Hyperspectral imaging and spectral unmixing for improving whole-body fluorescence cryo-imaging

Comparing spatial distributions of ALA-PpIX and indocyanine green in a whole pig brain glioma model using 3D fluorescence cryotomography

Significance

ALA-PpIX and second-window indocyanine green (ICG) have been studied widely for guiding the resection of high-grade gliomas. These agents have different mechanisms of action and uptake characteristics, which can affect their performance as surgical guidance agents. Elucidating these differences in animal models that approach the size and anatomy of the human brain would help guide the use of these agents. Herein, we report on the use of a new pig glioma model and fluorescence cryotomography to evaluate the 3D distributions of both agents throughout the whole brain.

Aim

We aim to assess and compare the 3D spatial distributions of ALA-PpIX and second-window ICG in a glioma-bearing pig brain using fluorescence cryotomography.

Approach

A glioma was induced in the brain of a transgenic Oncopig via adeno-associated virus delivery of Cre-recombinase plasmids. After tumor induction, the pro-drug 5-ALA and ICG were administered to the animal 3 and 24 h prior to brain harvest, respectively. The harvested brain was imaged using fluorescence cryotomography. The fluorescence distributions of both agents were evaluated in 3D in the whole brain using various spatial distribution and contrast performance metrics.

Results

Significant differences in the spatial distributions of both agents were observed. Indocyanine green accumulated within the tumor core, whereas ALA-PpIX appeared more toward the tumor periphery. Both ALA-PpIX and second-window ICG provided elevated tumor-to-background contrast (13 and 23, respectively).

Conclusions

This study is the first to demonstrate the use of a new glioma model and large-specimen fluorescence cryotomography to evaluate and compare imaging agent distribution at high resolution in 3D.

Whole-body fluorescence cryotomography identifies a fast-acting, high-contrast, durable contrast agent for fluorescence-guided surgery

Imaging of tumor-specific fluorescent contrast agents to guide tumor removal has been shown to improve outcomes and is now standard practice for some neurosurgical procedures. However, many agents require administration hours before surgery, a practical challenge, and may exhibit inconsistent concordance with contrast-enhanced MRI (CE-MRI), the current standard for diagnosing and guiding glioma removal. A fluorescent agent that accurately marks tumor shortly after administration and is otherwise similar to CE-MRI would help overcome these shortcomings. Methods: We used whole-body 3-D fluorescence cryo-imaging and co-registered CE-MRI volumes to evaluate several fluorescent contrast agent candidates for diagnostic performance and concordance with CE-MRI. Mice with brain tumors were administered a cocktail of fluorescent agent candidates and a MRI contrast agent, and then imaged with MRI and fluorescence cryo-imaging at several timepoints after administration. The high-resolution 3-D cryo-imaging volumes of the fluorescent agents were used to determine diagnostic performance metrics and correlation with CE-MRI. Results: While all agents showed positive metrics, one agent, tetramethylrhodamine conjugated to a small polyethylene glycol chain (TMR-PEG1k), outperformed the others, exhibiting minimal normal brain signal, high tumor-to-background-ratio, diagnostic accuracy, and cross-correlation to CE-MRI at all post-administration timepoints (10-90 min) and tumor lines examined. Conclusion: These favorable properties establish TMR-PEG1k as a promising candidate for surgical guidance.

Dynamic oxygen assessment techniques enable determination of anesthesia's impact on tissue

Tissue oxygenation is well understood to impact radiosensitivity, with reports demonstrating a significant effect of breathing condition and anesthesia type on tissue oxygenation levels and radiobiological response. However, the temporal kinetics of intracellular and extracellular oxygenation have never been quantified, on the timescale that may affect radiotherapy studies. C57BL/6 mice were anesthetized using isoflurane at various percentages or ketamine/xylazine (ket/xyl: 100/10 mg/kg) (N = 48). Skin pO2 was measured using Oxyphor PdG4 and tracked after anesthetization began. Oxyphor data was validated with relative measurements of intracellular oxygen via protoporphyrin IX (PpIX) delayed fluorescence (DF) imaging. Ex vivo localization of both PdG4 Oxyphor and PpIX were quantified. Under all isoflurane anesthesia conditions, leg skin pO2 levels significantly increased from 12-15 mmHg at the start of anesthesia induction (4-6 minutes) to 24-27 mmHg after 10 minutes (p < 0.05). Ketamine/xylazine anesthesia led to skin pO2 maintained at 15-16 mmHg throughout the 10-minute study period (p < 0.01). An increase of pO2 in mice breathing isoflurane was demonstrated with Oxyphor and PpIX DF, indicating similar intracellular and extracellular oxygenation. These findings demonstrate the importance of routine anesthesia administration, where consistency in the timing between induction and irradiation may be crucial to minimizing variability in radiation response.

Two-color diffuse in vivo flow cytometer

Significance

Hematogenous metastasis is mediated by circulating tumor cells (CTCs) and CTC clusters (CTCCs). We recently developed "diffuse in vivo flow cytometry" (DiFC) to detect fluorescent protein (FP) expressing CTCs in small animals. Extending DiFC to allow detection of two FPs simultaneously would allow concurrent study of different CTC sub-populations or heterogeneous CTCCs in the same animal.

Aim

The goal of this work was to develop and validate a two-color DiFC system capable of non-invasively detecting circulating cells expressing two distinct FPs.

Approach

A DiFC instrument was designed and built to detect cells expressing either green FP (GFP) or tdTomato. We tested the instrument in tissue-mimicking flow phantoms in vitro and in multiple myeloma bearing mice in vivo.

Results

In phantoms, we could accurately differentiate GFP+ and tdTomato+ CTCs and CTCCs. In tumor-bearing mice, CTC numbers expressing both FPs increased during disease. Most CTCCs (86.5%) expressed single FPs with the remainder both FPs. These data were supported by whole-body hyperspectral fluorescence cryo-imaging of the mice.

Conclusions

We showed that two-color DiFC can detect two populations of CTCs and CTCCs concurrently. This instrument could allow study of tumor development and response to therapies for different sub-populations in the same animal.

Comparing fluorescent contrast agents for fluorescence guided surgery using 3-D cryo-imaging

Fluorescence cryo-imaging is a high-resolution optical imaging technique that produces 3-D whole-body biodistributions of fluorescent molecules within an animal specimen. To accomplish this, animal specimens are administered a fluorescent molecule or reporter and are frozen to be autonomously sectioned and imaged at a temperature of -20°C or below. Thus, to apply this technique effectively, administered fluorescent molecules should be relatively invariant to low temperature conditions for cryo-imaging and ideally the fluorescence intensity should be stable and consistent in both physiological and cryo-imaging conditions. Herein, we assessed the mean fluorescence intensity of 11 fluorescent contrast agents as they are frozen in a tissue-simulating phantom experiment and show an example of a tested fluorescent contrast agent in a cryo-imaged whole pig brain. Most fluorescent contrast agents were stable within ~25% except for FITC and PEGylated FITC derivatives, which showed a dramatic decrease in fluorescence intensity when frozen.

SpeCamX: mobile app that turns unmodified smartphones into multispectral imagers

We present the development of SpeCamX, a mobile application that enables an unmodified smartphone into a multispectral imager. Multispectral imaging provides detailed spectral information about objects or scenes, but its accessibility has been limited due to its specialized requirements for the device. SpeCamX overcomes this limitation by utilizing the RGB photographs captured by smartphones and converting them into multispectral images spanning a range of 420 to 680 nm without a need for internal modifications or external attachments. The app also includes plugin functions for extracting medical information from the resulting multispectral data cube. In a clinical study, SpeCamX was used to implement an augmented smartphone bilirubinometer, predicting blood bilirubin levels (BBL) with superior performance in accuracy, efficiency and stability compared to default smartphone cameras. This innovative technology democratizes multispectral imaging, making it accessible to a wider audience and opening new possibilities for both medical and non-medical applications.

Current and Future Applications of Fluorescence Guidance in Orthopaedic Surgery

Fluorescence-guided surgery (FGS) is an evolving field that seeks to identify important anatomic structures or physiologic phenomena with helpful relevance to the execution of surgical procedures. Fluorescence labeling occurs generally via the administration of fluorescent reporters that may be molecularly targeted, enzyme-activated, or untargeted, vascular probes. Fluorescence guidance has substantially changed care strategies in numerous surgical fields; however, investigation and adoption in orthopaedic surgery have lagged. FGS shows the potential for improving patient care in orthopaedics via several applications including disease diagnosis, perfusion-based tissue healing capacity assessment, infection/tumor eradication, and anatomic structure identification. This review highlights current and future applications of fluorescence guidance in orthopaedics and identifies key challenges to translation and potential solutions.

A new candidate agent for fluorescence guided neurosurgery produces high, persistent tumor contrast shortly after administration

Neurosurgical fluorescence guidance relies on contrast agents to identify tumor regions to aid in increasing the extent of resection. Existing contrast agents for this indication each have their own limitation: unpredictable contrast from tumor heterogeneity, significant extravasation into the background brain and long incubation times. An ideal contrast agent should have high and rapid contrast that persists well into the surgical procedure. By using a whole animal hyperspectral cryo-imaging system several CAs were screened for these favorable properties and compared to the gold standard of gadolinium enhanced MR. Herein, we briefly report on the leading candidate Rd-PEG1k, which shows high contrast within minutes of administration that persists for at least 90 minutes.

Super-multiplexing excitation spectral microscopy with multiple fluorescence bands

Fluorescence microscopy, with high molecular specificity and selectivity, is a valuable tool for studying complex biological systems and processes. However, the ability to distinguish a large number of distinct subcellular structures in a single sample is impeded by the broad spectra of molecular fluorescence. We have recently shown that excitation spectral microscopy provides a powerful means to unmix up to six fluorophores in a single fluorescence band. Here, by working with multiple fluorescence bands, we extend this approach to the simultaneous imaging of up to ten targets, with the potential for further expansions. By covering the excitation/emission bandwidth across the full visible range, an ultra-broad 24-wavelength excitation scheme is established through frame-synchronized scanning of the excitation wavelength from a white lamp via an acousto-optic tunable filter (AOTF), so that full-frame excitation-spectral images are obtained every 24 camera frames, offering superior spectral information and multiplexing capability. With numerical simulations, we validate the concurrent imaging of 10 fluorophores spanning the visible range to achieve exceptionally low (∼0.5%) crosstalks. For cell imaging experiments, we demonstrate unambiguous identification of up to eight different intracellular structures labeled by common fluorophores of substantial spectral overlap with minimal color crosstalks. We thus showcase an easy-to-implement, cost-effective microscopy system for visualizing complex cellular components with more colors and lower crosstalks.

Denoising multiplexed microscopy images in n-dimensional spectral space

Hyperspectral fluorescence microscopy images of biological specimens frequently contain multiple observations of a sparse set of spectral features spread in space with varying intensity. Here, we introduce a spectral vector denoising algorithm that filters out noise without sacrificing spatial information by leveraging redundant observations of spectral signatures. The algorithm applies an n-dimensional Chebyshev or Fourier transform to cluster pixels based on spectral similarity independent of pixel intensity or location, and a denoising convolution filter is then applied in this spectral space. The denoised image may then undergo spectral decomposition analysis with enhanced accuracy. Tests utilizing both simulated and empirical microscopy data indicate that denoising in 3 to 5-dimensional (3D to 5D) spectral spaces decreases unmixing error by up to 70% without degrading spatial resolution.

Automated multicolor mesoscopic imaging for the 3-dimensional reconstruction of fluorescent biomarker distribution in large tissue specimens

Research in translational medicine often requires high-resolution characterization techniques to visualize or quantify the fluorescent probes. For example, drug delivery systems contain fluorescent molecules enabling in vitro and in vivo tracing to determine biodistribution or plasma disappearance. Albeit fluorescence imaging systems with sufficient resolution exist, the sample preparation is typically too complex to image a whole organism of the size of a mouse. This article established a mesoscopic imaging technique utilizing a commercially available cryo-microtome and an in-house built episcopic imaging add-on to perform imaging during serial sectioning. Here we demonstrate that our automated red, green, blue (RGB) and fluorescence mesoscope can generate sequential block-face and 3-dimensional anatomical images at variable thickness with high quality of 6 µm × 6 µm pixel size. In addition, this mesoscope features a numerical aperture of 0.10 and a field-of-view of up to 21.6 mm × 27 mm × 25 mm (width, height, depth).

Trends in hyperspectral imaging: from environmental and health sensing to structure-property and nano-bio interaction studies

Hyperspectral imaging (HSI) is a technique that allows for the simultaneous acquisition of both spatial and spectral information. While HSI has been known for years in the field of remote sensing, for instance in geology, cultural heritage, or food industries, it recently emerged in the fields of nano- and micromaterials as well as bioimaging and -sensing. Herein, the attractiveness of HSI arises from the suitability for generating knowledge about environment-specific optical properties, such as photoluminescence of optical probes in a biological sample or at a single-crystal/particle level, to be leveraged into better understanding of structure-property relationships and nano-bio interactions, respectively. Moreover, given its excellent spectral resolution, HSI is highly suitable for optical multiplexing in multiple dimensions, as sought after for, e.g., high throughput biological imaging by simultaneous tracking of multiple targets. Overall, HSI is an emerging technique that has the potential to transform analytical approaches from biomedicine to advanced materials research. This Trends Article provides insight into the potential of HSI, highlighting selected examples from well-established fields including environmental monitoring and food quality control to set the stage for the discussion of emerging opportunities at the micro- and nanoscale. Herein, special focus is set on photoluminescent micro- and nanoprobes for health and spectral conversion applications.

Hyperspectral Imaging for Clinical Applications

Measuring morphological and biochemical features of tissue is crucial for disease diagnosis and surgical guidance, providing clinically significant information related to pathophysiology. Hyperspectral imaging (HSI) techniques obtain both spatial and spectral features of tissue without labeling molecules such as fluorescent dyes, which provides rich information for improved disease diagnosis and treatment. Recent advances in HSI systems have demonstrated its potential for clinical applications, especially in disease diagnosis and image-guided surgery. This review summarizes the basic principle of HSI and optical systems, deep-learning-based image analysis, and clinical applications of HSI to provide insight into this rapidly growing field of research. In addition, the challenges facing the clinical implementation of HSI techniques are discussed.

Whole-brain MR-registered cryo-imaging of a porcine-human glioma model to compare contrast agent biodistributions

As rapidly accelerating technology, fluorescence guided surgery (FGS) has the potential to place molecular information directly into the surgeon's field of view by imaging administered fluorescent contrast agents in real time, circumnavigating pre-operative MR registration challenges with brain deformation. The most successful implementation of FGS is 5-ALA-PpIX guided glioma resection which has been linked to improved patient outcomes. While FGS may offer direct in-field guidance, fluorescent contrast agent distributions are not as familiar to the surgical community as Gd-MRI uptake, and may provide discordant information from previous Gd-MRI guidance. Thus, a method to assess and validate consistency between fluorescence-labeled tumor regions and Gd-enhanced tumor regions could aid in understanding the correlation between optical agent fluorescence and Gd-enhancement. Herein, we present an approach for comparing whole-brain fluorescence biodistributions with Gd-enhancement patterns on a voxel-by-voxel basis using co-registered fluorescent cryo-volumes and Gd-MRI volumes. In this initial study, a porcine-human glioma xenograft model was administered 5-ALA-PpIX, imaged with MRI, and euthanized 22 hours following 5-ALA administration. Following euthanization, the extracted brain was imaged with the cryo-macrotome system. After image processing steps and non-rigid, point-based registration, the fluorescence cryo-volume and Gd-MRI volume were compared for similarity metrics including: image similarity, tumor shape similarity, and classification similarity. This study serves as a proof-of-principle in validating our screening approach for quantitatively comparing 3D biodistributions between optical agents and Gd-based agents.

A method for validating depth-resolved biodistributions in topically-stained specimen with multi-channel fluorescence cryo-imaging

Fluorescent contrast agents targeted to cancer biomarkers are increasingly being explored for cancer detection, surgical guidance, and response monitoring. Efforts have been underway to topically apply such biomarker-targeted agents to freshly excised specimen for detecting cancer cell receptors on the surface as a method for intraoperative surgical margin assessment, including dual-probe staining methods introduce a second 'non-specific' optical agent as a control to help compensate for heterogeneous uptake and normalize the imaging field. Still, such specimen staining protocols introduce multifaceted complexity with unknown variables, such as tissue-specific diffusion, cell-specific binding and disassociation rates, and other factors, affecting the interpreted cancer-biomarker distribution across the specimen surface. The ability to recover three-dimensional dual-probe biodistributions throughout whole-specimens could offer a ground-truth validation method for examining topical staining uptake behaviors. Herein, we report on a novel method for characterizing dual-probe accumulation with 3D depth-profiles observed from a dual-probe fresh-specimen staining experiment.

Ratiometric Fluorescence Monitoring of Antibody-Guided Drug Delivery to Cancer Cells

Ratiometric measurements utilizing two independent fluorescence signals from a dual-dye molecular system help to improve the detection sensitivity and quantification of many analytical, bioanalytical, and pharmaceutical assays, including drug delivery monitoring. Nevertheless, these dual-dye conjugates have never been utilized for ratiometric monitoring of antibody (Ab)-guided targeted drug delivery (TDD). Here, we report for the first time on the new, dual-dye TDD system, Cy5s-Ab-Flu-Aza, comprising the switchable fluorescein-based dye (Flu) linked to the anticancer drug azatoxin (Aza), reference pentamethine cyanine dye (Cy5s), and Her2-specific humanized monoclonal Trastuzumab (Herceptin) antibody. The ability of ratiometric fluorescence monitoring of drug release was demonstrated with this model system in vitro in the example of the human breast cancer SKBR3 cell line overexpressing Her2 receptors. The proposed approach for designing ratiometric, antibody-guided TDD systems, where a "drug-switchable dye" conjugate and a reference dye are independently linked to an antibody, can be expanded to other drugs, dyes, and antibodies. Replacement of the green-emitting dye Flu, which was found not detectable in vivo, with a longer-wavelength (red or near-IR) switchable fluorophore should enable quantification of drug release in the body.

A hyperspectral approach for recovering agent excretion biodistributions using whole-body fluorescence cryo-imaging

Understanding the uptake and clearance kinetics of new drugs and contrast agents is an important aspect of drug development that typically involves a combination of imaging and analysis of harvested organs. Although these techniques are well-established and can be quantitative, they generally do not preserve high resolution biodistribution information. In this context, fluorescence whole-body cryo-imaging is a promising technique for recovering 3D drug/agent biodistributions at a high resolution throughout an entire study animal at specific time points. A common challenge associated with fluorescence imaging in tissue is that agent signal can be confounded by endogenous fluorescence signal which is often observed in the visible window. One method to address this issue is to acquire hyperspectral images and spectrally unmix agent signal from confounding autofluorescence signals using known spectral bases. Herein, we apply hyperspectral whole-body cryo-imaging and spectral unmixing to examine the distribution of multiple fluorescent agents in excretion organ regions.