Vital signs reflect circulatory function and hence hemodynamics on a macroscopic scale and are often unreliable or late indicators of hemodynamic instability. Previous studies support that alterations in the microcirculation may provide early indicators of deterioration and impending shock. Microcirculation is also restored late in the recovery process. Hence, monitoring microcirculation is important since treatments based on normalizing classical vital signs will not always restore microvascular hemodynamics and the microcirculation may remain in shock although e.g., blood pressure seems normal.

The aim of this study was to investigate alterations in skin microcirculation dynamics during lower body negative pressure as a model of shock and central hypovolemia. By using spatial frequency domain imaging (SFDI) and polarized reflectance imaging, we investigated the association between micro- and macrovascular function during these conditions. Furthermore, we evaluated microvascular reactivity using the capillary refill test.

Significance: Spatial frequency domain imaging (SFDI) is a diffuse optical measurement technique that can quantify tissue optical absorption (μ_a) and reduced scattering (μs′) on a pixel-by-pixel basis. Measurements of μ_a at different wavelengths enable the extraction of molar concentrations of tissue chromophores over a wide field, providing a noncontact and label-free means to assess tissue viability, oxygenation, microarchitecture, and molecular content. We present here openSFDI: an open-source guide for building a low-cost, small-footprint, three-wavelength SFDI system capable of quantifying μ_a and μs′ as well as oxyhemoglobin and deoxyhemoglobin concentrations in biological tissue. The companion website provides a complete parts list along with detailed instructions for assembling the openSFDI system.

Aim: We describe the design of openSFDI and report on the accuracy and precision of optical property extractions for three different systems fabricated according to the instructions on the openSFDI website.

Approach: Accuracy was assessed by measuring nine tissue-simulating optical phantoms with a physiologically relevant range of μ_a and μs′ with the openSFDI systems and a commercial SFDI device. Precision was assessed by repeatedly measuring the same phantom over 1 h.

Results: The openSFDI systems had an error of 0  ±  6  %   in μ_a and −2  ±  3  %   in μs′, compared to a commercial SFDI system. Bland–Altman analysis revealed the limits of agreement between the two systems to be   ±  0.004  mm^−  1 for μ_a and −0.06 to 0.1  mm^−  1 for μs′. The openSFDI system had low drift with an average standard deviation of 0.0007  mm^−  1 and 0.05  mm^−  1 in μ_a and μs′, respectively.

Conclusion: The openSFDI provides a customizable hardware platform for research groups seeking to utilize SFDI for quantitative diffuse optical imaging.