Detection limits of multi-spectral optical imaging under the skin surface

Analysis of Light Penetration Depth in Apple Tissues by Depth-Resolved Spatial-Frequency Domain Imaging

Spatial-frequency domain imaging (SFDI) has been developed as an emerging modality for detecting early-stage bruises of fruits, such as apples, due to its unique advantage of a depth-resolved imaging feature. This paper presents theoretical and experimental analyses to determine the light penetration depth in apple tissues under spatially modulated illumination. Simulation and practical experiments were then carried out to explore the maximum light penetration depths in 'Golden Delicious' apples. Then, apple experiments for early-stage bruise detection using the estimated reduced scattering coefficient mapping were conducted to validate the results of light penetration depths. The results showed that the simulations produced comparable or a little larger light penetration depth in apple tissues (~2.2 mm) than the practical experiment (~1.8 mm or ~2.3 mm). Apple peel further decreased the light penetration depth due to the high absorption properties of pigment contents. Apple bruises located beneath the surface peel with the depth of about 0-1.2 mm could be effectively detected by the SFDI technique. This study, to our knowledge, made the first effort to investigate the light penetration depth in apple tissues by SFDI, which would provide useful information for enhanced detection of early-stage apple bruising by selecting the appropriate spatial frequency.

Indocyanine Green Angiography Overpredicts Postoperative Necrosis Compared to Multispectral Reflectance Imaging

BACKGROUND

Sufficient perfusion is foundational to successful reconstructive surgery. Various technologies have been developed to help determine whether tissue is adequately perfused, or whether it will be prone to necrosis postoperatively. Indocyanine green (ICG) angiography is one such method that uses fluorescence and analyzes tissue perfusion. Multispectral reflectance imaging (MSRI) is an alternative technology that analyzes optical properties of oxygenated and deoxygenated hemoglobin to determine tissue viability. Because tissue in low-perfusion states may still survive because of sufficient oxygenation, the authors hypothesized that compared to MSRI, ICG angiography overpredicts necrosis, potentially resulting in unnecessary resection of viable tissue. This study expands on preliminary work to investigate this hypothesis.

METHODS

This was a prospective cohort of patients undergoing prepectoral direct implant reconstruction at a single institution. Each patient was examined intraoperatively with both ICG angiography and MSRI. Decisions to resect tissue were made in conjunction with MSRI and ICG images collected purely for data analysis. Patients were followed postoperatively for at least 2 months for signs of postoperative necrosis.

RESULTS

Fifty-three cases were included. ICG angiography accurately predicted viability in 40 of 40 patients (100%) and incorrectly predicted necrosis in 11 of 13 patients (84.6%). Simultaneously, MSRI predicted necrosis in zero patients and accurately predicted viability in 51 of 53 patients (96.2%). There was no statistically significant difference in demographic data among patients predicted to experience necrosis by means of ICG angiography versus those predicted to have entirely viable tissue.

CONCLUSION

This study suggests that ICG angiography is prone to overpredicting postoperative necrosis in comparison to MSRI.

CLINICAL RELEVANCE STATEMENT

This study suggests that multispectral reflectance imaging may benefit practicing plastic surgeons in determining the likelihood of postoperative necrosis.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Diagnostic, II.

Physiological wound assessment from coregistered and segmented tissue hemoglobin maps

A handheld near-infrared optical scanner (NIROS) was recently developed to map for effective changes in oxy- and deoxyhemoglobin concentration in diabetic foot ulcers (DFUs) across weeks of treatment. Herein, a coregistration and image segmentation approach was implemented to overlay hemoglobin maps onto the white light images of ulcers. Validation studies demonstrated over 97% accuracy in coregistration. Coregistration was further applied to a healing DFU across weeks of healing. The potential to predict changes in wound healing was observed when comparing the coregistered and segmented hemoglobin concentration area maps to the visual area of the wound.

Rapid dissemination of light transport models on the web

Light transport models are employed in applications in such varied areas as realistic image synthesis, noninvasive treatment of diseases, and remote sensing of natural resources. Openly accessible research resources can lead to significant advances involving these applications by fostering the cross-fertilization of different scientific disciplines. However, few light transport models have their source code openly available for download. Moreover, simply making the code available might not be enough; these models' complexity usually prevents their use beyond the research groups that developed them. The NPSGD (Natural Phenomena Simulation Group Distributed) framework makes light transport models easily accessible for online use. NPSGD acts a front end, connecting model implementations to the Web. It lets researchers perform predictive and time-intensive light transport simulations in a user-friendly, fault-tolerant way. More important, as a proof of concept, NPSGD demonstrates that the reproducibility of research results through model transparency is feasible. Such reproducibility can result in fruitful collaborations between model developers and users, regardless of their field of expertise.

Modeling of reflectometric and ellipsometric spectra from the skin in the terahertz and submillimeter waves region

The human skin is modeled as a stack of homogeneous layers in the terahertz and submillimeter waves regions with some anisotropy due to the helical sweat glands and other elongated entities. A dielectric model for the skin is presented, valid for a wider frequency range (up to the terahertz region) taking into account the dispersive nature of the effective conductivity. Polarized reflectivity and generalized ellipsometric parameters are calculated versus angle and wavelength. Recent studies have claimed that the helical sweat ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the sub-terahertz region. We found that water absorption, dispersion and multiple interference effects play the major role in shaping the spectrum without the need for the assumption of the sweat ducts acting as low-Q helical antennae. High sensitivities to the water content are found particularly in the ellipsometric parameters at large incidence angles. Hence a new methodology is proposed to detect skin cancer using variable angle ellipsometry or polarized reflectometry. The parameter found with the highest sensitivity to water content is cos Δ(pp) with Δ(pp) being the phase of the on-diagonal reflection matrix ratio between p-to-p polarization.

Quantitative principal component model for skin chromophore mapping using multi-spectral images and spatial priors

We describe a novel reconstruction algorithm based on Principal Component Analysis (PCA) applied to multi-spectral imaging data. Using numerical phantoms, based on a two layered skin model developed previously, we found analytical expressions, which convert qualitative PCA results into quantitative blood volume and oxygenation values, assuming the epidermal thickness to be known. We also evaluate the limits of accuracy of this method when the value of the epidermal thickness is not known. We show that blood volume can reliably be extracted (less than 6% error) even if the assumed thickness deviates 0.04mm from the actual value, whereas the error in blood oxygenation can be as large as 25% for the same deviation in thickness. This PCA based reconstruction was found to extract blood volume and blood oxygenation with less than 8% error, if the underlying structure is known. We then apply the method to in vivo multi-spectral images from a healthy volunteer's lower forearm, complemented by images of the same area using Optical Coherence Tomography (OCT) for measuring the epidermal thickness. Reconstruction of the imaging results using a two layered analytical skin model was compared to PCA based reconstruction results. A point wise correlation was found, showing the proof of principle of using PCA based reconstruction for blood volume and oxygenation extraction.

Does human skin truly behave as an array of helical antennae in the millimeter and terahertz wave ranges?

The sweat ducts of the human perspiration system are helically shaped tubes, filled with a conductive aqueous solution. Recent studies have claimed that these ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the subterahertz region. Using local homogenization theory for the skin embedded with sweat ducts, we found that multiple interference effects from the skin layers play the major role in determining the skin electromagnetic characteristics in the millimeter and terahertz regions without the need for the assumption of the sweat ducts acting as low-Q helical antennae.

Use of a multi-spectral camera in the characterization of skin wounds

Skin breakdown is a prevalent and costly medical condition worldwide, with the etiologic and healing processes being complex and multifactorial. Quantitative assessment of wound healing is challenging due to the subjective measurement of wound size and related characteristics. We propose that in vivo spectral reflectance measurements can serve as valuable clinical monitoring tool/device in the study of wound healing. We have designed a multi spectral camera able to acquire 18 wavelength sensitive images in a single snapshot. A lenslets array in front of a digital camera is combined with narrowband filters (bandwidth 10 nm) ranging from 460 to 886 nm. Images taken with the spectroscopic camera are composed of 18 identical sub-images, each carrying different spectral information, that can be used in the assessment of skin chromophores. A clinical trial based on a repeated measures design was conducted at the National Rehabilitation Hospital on 15 individuals to assess whether Poly Carboxy Methyl Glucose Sulfate (PCMGS, CACIPLIQ20), a bio-engineered component of the extracellular matrix of the skin, is effective at promoting healing of a variety of wounds. Multi spectral images collected at different wavelengths combined with optical skin models were used to quantify skin oxygen saturation and its relation to the traditional measures of wound healing.

Spectral imaging reveals microvessel physiology and function from anastomoses to thromboses

Abnormal microvascular physiology and function is common in many diseases. Numerous pathologies include hypervascularity, aberrant angiogenesis, or abnormal vascular remodeling among the characteristic features of the disease, and quantitative imaging and measurement of microvessel function can be important to increase understanding of these diseases. Several optical techniques are useful for direct imaging of microvascular function. Spectral imaging is one such technique that can be used to assess microvascular oxygen transport function with high spatial and temporal resolution in microvessel networks through measurements of hemoglobin saturation. We highlight novel observation made with our intravital microscopy spectral imaging system employed with mouse dorsal skin-fold window chambers for imaging hemoglobin saturation in microvessel networks. Specifically, we image acute oxygenation fluctuations in a tumor microvessel network, the development of arteriovenous malformations in a mouse model of hereditary hemorrhagic telangiectasia, and the formation of spontaneous and induced microvascular thromboses and occlusions.

In vivo optical properties of melanocytic skin lesions: common nevi, dysplastic nevi and malignant melanoma

We present an in vivo study of the optical properties of common nevi, dysplastic nevi and malignant melanoma skin lesions in human subjects. Reflectance spectra were measured on 1379 skin lesions, in the visible and near-infrared spectral regions, using a spectral imaging system, in a clinical setting. Analysis of the data using a reflectance model revealed differences between the optical properties of melanin present in nevi and melanoma lesions. These differences, which are in agreement with our previous observations on average reflectance spectra, may be potentially useful for the noninvasive characterization of pigmented skin lesions and the early diagnosis of melanoma.

Full-field laser-Doppler imaging and its physiological significance for tissue blood perfusion

Using Monte Carlo simulations for a semi-infinite medium representing a skeletal muscle tissue, it is demonstrated that the zero- and first-order moments of the power spectrum for a representative pixel of a full-field laser-Doppler imager behave differently from classical laser-Doppler flowmetry. In particular, the zero-order moment has a very low sensitivity to tissue blood volume changes, and it becomes completely insensitive if the probability for a photon to interact with a moving red blood cell is above 0.05. It is shown that the loss in sensitivity is due to the strong forward scatter of the propagating photons in biological tissues (i.e., anisotropy factor g = 0.9). The first-order moment is linearly related to the root mean square of the red blood cell velocity (the Brownian component), and there is also a positive relationship with tissue blood volume. The most common physiological interpretation of the first-order moment is as tissue blood volume times expectation of the blood velocity (in probabilistic terms). In this sense, the use of the first-order moment appears to be a reasonable approach for qualitative real-time blood flow monitoring, but it does not allow us to obtain information on blood velocity or volume independently. Finally, it is shown that the spatial and temporal resolution trade-off imposed by the CMOS detectors, used in full-field laser-Doppler hardware, may lead to measurements that vary oppositely with the underlying physiological quantities. Further improvements on detectors' sampling rate will overcome this limitation.