The emerging technique of three-dimensional (3D) printing provides a revolutionary way to fabricate objects with biologically realistic geometries. Previously we have performed optical and morphological characterization of basic 3D printed tissue-simulating phantoms and found them suitable for use in evaluating biophotonic imaging systems. In this study we assess the potential for printing phantoms with irregular, image-defined vascular networks that can be used to provide clinically-relevant insights into device performance. A previously acquired fundus camera image of the human retina was segmented, embedded into a 3D matrix, edited to incorporate the tubular shape of vessels and converted into a digital format suitable for printing. A polymer with biologically realistic optical properties was identified by spectrophotometer measurements of several commercially available samples. Phantoms were printed with the retinal vascular network reproduced as ~1.0 mm diameter channels at a range of depths up to ~3 mm. The morphology of the printed vessels was verified by volumetric imaging with μ-CT. Channels were filled with hemoglobin solutions at controlled oxygenation levels, and the phantoms were imaged by a near-infrared hyperspectral reflectance imaging system. The effect of vessel depth on hemoglobin saturation estimates was studied. Additionally, a phantom incorporating the vascular network at two depths was printed and filled with hemoglobin solution at two different saturation levels. Overall, results indicated that 3D printed phantoms are useful for assessing biophotonic system performance and have the potential to form the basis of clinically-relevant standardized test methods for assessment of medical imaging modalities.
Features of the tumor microenvironment (TME), such as hemoglobin saturation (HbSat), can provide valuable information on early development and progression of tumors. HbSat correlates with high metabolism and precedes the formation of angiogenic tumors; therefore, changes in HbSat profile can be used as a biomarker for early cancer detection. In this project, we develop a methodology to evaluate HbSat for forecasting early tumor development in a mouse model. We built a delta (δ) cumulative feature that includes spatial and temporal distribution of HbSat for classifying tumor/normal areas. Using a two-class (normal and tumor) logistic regression, the δ feature successfully forecasts tumor areas in two window chamber mice (AUC=0.90 and 0.85). To assess the performance of the logistic regression-based classifier utilizing the δ feature of each region, we conduct a 10-fold cross-validation analysis (AUC of the ROC=0.87). These results show that the TME features based on HbSat can be used to evaluate tumor progression and forecast new occurrences of tumor areas.
Fluorescence spectroscopy has been widely investigated as a technique for identifying pathological tissue; however, unrelated subject-to-subject variations in spectra complicate data analysis and interpretation. We describe and evaluate a new biosensing technique, differential laser-induced perturbation spectroscopy (DLIPS), based on deep ultraviolet (UV) photochemical perturbation in combination with difference spectroscopy. This technique combines sequential fluorescence probing (pre- and post-perturbation) with sub-ablative UV perturbation and difference spectroscopy to provide a new spectral dimension, facilitating two improvements over fluorescence spectroscopy. First, the differential technique eliminates significant variations in absolute fluorescence response within subject populations. Second, UV perturbations alter the extracellular matrix (ECM), directly coupling the DLIPS response to the biological structure. Improved biosensing with DLIPS is demonstrated in vivo in a murine model of chemically induced skin lesion development. Component loading analysis of the data indicates that the DLIPS technique couples to structural proteins in the ECM. Analysis of variance shows that DLIPS has a significant response to emerging pathology as opposed to other population differences. An optimal likelihood ratio classifier for the DLIPS dataset shows that this technique holds promise for improved diagnosis of epithelial pathology. Results further indicate that DLIPS may improve diagnosis of tissue by augmenting fluorescence spectra (i.e. orthogonal sensing).
The formation of new microvasculature is essential for a tumor mass to grow. Vascular targeting agents (VTAs),
including anti-angiogenic drugs and vascular disrupting agents, aim to either inhibit new vasculature growth or destroy
existing vasculature, respectively. Because the mechanisms for anti-angiogenic drugs and vascular disrupting agents are
complementary, analysis of these drugs used together is under investigation for the enhanced treatment of tumors in
comparison to each treatment alone. The preclinical evaluation of the effects of VTAs on tumor growth in small animal
models is vital for the development of effective drugs for clinical use. In vivo hyperspectral imaging microscopy of
hemoglobin saturation has been used previously to investigate the efficacy of VTAs through analysis of tumor
microvessel oxygenation after drug administration. Combining this imaging modality with first-pass fluorescence
angiographic imaging can give additional important information about the vessel morphology and blood flow changes
that occur after VTA treatment, thus elucidating the relationship between microvessel structure changes and
oxygenation. In this study, we report the combined use of hyperspectral and first pass fluorescence angiographic
imaging to examine the relationship between vessel morphology and oxygenation of human prostate cancer tumors in
mice following treatment with vascular disrupting agents, OXi4503, and anti-VEGF angiogenesis inhibitor, cediranib.
Imaging of the tumors is completed before treatment as well as in the days following treatment.
Chemotherapy is a standard treatment for metastatic cancer. However drug toxicity limits the dosage that can safely
be used, thus reducing treatment efficacy. Drug carrier particles, like liposomes, can help reduce toxicity by
shielding normal tissue from drug and selectively depositing drug in tumors. Over years of development, liposomes
have been optimized to avoid uptake by the Reticuloendothelial System (RES) as well as effectively retain their drug
content during circulation. As a result, liposomes release drug passively, by slow leakage, but this uncontrolled drug
release can limit treatment efficacy as it can be difficult to achieve therapeutic concentrations of drug at tumor sites
even with tumor-specific accumulation of the carriers. Lipid membranes can be photochemically lysed by both
Type I (photosensitizer-substrate) and Type II (photosensitizer-oxygen) reactions. It has been demonstrated in red
blood cells (RBCs) in vitro that these photolysis reactions can occur in two distinct steps: a light-initiated reaction
followed by a thermally-initiated reaction. These separable activation steps allow for the delay of photohemolysis in
a controlled manner using the irradiation energy, temperature and photosensitizer concentration. In this work we
have translated this technique from RBCs to liposomal nanoparticles. To that end, we present in vitro data
demonstrating this delayed bolus release from liposomes, as well as the ability to control the timing of this event.
Further, we demonstrate for the first time the improved delivery of bioavailable cargo selectively to target sites in
vivo.
Abnormal microvascular function and angiogenesis are key components of various diseases that can contribute to
the perpetuation of the disease. Several skin diseases and ophthalmic pathologies are characterized by
hypervascularity, and in cancer the microvasculature of tumors is structurally and functionally abnormal. Thus, the
microvasculature can be an important target for treatment of diseases characterized by abnormal microvasculature.
Motivated largely by cancer research, significant effort has been devoted to research on drugs that target the
microvasculature. Several vascular targeting drugs for cancer therapy are in clinical trials and approved for clinical
use, and several off-label uses of these drugs have been reported for non-cancer diseases. The ability to image and
measure parameters related to microvessel function preclinically in laboratory animals can be useful for
development and comparison of vascular targeting drugs. For example, blood supply time measurements give
information related to microvessel morphology and can be measured with first-pass fluorescence imaging.
Hemoglobin saturation measurements give an indication of microvessel oxygen transport and can be measured with
spectral imaging. While each measurement individually gives some information regarding microvessel function, the
measurements together may yield even more information since theoretically microvessel morphology can influence
microvessel oxygenation, especially in metabolically active tissue like tumors. However, these measurements have
not yet been combined. In this study, we report the combination of blood supply time imaging and hemoglobin
saturation imaging of microvessel networks in tumors using widefield fluorescence and spectral imaging,
respectively. The correlation between the measurements in a mouse mammary tumor is analyzed.
KEYWORDS: Optical coherence tomography, Microelectromechanical systems, Mirrors, Actuators, Doppler tomography, In vivo imaging, 3D image processing, Endoscopy, Ear, Cancer
Most cancers occur inside human body, so endoscopic high-resolution imaging modalities are required for early cancer
detection and surgical removal. This paper reports in vivo endoscopic 3D imaging based on optical coherence
tomography (OCT). Endoscopic imaging is enabled by integrating rapid-scanning MEMS mirror into a miniature
imaging probe. The MEMS mirror has an aperture size of 1 mm by 1 mm and a chip size of 2 mm by 2 mm. The optical
scan angle exceeds ±25 V at 6 Vdc, and thus large, constant-velocity, linear scan can be realized. The outer diameter of
the probe is only 5 mm. The axial resolution is about 10 μm and the imaging speed is 2.5 frames per second. Doppler
OCT imaging has also been demonstrated.
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.
Imaging of immune system and tissue response to immunogenic agents can be important to the development of new
biomaterials. Additionally, quantitative functional imaging can be useful for testing and evaluation of methods to alter
or control the immune system response to implanted materials. In this preliminary study, we employ spectral imaging
and fluorescence imaging to measure immune system and tissue response to implanted immunogenic agents. Poly (D,L
lactide-co-glycolide) (PLGA) with a 50:50 composition was used to create immunogenic microparticles (MPs).
Lipopolysaccharide (LPS) encapsulated in the MPs was used to provoke a tissue immune response in mice and
encapsulated fluorescein isothiocyanate (FITC) was used to fluorescently label the MPs for imaging. Control MPs did
not contain LPS. The MPs were delivered at 50 particles/μL in a total volume of 20μL by subcutaneous injection in the
skin of a nude mouse in a dorsal skin-fold window chamber preparation. Cultured immune cells from a mouse
leukemic monocyte macrophage cell line were exogenously labeled with the fluorescent dye DiD in solution at a
concentration of 8000cells/μL. Immediately after window chamber surgery and implantation of the MPs, 100μL of the
fluorescent macrophage solution was administered via the tail vein. Fluorescence imaging was used to track MPs and
macrophages while spectral imaging was used for imaging and measurement of hemoglobin saturation in the tissue
microvasculature. Imaging was performed periodically over about three days. The spectral and fluorescence imaging
combination enabled detailed observations of the macrophage response and functional effects on the tissue.
Tumors are highly metabolically active and thus require ample oxygen and nutrients to proliferate. Neovasculature
generated by angiogenesis is required for tumors to grow beyond a size of about 1-2mm. Functional tumor vasculature
also provides an access point for development of distant metastases. Due to the importance of the microvasculature for
tumor growth, proliferation, and metastasis, the microvasculature has emerged as a therapeutic target for treatment of
solid tumors. We employed spectral imaging in a rodent window chamber model to observe and measure the oxygen
transport function of tumor microvasculature in a human renal cell carcinoma after treatment with a fast acting vascular
disrupting agent. Human Caki-1 cells were grown in a dorsal skin-fold window chamber in athymic nude mice.
Spectral imaging was used to measure hemoglobin saturation immediately before, immediately after and also at 2, 4, 6,
8, 24 and 48 hours after administration of the tubulin binding agent OXi4503. Up to 4 hours after treatment, tumor
microvasculature was disrupted from the tumor core towards the periphery as seen in deoxygenation as well as
structural changes of the vasculature. Reoxygenation and neovascularization commenced from the periphery towards
the core from 6 - 48 hours after treatment. The timing of the effects of vascular disrupting agents can influence
scheduling of repeat treatments and combinatorial treatments such as chemotherapy and radiation therapy. Spectral
imaging can potentially provide this information in certain laboratory models from endogenous signals with microvessel
resolution.
Abnormal microvasculature contributes to the pathophysiologic microenvironment of tumors. Understanding microvascular tumor oxygen transport is necessary to comprehend the factors that influence tumor biology, physiology, and therapy. Previously, we described an in vivo spectral imaging microscopy system for measurements of microvessel hemoglobin saturation (HbSat). We measure temporal fluctuations and spatial gradients in tumor microvessel oxygenation and identify instances of anastomoses between vessels with significantly different oxygenations. Slow periodic fluctuations in HbSat <0.2 cycles per minute were observed. These measurements are consistent with microelectrode measurements of fluctuating tumor oxygenation. Gradients in HbSat along individual tumor microvessels were measured that were larger in magnitude than normal tissue microvessels. Images were captured of anastomoses of tumor microvessels with diameters 100 µm and significantly different HbSat values (>20%). Shunting of inspired oxygen, presumably due to arteriovenous anastomoses, from tumor feeding arterioles to adjacent venules was imaged. This effect was confined to a region around the tumor and was not observed in nearby normal microvessels. Imaging measurements of tumor microvessel oxygen transport may offer insight to current questions regarding oxygen-related tumor biology and treatment responses, and spectral imaging may be a useful research tool in this regard.
It is well established that hypoxia can influence tumor biology and physiology, gene expression, metastatic potential, treatment efficacy, and patient survival. Most human solid tumors have been shown to have some hypoxic regions, thus there is a strong motivation to understand the various causes of hypoxia. One key to understanding tumor hypoxia involves the study of oxygen transport to tumors, and the connection between hypoxia, tumor microvasculature, and the tumor microenvironment. Recent research has suggested that the causes of tumor hypoxia are much more complex than indicated by the classical paradigms ("chronic" and "acute" hypoxia), and several potential factors have been identified. Two such factors are temporal fluctuations in tissue pO2 and longitudinal gradients in oxygen transport. Research has shown the existence of low frequency (<2 cycles per minute) fluctuations in tumor pO2 without cessation of blood flow that can lead to transient hypoxia. In addition, longitudinal gradients in tumor pO2 along the arteriolar afferent direction have been documented in window chamber tumors. However, the causes of the pO2 temporal fluctuations and longitudinal gradients are not exactly known, and the clinical significance of these observations is not well understood. In this preliminary study, we demonstrate the potential of optical imaging measurements of hemoglobin saturation to add new information in these areas. Slow temporal fluctuations of hemoglobin saturation (HbSat) and gradients in the average HbSat were observed in some 4T1 mouse mammary carcinoma microvessels. With additional research, the mechanisms behind these phenomena and insights into their clinical significance may be revealed.
Tumor hypoxia has been shown to have prognostic value in clinical trials involving radiation, chemotherapy, and surgery. Tumor oxygenation studies at microvascular levels can provide understanding of oxygen transport on scales at which oxygen transfer to tissue occurs. To fully grasp the significance of blood oxygen delivery and hypoxia at microvascular levels during tumor growth and angiogenesis, the spatial and temporal relationship of the data must be preserved and mapped. Using tumors grown in window chamber models, hyperspectral imaging can provide serial spatial maps of blood oxygenation in terms of hemoglobin saturation at the microvascular level. We describe our application of hyperspectral imaging for in vivo microvascular tumor oxygen transport studies using red fluorescent protein (RFP) to identify all tumor cells, and hypoxia-driven green fluorescent protein (GFP) to identify the hypoxic fraction. 4T1 mouse mammary carcinoma cells, stably transfected with both reporter genes, are grown in dorsal skin-fold window chambers. Hyperspectral imaging is used to create image maps of hemoglobin saturation, and classify image pixels where RFP alone is present (tumor cells), or both RFP and GFP are present (hypoxic tumor cells). In this work, in vivo calibration of the imaging system is described and in vivo results are shown.
Tumor hypoxia has been shown to be of prognostic value in several clinical trials involving radiation, chemotherapy, and surgery. Studies of tumor oxygenation at the microvascular and microregional levels can provide understanding of tumor oxygen transport on scales comparable to the diffusion distance of oxygen in tissue. To fully grasp the significance of blood oxygen delivery and hypoxia at the microvascular level, the spatial and temporal relationship of blood oxygenation data must be preserved and mapped. Using tumors grown in dorsal skin-fold window chamber models, hyperspectral imaging can provide spatial maps of blood oxygenation in terms of hemoglobin saturation at the microvascular level, and these measurements can be performed serially in the same animal in a non-invasive fashion with relative technical ease. A hyperspectral imaging system has been constructed to create image maps of hemoglobin saturation in microvasculature of tumors grown in dorsal skin-fold window chambers. Preliminary baseline studies of early tumor development using 4T1 mouse mammary carcinomas are currently being conducted with the system.
Previous in vitro laser-tissue welding research demonstrated a technique to improve the acute breaking strength of liquid albumin solder using biodegradable polymer film reinforcement. This study’s purpose was to explore what problems could arise in vivo that were not evident in vitro. Full thickness incisions were created on the dorsum of Sprague-Dawley rats. A poly(lactic-co-glycolic acid) (PLGA) film was subcutaneously implanted in the incision (controls had no film). The incisions were closed by laser-tissue welding: albumin solder with Indocyanine Green dye was inserted between the incision edges and coagulated with a diode laser. Specimens were harvested at 0, 3, 7, and 14 days for strength testing and histologic analysis. Histology indicated that the PLGA film was relatively well tolerated by the tissue. Breaking strengths of controls at 0 and 14 days were stronger than specimens with implanted films (t-test, P < 0.05). Difficulty apposing the incision edges due to the film presence probably contributed to low acute strengths. Interference with wound contraction by the films possibly contributed to lower breaking strengths at 14 days. Polymer films for patch reinforcement of solder may be clinically feasible; however, application specific problems can occur and should be considered when developing a clinical technique.
Previous work demonstrated increased breaking strengths of tissue repaired with liquid albumin solder reinforced with a biodegradable polymer film compared to unreinforced control specimens. It was hypothesized that the breaking strength increase was due to reinforcement of the liquid solder cohesive strength. Immersion in a moist environment can decrease the adhesion of solder to tissue and negate any strength benefits gained from reinforcement. The purpose of this study was to determine if hydrated specimens repaired with reinforced solder would still be stronger than unreinforced controls. A 50%(w/v) bovine serum albumin solder with 0.5 mg/mL Indocyanine Green dye was used to repair an incision in bovine aorta. The solder was coagulated with 806-nm diode laser light. A poly(DL-lactic- co-glycolic acid) film was used to reinforce the solder (the controls had no reinforcement). The repaired tissues were immersed in phosphate buffered saline for time periods of 1 and 2 days. The breaking strengths of all of the hydrated specimens decreased compared to the acute breaking strengths. However, the reinforced specimens still had larger breaking strengths than the unreinforced controls. These results indicate that reinforcement of a liquid albumin solder may have the potential to improve the breaking strength in a clinical setting.
The purpose of this study was to determine if solid material reinforcement of a liquid albumin solder coagulum could improve the cohesive strength of the solder and thus the ultimate breaking strength of the incision repair in vitro. A 50%(w/v) bovine serum albumin solder with 0.5 mg/mL Indocyanine Green (ICG) dye was used to repair an incision in bovine aorta. The solder was coagulated with an 806 nm CW diode laser. A 50 micrometer thick poly(DL-lactic-co-glycolic acid) film was used to reinforce the solder (the controls had no reinforcement). Acute breaking strengths were measured and the data were analyzed by one-way ANOVA (P less than 0.05). Multiple comparisons of means were performed using the Newman- Keuls test. Observations of the failure modes indicated cohesive strength reinforcement of the test specimens versus the controls. At the higher laser powers used in this study (400 and 450 mW), the reinforced solder was consistently stronger than the controls. Reinforcement of liquid albumin solders in laser-assisted incision repair may have mechanical advantages in terms of acute breaking strength over conventional methods that do not reinforce the cohesive strength of the solder.
Newly developed light-activated surgical adhesives have been investigated as a substitute to traditional protein solders for vascular tissue fusion without the need for sutures. Canine femoral arteries (n equals 14), femoral veins (n equals 14) and carotid arteries (n equals 10) were exposed, and a 0.3 to 0.6 cm longitudinal incision was made in the vessel walls. The surgical adhesive, composed of a poly(L-lactic-co-glycolic acid) scaffold doped with the traditional protein solder mix of bovine serum albumin and indocyanine green dye, was used to close the incisions in conjunction with an 805 nm diode laser. Blood flow was restored to the vessels immediately after the procedure and the incision sites were checked for patency. The new adhesives were flexible enough to be wrapped around the vessels while their solid nature avoided the problems associated with 'runaway' of the less viscous liquid protein solders widely used by researchers. Assessment parameters included measurement of the ex vivo intraluminal bursting pressure one to two hours after surgery, as well as histology. The acute intraluminal bursting pressures were significantly higher in the laser-solder group (greater than 300 mmHg) compared to the suture control group (less than 150 mmHg) where four evenly spaced sutures were used to repair the vessel (n equals 4). Histological analysis showed negligible evidence of collateral thermal damage to the underlying tissue in the laser-solder repair group. These initial results indicated that laser-assisted vascular repair using the new adhesives is safe, easy to perform, and contrary to conventional suturing, provides an immediate leak-free closure. In addition, the flexible and moldable nature of the new adhesives should allow them to be tailored to a wide range of tissue geometries, thus greatly improving the clinical applicability of laser-assisted tissue repair.
The objectives of this study were to determine if the optical absorption properties of urinary calculi affect the threshold fluence for ablation or fragmentation and the ablation efficiency due to laser irradiation. The Vanderbilt free electron laser was tuned to selected wavelengths based on the absorption spectrum of various types of urinary calculi. The threshold fluences for ablation of the calculi were measured at different wavelengths. A preliminary study of the ablation efficiency (ablation depth per unit incidence fluence) was performed. The results were fond to be in agreement with a thermal ablation model for which the threshold fluences were proportional to l/(mu) a. The ablation efficiencies were higher in regions of the infrared spectra in which absorption was higher. For a fixed laser irradiation, the lower threshold fluences within regions of high optical absorption allowed more energy to enhance calculus ablation. This study provided insight into determining the optimum wavelengths for ablation and laser lithotripsy.
The purpose of this study was to explore the feasibility of using a free-electron laser (FEL) to photothermally coagulate an albumin solder for laser-assisted incision closure. A 50%(w/v) bovine serum albumin solder was used to repair an incision in bovine aorta. The solder was coagulated by targeting absorption peaks in the solder infrared absorption spectrum using the FEL. Acute breaking strengths of repaired incisions were measured and the data analyzed by one-way ANOVA (P < 0.05). Multiple comparisons of means were performed using the Newman-Keuls test. The solder absorption spectrum from 2 - 10 microns was similar to water with an additional peak at 6.45 microns (amide II) due to the albumin. Preliminary results indicated that wavelengths at or very close to the absorption peaks were excessively absorbed, resulting in only the top surface of the solder being coagulated. Using wavelengths at points of weak absorption on the water absorption curve yielded better results.
Recent laser-tissue soldering work in our lab has demonstrated the feasibility of building a solder bond from individually coagulated small droplets using a precision pipette for the deposition of the solder droplets. This method of using small, precise volumes of solder to build a bond may result in stronger and more reproducible bonds than coagulating an equivalent large volume of solder all at once. We have investigated the technique further in this study. The solder was dispensed onto the intimal side of a bovine aorta substrate and irradiated with an 808nm diode laser. A bond was created across an incision in the tissue substrate by alternately dispensing and coagulating each small volume of solder, or by coagulating a single large equivalent volume. Acute strength analysis was performed on the solder bond. Future work will concentrate on testing a bench-top solder dispensing device and investigating the feasibility of turning the deice into a prototype tool for clinical applications.
An in vitro study was performed using an 808nm-diode laser in conjunction with indocyanine green-doped albumin protein solders to repair bovine aorta specimens. Investigations were conducted to determine optimal solder and laser parameters for tissue repair in terms of tensile strength, temperature rise and damage and the microscopic nature of the bonds formed. Liquid and solid protein solders prepared from 25% and 60% bovine serum albumin (BSA), respectively, were compared.
The tensile strengths of the repairs were greatly improved with an increase in BSA concentration from 25% to 60% and a reduction in ICG dye concentration from 2.5 mg/ml to 0.25 mg/ml. Increasing the later irradiance and thus surface temperature resulted in an increased severity of histological injury. Thermal denaturation of the tissue substrate increased laterally and in depth with higher temperatures. Optimal repairs in terms of bond strength and thermal damage were achieved by denaturing a solid protein solder composed of 60% BSA and .025mg/ml ICG with an irradiance of 6.4 W/cm2. Using this combination of solder and laser parameters, surface temperatures were observed to reach 85±5°C with an average temperature difference across the solder strips of 15°C across a thickness of 150 μm. Histological examination of the repairs formed using these parameters showed negligible evidence of collateral thermal damage to the underlying tissue. Scanning electron microscopy suggested albumin intertwining within the itssue collagen matrix and subsequent fusion with the collagen as the mechanism for laser tissue soldering.
Variations in laser irradiance, exposure time, solder composition, chromophore type and concentration have led to inconsistencies in published results of laser-solder repair of tissue. To determine optimal parameters for laser tissue soldering, an in vitro study was performed using an 808-nm diode laser in conjunction with an indocyanine green (ICG)- doped albumin protein solder to weld bovine aorta specimens. Liquid and solid protein solders prepared from 25% and 60% bovine serum albumin (BSA), respectively, were compared. The effects of laser irradiance and exposure time on tensile strength of the weld and temperature rise as well as the effect of hydration on bond stability were investigated. Optimum irradiance and exposure times were identified for each solder type. Increasing the BSA concentration from 25% to 60% greatly increased the tensile strength of the weld. A reduction in dye concentration from 2.5 mg/ml to 0.25 mg/ml was also found to result in an increase in tensile strength. The strongest welds were produced with an irradiance of 6.4 W/cm2 for 50 s using a solid protein solder composed of 60% BSA and 0.25 mg/ml ICG. Steady-state solder surface temperatures were observed to reach 85 plus or minus 5 degrees Celsius with a temperature gradient across the solid protein solder strips of between 15 and 20 degrees Celsius. Finally, tensile strength was observed to decrease significantly (20 to 25%) after the first hour of hydration in phosphate-buffered saline. No appreciable change was observed in the strength of the tissue bonds with further hydration.
Tissue optical properties are necessary parameters for prescribing light dosimetry in photomedicine. In many diagnostic or therapeutic applications where optical fiber probes are used, pressure is often applied to the tissue to reduce index mismatch and increase light transmittance. In this study, we have measured in vitro optical properties as a function of pressure with a visible-IR spectrophotometer. A spectral range of 400 - 1800 nm with a spectral resolution of 5 nm was used for all measurements. Skin specimens of two Hispanic donors and three caucasian donors were obtained from the tissue bank. Each specimen, sandwiched between microscope slides, was compressed by a spring-loaded apparatus. Then diffuse reflectance and transmittance of each sample were measured at no load and at approximately 0.1 and 1 kgf/cm2. Under compression, tissue thicknesses were reduced up to 78%. Generally, reflectance decreased while the overall transmittance increased under compression. The absorption and reduced scattering coefficients were calculated using the inverse adding doubling method. Compared with the no-load controls, there was an increase in the absorption and scattering coefficients among most of the compressed specimens.
KEYWORDS: Monte Carlo methods, Tissue optics, Blood vessels, Tissues, 3D modeling, Photon transport, Skin, Optical simulations, Data modeling, Absorption
In order to reach a higher level of accuracy in simulation of port wine stain treatment, we propose to discard the typical layered geometry and cylindrical blood vessel assumptions made in optical models and use imaging techniques to define actual tissue geometry. Two main additions to the typical 3D, weighted photon, variable step size Monte Carlo routine were necessary to achieve this goal. First, optical low coherence reflectometry (OLCR) images of rat skin were used to specify a 3D material array, with each entry assigned a label to represent the type of tissue in that particular voxel. Second, the Monte Carlo algorithm was altered so that when a photon crosses into a new voxel, the remaining path length is recalculated using the new optical properties, as specified by the material array. The model has shown good agreement with data from the literature. Monte Carlo simulations using OLCR images of asymmetrically curved blood vessels show various effects such as shading, scattering-induced peaks at vessel surfaces, and directionality-induced gradients in energy deposition. In conclusion, this augmentation of the Monte Carlo method can accurately simulate light transport for a wide variety of nonhomogeneous tissue geometries.
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