Diagnostic optical modalities detect and analyze backscattered photons from illuminated tissue. The common principle for these techniques is that the detected light contains information on tissue scattering and absorption from the interrogated tissue volume. To untangle this information, one needs to understand how the optical properties are related to photon propagation in tissue. Therapeutic modalities on the other hand, depositing light energy in tissue, are also influenced by the means for light delivery and tissue light transport governed by the optical properties.

Skin and kidney microvascular functions may be affected independently in diabetes mellitus. We investigated skin microcirculatory function in 79 subjects with diabetes type 2, where 41 had microalbuminuria and 38 not, and in 41 age-matched controls. The oxygen saturation, fraction of red blood cells and speed-resolved microcirculatory perfusion (% red blood cells × mm/s) divided into three speed regions: 0–1, 1–10 and above 10 mm/s, were assessed during baseline and after local heating of the foot with a new device integrating diffuse reflectance spectroscopy and laser Doppler flowmetry. Arterial stiffness was assessed as carotid-femoral pulse wave velocity. Subjects with diabetes and microalbuminuria had significantly higher carotid-femoral pulse wave velocity compared to subjects without microalbuminuria and to controls. The perfusion for speeds 0–1 mm/s and red blood cell tissue fraction were reduced in subjects with diabetes at baseline and after heating, independent of microalbuminuria. These parameters were correlated to HbA1c. In conclusion, the reduced nutritive perfusion and red blood cell tissue fraction in type 2 diabetes were related to long-term glucose control but independent of microvascular changes in the kidneys and large-vessel stiffness. This may be due to different pathogenic pathways in the development of nephropathy, large-vessel stiffness and cutaneous microvascular impairment.

The objective of this study was to assess normative values for comprehensive forearm skin microcirculatory function: oxygen saturation, tissue fraction of red blood cells (RBCs), and speed-resolved perfusion. Furthermore, to examine the influence of age and sex on microcirculatory function. Measurements were performed using a noninvasive probe-based system, including diffuse reflectance spectroscopy and laser-Doppler flowmetry, yielding output data in absolute units. The study was conducted within the Swedish CArdioPulmonary BioImage Study (SCAPIS) and included 1,765 men and women aged 50–65 yr from the Linköping general population. Normative values are given at baseline, at the end of a 5-min occlusion of the brachial artery and during hyperemia after occlusion release. We found a consistent age distribution, in which the oldest individuals had the lowest peak oxygen saturation ( P < 0.001) and the highest baseline low-speed perfusion ( P < 0.001). Women had higher peak oxygen saturation ( P < 0.001), lower RBC tissue fraction, in general ( P < 0.001), lower baseline perfusion in all speed regions ( P = 0.01), and lower peak high-speed perfusion at hyperemia ( P < 0.001). The normative data can be used as reference values in future studies of disease-specific populations. The results show that age and sex are important aspects to consider in studies of microvascular function. Women and younger age were factors associated with higher peak oxygen saturation after ischemia. This is a novel parameter that reflects overall microcirculatory function associated with vascular dilation capacity. NEW & NOTEWORTHY This study expands experimental microcirculatory research to clinical use by providing normative values on microcirculatory function in a large population-based cohort. Women and younger age were factors associated with higher peak oxygen saturation after ischemia, which implies that age and sex are important aspects to consider in studies of microvascular function. This study is the first step toward using microcirculatory assessment as a tool to improve diagnosis, prognosis, and treatment in disease-specific populations.

Laser Doppler flowmetry (LDF) can be used to measure perfusion [fraction of red blood cells (RBCs) times their average speed] in the microcirculation. Conventional LDF measures perfusion in arbitrary units, by calculating the first moment of the Doppler spectrum.¹ By considering the complete distribution of Doppler frequencies and not only the first moment, a speed-resolved perfusion measure can be obtained. Using this approach typically involves a decomposition of the LDF photodetector power spectrum. Liebert et al.² developed a calculation scheme based on light transport simulations where the scattering phase function anisotropy value of the medium was varied until the modeled and measured spectra matched. The calculations were evaluated in the single Doppler-shift regime using highly diluted milk flowing in tubes.³ No independent validation of the anisotropy value was performed. Later, this model was expanded to account for multiple Doppler shifts.⁴ However, evaluation measurements in the multiple Doppler-shift regimes using a flow phantom showed that it was not possible to estimate the speed distribution in detail due to system noise.

Biomedical imaging using light can be a powerful, noninvasive, and inexpensive tool to detect biological markers associated with metabolic functions or diseases.¹ Spectral imaging can be used to gain structural information about the tissue from its diffuse scattering contrast and chemical information by analyzing its absorbance spectrum. Spatial frequency domain imaging (SFDI) is a technique to perform quantitative spectral measurements over an adjustable field of view (FOV) (in the order of 1 to 10 cm), and it is rapidly gathering attention from the scientific community.^2,3 The main advantage of SFDI compared to other diffuse optical techniques is the ability to separate the effects of scattering and absorption based on their spatial frequency-dependent effect on reflectance. It also allows one to quantitatively measure chromophores concentrations in turbid media, such as tissues and tissue-simulating phantoms.⁴ Another advantage of this technique is its depth sensitivity depending on the spatial frequencies employed, which allows one to reconstruct a three-dimensional map of optical properties.⁵ SFDI has been employed in clinical studies for a number of different applications, such as vascular assessment in patients with diabetes,⁶ evaluation of burn wounds severity,^7,8 enhancement of cancer detection in vitro,^{9 – 11} and skin characterization for cosmetic surgery.^{12 – 14}