Noninvasive optical imaging of the subarachnoid space and cerebrospinal fluid pathways based on near-infrared fluorescence

Applications of Microscope-Integrated Indocyanine Green Videoangiography in Cerebral Revascularization Procedures

Indocyanine green videoangiography (ICG-VA) is a near-infrared range fluorescent marker used for intraoperative real-time assessment of flow in cerebrovascular surgery. Given its high spatial and temporal resolution, ICG-VA has been widely established as a useful technique to perform a qualitative analysis of the graft patency during revascularization procedures. In addition, this fluorescent modality can also provide valuable qualitative and quantitative information regarding the cerebral blood flow within the bypass graft and in the territories supplied. Digital subtraction angiography (DSA) is considered to be the gold standard diagnostic modality for postoperative bypass graft patency assessment. However, this technique is time and labor intensive and an expensive interventional procedure. In contrast, ICG-VA can be performed intraoperatively with no significant addition to the total operative time and, when used correctly, can accurately show acute occlusion. Such time-sensitive ischemic injury detection is critical for flow reestablishment through direct surgical management. In addition, ICG has an excellent safety profile, with few adverse events reported in the literature. This review outlines the chemical behavior, technical aspects, and clinical implications of this tool as an intraoperative adjunct in revascularization procedures.

In Vivo Tumor Angiogenesis Imaging Using Peptide-Based Near-Infrared Fluorescent Probes

Near-infrared fluorescence (NIRF) imaging is an emerging imaging technique for studying diseases at the molecular level. Optical imaging with a near-infrared emitting fluorophore for targeting tumor angiogenesis offers a noninvasive method for early tumor detection and efficient monitoring of tumor response to anti-angiogenesis therapy. CD13 receptor, a zinc-dependent membrane-bound ectopeptidase, plays important roles in regulating tumor angiogenesis and the growth of new blood vessels. In this chapter, we use CD13 receptor as an example to demonstrate how to construct CD13-specific NGR-containing peptides via bioorthogonal click chemistry for visualizing and quantifying the CD13 receptor expression in vivo by means of NIRF optical imaging.

Experimental validation of an inverse fluorescence Monte Carlo model to extract concentrations of metabolically relevant fluorophores from turbid phantoms and a murine tumor model

An inverse Monte Carlo based model has been developed to extract intrinsic fluorescence from turbid media. The goal of this work was to experimentally validate the model to extract intrinsic fluorescence of three biologically meaningful fluorophores related to metabolism from turbid media containing absorbers and scatterers. Experimental studies were first carried out on tissue-mimicking phantoms that contained individual fluorophores and their combinations, across multiple absorption, scattering, and fluorophore concentrations. The model was then tested in a murine tumor model to determine both the kinetics of fluorophore uptake as well as overall tissue fluorophore concentration through extraction of the intrinsic fluorescence of an exogenous contrast agent that reports on glucose uptake. Results show the model can be used to recover the true intrinsic fluorescence spectrum with high accuracy (R(2)=0.988) as well as accurately compute fluorophore concentration in both single and multiple fluorophores phantoms when appropriate calibration standards are available. In the murine tumor, the model-corrected intrinsic fluorescence could be used to differentiate drug dose injections between different groups. A strong linear correlation was observed between the extracted intrinsic fluorescence intensity and injected drug dose, compared with the distorted turbid tissue fluorescence.

Intraoperative EC-IC bypass blood flow assessment with indocyanine green angiography in moyamoya and non-moyamoya ischemic stroke

OBJECTIVE

Superficial temporal artery-middle cerebral artery (STA-MCA) anastomosis has been used in moyamoya disease (MD) and non-moyamoya ischemic stroke (non-MD). It is important to monitor hemodynamic changes caused by bypass surgery for postoperative management. We evaluated the bypass blood flow during STA-MCA anastomosis by using indocyanine green (ICG) fluorescence angiography.

METHODS

We evaluated the bypass blood flow in 13 MD and 21 non-MD patients during STA-MCA anastomosis by means of ICG angiography with injection of ICG into the anastomosed STA. The ICG perfusion area was calculated when the ICG fluorescence intensity reached maximum. We measured cortical oxygen saturation before anastomosis by means of visual light spectroscopy.

RESULTS

ICG angiography demonstrated bypass blood flow from the anastomosed STA to the cortical vessels in all patients. The ICG perfusion area in MD (20.7 ± 6.6 cm(2)) was significantly larger than that in non-MD (8.4 ± 9.1 cm(2), P < 0.05). The cortical oxygen saturation (58.9% ± 8.3%) in MD was significantly lower than that in non-MD (73.4% ± 9.5%, P < 0.05).

CONCLUSIONS

ICG angiography with injection of ICG into the bypass artery allowed quantitative assessment of bypass blood flow. The bypass supplies blood flow to a greater extent in MD than in non-MD during surgery. This might be caused by a larger pressure gradient between the anastomosed STA and recipient vessels in MD. These observations indicate that MD requires careful control of systemic blood pressure after surgery to avoid cerebral hyperperfusion syndrome. ICG angiography is considered useful for facilitating safe and accurate bypass surgery and providing information for postoperative management.

Near-infrared fluorophores containing benzo[c]heterocycle subunits

The syntheses and spectroscopic properties of eight new push-pull-type near-infrared fluorophores that contain either isobenzofuran or isothianaphthene subunits are presented. The isobenzofuran dyes demonstrate significantly red-shifted absorption compared with their isothianaphthene counterparts, which is attributed to isobenzofuran's more potent pro-quinoidal character.

Molecular pathology of malignant gliomas

Malignant gliomas, the most common type of primary brain tumor, are a spectrum of tumors of varying differentiation and malignancy grades. These tumors may arise from neural stem cells and appear to contain tumor stem cells. Early genetic events differ between astrocytic and oligodendroglial tumors, but all tumors have an initially invasive phenotype, which complicates therapy. Progression-associated genetic alterations are common to different tumor types, targeting growth-promoting and cell cycle control pathways and resulting in focal hypoxia, necrosis, and angiogenesis. Knowledge of malignant glioma genetics has already impacted clinical management of these tumors, and researchers hope that further knowledge of the molecular pathology of malignant gliomas will result in novel therapies.

Molecular imaging using visible light to reveal biological changes in the brain

Advances in imaging have enabled the study of cellular and molecular processes in the context of the living body that include cell migration patterns, location and extent of gene expression, degree of protein-protein interaction, and levels of enzyme activity. These tools, which operate over a range of scales, resolutions, and sensitivities, have opened up broad new areas of investigation where the influence of organ systems and functional circulation is intact. There are a myriad of imaging modalities available, each with its own advantages and disadvantages, depending on the specific application. Among these modalities, optical imaging techniques, including in vivo bioluminescence imaging and fluorescence imaging, use visible light to interrogate biology in the living body. Optimal imaging with these modalities require that the appropriate marker be used to tag the process of interest to make it uniquely visible using a particular imaging technology. For each optical modality, there are various labels to choose from that range from dyes that permit tissue contrast and dyes that can be activated by enzymatic activity, to gene-encoding proteins with optical signatures that can be engineered into specific biological processes. This article provides and overview of optical imaging technologies and commonly used labels, focusing on bioluminescence and fluorescence, and describes several examples of how these tools are applied to biological questions relating to the central nervous system.

Imaging of cerebrospinal fluid space and movement in mice using near infrared fluorescence

We developed an optical method for imaging the cerebrospinal fluid (CSF) space and the movement of CSF in mice using a near infrared fluorescence imaging methodology. Indocyanine green bound to high-density lipoprotein (ICG-HDL) was injected into the lumber subarachnoid space of nude mice. The time course of CSF movement was followed over 48 h. The imaging system was configured for epi-fluorescence measurements at near infrared wavelengths using the illumination light and narrow band excitation filtration with central wavelength of 755 nm. Emission light was filtered with a long pass filter with a cutoff at 798 nm. After the injection of ICG-HDL, a strong fluorescence signal clearly delineated the cisterna magna, bilateral supracerebellar/cerebello-pontine angle cistern, and interhemispheric cistern. Much of the fluorescent tracer was washed out within 24h after the injection. This study has therefore demonstrated that an optical method, employing near infrared fluorescence imaging, can determine the CSF space and the movement of CSF in nude mice.

Improvement of transcutaneous fluorescent images with a depth-dependent point-spread function

The point-spread function (PSF) for transcutaneous fluorescent imaging was obtained as an analytical solution in a closed form. It is applicable to cases in which the optical property of the image-blurring turbid medium is considered to be fairly homogeneous. We proposed a technique to improve a transcutaneous image by using depth-dependent PSF. Contrast of the fluorescent image was improved for depths of 1-15 mm in a scattering medium (micro(s)' = 1/mm). The visible depth was more than doubled with this technique. An experiment with a rat demonstrated considerable improvement of a transcutaneous image of the cerebral vein at a specified depth. The spread image of the heart was reduced to the correct size by use of the PSF with the actual depth of the heart.

Sensitivity and depth penetration of continuous wave versus frequency-domain photon migration near-infrared fluorescence contrast-enhanced imaging

The development of near-infrared fluorescent contrast agents and imaging techniques depends on the deep penetration of excitation light through several centimeters of tissue and the sensitive collection of the re-emitted fluorescence. In this contribution, the sensitivity and depth penetration of various fluorescence-enhanced imaging studies is surveyed and compared with current studies using continuous wave (CW) and frequency-domain photon migration (FDPM) measurements with planar wave illumination of modulated excitation light at 100 MHz and area collection of reemitted fluorescent light using a previously developed modulated intensified charge-coupled device camera system. Fluorescence was generated from nanomolar to micromolar solutions of indocyanine green (ICG) in a 100 microL volume submerged at 1-4 cm depths in a 1% Liposyn solution to mimic tissue scattering properties. Enhanced depth penetration and sensitivity are achieved with optimal filter rejection of excitation light, and FDPM rejection of background light is not achieved using CW methods. We show the ability to detect as few as 100 fmol of ICG from area illumination of 785 nm light (5.5 mW/cm2) and FDPM area collection of 830 nm fluorescent light generated from 3 cm below the phantom surface. The lowered noise floor of FDPM measurements enables greater sensitivity and penetration depth than comparable CW measurements.

Fluorescence-enhanced, near infrared diagnostic imaging with contrast agents

The deep tissue propagation of near-infrared (NIR) light between 700-900 nm offers new opportunities for diagnostic imaging when employing sensitive detection techniques and NIR excitable fluorescent agents that target and report disease and metabolism. Herein, we highlight approaches for illuminating tissues and monitoring the re-emitted fluorescence for tomographic reconstruction, strategies for developing fluorescent dye constructs, and clinical opportunities for fluorescence-enhanced NIR optical imaging.

Cellular activation of the self-quenched fluorescent reporter probe in tumor microenvironment

The effect of intralysosomal proteolysis of near-infrared fluorescent (NIRF) self-quenched macromolecular probe (PGC-Cy5.5) has been previously reported and used for tumor imaging. Here we demonstrate that proteolysis can be detected noninvasively in vivo at the cellular level. A codetection of GFP fluorescence (using two-photon excitation) and NIRF was performed in tumor-bearing animals injected with PGC-Cy5.5. In vivo microscopy of tumor cells in subdermal tissue layers (up to 160 microm) showed a strong Cy5.5 dequenching effect in GFP-negative cells. This observation was corroborated by flow cytometry, sorting, and reverse transcription polymerase chain reaction analysis of tumor-isolated cells. Both GFP-positive (81% total) and GFP-negative (19% total) populations contained Cy5.5-positive cells. The GFP-negative cells were confirmed to be host mouse cells by the absence of rat cathepsin mRNA signal. The subfraction of GFP-negative cells (2.5-3.0%) had seven times higher NIRF intensity than the majority of GFP-positive or GFP-negative cells (372 and 55 AU, respectively). Highly NIRF-positive, FP-negative cells were CD45- and MAC3-positive. Our results indicate that: 1) intracellular proteolysis can be imaged in vivo at the cellular level using cathepsin-sensitive probes; 2) tumor-recruited cells of hematopoetic origin participate most actively in uptake and degradation of long-circulating macromolecular probes.

Three-dimensional unconstrained and constrained image-reconstruction techniques applied to fluorescence, frequency-domain photon migration

The development of near-infrared (NIR) optical imaging for biomedical optical imaging is hampered by the computational intensiveness of large-scale three-dimensional (3-D) image reconstruction and the potential lack of endogenous contrast for detection of relevant tissue features. In this contribution the inverse optical imaging problem is formulated in three dimensions in a noncompressive geometry as a simple-bound constrained minimization problem in order to recover the interior fluorescence properties of exogenous contrast agent from frequency-domain photon migration measurements at the boundary. The solution of the forward optical diffusion problem for the frustum shape containing fluorescence inclusions of 10:1 contrast is accomplished by use of the Galerkin finite-element formulation. The inverse approach employs the truncated Newton method with trust region and a modification of automatic reverse differentiation to speed the computation of the optimization problem. The image-reconstruction results confirm that the constrained minimization may offer a more logical approach for the 3-D optical imaging problem than unconstrained optimization.

Developments toward diagnostic breast cancer imaging using near-infrared optical measurements and fluorescent contrast agents

The use of near-infrared (NIR) light to interrogate deep tissues has enormous potential for molecular-based imaging when coupled with NIR excitable dyes. More than a decade has now passed since the initial proposals for NIR optical tomography for breast cancer screening using time-dependent measurements of light propagation in the breast. Much accomplishment in the development of optical mammography has been demonstrated, most recently in the application of time-domain, frequency-domain, and continuous-wave measurements that depend on endogenous contrast owing to angiogenesis and increased hemoglobin absorbance for contrast. Although exciting and promising, the necessity of angiogenesis-mediated absorption contrast for diagnostic optical mammography minimizes the potential for using NIR techniques to assess sentinel lymph node staging, metastatic spread, and multifocality of breast disease, among other applications. In this review, we summarize the progress made in the development of optical mammography, and focus on the emerging work underway in the use of diagnostic contrast agents for the molecular-based, diagnostic imaging of breast.