Methodology for enhanced optical signal acquisition in wearable cardiovascular monitoring: Initial findings

Background: Non-invasive optical measurements such as diffuse correlation spectroscopy and photoplethysmography provide critical physiological information, including cardiovascular parameters. Compact and wearable optical devices enable point-of-care and daily monitoring of cardiovascular signals. Objective: In this study, we propose a comprehensive methodology for informed design of optical transceivers to optimize signal acquisition. Specifically, we investigated the dependence of depth sensitivity on scattering as a function of source-detector distance (SDD). Methods: Speckle variance optical coherence tomography was performed on healthy adult volunteers (3 female, 3 male) to obtain three-dimensional angiograms of the skin microvascular network. Using machine vision algorithms, we quantified microvascular parameters including average depth, width, and volumetric density. These parameters were incorporated into a multi-layer skin digital twin model, and Monte Carlo simulations of light transport at 660 nm were performed across a range of SDD values. By analyzing scattering events in each skin layer, we quantified the SDD-dependent depth sensitivity. Results: Our results indicate that at short SDDs (i.e., 0.15 mm), scattering predominantly occurs in the upper dermis (i.e., 49%), whereas at longer SDDs (i.e., 4 mm), the hypodermis becomes dominant (i.e., 41%). With an average microvascular depth of 130±30μm (within the upper dermis), we identified an optimal SDD of 0.9 mm, yielding a maximum scattering contribution of 72% for the studied population. Conclusion: Our methodology establishes a foundation for patient-specific optimization of optical signal acquisition, with potential applications in diverse populations, including hypertensive elderly patients. Significance: Our study enables patient-specific device design addressing physiological variations across individuals (e.g. differences in microvascular networks and skin tone).