still lacking. Studies have been performed by functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) on brain activations during imaginary walking tasks, the only way to simulate gaiting due to the severe movement limitations of these techniques. Less can be found on brain hemodynamic monitoring during real gaiting in ecological experimental conditions. Different fNIRS and fMRI studies have shown how goal-directed locomotion, such as walking on assigned steps, given path-lengths, or walking on a straight line (real or imagined), significantly affects prefrontal cortex hemodynamics, reason why many experiments have monitored these areas. However, the involvement of motor cortex areas and the comparison of goal-directed locomotion with the more natural forward walking is not clear yet.

Successful demonstration of a unique portable CW-TDDOS system for accurate and multiwavelength retrieval of tissue optical properties. Determining these properties has potential to improve the diagnosis and treatment outcomes in clinical and sports settings.