Predicting in vivo fluorescence lifetime behavior of near-infrared fluorescent contrast agents using in vitro measurements

Synthesis of novel cyanine dyes as antitumor agents

In this study, some novel cyanine dyes, 1, 3, and 5-15, were synthesized by a one-pot step reaction of pyridinium salts 2 and/or 4 with benzenaminium salt 1. N-{[1-Chloro-3,4-dihydronaphthalen-2-yl)methylene]benzenaminium} chloride 1 was obtained by the reaction of α-tetralone with Vilsmeier-Haack reagent, followed by a mixture of an equimolar ratio of anilin/ethanol (1:1). All new cyanine dyes were evaluated in vitro for their anticancer activity against two cell lines, that is, HepG2 (human hepatocellular liver carcinoma) and MCF-7 (breast cancer). The obtained results were compared with human lung fibroblasts (WI-38) and Vero cells (derived from the kidney of an African green monkey) as normal cells. In particular, some of these compounds, 6, 9, 13, and 14, were found to be the most potent derivatives against all the cancer cell lines, without effect on the normal cells. According to the structure-activity relationship, compound 13 (IC₅₀  = 8.8 µg/ml) exhibited a higher activity against HepG2 cells, as it contains the azo group and two phenyl rings and due to the presence of the π-conjugated system attached to the two pyridine rings. Compound 6 (IC₅₀  = 8 µg/ml) exhibited a higher activity against MCF-7 cells, as it contains two chlorine atoms and the π-conjugated system of the pyridine rings.

Isoflurane increases cell membrane fluidity significantly at clinical concentrations

There is an on-going debate whether anesthetic drugs, such as isoflurane, can cause meaningful structural changes in cell membranes at clinical concentrations. In this study, the effects of isoflurane on lipid membrane fluidity were investigated using fluorescence anisotropy and spectroscopy. In order to get a complete picture, four very different membrane systems (erythrocyte ghosts, a 5-lipid mixture that mimics brain endothelial cell membrane, POPC/Chol, and pure DPPC) were selected for the study. In all four systems, we found that fluorescence anisotropies of DPH-PC, nile-red, and TMA-DPH decrease significantly at the isoflurane concentrations of 1 mM and 5 mM. Furthermore, the excimer/monomer (E/M) ratio of dipyrene-PC jumps immediately after the addition of isoflurane. We found that isoflurane is quite effective to loosen up highly ordered lipid domains with saturated lipids. Interestingly, 1 mM isoflurane causes a larger decrease of nile-red fluorescence anisotropy in erythrocyte ghosts than 52.2 mM of ethanol, which is three times the legal limit of blood alcohol level. Our results paint a consistent picture that isoflurane at clinical concentrations causes significant and immediate increase of membrane fluidity in a wide range of membrane systems.

In vitro and in vivo phasor analysis of stoichiometry and pharmacokinetics using short-lifetime near-infrared dyes and time-gated imaging

We introduce a simple new approach for time-resolved multiplexed analysis of complex systems using near-infrared (NIR) dyes, applicable to in vitro and in vivo studies. We show that fast and precise in vitro quantification of NIR fluorophores' short (subnanosecond) lifetime and stoichiometry can be done using phasor analysis, a computationally efficient and user-friendly representation of complex fluorescence intensity decays obtained with pulsed laser excitation and time-gated camera imaging. We apply this approach to the study of binding equilibria by Förster resonant energy transfer using two different model systems: primary/secondary antibody binding in vitro and ligand/receptor binding in cell cultures. We then extend it to dynamic imaging of the pharmacokinetics of transferrin engagement with the transferrin receptor in live mice, elucidating the kinetics of differential transferrin accumulation in specific organs, straightforwardly differentiating specific from nonspecific binding. Our method, implemented in a freely-available software, has the advantage of time-resolved NIR imaging, including better tissue penetration and background-free imaging, but simplifies and considerably speeds up data processing and interpretation, while remaining quantitative. These advances make this method attractive and of broad applicability for in vitro and in vivo molecular imaging and could be extended to applications as diverse as image-guided surgery or optical tomography.

Perfusion-based fluorescence imaging method delineates diverse organs and identifies multifocal tumors using generic near-infrared molecular probes

Rapid detection of multifocal cancer without the use of complex imaging schemes will improve treatment outcomes. In this study, dynamic fluorescence imaging was used to harness differences in the perfusion kinetics of near-infrared (NIR) fluorescent dyes to visualize structural characteristics of different tissues. Using the hydrophobic nontumor-selective NIR dye cypate, and the hydrophilic dye LS288, a high tumor-to-background contrast was achieved, allowing the delineation of diverse tissue types while maintaining short imaging times. By clustering tissue types with similar perfusion properties, the dynamic fluorescence imaging method identified secondary tumor locations when only the primary tumor position was known, with a respective sensitivity and specificity of 0.97 and 0.75 for cypate, and 0.85 and 0.81 for LS288. Histological analysis suggests that the vasculature in the connective tissue that directly surrounds the tumor was a major factor for tumor identification through perfusion imaging. Although the hydrophobic dye showed higher specificity than the hydrophilic probe, use of other dyes with different physical and biological properties could further improve the accuracy of the dynamic imaging platform to identify multifocal tumors for potential use in real-time intraoperative procedures.

High-speed time-resolved laser-scanning microscopy using the line-to-pixel referencing method

Fluorescence lifetime imaging microscopy (FLIM) is a powerful technique to visualize photophysical characteristics of biological targets. However, conventional FLIM methods have some limitations that restrict obtaining high-precision images in real time. Here, we propose a high-speed time-resolved laser-scanning microscopy by incorporating a novel line-to-pixel referencing method into the previously suggested analog mean-delay (AMD) method. The AMD method dramatically enhances the photon accumulation speed for achieving the certain precision compared to the time-correlated single-photon counting (TCSPC) method while maintaining high photon efficiency. However, its imaging pixel rate can still be restricted by the rearm time of the digitizer when it is triggered by laser pulses. With our line-to-pixel referencing method, the pulse train repeats faster than the trigger rearm time can be utilized by generating a line trigger, which is phase-locked with only the first pulse in each horizontal line composing an image. Our proposed method has been tested with a pulsed laser with 40 MHz repetition rate and a commercial digitizer with a 500 ns trigger rearm time, and a frame rate of 3.73 fps with a pixel rate of 3.91 MHz was accomplished while maintaining the measurement precision under 20 ps.

Fluorescence Lifetime Imaging of Cancer In Vivo

Optical imaging of fluorescent reporters in animal models of cancer has become a common tool in oncologic research. Fluorescent reporters including fluorescent proteins, organic dyes, and inorganic photonic materials are used in fluorescence spectroscopy, microscopy, and whole body preclinical imaging. Fluorescence lifetime imaging provides additional, quantitative information beyond that of conventional fluorescence intensity signals, enabling signal multiplexing, background separation, and biological sensing unique to fluorescent materials.

Fluorescence-based method for assessment of blood-brain barrier disruption

We report on a fluorescence-based optical method for assessment of blood-brain barrier in humans. The technique is based on monitoring of fluorescence light excited in the dye circulating in the brain. Measurements were carried out in healthy volunteers and in patients with disruption of the blood-brain barrier with the use of time-resolved method during inflow and washout of indocyanine green after its intravenous injection. We show large differences in the fluorescence signals - in healthy subjects a fast washout of the dye can be observed whereas in patients the washout is significantly prolonged. We conclude that the monitoring of the fluorescence signals during injection of exogenous optical contrast agent can be used for the assessment of the condition of blood-brain barrier at the bedside. The technique may be of benefit for diagnosis of the patients suffering from damage of the blood-brain barrier and in monitoring of therapies used in such patients.

Ultrabright NIR fluorescent mesoporous silica nanoparticles

Near-infrared (NIR) water-dispersible fluorescent tags are of big importance for biomedical imaging. Bright, stable, biocompatible NIR fluorescent nanoparticles have great translation potential to improve diagnosis of early stages of different diseases. Here we report on the synthesis of exceptionally bright ("ultrabright") fluorescent meso(nano)porous silica nanoparticles of 28 ± 3 nm in diameter. The NIR fluorescent dye LS277 is encapsulated inside these silica nanoparticles. The wavelengths of the maximum excitation/fluorescence of the particles are 804/815 nm. The absorptivity coefficient of the particles is 2.1 × 10⁸ M^-1 cm^-1 at 805 nm and the quantum yield of the dye increased by a factor of 5 after encapsulating to 1.5%. The fluorescent brightness of these particles is more than 2000× higher than the fluorescence of one molecule of LS277 in water. When exited in NIR spectral region (>700 nm), these particles are up to 4× brighter than QD800 commercial quantum dots emitting at 800 nm. We demonstrate that the synthesized NIR mesoporous silica nanoparticles easily internalize 4T1luc breast tumor cells, and remain bright for more than 9 weeks whereas the dye is completely bleached by that time.

Near-infrared-emitting nanoparticles for lifetime-based multiplexed analysis and imaging of living cells

The increase in information content from bioassays and bioimaging requires robust and efficient strategies for the detection of multiple analytes or targets in a single measurement, thereby addressing current health and security concerns. For fluorescence techniques, an attractive alternative to commonly performed spectral or color multiplexing presents lifetime multiplexing and the discrimination between different fluorophores based on their fluorescence decay kinetics. This strategy relies on fluorescent labels with sufficiently different lifetimes that are excitable at the same wavelength and detectable within the same spectral window. Here, we report on lifetime multiplexing and discrimination with a set of nanometer-sized particles loaded with near-infrared emissive organic fluorophores chosen to display very similar absorption and emission spectra, yet different fluorescence decay kinetics in suspension. Furthermore, as a first proof-of-concept, we describe bioimaging studies with 3T3 fibroblasts and J774 macrophages, incubated with mixtures of these reporters employing fluorescence lifetime imaging microscopy. These proof-of-concept measurements underline the potential of fluorescent nanoparticle reporters in fluorescence lifetime multiplexing, barcoding, and imaging for cellular studies, cell-based assays, and molecular imaging.

Optical imaging of cell death in traumatic brain injury using a heat shock protein-90 alkylator

Traumatic brain injury is a major public health concern and is characterised by both apoptotic and necrotic cell death in the lesion. Anatomical imaging is usually used to assess traumatic brain injuries and there is a need for imaging modalities that provide complementary cellular information. We sought to non-invasively image cell death in a mouse model of traumatic brain injury using a near-infrared fluorescent conjugate of a synthetic heat shock protein-90 alkylator, 4-(N-(S-glutathionylacetyl) amino) phenylarsonous acid (GSAO). GSAO labels both apoptotic and necrotic cells coincident with loss of plasma membrane integrity. The optical GSAO specifically labelled apoptotic and necrotic cells in culture and did not accumulate in healthy organs or tissues in the living mouse body. The conjugate is a very effective imager of cell death in brain lesions. The optical GSAO was detected by fluorescence intensity and GSAO bound to dying/dead cells was detected from prolongation of the fluorescence lifetime. An optimal signal-to-background ratio was achieved as early as 3 h after injection of the probe and the signal intensity positively correlated with both lesion size and probe concentration. This optical GSAO offers a convenient and robust means to non-invasively image apoptotic and necrotic cell death in brain and other lesions.

High-sensitivity detection of breast tumors in vivo by use of a pH-sensitive near-infrared fluorescence probe

We investigated the potential of the pH-sensitive dye, CypHer5E, conjugated to Herceptin (pH-Her) for the sensitive detection of breast tumors in mice using noninvasive time-domain near-infrared fluorescence imaging and different methods of data analysis. First, the fluorescence properties of pH-Her were analyzed as function of pH and/or dye-to-protein ratio, and binding specificity was confirmed in cell-based assays. Subsequently, the performance of pH-Her in nude mice bearing orthotopic HER2-positive (KPL-4) and HER2-negative (MDA-MB-231) breast carcinoma xenografts was compared to that of an always-on fluorescent conjugate Alexa Fluor 647-Herceptin (Alexa-Her). Subtraction of autofluorescence and lifetime (LT)-gated image analyses were performed for background fluorescence suppression. In mice bearing HER2-positive tumors, autofluorescence subtraction together with the selective fluorescence enhancement of pH-Her solely in the tumor's acidic environment provided high contrast-to-noise ratios (CNRs). This led to an improved sensitivity of tumor detection compared to Alexa-Her. In contrast, LT-gated imaging using LTs determined in model systems did not improve tumor-detection sensitivity in vivo for either probe. In conclusion, pH-Her is suitable for sensitive in vivo monitoring of HER2-expressing breast tumors with imaging in the intensity domain and represents a promising tool for detection of weak fluorescent signals deriving from small tumors or metastases.

Two-photon optical properties of near-infrared dyes at 1.55 μm excitation

Two-photon (2P) optical properties of cyanine dyes were evaluated using a 2P fluorescence spectrophotometer with 1.55 μm excitation. We report the 2P characteristics of common NIR polymethine dyes, including their 2P action cross sections and the 2P excited fluorescence lifetime. One of the dyes, DTTC, showed the highest 2P action cross-section (∼103 ± 19 GM) and relatively high 2P excited fluorescence lifetime and can be used as a scaffold for the synthesis of 2P molecular imaging probes. The 2P action cross-section of DTTC and the lifetime were also highly sensitive to the solvent polarity, providing other additional parameters for its use in optical imaging and the mechanism for probing environmental factors. Overall, this study demonstrated the quantitative measurement of 2P properties of NIR dyes and established the foundation for designing molecular probes for 2P imaging applications in the NIR region.

Near-infrared fluorescence lifetime pH-sensitive probes

We report what we believe to be the first near-infrared pH-sensitive fluorescence lifetime molecular probe suitable for biological applications in physiological range. Specifically, we modified a known fluorophore skeleton, hexamethylindotricarbocyanine, with a tertiary amine functionality that was electronically coupled to the fluorophore, to generate a pH-sensitive probe. The pK(a) of the probe depended critically on the location of the amine. Peripheral substitution at the 5-position of the indole ring resulted in a compound with pK(a) ∼ 4.9 as determined by emission spectroscopy. In contrast, substitution at the meso-position shifted the pK(a) to 5.5. The resulting compound, LS482, demonstrated steady-state and fluorescence-lifetime pH-sensitivity. This sensitivity stemmed from distinct lifetimes for protonated (∼1.16 ns in acidic DMSO) and deprotonated (∼1.4 ns in basic DMSO) components. The suitability of the fluorescent dyes for biological applications was demonstrated with a fluorescence-lifetime tomography system. The ability to interrogate cellular processes and subsequently translate the findings in living organisms further augments the potential of these lifetime-based pH probes.

Assembly and targeting of liposomal nanoparticles encapsulating quantum dots

Quantum dots (QDs) are attracting intense interest as fluorescence labeling agents for biomedical imaging because biocompatible coatings and relatively nontoxic rare earth metal QDs have emerged as possible options. QD photoemissions are bright, of narrow wavelength range, and very stable. We sought to encapsulate QDs within targeted PEGylated liposomes to reduce their propensity for liver uptake and to amplify the already strong QD emission signal. A novel lipid-QD conjugate initialized a process by which lipids in solution coalesced around the QDs. The liposomal structure was confirmed with size measurements, SEM, and IR spectroscopy. PEGylated QD liposomes injected into a xenograft tumor model largely cleared from the body within 24 h. Residual liver labeling was low. Targeted QD liposomes exhibited robust tumor labeling compared with controls. This study highlights the potential of these near IR emitting QD liposomes for preclinical/clinical applications.

Wavelength-resolved measurements of fluorescence lifetime of indocyanine green

We study fluorescence lifetime of indocyanine green (ICG) using femtosecond laser and sensitive detection based on time-correlated single-photon counting. A time-resolved multichannel spectral system is constructed and applied for determination of the fluorescence lifetime of the ICG in different solvents. Emission properties of ICG in water, milk, and 1% intralipid solution are investigated. Fluorescence of the fluorophore of different concentrations (in a range of 1.7-160 μM) dissolved in different solutions is excited by femtosecond pulses generated with the use of Ti:Sa laser tuned within the range of 740-790 nm. It is observed that fluorescence lifetime of ICG in water is 0.166 ± 0.02 ns and does not depend on excitation and emission wavelengths. We also show that for the diffusely scattering solvents (milk and intralipid), the lifetime may depend on the dye concentration (especially for large concentrations of ICG). This effect should be taken into account when analyzing changes in the mean time of arrival of fluorescence photons excited in ICG dissolved in such optically turbid media.

Energy transfer cassettes in silica nanoparticles target intracellular organelles

Lipophilic energy transfer cassettes like 1 and 2 are more conveniently synthesized than the corresponding hydrophilic compounds, but they are not easily used in aqueous media. To overcome the latter issue, cassettes 1 and 2 were separately encapsulated in silica nanoparticles (ca. 22 nm) which freely disperse in aqueous media. Photophysical properties of the encapsulated dyes 1-SiO(2) and 2-SiO(2) were recorded. The nanoparticles 1-SiO(2) permeated into Clone 9 rat liver cells and targeted only the ER. A high degree of energy transfer was observed in this organelle such that most of the light fluoresced from the acceptor part, i.e. the particles appeared red. Silica nanoparticles 2-SiO(2) also permeated into Clone 9 rat liver cells and they targeted mitochondria but were also observed in endocytic vesicles (lysosomes or endosomes). In these organelles they fluoresced red and red/green respectively. Thus the cargo inside the nanoparticles influences where they localize in cells, and the environment of the nanoparticles in the cells changes the fluorescent properties of the encapsulated dyes. Neither of these findings were anticipated given that silica nanoparticles of this type are generally considered to be non-porous.

Rational approach to select small peptide molecular probes labeled with fluorescent cyanine dyes for in vivo optical imaging

We demonstrate that the structure of carbocyanine dyes, which are commonly used to label small peptides for molecular imaging and not the bound peptide, controls the rate of extravasation from blood vessels to tissue. By examining several near-infrared (NIR) carbocyanine fluorophores, we demonstrate a quantitative correlation between the binding of a dye to albumin, a model plasma protein, and the rate of extravasation of the probe into tissue. Binding of the dyes was measured by fluorescence quenching of the tryptophans in albumin and was found to be inversely proportional to the rate of extravasation. The rate of extravasation, determined by kurtosis from longitudinal imaging studies using rodent ear models, provided a basis for quantitative measurements. Structure-activity studies aimed at evaluating a representative library of NIR fluorescent cyanine probes showed that hydrophilic dyes with binding constants several orders of magnitude lower than their hydrophobic counterparts have much faster extravasation rate, establishing a foundation for rational probe design. The correlation provides a guideline for dye selection in optical imaging and a method to verify if a certain dye is optimal for a specific molecular imaging application.

Enhancing magnetic resonance imaging tumor detection with fluorescence intensity and lifetime imaging

Early detection is important for many solid cancers but the images provided by ultrasound, magnetic resonance imaging (MRI), and computed tomography applied alone or together, are often not sufficient for decisive early screening ∕ diagnosis. We demonstrate that MRI augmented with fluorescence intensity (FI) substantially improves detection. Early stage murine pancreatic tumors that could not be identified by blinded, skilled observers using MRI alone, were easily identified with MRI along with FI images acquired with photomultiplier tube detection and offset laser scanning. Moreover, we show that fluorescence lifetime (FLT) imaging enables positive identification of the labeling fluorophore and discriminates it from surrounding tissue autofluorescence. Our data suggest combined-modality imaging with MRI, FI, and FLT can be used to screen and diagnose early tumors.

Image-guided surgery using invisible near-infrared light: fundamentals of clinical translation

The field of biomedical optics has matured rapidly over the last decade and is poised to make a significant impact on patient care. In particular, wide-field (typically > 5 cm), planar, near-infrared (NIR) fluorescence imaging has the potential to revolutionize human surgery by providing real-time image guidance to surgeons for tissue that needs to be resected, such as tumors, and tissue that needs to be avoided, such as blood vessels and nerves. However, to become a clinical reality, optimized imaging systems and NIR fluorescent contrast agents will be needed. In this review, we introduce the principles of NIR fluorescence imaging, analyze existing NIR fluorescence imaging systems, and discuss the key parameters that guide contrast agent development. We also introduce the complexities surrounding clinical translation using our experience with the Fluorescence-Assisted Resection and Exploration (FLARE™) imaging system as an example. Finally, we introduce state-of-the-art optical imaging techniques that might someday improve image-guided surgery even further.

Lifetime-based tomographic multiplexing

Near-infrared (NIR) fluorescence tomography of multiple fluorophores has previously been limited by the bandwidth of the NIR spectral regime and the broad emission spectra of most NIR fluorophores. We describe in vivo tomography of three spectrally overlapping fluorophores using fluorescence lifetime-based separation. Time-domain images are acquired using a voltage-gated, intensified charge-coupled device (CCD) in free-space transmission geometry with 750 nm Ti:sapphire laser excitation. Lifetime components are fit from the asymptotic portion of fluorescence decay curve and reconstructed separately with a lifetime-adjusted forward model. We use this system to test the in vivo lifetime multiplexing suitability of commercially available fluorophores, and demonstrate lifetime multiplexing in solution mixtures and in nude mice. All of the fluorophores tested exhibit nearly monoexponential decays, with narrow in vivo lifetime distributions suitable for lifetime multiplexing. Quantitative separation of two fluorophores with lifetimes of 1.1 and 1.37 ns is demonstrated for relative concentrations of 1:5. Finally, we demonstrate tomographic imaging of two and three fluorophores in nude mice with fluorophores that localize to distinct organ systems. This technique should be widely applicable to imaging multiple NIR fluorophores in 3-D.

Low-frequency wide-field fluorescence lifetime imaging using a high-power near-infrared light-emitting diode light source

Fluorescence lifetime imaging (FLi) could potentially improve exogenous near-infrared (NIR) fluorescence imaging, because it offers the capability of discriminating a signal of interest from background, provides real-time monitoring of a chemical environment, and permits the use of several different fluorescent dyes having the same emission wavelength. We present a high-power, LED-based, NIR light source for the clinical translation of wide-field (larger than 5 cm in diameter) FLi at frequencies up to 35 MHz. Lifetime imaging of indocyanine green (ICG), IRDye 800-CW, and 3,3(')-diethylthiatricarbocyanine iodide (DTTCI) was performed over a large field of view (10 cm by 7.5 cm) using the LED light source. For comparison, a laser diode light source was employed as a gold standard. Experiments were performed both on the bench by diluting the fluorescent dyes in various chemical environments in Eppendorf tubes, and in vivo by injecting the fluorescent dyes mixed in Matrigel subcutaneously into CD-1 mice. Last, measured fluorescence lifetimes obtained using the LED and the laser diode sources were compared with those obtained using a state-of-the-art time-domain imaging system and with those previously described in the literature. On average, lifetime values obtained using the LED and the laser diode light sources were consistent, exhibiting a mean difference of 3% from the expected values and a coefficient of variation of 12%. Taken together, our study offers an alternative to laser diodes for clinical translation of FLi and explores the use of relatively low frequency modulation for in vivo imaging.

Long fluorescence lifetime molecular probes based on near infrared pyrrolopyrrole cyanine fluorophores for in vivo imaging

Fluorescence lifetime (FLT) properties of organic molecules provide a new reporting strategy for molecular imaging in the near infrared (NIR) spectral region. Unfortunately, most of the NIR fluorescent dyes have short FLT typically clustered below 1.5 ns. In this study, we demonstrate that a new class of NIR fluorescent dyes, pyrrolopyrrole cyanine dyes, have exceptionally long FLTs ranging from 3 to 4 ns, both in vitro (dimethyl sulfoxide and albumin/water solutions) and in vivo (mice). These results provide a new window for imaging molecular processes, rejecting backscattered light and autofluorescence, and multiplexing imaging information with conventional NIR fluorescent dyes that absorb and emit light at similar wavelengths.

Whole-body, real-time preclinical imaging of quantum dot fluorescence with time-gated detection

We describe a wide-field preclinical imaging system optimized for time-gated detection of quantum dot fluorescence emission. As compared to continuous wave measurements, image contrast was substantially improved by suppression of short-lifetime background autofluorescence. Real-time (8 frames/s) biological imaging of subcutaneous quantum dot injections is demonstrated simultaneously in multiple living mice.

Three-dimensional surface profile intensity correction for spatially modulated imaging

We describe a noncontact profile correction technique for quantitative, wide-field optical measurement of tissue absorption (microa) and reduced scattering (micros) coefficients, based on geometric correction of the sample's Lambertian (diffuse) reflectance intensity. Because the projection of structured light onto an object is the basis for both phase-shifting profilometry and modulated imaging, we were able to develop a single instrument capable of performing both techniques. In so doing, the surface of the three-dimensional object could be acquired and used to extract the object's optical properties. The optical properties of flat polydimethylsiloxane (silicone) phantoms with homogenous tissue-like optical properties were extracted, with and without profilometry correction, after vertical translation and tilting of the phantoms at various angles. Objects having a complex shape, including a hemispheric silicone phantom and human fingers, were acquired and similarly processed, with vascular constriction of a finger being readily detectable through changes in its optical properties. Using profilometry correction, the accuracy of extracted absorption and reduced scattering coefficients improved from two- to ten-fold for surfaces having height variations as much as 3 cm and tilt angles as high as 40 deg. These data lay the foundation for employing structured light for quantitative imaging during surgery.

Dynamic noninvasive monitoring of renal function in vivo by fluorescence lifetime imaging

Kidneys normally filter the blood of excess salts and metabolic products, such as urea, while retaining plasma proteins. In diseases such as multiple myeloma and diabetes mellitus, the renal function is compromised and protein escapes into the urine. In this study, we present the use of fluorescence lifetime imaging (FLI) to image excess serum protein in urine (proteinuria). The near-infrared fluorescent dye LS-288 has distinct lifetimes when bound to protein versus free in solution, providing contrast between the protein-rich viscera and the mostly protein-free bladder. FLI with LS-288 in mice revealed that fluorescence lifetime (FLT) differences in the bladder relative to surrounding tissues was due to the fractional contributions of the bound and unbound dye molecules. The FLT of LS-288 decreased in the case of proteinuria while fluorescence intensity was unchanged. The results show that FLI can be useful for the dynamic imaging of protein-losing nephropathy due to diabetes mellitus and other renal diseases and suggest the potential use of the FLI to distinguish tumors from fluid-filled cysts in the body.