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).

Optical wearable sensors provide crucial information on cardiovascular biomarkers used to estimate heart rate variability, blood oxygen saturation, and arterial blood pressure. However, environmental factors, including external contact pressure, can significantly affect the quality of the acquired signal but have been poorly studied. Objective: In this work, we investigate the influence of external contact pressure on the photoplethysmography (PPG) signal in reflection mode. By systematic application of external contact pressure to the fingertips of volunteers, we aim to examine how such pressure affects the morphological features of a representative cardiac cycle. Methods: We designed, and 3D printed a mounting system to apply controlled pressure to the fingertips of volunteers. This system generated a controllable force using a spring mechanism coupled with a rotating screw. First, we quantified the spring constant and the pressure it applies per revolution. Then, using a PPG sensor operating at a wavelength of 660 ± 20nm, we recorded raw photocurrent signals from three healthy adults. Using our proposed signal processing algorithm, we created ensemble-averaged representative cardiac cycles. Results: We calculated the spring's constant within the mounting system as 339.3N/m. Using this system, we applied external contact pressure values from 20 to 180mmHg. Our results show that the amplitude of systolic peak, dicrotic notch, and diastolic peak for this external contact pressure range continuously rise with a factor of 2.5, 5, and 2, respectively. Conclusion: Our 3D printed mounting system provided a reliable means of applying controlled and reproducible external contact pressure to the fingertips of adult volunteers. We conclude that such contact pressure substantially influences the amplitude of the obtained PPG photocurrent, being a crucial factor in optical wearable sensor design. Significance: Our findings pave the way for determining the optimal level of external contact pressure, as an environmental factor. Such optimal pressure should balance user comfort with the quality of the measured PPG signal, thereby supporting the reliable estimation of cardiovascular parameters.

Background: In reconstructive surgery, adequate perfusion of flaps is essential for successful outcomes. Assessing normal flap perfusion and differentiating it from problematic perfusion is important and time-consuming. Laser speckle contrast imaging (LSCI), an optical technique for quantitative microcirculation assessment, can be used to non-invasively monitor flap perfusion. We designed a wireless handheld LSCI device and investigated its clinical feasibility in Deep Inferior Epigastric Perforator (DIEP) flap reconstruction surgery. Methods: In a case series study with 15 patients and 20 DIEP flaps, perfusion was measured perioperatively across the 4 Hartrampf zones at 4 specific time points and on the first post-operative day. Results: In unilateral reconstructions, the perfusion in dissected flaps showed a perfusion gradient across the zones, with the highest perfusion in zone I and lowest perfusion in zone IV. The decrease in perfusion between unilateral flap elevation and temporary occlusion measurements was detected in unilateral flaps, but not in bilateral flaps. The device could detect flap failure in 2 cases by measuring anomalous perfusion values. Conclusions: The results indicate that our device is potentially valuable for flap monitoring. It detected differences in perfusion throughout the flap zones, a decrease in perfusion when clamping the pedicle and anomalous perfusion in flaps that failed. Motion artefact correction is needed to measure reliably during motion caused by patient breathing, pulse and operator motion. Further studies are needed to determine whether the wireless perfusion imager enables the early detection of complications, which could aid in prevention or prompt reintervention to salvage a flap.

Background: Handheld laser speckle contrast imaging (LSCI) is crucial in clinical settings, but motion artifacts (MA) can compromise perfusion image reliability. Current prevention and suppression methods are often impractical or complex. Machine vision techniques, promising in medical imaging, could improve signal quality, but their use in suppressing MA is still unexplored. Objective: We propose an innovative method based on linear regression for MA correction (MAC) in LSCI and validate it in vivo. Methods: We performed paired handheld and mounted LSCI measurements on 14 subjects with psoriasis using the previously validated handheld perfusion imager (HAPI). By marking lesion boundaries for clinical purposes, the HAPI used a monochromatic camera for both speckle imaging and motion detection, simplifying hardware requirements. Accurate estimation of relative displacements between the test object and LSCI probe allowed us to apply MAC to the perfusion images. Results: Local perfusion values correlated with applied speed were used to calculate and correct MA. The difference between mean perfusion in handheld and mounted modes after MAC significantly decreased (median error 14.2 perfusion units (p.u.) on lesions before correction (p< 0.0005) and 0.5 p.u. after correction (p=0.2)). Conclusion: The findings provide evidence for robust handheld LSCI and validate the MA technique in psoriasis case. Of the two causes of MA-on-surface speeds and wavefront tilt-we address the former and correct mean perfusion, assuming constant temporal perfusion at each location. Significance: We describe a practical, non-contact, marker-free technique for reliable handheld perfusion imaging, supporting further clinical translation in plastic surgery and burns.

Perfusion of the flap is essential in Deep Inferior Epigastric Perforator (DIEP) flap breast reconstruction surgery for flap survival, yet perioperative assessment of flap perfusion to detect perfusion-related problems is challenging. Laser Speckle Contrast Imaging (LSCI) is an optical technique for quantitative microcirculation assessment. Mounted LSCI devices are bulky and impractical to use during surgery, or in other clinical settings where investigation of microcirculation is needed. Therefore, a handheld wireless perfusion imager (WIPI) was developed including a 660 nm laser and an RGB camera for image registration. With this device, flap perfusion during surgery (at baseline, after fully raising the flap, and during ischemia) was compared for different Hartrampf zones. The results indicate that poorly perfused DIEP flap zones may be detected in an early stage using a handheld LSCI device.

Significance: In handheld laser speckle contrast imaging (LSCI), motion artifacts (MA) are inevitable. Suppression of MA leads to a valid and objective assessment of tissue perfusion in a wide range of medical applications including dermatology and burns. Our study shines light on the sources of these artifacts, which have not yet been explored.We propose a model based on optical Doppler effect to predict speckle contrast drop as an indication of MA. Aim: We aim to theoretically model MA when an LSCI system measuring on static scattering media is subject to translational displacements.We validate the model using both simulation and experiments. This is the crucial first step toward creating robustness against MA. Approach: Our model calculates optical Doppler shifts in order to predict intensity correlation function and contrast of the time-integrated intensity as functions of applied speed based on illumination and detection wavevectors. To validate the theoretical predictions, computer simulation of the dynamic speckles has been carried out. Then experiments are performed by both high-speed and low-framerate imaging. The employed samples for the experiments are a highly scattering matte surface and a Delrin plate of finite scattering level in which volume scattering occurs. Results: An agreement has been found between theoretical prediction, simulation, and experimental results of both intensity correlation functions and speckle contrast. Coefficients in the proposed model have been linked to the physical parameters according to the experimental setups. Conclusions: The proposed model provides a quantitative description of the influence of the types of illumination and media in the creation of MA. The accurate prediction of MA caused by translation based on Doppler shifts makes our model suitable to study the influence of rotation. Also the model can be extended for the case of dynamic media, such as live tissue.

Laser speckle contrast imaging (LSCI) is an optical technique for noninvasive assessment of microcirculatory blood flow. LSCI has a broad application in medicine including dermatology. Since laser speckles are the basis for this imaging modality, any external motions during a measurement from both patient and operator affect the blood flow images. This challenge is called motion artefacts (MA). Here, we propose a complete procedure for analysis of speckles, that is, pre-segmentation, segmentation, motion detection, spatial alignment, perfusion map calculation and MA suppression. The handheld perfusion imager (HAPI) operated in both handheld and mounted schemes, has been used for measurements on 14 psoriasis subjects. The advantage of HAPI is use of a single monochromatic camera for both speckle imaging and motion detection. We make use of the black marker dots (made by the clinical investigator to determine visual psoriasis lesion boundary) for calculation of two-dimensional displacements of HAPI during each measurement (i.e. on-surface displacements). These on-surface displacements are integrated to translate each speckle image back to the initial position at the start of the measurement (i.e. spatial alignment). Furthermore, in handheld measurements, MA corrected blood flow maps (also called perfusion maps) are formed by extrapolation of a linear fit from local perfusion versus detected speed to the zero speed, that is, a value ideally always lower than the local mean perfusion. We show that our MA suppression technique makes handheld perfusion maps more similar to the associated mounted perfusion maps in term image histograms and mean values.

Progress has been made in laser speckle contrast imaging (LSCI) of microcirculatory blood flow for biology and medicine. However, the underlying reason for occurrence of movement artefacts (MA) that compromises effective use of LSCI remains largely unexplored. Here, employing a dual-camera setup for both speckle imaging and movement tracking, we validate our analytical model that is based on optical Doppler effect for predication of speckle contrast drop as a function of applied translational speed. We perform both motorized and handheld experiments where planar and scrambled wave illumination schemes have been examined. Experimental data points fairly match the theoretical predictions. These findings indicate that the vision-based movement detection during handheld LSCI is a preferable option. Moreover, the proposed analytical model is promising for further exploration of MA in order to realize a reliable handheld LSCI.

Movement artefacts distort handheld measurements of laser speckle contrast imaging (LSCI). Enabling a robust LSCI in handheld use brings convenience for both patients and clinical staff. However, there is a lack of a comprehensive model that can predict and potentially compensate the amount of movement artefacts occurring during a handheld LSCI measurement. Here, we propose an analytical-numerical model based on the optical Doppler effect for handheld LSCI in case of translation on a high scattering static surface. The model incorporates the type of illumination as well as the imaging geometry by taking into account the spread of wavevectors for illumination and detection. We validate the theoretical model by simulated dynamic speckles and experiments for the cases of (1) planar and spherical waves illumination and (2) scrambled waves illumination. Results of the speckle simulation are in agreement with predictions of the numerical model for semi-circular form of the density functions of the incoming and outgoing wavevectors.

Background: Skin microvasculature changes are crucial in psoriasis development and correlate with perfusion. The noninvasive Handheld Perfusion Imager (HAPI) examines microvascular skin perfusion in large body areas using laser speckle contrast imaging (LSCI). Objectives: To (i) assess whether increased perilesional perfusion and perfusion inhomogeneity are predictors for expansion of psoriasis lesions and (ii) assess feasibility of the HAPI system in a mounted modality. Methods: In this interventional pilot study in adults with unstable plaque psoriasis, HAPI measurements and color photographs were performed for lesions present on one body region at week 0, 2, 4, 6 and 8. The presence of increased perilesional perfusion and perfusion inhomogeneity was determined. Clinical outcome was categorized as increased, stable or decreased lesion surface between visits. Patient feedback was collected on a 10-point scale. Results: In total, 110 lesions with a median follow-up of 6 (IQR 6.0) weeks were assessed in 6 patients with unstable plaque psoriasis. Perfusion data was matched to 281 clinical outcomes after two weeks. A mixed multinomial logistic regression model revealed a predictive value of perilesional increased perfusion (OR 9.90; p < 0.001) and perfusion inhomogeneity (OR 2.39; p = 0.027) on lesion expansion after two weeks compared to lesion stability. HAPI measurements were considered fast, patient-friendly and important by patients. Conclusion: Visualization of increased perilesional perfusion and perfusion inhomogeneity by noninvasive whole field LSCI holds potential for prediction of psoriatic lesion expansion. Furthermore, the HAPI is a feasible and patient-friendly tool.

We assessed the reliability of handheld laser speckle contrast perfusion imaging by evaluating mounted/handheld measurement pairs operated on psoriasis lesions in three steps. First, we made a denoised perfusion map per measurement based on spatial alignment of raw speckle frames and temporal averaging of perfusion frames. Second, we used the measured on-surface speed information to compensate the movement-induced perfusion by extrapolation of the local perfusion values to the value corresponds to zero on-surface speed. Third, we compared mounted/handheld measurement pairs based on perfusion inhomogeneity and increased perilesional perfusion criteria independent of the movement artefact compensation mentioned in the second step. We conclude that after proper post-processing, handheld LSCI measurements can be as reliable as mounted measurements in terms of geometrical distorting, but with challenges to be overcome for correcting perfusion values.

Fluid flow shear stresses are strong regulators for directing the organization of vascular networks. Knowledge of structural and flow dynamics information within complex vasculature is essential for tuning the vascular organization within engineered tissues, by manipulating flows. However, reported investigations of vascular organization and their associated flow dynamics within complex vasculature over time are limited, due to limitations in the available physiological pre-clinical models, and the optical inaccessibility and aseptic nature of these models. Here, we developed laser speckle contrast imaging (LSCI) and side-stream dark field microscopy (SDF) systems to map the vascular organization, spatio-temporal blood flow fluctuations as well as erythrocytes movements within individual blood vessels of developing chick embryo, cultured within an artificial eggshell system. By combining imaging data and computational simulations, we estimated fluid flow shear stresses within multiscale vasculature of varying complexity. Furthermore, we demonstrated the LSCI compatibility with bioengineered perfusable muscle tissue constructs, fabricated via molding techniques. The presented application of LSCI and SDF on perfusable tissues enables us to study the flow perfusion effects in a non-invasive fashion. The gained knowledge can help to use fluid perfusion in order to tune and control multiscale vascular organization within engineered tissues.

Laser speckle contrast imaging (LSCI) is a non-invasive and affordable technique to visualize skin perfusion. Handheld use of the system facilitates measurements on various skin areas in a flexible manner. However, movement artefacts caused by handheld operation or test subject movements hamper its performance. In this work, we study the influence of the laser beam type in handheld-LSCI by evaluating the speckle contrast on static objects for beams with planar, spherical or scrambled wavefronts, and for movement artefacts caused by tilting or translation of wavefronts. We show that the scrambled waves made by often-used engineered diffusers lead to significantly larger movement artefacts than planar or spherical waves.