SignificanceFluorescence sensing within tissue is an effective tool for tissue characterization; however, the modality and geometry of the image acquisition can alter the observed signal.AimWe introduce a novel optical fiber-based system capable of measuring two fluorescent contrast agents through 2 cm of tissue with simple passive electronic switching between the excitation light, simultaneously acquiring fluorescence and excitation data. The goal was to quantify indocyanine green (ICG) and protoporphyrin IX (PpIX) within tissue, and the sampling method was compared with wide-field surface imaging to contrast the value of deep sensing versus surface imaging.ApproachThis was achieved by choosing filters for specific wavelengths that were mutually exclusive between ICG and PpIX and coupling these filters to two separate detectors, which allows for direct swapping of the excitation and emission channels by switching the on-time of each excitation laser between 780- and 633-nm wavelengths.ResultsThis system was compared with two non-contact surface imaging systems for both ICG and PpIX, which revealed that the fluorescence depth sensing system was superior in its ability to resolve kinetics differences in deeper tissues that would normally be dominated by strong signals from skin and other surface tissues. Specifically, the system was tested using pancreatic adenocarcinoma tumors injected into murine models, which were imaged at several time points throughout tumor growth to its ∼6-mm diameter. This demonstrated the system’s capability to track longitudinal changes in ICG and PpIX kinetics that result from tumor growth and development, with larger tumors showing sluggish uptake and clearance of ICG, which was not observable with surface imaging. Similarly, PpIX was quantified, which showed slower kinetics over different time points, and was further compared with the wide-filed imager. These results were further validated through depth measurements in tissue phantoms and model-based interpretation.ConclusionThis fluorescence depth sensing system can be used to sample the interior blood flow characteristics by ICG sensing of tissue as deep as 20 mm into the tissue with sensitivity to kinetics that are superior to surface imaging and may be combined with other imaging modalities such as ultrasound to provide guided deep fluorescence measurements.
Hydrogels and hydrogel-based materials, thanks to their biocompatibility and biodegradability, are widely used as a supporting matrix for embedding various kinds of luminescent probes for biological sensing applications. Here we describe a family of phosphorescent hydrogels, termed Oxygels, which were designed specifically for local sensing of oxygen by means of Cherenkov-Excited Luminescence Imaging (CELI) in and around tumors during application of radiation therapy. Previously, our group has developed soluble phosphorescent probes, known as Oxyphors, and demonstrated their performance in CELI of oxygen. Oxyphors comprise phosphorescent metalloporphyrins encapsulated inside hydrophobic dendrimers, whose periphery is modified with polyethyleneglycol (PEG) residues. The PEG layer creates a hydrophilic jacket around the dendrimer, precluding interactions of the probe with biomacromolecules. As a result, Oxyphors retain stable calibration parameters, enabling quantitative imaging of oxygen in in vivo. However, locally delivered Oxyphors rapidly diffuse away from the injection sites and spread throughout the body, posing challenges to local oxygen quantification as well as raising concerns in terms of regulatory (FDA) approval. To this end, hydrogel-supported phosphorescent sensors implanted into tissue should allow for continuous local monitoring of oxygen during RT, aiding optimization of treatment protocols and facilitating the development of new types of RT treatment.
Fluorescent markers can make surgery more specific by enhancing contrast during tissue resection in certain types of disease. Pressure-Enhanced Sensing Surgery (PRESS) uses a commonly available human precursor molecule, 5-Aminolevulinic Acid, to stimulate immediate fluorescence when there is hypoxia present. This pre-contrast agent is metabolized into heme in most human cells, but produces a red fluorescent molecule, protoporphyrin IX, as an intermediate contrast agent. PpIX delayed fluorescence is amplified in low oxygen environment of tissue. PRESS contrast can be used through tissue palpation, leading to contrast greater than 5 in pancreatic, brain, ovarian and head & neck tumors. PRESS imaging is the first real-time widefield acquisition of palpation response in vivo, making it a valuable tool for highlighting hypoxic tissues and guiding oncologic surgical resection.
Multiple FGS devices have been FDA cleared for use in open and laparoscopic surgery. Despite the rapid growth of the field, there has been a lack of standardization methods. We propose a system evaluation pipeline through the use of photo-stable ICG fluorescence phantoms. The approach is validated across five different FDA-approved open-FGS systems which are characterized for: spatial resolution, sensitivity and linearity, imaging depth, depth of field, uniformity, spatial distortion, signal-background ratio, excitation wavelength bands and power. The results highlight how such a standardization approach can be successfully implemented for inter-system comparisons and to better understand features within each device.
Real-time visualization of burn severity and hypoxic response is important in understanding wound healing outcomes. Herein we propose a novel approach to macroscopically image burn induced hypoxia through endogenous PpIX delayed fluorescence. The approach is validated in-vivo using athymic nude mice with induced full-thickness burns. PpIX delayed fluorescence imaging was performed longitudinally for 3 days post injury. The localization of PpIX signals was also correlated to Indocyanine Green fluorescence imaging, a technique for blood perfusion and tissue injury in the burn wound. The current results highlight the capability of the approach for real-time in-vivo quantification of hypoxic response to burn wounds.
Imaging of indocyanine green (ICG) can reveal vascular permeability, and it has been previously demonstrated in pancreatic adenocarcinoma tumors [1]. The relevance of this to clinical use has remained speculative, although it is likely that these vascular permeability measurements could be used for resection guidance or could also be predictive of drug retention or immune infiltration in dysplastic tissues in non-surgical tumors. Second-Window Indocyanine Green (SWIG) imaging, in which a high ICG dose is administered followed by imaging at hours or days post-injection, has been shown to have potential in several oncologic indications [2,3], and is dependent upon dysplastic tissues having a higher degree of bulk tissue retention. Herein, we evaluated the capability of early phase vascular permeability estimates within minutes after ICG injection, and how they may be related to the degree of ICG retention in SWIG imaging.
Pancreatic cell lines, AsPC1 or BxPC3 were grown into tumors in nude mice, providing models that display different capillary network morphologies. Using a clinical surgical fluorescence imaging system, mice were imaged for 10 minutes following bolus IV injection of 4mg/kg ICG. Mice were subsequently imaged 24 hours after the initial injection to measure the intensity of the tumor relative to a muscle tissue reference for SWIG images. The temporal slope of tissue uptake within the first few minutes was used to estimate vascular permeability.
Initial vascular permeability estimates from flow kinetics imaging were not predictive of the ICG retention in SWIG imaging. This would indicate that lymphatics or other factors likely play a larger role in determining retention.
SignificanceFluorescence guided surgery (FGS) has demonstrated improvements in decision making and patient outcomes for a wide range of surgical procedures. Not only can FGS systems provide a higher level of structural perfusion accuracy in tissue reconstruction cases but they can also serve for real-time functional characterization. Multiple FGS devices have been Food and Drug administration (FDA) cleared for use in open and laparoscopic surgery. Despite the rapid growth of the field, there has been a lack standardization methods.AimThis work overviews commonalities inherent to optical imaging methods that can be exploited to produce such a standardization procedure. Furthermore, a system evaluation pipeline is proposed and executed through the use of photo-stable indocyanine green fluorescence phantoms. Five different FDA-approved open-field FGS systems are used and evaluated with the proposed method.ApproachThe proposed pipeline encompasses the following characterization: (1) imaging spatial resolution and sharpness, (2) sensitivity and linearity, (3) imaging depth into tissue, (4) imaging system DOF, (5) uniformity of illumination, (6) spatial distortion, (7) signal to background ratio, (8) excitation bands, and (9) illumination wavelength and power.ResultsThe results highlight how such a standardization approach can be successfully implemented for inter-system comparisons as well as how to better understand essential features within each FGS setup.ConclusionsDespite clinical use being the end goal, a robust yet simple standardization pipeline before clinical trials, such as the one presented herein, should benefit regulatory agencies, manufacturers, and end-users to better assess basic performance and improvements to be made in next generation FGS systems.
SignificancePancreatic cancer tumors are known to be avascular, but their neovascular capillaries are still chaotic leaky vessels. Capillary permeability could have significant value for therapy assessment, and its quantification might be possible with macroscopic imaging of indocyanine green (ICG) kinetics in tissue.AimThe capacity of using standard fluorescence surgical systems for ICG kinetic imaging as a probe for capillary leakage was evaluated using a clinical surgical fluorescence imaging system, as interpreted through vascular permeability modeling.ApproachXenograft pancreatic adenocarcinoma models were imaged in mice during bolus injection of ICG to capture the kinetics of uptake. Image analysis included ratiometric data, normalization, and match to theoretical modeling. Kinetic data were converted into the extraction fraction of the capillary leakage.ResultsPancreatic tumors were usually less fluorescent than the surrounding healthy tissues, but still the rate of tumor perfusion could be assessed to quantify capillary extraction. Model simulations showed that flow kinetics stabilized after about 1 min beyond the initial bolus injection and that the relative extraction fraction model estimates matched the experimental data of normalized uptake within the tissue. The kinetics in the time period of 1 to 2 min post-injection provided optimal differential data between AsPC1 and BxPC3 tumors, although high individual variation exists between tumors.ConclusionsICG kinetic imaging during the initial leakage phase was diagnostic for quantitative vascular permeability within pancreatic tumors. Methods for autogain correction and normalized model-based interpretation allowed for quantification of extraction fraction and difference identification between tumor types in early timepoints.
This conference presentation was prepared for the Molecular-Guided Surgery: Molecules, Devices, and Applications IX conference at SPIE BiOS, SPIE Photonics West 2023.
Reconstructions in 3D widefield Diffuse Optical Tomography (DOT) suffer from poor spatial resolution. Therefore, widefield DOT techniques benefit from incorporating structural priors from a complementary modality, such as the micro-CT. Unfortunately, traditional Laplacian-based methods to integrate the priors in the inverse problem are highly time-consuming. Therefore, we propose a Deep Neural Network based end-to-end inverse solver that combines features from AUTOMAP and Z-net and utilizes the micro-CT priors in the training stage. Initial in silico and experimental phantom results demonstrate that the proposed network accurately reconstructs, in 3D, the absorption contrast with a high resolution.
KEYWORDS: Monte Carlo methods, Data modeling, Luminescence, Computer simulations, Reflectivity, Sensors, In vivo imaging, Absorption, Diffuse optical tomography, 3D modeling
Significance: Deep learning (DL) models are being increasingly developed to map sensor data to the image domain directly. However, DL methodologies are data-driven and require large and diverse data sets to provide robust and accurate image formation performances. For research modalities such as 2D/3D diffuse optical imaging, the lack of large publicly available data sets and the wide variety of instrumentation designs, data types, and applications leads to unique challenges in obtaining well-controlled data sets for training and validation. Meanwhile, great efforts over the last four decades have focused on developing accurate and computationally efficient light propagation models that are flexible enough to simulate a wide variety of experimental conditions.
Aim: Recent developments in Monte Carlo (MC)-based modeling offer the unique advantage of simulating accurately light propagation spatially, temporally, and over an extensive range of optical parameters, including minimally to highly scattering tissue within a computationally efficient platform. Herein, we demonstrate how such MC platforms, namely “Monte Carlo eXtreme” and “Mesh-based Monte Carlo,” can be leveraged to generate large and representative data sets for training the DL model efficiently.
Approach: We propose data generator pipeline strategies using these platforms and demonstrate their potential in fluorescence optical topography, fluorescence optical tomography, and single-pixel diffuse optical tomography. These applications represent a large variety in instrumentation design, sample properties, and contrast function.
Results: DL models trained using the MC-based in silico datasets, validated further with experimental data not used during training, show accurate and promising results.
Conclusion: Overall, these MC-based data generation pipelines are expected to support the development of DL models for rapid, robust, and user-friendly image formation in a wide variety of applications.
Significance: Biomedical optics system design, image formation, and image analysis have primarily been guided by classical physical modeling and signal processing methodologies. Recently, however, deep learning (DL) has become a major paradigm in computational modeling and has demonstrated utility in numerous scientific domains and various forms of data analysis.
Aim: We aim to comprehensively review the use of DL applied to macroscopic diffuse optical imaging (DOI).
Approach: First, we provide a layman introduction to DL. Then, the review summarizes current DL work in some of the most active areas of this field, including optical properties retrieval, fluorescence lifetime imaging, and diffuse optical tomography.
Results: The advantages of using DL for DOI versus conventional inverse solvers cited in the literature reviewed herein are numerous. These include, among others, a decrease in analysis time (often by many orders of magnitude), increased quantitative reconstruction quality, robustness to noise, and the unique capability to learn complex end-to-end relationships.
Conclusions: The heavily validated capability of DL’s use across a wide range of complex inverse solving methodologies has enormous potential to bring novel DOI modalities, otherwise deemed impractical for clinical translation, to the patient’s bedside.
Diffuse optical tomography, including fluorescence molecular tomography (FMT) have been greatly facilitated by the implementation of structured illumination (SI) strategies in recent years. In this work, we investigate the inverse problem in k-space reflectance fluorescence tomography. This in silico investigation leverages MCX, a Monte Carlo based platform, to generate large data sets for comparison between dAUTOMAP, a deep learning architecture, and commonly employed iterative solvers. We show that the proposed dAUTOMAP-based technique outperforms the traditional reconstruction algorithms. This new image formation approach is expected to facilitate imaging of sub-cutaneous tumors in live animals with enhanced resolution compared to the current gold standard.
KEYWORDS: In vivo imaging, Auto-fluorescence imaging, Tissues, In vitro testing, Visible radiation, Structured light, Spectrophotometry, Single photon, Sensors, Near infrared
We propose a Single-Pixel Macroscopic Autofluorescence Imaging platform with supercontinuum excitation (440-690nm) and 16 parallel wavelength detection (475-1000nm) through a spectrophotometer coupled Single Photon Counting PMT. Recorded decays of FAD, POPOP and PPIX commercial auto-fluorophores serve to simulate training samples for UNMIX-ME, a deep learning algorithm that disentangles spectral overlaps. In silico mixed samples are reconstructed as a proof of concept and mixed in vitro samples prepared, measured and reconstructed to unmixed intensity and lifetime images. The results highlight the utility of the platform to macroscopically quantify autofluorescence lifetime in vitro and its future potential for in vivo autofluorescence imaging.
Human EGF receptor 2 (HER2) is an important oncogene and marker of aggressive metastatic cancer, found in up to 20% of oncologic patients. Anti-HER2 humanized monoclonal antibody trastuzumab (TZM) has been successfully used over the last two decades. However, both primary and acquired TZM resistance calls for the deeper investigation on TZMHER2 binding, internalization and trafficking/degradation in cancer cells in vitro and in vivo. Fluorescence lifetime FRET imaging (FLIM FRET) offers a unique approach to monitor TZM-HER2 binding followed by their uptake into target cells via the reduction of donor fluorophore lifetime. In this study, we characterized for the first time TZM-AF700 uptake and its relation to HER2 expression in AU565 human breast cancer cell line using confocal microscopy. Further, we have quantified the dimerization of HER2 via NIR FLIM FRET in vitro microscopy. Extensive analysis confirmed high specificity and efficiency of TZM FRET signal. Interestingly, we observed a significant heterogeneity of FRET within the cells: the highest TZM FRET levels occurred at the plasma membrane, whereas less if any donor lifetime reduction was registered in the perinuclear endosomes. These results suggest that HER2 dimers undergo dissociation or degradation upon TZM binding and trafficking. Overall, this study provides a good foundation for in vivo TZM FRET imaging of target engagement in preclinical studies.
Pattern generalization was proposed recently as an avenue to increase the acquisition speed of single-pixel imaging setups. This approach consists of designing some positive patterns that reproduce the target patterns with negative values through linear combinations. This avoids the typical burden of acquiring the positive and negative parts of each of the target patterns, which doubles the acquisition time. In this study, we consider the generalization of the Daubechies wavelet patterns and compare images reconstructed using our approach and using the regular splitting approach. Overall, the reduction in the number of illumination patterns should facilitate the implementation of compressive hyperspectral lifetime imaging for fluorescence-guided surgery.
KEYWORDS: Fluorescence resonance energy transfer, Luminescence, Near infrared, In vivo imaging, Multiplexing, Confocal microscopy, Fluorescence lifetime imaging, Resonance energy transfer, Organisms, Absorbance
Fluorescence lifetime imaging (FLI) is widely regarded as the most robust means to utilize Förster resonance energy transfer (FRET) to study protein-protein interactions. Upon donor excitation, FLI estimates FRET occurrence by determining the reduction of the fluorescence lifetime of the donor when in close proximity (2-10nm) of an acceptor. Recently, macroscopic FLI-FRET (MFLI-FRET) in living mice has been attained by using a near-infrared (NIR)-labeled transferrin (Tf) FRET pair. To harness the potential of multiplexing FLI-FRET in live organisms, it is necessary to employ NIR dark acceptor fluorophores to avoid spectral cross-contamination. IRDye QC-1 (QC-1, LI-COR) is a dark quencher that has a broad absorbance spectrum encompassing the NIR range. Herein, we demonstrate that QC-1 is an effective acceptor for quenching of Alexa Fluor 700 (AF700) via FRET in IgG antibody interactions. Additionally, we characterized the cellular uptake of Tf conjugated to QC-1 using confocal microscopy, NIR FLI microscopy, and wide-field MFLI imaging. The AF700/QC-1 FRET pair exhibits a linear trend in FRET with increasing A:D ratio. In vivo MFLI-FRET imaging was performed under reflectance geometry to compare Tf AF700/AF750 and Tf AF700/QC1 at A:D ratio 2:1 2, 6, and 24h post-injection. FRET was detected in the liver, an important organ for pharmacokinetic studies that shows elevated expression of transferrin receptor (TfR), but not in the bladder, an important organ for drug clearance. Although we observed slightly less FRET using AF700/QC-1 compared to AF700/AF750, both in vitro and in vivo, we found that QC-1 is suitable for FRET imaging and multiplexing approaches.
Diffuse optical imaging probes deep living tissue enabling structural, functional, metabolic, and molecular imaging. Recently, due to the availability of spatial light modulators, wide-field quantitative diffuse optical techniques have been implemented, which benefit greatly from structured light methodologies. Such implementations facilitate the quantification and characterization of depth-resolved optical and physiological properties of thick and deep tissue at fast acquisition speeds. We summarize the current state of work and applications in the three main techniques leveraging structured light: spatial frequency-domain imaging, optical tomography, and single-pixel imaging. The theory, measurement, and analysis of spatial frequency-domain imaging are described. Then, advanced theories, processing, and imaging systems are summarized. Preclinical and clinical applications on physiological measurements for guidance and diagnosis are summarized. General theory and method development of tomographic approaches as well as applications including fluorescence molecular tomography are introduced. Lastly, recent developments of single-pixel imaging methodologies and applications are reviewed.
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