Ex vivo optical measurements of glucose diffusion kinetics in native and diabetic mouse skin

Optical clearing of human skin: Molecular modeling and in vivo OCT study

The results of in vivo immersion optical clearing of human skin under the action of two different optical clearing agents (OCAs), such as an aqueous sucrose solution and a radiographic contrast agent Omnipaque™ 300 (iohexol), were obtained with the use of optical coherence tomography (OCT) method. The rate of reduction of light scattering coefficient, obtained through an averaged A-scan of the OCT image in the region of dermis within the depths from 350 to 700 μm, were determined to evaluate the efficiency of optical clearing (EOC). The correlations between the EOC and the energy of intermolecular interaction of OCAs with a fragment of collagen peptide have been established as a result of molecular modeling by quantum chemistry methods HF/STO3G/DFT/B3LYP/6-311G(d) of a number of OCAs (glycerol, iohexol, sucrose, ribose, fructose, glucose) with mimetic peptide of collagen (GPH)3 .

MR and fluorescence imaging of gadobutrol-induced optical clearing of red fluorescent protein signal in an in vivo cancer model

Multimodality registration of optical and MR images in the same tissue volume in vivo may be enabled by MR contrast agents with an optical clearing (OC) effect. The goals of this study were to (a) investigate the effects of clinical MR contrast agent gadobutrol (GB) and its combinations as a potential OC agent assisting in fluorescence intensity (FI) imaging in vivo and (b) evaluate MRI as a tool for imaging of topical or systemic application of GB for the purpose of OC. Subcutaneous tumor xenografts expressing red fluorescent marker protein were used as disease models. MRI was performed at 1 T 1 H MRI using T1 -weighted 3D gradient-echo (T1w-3D GRE) sequences to measure time-dependent MR signal intensity changes by region of interest analysis after image segmentation. Topical application of 1.0 M or 0.7 M GB-containing OC mixture with water and dimethyl sulfoxide showed similar 30-40% increases of tumor FI during the initial 15 min. Afterwards, the OC effect of GB on FI and tumor/background FI ratio showed a decrease over time in the case of 1.0 M GB, unlike the 0.7 M GB mixture, which resulted in a steady increase of FI and tumor/background ratio for 15-60 min. The use of T1w-3D GRE MR pulse sequences showed that concentrated 1.0 M GB resulted in MR signal loss of the skin due to high magnetic susceptibility and that signal loss coincided with the OC effect. Intravenous injection of 0.3 mmol GB/kg resulted in a rapid but transient 40% increase of FI of the tumors. Overall, 1 T MRI enabled tracking of GB-containing OC compositions on the skin surface and tumor tissue, supporting the observation of a time-dependent FI increase in vivo.

Magnetic resonance contrast agents in optical clearing: Prospects for multimodal tissue imaging

Skin optical clearing effect ex vivo and in vivo was achieved by topical application of low molecular weight paramagnetic magnetic resonance contrast agents. This novel feature has not been explored before. By using collimated transmittance the diffusion coefficients of three clinically used magnetic resonance contrast agents, that is Gadovist, Magnevist and Dotarem as well as X-ray contrast agent Visipaque in mouse skin were determined ex vivo as (4.29 ± 0.39) × 10^-7 cm² /s, (5.00 ± 0.72) × 10^-7 cm² /s, (3.72 ± 0.67) × 10^-7 cm² /s and (1.64 ± 0.18) × 10^-7 cm² /s, respectively. The application of gadobutrol (Gadovist) resulted in efficient optical clearing that in general, was superior to other contrast agents tested and allowed to achieve: (a) more than 12-fold increase of transmittance over 10 minutes after application ex vivo; (b) markedly improved images of skin architecture obtained with optical coherence tomography; (c) an increase of the fluorescence intensity/background ratio in TagRFP-red fluorescent marker protein expressing tumor by five times after 15 minutes application into the skin in vivo. The obtained results have immediate implications for multimodality imaging because many contrast agents are capable of simultaneously enhancing the contrast of multiple imaging modalities.

Methods for Optical Skin Clearing in Molecular Optical Imaging in Dermatology

This short review describes recent progress in using optical clearing (OC) technique in skin studies. Optical clearing is an efficient tool for enhancing the probing depth and data quality in multiphoton microscopy and Raman spectroscopy. Here, we discuss the main mechanisms of OC, its safety, advantages, and limitations. The data on the OC effect on the skin water content are presented. It was demonstrated that 70% glycerol and 100% Omnipaque^TM 300 reduce the water content in the skin. Both OC agents (OCAs) significantly affect the strongly bound and weakly bound water. However, Omnipaque^TM 300 causes considerably less skin dehydration than glycerol. In addition, the results of examination of the OC effect on autofluorescence in two-photon excitation and background fluorescence in Raman scattering at different skin depths are presented. It is shown that Omnipaque^TM 300 is a promising OCA due to its ability to reduce background fluorescence in the upper skin layers. The possibility of multimodal imaging combining optical methods and OC technique is discussed.

Multimodal Optical Diagnostics of Glycated Biological Tissues

Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia accompanied by the disruption of carbohydrate, lipid, and proteins metabolism and development of long-term microvascular, macrovascular, and neuropathic changes. This review presents the results of spectroscopic studies on the glycation of tissues and cell proteins in organisms with naturally developing and model diabetes and in vitro glycated samples in a wide range of electromagnetic waves, from visible light to terahertz radiation. Experiments on the refractometric measurements of glycated and oxygenated hemoglobin in broad wavelength and temperature ranges using digital holographic microscopy and diffraction tomography are discussed, as well as possible application of these methods in the diabetes diagnostics. It is shown that the development and implementation of multimodal approaches based on a combination of phase diagnostics with other methods is another promising direction in the diabetes diagnostics. The possibilities of using optical clearing agents for monitoring the diffusion of substances in the glycated tissues and blood flow dynamics in the pancreas of animals with induced diabetes have also been analyzed.

Hydrogen bound water profiles in the skin influenced by optical clearing molecular agents-Quantitative analysis using confocal Raman microscopy

Confocal Raman microscopy has been used to measure depth-dependent profiles of porcine skin ex vivo in the high wavenumber region after application of molecular optical clearing agents (OCAs). Glycerol (70%) and iohexol (100% Omnipaque [300]) water solutions were used as OCAs and topically applied to porcine ear skin for 30 and 60 minutes. Using Gaussian function-based deconvolution, the changes of hydrogen bound water molecule types have been microscopically analyzed down to the depth of 200 μm. Results show that both OCAs induced skin dehydration (reduction of total water), which is 51.3% for glycerol (60 minutes), 33.1% for glycerol (30 minutes), 8.3% for Omnipaque (60 minutes) and 4.4% for Omnipaque (30 minutes), on average for the 40 to 200 μm depths. Among the water types in the skin, the following reduction was observed in concentration of weakly bound (51.1%, 33.2%, 7.5% and 4.6%), strongly bound (50.4%, 33.0%, 7.9% and 3.4%), tightly bound (63.6%, 42.3%, 26.1% and 12.9%) and unbound (55.4%, 28.7%, 10.1% and 5.9%) water types on average for the 40 to 200 μm depths, post application of glycerol (60 minutes), glycerol (30 minutes), Omnipaque (60 minutes) and Omnipaque (30 minutes), respectively. As most concentrated in the skin, weakly and strongly bound water types are preferentially involved in the OCA-induced water flux in the skin, and thus, are responsible for optical clearing efficiency.

A robust ex vivo method to evaluate the diffusion properties of agents in biological tissues

A robust method is presented for evaluating the diffusion properties of chemicals in ex vivo biological tissues. Using this method that relies only on thickness and collimated transmittance measurements, the diffusion properties of glycerol, fructose, polypropylene glycol and water in muscle tissues were evaluated. Amongst other results, the diffusion coefficient of glycerol in colorectal muscle was estimated with a value of 3.3 × 10^-7 cm² /s. Due to the robustness and simplicity of the method, it can be used in other fields of biomedical engineering, namely in organ cryoprotection and food industry.

Noninvasive assessments of skin glycated proteins by fluorescence and Raman techniques in diabetics and nondiabetics

Diabetes is a complex metabolic disease and has chronic complications. It has been considered a serious public health problem. The aim of the current study was to evaluate skin glycated proteins through fluorescence and Raman techniques. One hundred subjects were invited to participate in the study. Six volunteers did not attend due to exclusion criteria or a change of mind about participating. Therefore, 94 volunteers were grouped according to age range (20-80 years), health condition (nondiabetic, with insulin resistance [IR] and/or diabetic) and Fitzpatrick skin type (I-VI). The fluorescence spectrometer and the portable Raman spectroscopy system were used to measure glycated proteins from the skin. There was elevated skin autofluorescence in healthy middle-aged and elderly subjects, as well as in patients with IR and/or diabetes. Regarding Raman spectroscopy, changes in the skin hydration state, degradation of type I collagen and greater glycation were related for diabetes and chronological aging. Weak and positive correlation between the skin autofluorescence and the Raman peaks ratio (855/876) related to the glycated proteins was also found. Raman spectroscopy shows several bands for spectral analyses, complementing the fluorescence data. Therefore, this study contributes to understanding of the optical of human skin for noninvasive diabetes screening.

Optimizing tissue-clearing conditions based on analysis of the critical factors affecting tissue-clearing procedures

Tissue-clearing techniques have received great attention for volume imaging and for the potential to be applied in optical diagnosis. In principle, tissue clearing is achieved by reducing light scattering through a combination of lipid removal, size change, and matching of the refractive index (RI) between the imaging solution and the tissue. However, the contributions of these major factors in tissue clearing have not been systematically evaluated yet. In this study, we experimentally measured and mathematically calculated the contribution of these factors to the clearing of four organs (brain, liver, kidney, and lung). We found that these factors differentially influence the maximal clearing efficacy of tissues and the diffusivity of materials inside the tissue. We propose that these physical properties of organs can be utilized for the quality control (Q/C) process during tissue clearing, as well as for the monitoring of the pathological changes of tissues.

Current status, pitfalls and future directions in the diagnosis and therapy of lymphatic malformation

Lymphatic malformations are complex congenital vascular lesions composed of dilated, abnormal lymphatic channels of varying size that can result in significant esthetic and physical impairment due to relentless growth. Lymphatic malformations comprised of micro-lymphatic channels (microcystic) integrate and infiltrate normal soft tissue, leading to a locally invasive mass. Ultrasonography and magnetic resonance imaging assist in the diagnosis but are unable to detect microvasculature present in microcystic lymphatic malformations. In this review, we examine existing tools and elaborate on alternative diagnostic methods in assessing lymphatic malformations. In particular, photoacoustics, low-toxicity nanoparticles and optical clearing can overcome existing challenges in the examination of lymphatic channels in vivo. In combination with photothermal scanning and flow cytometry, Photoacoustic techniques may provide a versatile tool for lymphatic-related clinical applications, potentially leading to a single diagnostic and therapeutic platform to overcome limitations in current imaging techniques and permit targeted theranostics of microcystic lymphatic malformations.

Recent progress in tissue optical clearing for spectroscopic application

This paper aims to review recent progress in optical clearing of the skin and over naturally turbid biological tissues and blood using this technique in vivo and in vitro with multiphoton microscopy, confocal Raman microscopy, confocal microscopy, NIR spectroscopy, optical coherence tomography, and laser speckle contrast imaging. Basic principles of the technique, its safety, advantages and limitations are discussed. The application of optical clearing agent on a tissue allows for controlling the optical properties of tissue. Optical clearing-induced reduction of tissue scattering significantly facilitates the observation of deep-located tissue regions, at the same time improving the resolution and image contrast for a variety of optical imaging methods suitable for clinical applications, such as diagnostics and laser treatment of skin diseases, mucosal tumor imaging, laser disruption of pathological abnormalities, etc.

In vivo imaging the motility of monocyte/macrophage during inflammation in diabetic mice

Diabetes, as a chronic metabolic disease, can impair the immune function of monocytes/macrophages (MMs). However, it is unclear how MM immune response to inflammation with the development of diabetes, and whether immune response around the inflammatory foci depends on the depth in tissue. Footpad provides a classical physiological site for monitoring cellular behavior during inflammation, but limited to the superficial dermis due to the strong scattering of footpad. Herein, we used confocal microscopy to monitor the motility of MMs in deeper tissue around inflammatory foci with the development of type 1 diabetic (T1D) mice through a switchable footpad skin optical clearing window. Delayed-type hypersensitivity (DTH) model was elicited on the footpad of T1D. Results demonstrated that progressive T1D led to the gradually potentiated MM recruitment and increased expression of monocyte chemoattractant protein-1 during DTH, but MM migration displacement, motion velocity and motility coefficient were significantly attenuated. Besides, MMs from the deeper dermis had a much larger migration displacement than those from superficial dermis at early stages of DTH but an opposite tendency at late stages for non-T1D, while progressive T1D obscured this difference gradually. This study will be helpful for investigating the influences of progressive metabolic diseases on immune response. MM motion trajectory at depth of superficial dermis and the deeper dermis at AOVA (heat-aggregated ovalbumin)-4 hours and AOVA-72 hours on non-T1D (A) and T1D-4 weeks (B). Mean motility coefficient (C) at the 2 depths. Data were pooled from 6 mice per group. *P < .05 and **P < .01 compared among different T1D disease durations. #P < .05 compared between different depths.

Glycerol dehydration of native and diabetic animal tissues studied by THz-TDS and NMR methods

The optical clearing method has been widely used for different spectral ranges where it provides tissue transparency. In this work, we observed the enhanced penetration of the terahertz waves inside biological samples (skin, kidney, and cornea) treated with glycerol solutions inducing changes of optical and dielectric properties. It was supported by the observed trend of free-to-bound water ratio measured by the nuclear magnetic resonance (NMR) method. The terahertz clearing efficiency was found to be less for diabetic samples than for normal ones. Results of the numerical simulation proved that pulse deformation is due to bigger penetration depth caused by the reduction of absorption and refraction at optical clearing.

A comparative study of ex vivo skin optical clearing using two-photon microscopy

Multiphoton tomography (MPT) is a prospective tool for imaging the skin structure. Aiming to increase the probing depth, a comparative ex vivo study of optical clearing of porcine ear skin was performed by using two optical clearing agents (OCAs), i.e., glycerol and iohexol (Omnipaque^TM ) at different concentrations, which exhibit different osmotic properties. The results show that a topical application of glycerol or Omnipaque^TM solutions onto the skin for 60 min significantly improved the depth and contrast of the MPT signals. By utilizing 40%, 60% and 100% glycerol, and 60% and 100% Omnipaque^TM it was demonstrated that both agents improve autofluorescence and SHG (second harmonic generation) signals from the skin. At the applied concentrations and agent time exposure, glycerol is more effective than Omnipaque^TM . However, tissue shrinkage and cell morphology changes were found for highly concentrated glycerol solutions. Omnipaque^TM , on the contrary, increases the safety and has no or minimal tissue shrinkage during the optical clearing process. Moreover Omnipaque^TM allows for robust multimodal optical/X-ray imaging with automatically matched optically cleared and X-ray contrasted tissue volumes. These findings make Omnipaque^TM more prospective than glycerol for some particular application.

A useful way to develop effective in vivo skin optical clearing agents

Skin optical clearing has shown tremendous potential in improving various optical imaging performances, but there is some certain blindness in screening out high-efficiency in vivo optical clearing methods. In this work, three optical clearing agents: sucrose (Suc), fructose (Fruc) and PEG-400 (PEG), and two chemical penetration enhancers: propylene glycol (PG) and thiazone (Thiaz) were used. PEG was firstly mixed with the two penetration enhancers, respectively, and then mixed with Fruc and Suc, respectively, to obtain six kinds of skin optical clearing agents (SOCAs). Optical coherence tomography angiography was applied to monitor SOCAs-induced changes in imaging performances, skin optical properties, refractive index mismatching extent, and permeability rate. Experimental results demonstrated that PEG+Thiaz+Suc has the optimal capacity in enhancing the imaging performances, decreasing the scattering and the refractive index mismatching since Thiaz is superior to PG, and Suc is superior to Fruc. This study indicates that the optimal SOCA can be obtained directly by means of additionally adding or replacing the similar category substance in preexisting SOCAs with some more effective reagents. It not only provides an optimal SOCA, but also provides a useful way to develop more effective SOCAs. Cross-section skin structural texture (a), reconstructed blood flow distribution information (b), before or after treated with different SOCAs.