Noninvasive measurement of fluorophore concentration in turbid media with a simple fluorescence /reflectance ratio technique

Miniature, multi-dichroic instrument for measuring the concentration of multiple fluorophores

Identification of tumour margins during resection of the brain is critical for improving the post-operative outcomes. Due to the highly infiltrative nature of glioblastoma multiforme (GBM) and limited intraoperative visualization of the tumour margin, incomplete surgical resection has been observed to occur in up to 80 % of GBM cases, leading to nearly universal tumour recurrence and overall poor prognosis of 14.6 months median survival. This research presents a miniaturized, SiPMT-based optical system for simultaneous measurement of powerful DRS and weak auto-fluorescence for brain tumour detection. The miniaturisation of the optical elements confined the spatial separation of eight select wavelengths into footprint measuring 1.5 × 2 × 16 mm. The small footprint enables this technology to be integrated with existing surgical guidance instruments in the operating room. It's dynamic ability to subtract any background illumination and measure signal intensities across a broad range from pW to mWs make this design much more suitable for clinical environments as compared to spectrometer-based systems with limited dynamic ranges and high integration times. Measurements using optical tissue phantoms containing mixed fluorophores demonstrate correlation coefficients between the fitted response and actual concentration using PLS regression being 0.95, 0.87 and 0.97 for NADH, FAD and PpIX , respectively. These promising results indicate that our proposed miniaturized instrument could serve as an effective alternative in operating rooms, assisting surgeons in identifying brain tumours to achieving positive surgical outcomes for patients.

Novel Combined Spectral Indices Derived from Hyperspectral and Laser-Induced Fluorescence LiDAR Spectra for Leaf Nitrogen Contents Estimation of Rice

Spectra of reflectance (Sr) and fluorescence (Sf) are significant for crop monitoring and ecological environment research, and can be used to indicate the leaf nitrogen content (LNC) of crops indirectly. The aim of this work is to use the Sr-Sf features obtained with hyperspectral and laser-induced fluorescence LiDAR (HSL, LIFL) systems to construct novel combined spectral indices (NCIH-F) for multi-year rice LNC estimation. The NCIH-F is in a form of FWs* Φ + GSIs* Φ , where Φ is the Sr-Sf features, and FWs and GSIs are the feature weights and global sensitive indices for each characteristic band. In this study, the characteristic bands were chosen in different ways. Firstly, the Sr-Sf characteristics which can be the intensity or derivative variables of spectra in 685 and 740 nm, have been assigned as the Φ value in NCIH-F formula. Simultaneously, the photochemical reflectance index (PRI) formed with 531 and 570 nm was modified based on a variant spectral index, called PRIfraction, with the Sf intensity in 740 nm, and then compared its potential with NCIH-F on LNC estimation. During the above analysis, both NCIH-F and PRIfraction values were utilized to model rice LNC based on the artificial neural networks (ANNs) method. Subsequently, four prior bands were selected, respectively, with high FW and GSI values as the ANNs inputs for rice LNC estimation. Results show that FW- and GSI-based NCIH-F are closely related to rice LNC, and the performance of previous spectral indices used for LNC estimation can be greatly improved by multiplying their FWs and GSIs. Thus, it can be included that the FW- and GSI-based NCIH-F constitutes an efficient and reliable constructed form combining HSL (Sr) and LIFL (Sf) data together for rice LNC estimation.

Quantitative Wide-Field Imaging Techniques for Fluorescence Guided Neurosurgery

Fluorescence guided surgery (FGS) has fueled the development of novel technologies aimed at maximizing the utility of fluorescence imaging to help clinicians diagnose and in certain cases treat diseases across a breadth of disciplines such as dermatology, gynecology, oncology, ophthalmology, and neurosurgery. In neurosurgery, the goal of FGS technologies is to provide the neurosurgeon with additional information which can serve as a visual aid to better identify tumor tissue and associated margins. Yet, current clinical FGS technologies are qualitative in nature, limiting the ability to make accurate, reliable, and repeatable measurements. To this end, developments in fluorescence quantification are needed to overcome current limitations of FGS. Here we present an overview of the recent developments in quantitative fluorescence guidance technologies and conclude with the most recent developments aimed at wide-field quantitative fluorescence imaging approaches in neurosurgery.

Recommendations for reporting on emerging optical imaging agents to promote clinical approval

Intraoperative fluorescence imaging is particularly well-suited for surgical applications due to its inherently high sensitivity, resolution, and ability to provide images in real-time. To date, the intraoperative observation of fluorescence has largely been subjective. With the need to show objective evidence in order to demonstrate the benefit of this technique, quantitative data needs to be provided to overseeing regulatory bodies. Standardization of fluorescence imaging protocols would improve reproducibility and minimize inter- and intra-institution variance. This would allow studies to be conducted using the same injection techniques, imaging times, reconstruction methods, and analyses. Here, we provide recommendations for standardized methodologies with the goal of setting a minimum requirement for reporting fluorescence-guided surgery results based on both qualitative and (semi-) quantitative data collection. Clinical trials using fluorescence-guided surgery should present results of three critical elements; 1) intra-operative imaging, 2) specimen mapping and pathology correlation, and 3) target validation. Qualitative analyses should consist of a bright field image, black-and-white fluorescence image, pseudo-colored fluorescence overlay image, and/or heat-map whereby fluorescence signal intensity differences are displayed on a color spectrum. Quantitative analyses should include 1) intraoperative data (consisting of images or video, raw numeric values and ratios); 2) specimen mapping, for correlation of fluorescence with the presence of disease (performed using fresh tissue); and 3) target validation (designed to determine fluorescence intensity relative to receptor density of a specific area). Including the aforementioned methods of both qualitative and quantitative analyses will ensure that trial results are comparable and could be collated in future studies to expedite FDA approval.

Visible light OCT-based quantitative imaging of lipofuscin in the retinal pigment epithelium with standard reference targets

We developed a technology for quantitative retinal autofluorescence (AF, or FAF for fundus AF) imaging for quantifying lipofuscin in the retinal pigment epithelium (RPE). The technology is based on simultaneous visible light optical coherence tomography (VIS-OCT) and AF imaging of the retina and a pair of reference standard targets at the intermediate retinal imaging plane with known reflectivity for the OCT and fluorescence efficiency for the FAF. The technology is able to eliminate the pre-RPE attenuation in FAF imaging by using the simultaneously acquired VIS-OCT image. With the OCT and fluorescence images of the reference targets, the effects of illumination power and detector sensitivity can be eliminated.

Tissue intrinsic fluorescence recovering by an empirical approach based on the PSO algorithm and its application in type 2 diabetes screening

In order to reduce the influence of scattering and absorption on tissue fluorescence spectra, after tissue fluorescence and diffuse reflectance in different tissue optical properties were simulated by the Monte Carlo method, a tissue intrinsic fluorescence recovering algorithm making use of diffuse reflectance spectrum was developed. The empirical parameters in the tissue intrinsic fluorescence recovering algorithm were coded as a particle in the solution domain, the classification performance was defined as the fitness, and then a particle swarm optimization (PSO) algorithm was established for empirical parameters optimization. The skin autofluorescence and diffuse reflectance spectra of 327 subjects were collected in Anhui Provincial Hospital. The skin intrinsic autofluorescence spectra were recovered by using the empirical approach and the integration area of the spectra were calculated as fluorescence intensity. Receiver operating characteristic (ROC) analysis for fluorescence intensity was applied to evaluate the classification performance in type 2 diabetes screening. In addition, a support vector machine (SVM) method was implemented to improve the performance of the classification. The results showed that the sensitivity and specificity were 32% and 76% respectively, and the area under the curve was 0.54 before recovering, while the sensitivity and specificity were 72% and 86% respectively, and the area under the curve was 0.86 after recovering. Furthermore, the sensitivity and specificity increased to 83% and 86% respectively when using linear SVM while 84% and 88%, respectively, when using nonlinear SVM. The results indicate that using the tissue fluorescence spectrum recovery algorithm based on PSO can improve the application of tissue fluorescence spectroscopy effectively.

A dual-channel endoscope for quantitative imaging, monitoring, and triggering of doxorubicin release from liposomes in living mice

Doxorubicin (Dox) is approved for use in liposomal form for the treatment of ovarian cancer. We previously developed a long-circulating Dox formulation in liposomes containing small amounts of porphyrin-phospholipid, which enables on-demand drug release with near-infrared irradiation. In this study, we present and evaluate a dual-modal, dual-channel light endoscope that allows quantitative reflectance and fluorescence imaging for monitoring of local Dox concentrations in target areas. The endoscope consists of two flexible imaging fibers; one to transmit diagnostic and therapeutic light to the target, and the other to detect fluorescent and reflected light. Thus, the endoscope serves for imaging, for light delivery to trigger drug release, and for monitoring drug concentration kinetics during drug release. We characterized the performance of this endoscope in tissue phantoms and in an in vivo model of ovarian cancer. This study demonstrates the feasibility of non-invasive, quantitative mapping of Dox distribution in vivo via endoscopic imaging.

Fluorescence-guided resection of brain tumor: review of the significance of intraoperative quantification of protoporphyrin IX fluorescence

Surgical removal of tumor mass is a common approach in the management of brain tumors. However, the precise delineation of normal tissue from tumor tissue for a complete resection of tumor mass in brain tumor surgery remains a difficult task for neurosurgeons. Aminolevulinic acid (ALA)-mediated exgogenous fluorescence of protoporphyrin IX (PpIX) is a sensitive approach for tumor imaging. Recent studies suggest that the use of ALA/PpIX-mediated fluorescence-guided resection (FGR) or fluorescence-guided surgery can enable more accurate and complete resection of brain tumors, especially when used in quantitative fashion. This review will highlight the current progress in PpIX-mediated FGR and discuss technical challenges in intraoperative quantification of intracellular PpIX fluorescence during FGR of brain tumor.

Quantitative Fluorescence Sensing Through Highly Autofluorescent, Scattering, and Absorbing Media Using Mobile Microscopy

Compact and cost-effective systems for in vivo fluorescence and near-infrared imaging in combination with activatable reporters embedded inside the skin to sample interstitial fluid or blood can enable a variety of biomedical applications. However, the strong autofluorescence of human skin creates an obstacle for fluorescence-based sensing. Here we introduce a method for quantitative fluorescence sensing through highly autofluorescent, scattering, and absorbing media. For this, we created a compact and cost-effective fluorescence microscope weighing <40 g and used it to measure various concentrations of a fluorescent dye embedded inside a tissue phantom, which was designed to mimic the optical characteristics of human skin. We used an elliptical Gaussian beam excitation to digitally separate tissue autofluorescence from target fluorescence, although they severely overlap in both space and optical spectrum. Using ∼10-fold less excitation intensity than the safety limit for skin radiation exposure, we successfully quantified the density of the embedded fluorophores by imaging the skin phantom surface and achieved a detection limit of ∼5 × 10(5) and ∼2.5 × 10(7) fluorophores within ∼0.01 μL sample volume that is positioned 0.5 and 2 mm below the phantom surface, corresponding to a concentration of 105.9 pg/mL and 5.3 ng/mL, respectively. We also confirmed that this approach can track the spatial misalignments of the mobile microscope with respect to the embedded target fluorescent volume. This wearable microscopy platform might be useful for designing implantable biochemical sensors with the capability of spatial multiplexing to continuously monitor a panel of biomarkers and chronic conditions even at patients' home.

Quantitative imaging of light-triggered doxorubicin release

The efficacy of chemotherapy is related, in large part, to the concentration of drug that reaches tumor sites. Doxorubicin (DOX) is a common anti-cancer drug that is also approved for use in liposomal form for the treatment of ovarian cancer. We recently developed a porphyrin-phospholipid (PoP)-liposome system that enables on demand release of DOX from liposomes using near infrared irradiation to improve DOX bioavailability. Owing to its intrinsic fluorescence, it is possible, and desirable, to quantify DOX concentration and distribution, preferably noninvasively. Here we quantified DOX distribution following light-triggered drug release in phantoms and an animal carcass using spatial frequency domain imaging. This study demonstrates the feasibility of non-invasive quantitative mapping of DOX distributions in target areas.

Assessment and correction of turbidity effects on Raman observations of chemicals in aqueous solutions

Improvements in diode laser, fiber optic, and data acquisition technologies are enabling increased use of Raman spectroscopic techniques for both in lab and in situ water analysis. Aqueous media encountered in the natural environment often contain suspended solids that can interfere with spectroscopic measurements, yet removal of these solids, for example, via filtration, can have even greater adverse effects on the extent to which subsequent measurements are representative of actual field conditions. In this context, this study focuses on evaluation of turbidity effects on Raman spectroscopic measurements of two common environmental pollutants in aqueous solution: ammonium nitrate and trichloroethylene. The former is typically encountered in the runoff from agricultural operations and is a strong scatterer that has no significant influence on the Raman spectrum of water. The latter is a commonly encountered pollutant at contaminated sites associated with degreasing and cleaning operations and is a weak scatterer that has a significant influence on the Raman spectrum of water. Raman observations of each compound in aqueous solutions of varying turbidity created by doping samples with silica flour with grain sizes ranging from 1.6 to 5.0 μm were employed to develop relationships between observed Raman signal strength and turbidity level. Shared characteristics of these relationships were then employed to define generalized correction methods for the effect of turbidity on Raman observations of compounds in aqueous solution.

Dual-channel red/blue fluorescence dosimetry with broadband reflectance spectroscopic correction measures protoporphyrin IX production during photodynamic therapy of actinic keratosis

Dosimetry for aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) photodynamic therapy of actinic keratosis was examined with an optimized fluorescence dosimeter to measure PpIX during treatment. While insufficient PpIX generation may be an indicator of incomplete response, there exists no standardized method to quantitate PpIX production at depths in the skin during clinical treatments. In this study, a spectrometer-based point probe dosimeter system was used to sample PpIX fluorescence from superficial (blue wavelength excitation) and deeper (red wavelength excitation) tissue layers. Broadband white light spectroscopy (WLS) was used to monitor aspects of vascular physiology and inform a correction of fluorescence for the background optical properties. Measurements in tissue phantoms showed accurate recovery of blood volume fraction and reduced scattering coefficient from WLS, and a linear response of PpIX fluorescence versus concentration down to 1.95 and 250 nM for blue and red excitations, respectively. A pilot clinical study of 19 patients receiving 1-h ALA incubation before treatment showed high intrinsic variance in PpIX fluorescence with a standard deviation/mean ratio of > 0.9. PpIX fluorescence was significantly higher in patients reporting higher pain levels on a visual analog scale. These pilot data suggest that patient-specific PpIX quantitation may predict outcome response.

Recovery of intrinsic fluorescence from single-point interstitial measurements for quantification of doxorubicin concentration

BACKGROUND AND OBJECTIVE

We developed a method for the recovery of intrinsic fluorescence from single-point measurements in highly scattering and absorbing samples without a priori knowledge of the sample optical properties. The goal of the study was to demonstrate accurate recovery of fluorophore concentration in samples with widely varying background optical properties, while simultaneously recovering the optical properties.

MATERIALS AND METHODS

Tissue-simulating phantoms containing doxorubicin, MnTPPS, and Intralipid-20% were created, and fluorescence measurements were performed using a single isotropic probe. The resulting spectra were analyzed using a forward-adjoint fluorescence model in order to recover the fluorophore concentration and background optical properties.

RESULTS

We demonstrated recovery of doxorubicin concentration with a mean error of 11.8%. The concentration of the background absorber was recovered with an average error of 23.2% and the scattering spectrum was recovered with a mean error of 19.8%.

CONCLUSION

This method will allow for the determination of local concentrations of fluorescent drugs, such as doxorubicin, from minimally invasive fluorescence measurements. This is particularly interesting in the context of transarterial chemoembolization (TACE) treatment of liver cancer.

Gadolinium- and 5-aminolevulinic acid-induced protoporphyrin IX levels in human gliomas: an ex vivo quantitative study to correlate protoporphyrin IX levels and blood-brain barrier breakdown

In recent years, 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) fluorescence guidance has been used as a surgical adjunct to improve the extent of resection of gliomas. Exogenous administration of ALA before surgery leads to the accumulation of red fluorescent PpIX in tumor tissue that the surgeon can visualize and thereby discriminate between normal and tumor tissue. Selective accumulation of PpIX has been linked to numerous factors, of which blood-brain barrier breakdown has been suggested to be a key factor. To test the hypothesis that PpIX concentration positively correlates with gadolinium (Gd) concentrations, we performed ex vivo measurements of PpIX and of Gd using inductively coupled plasma mass spectrometry, the latter as a quantitative biomarker of blood-brain barrier breakdown; this was corroborated with immunohistochemistry of microvascular density in surgical biopsies of patients undergoing fluorescence-guided surgery for glioma. We found positive correlations between PpIX concentration and Gd concentration (r = 0.58, p < 0.0001) and between PpIX concentration and microvascular density (r = 0.55, p < 0.0001), suggesting a significant, yet limited, association between blood-brain barrier breakdown and ALA-induced PpIX fluorescence. To our knowledge, this is the first time that Gd measurements by inductively coupled plasma mass spectrometry have been used in human gliomas.

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.

Scattering phase function spectrum makes reflectance spectrum measured from Intralipid phantoms and tissue sensitive to the device detection geometry

Reflectance spectra measured in Intralipid (IL) close to the source are sensitive to wavelength-dependent changes in reduced scattering coefficient ([Formula: see text]) and scattering phase function (PF). Experiments and simulations were performed using device designs with either single or separate optical fibers for delivery and collection of light in varying concentrations of IL. Spectral reflectance is not consistently linear with varying IL concentration, with PF-dependent effects observed for single fiber devices with diameters smaller than ten transport lengths and for separate source-detector devices that collected light at less than half of a transport length from the source. Similar effects are thought to be seen in tissue, limiting the ability to quantitatively compare spectra from different devices without compensation.

Semi-empirical model of the effect of scattering on single fiber fluorescence intensity measured on a turbid medium

Quantitative determination of fluorophore content from fluorescence measurements in turbid media, such as tissue, is complicated by the influence of scattering properties on the collected signal. This study utilizes a Monte Carlo model to characterize the relationship between the fluorescence intensity collected by a single fiber optic probe (F(SF)) and the scattering properties. Simulations investigate a wide range of biologically relevant scattering properties specified independently at excitation (λ(x)) and emission (λ(m)) wavelengths, including reduced scattering coefficients in the range μ'(s)(λ(x)) ∈ [0.1 - 8]mm(-1) and μ'(s)(λ(m)) ∈ [0.25 - 1] × μ'(s)(λ(x)). Investigated scattering phase functions (P(θ)) include both Henyey-Greenstein and Modified Henyey-Greenstein forms, and a wide range of fiber diameters (d(f) ∈ [0.2 - 1.0] mm) was simulated. A semi-empirical model is developed to estimate the collected F(SF) as the product of an effective sampling volume, and the effective excitation fluence and the effective escape probability within the effective sampling volume. The model accurately estimates F(SF) intensities (r=0.999) over the investigated range of μ'(s)(λ(x)) and μ'(s)(λ(m)), is insensitive to the form of the P(θ), and provides novel insight into a dimensionless relationship linking F(SF) measured by different d(f).

Combined fluorescence and reflectance spectroscopy for in vivo quantification of cancer biomarkers in low- and high-grade glioma surgery

Biomarkers are indicators of biological processes and hold promise for the diagnosis and treatment of disease. Gliomas represent a heterogeneous group of brain tumors with marked intra- and inter-tumor variability. The extent of surgical resection is a significant factor influencing post-surgical recurrence and prognosis. Here, we used fluorescence and reflectance spectral signatures for in vivo quantification of multiple biomarkers during glioma surgery, with fluorescence contrast provided by exogenously-induced protoporphyrin IX (PpIX) following administration of 5-aminolevulinic acid. We performed light-transport modeling to quantify multiple biomarkers indicative of tumor biological processes, including the local concentration of PpIX and associated photoproducts, total hemoglobin concentration, oxygen saturation, and optical scattering parameters. We developed a diagnostic algorithm for intra-operative tissue delineation that accounts for the combined tumor-specific predictive capabilities of these quantitative biomarkers. Tumor tissue delineation achieved accuracies of up to 94% (specificity = 94%, sensitivity = 94%) across a range of glioma histologies beyond current state-of-the-art optical approaches, including state-of-the-art fluorescence image guidance. This multiple biomarker strategy opens the door to optical methods for surgical guidance that use quantification of well-established neoplastic processes. Future work would seek to validate the predictive power of this proof-of-concept study in a separate larger cohort of patients.

Quantification of in vivo fluorescence decoupled from the effects of tissue optical properties using fiber-optic spectroscopy measurements

We present a method for tissue fluorescence quantification in situ using a handheld fiber optic probe that measures both the fluorescence and diffuse reflectance spectra. A simplified method to decouple the fluorescence spectrum from distorting effects of the tissue optical absorption and scattering is developed, with the objective of accurately quantifying the fluorescence in absolute units. The primary motivation is measurement of 5-aminolevulinic acid-induced protoporphyrin IX (ALA-PpIX) concentration in tissue during fluorescence-guided resection of malignant brain tumors. This technique is validated in phantoms and ex vivo mouse tissues, and tested in vivo in a rabbit brain tumor model using ALA-PpIX fluorescence contrast.

Estimation of indocyanine green concentration in blood from fluorescence emission: application to hemodynamic assessment during hemodialysis

There is considerable interest in assessing cardiovascular function noninvasively in patients receiving hemodialysis. A possible approach is to measure the blood concentration of bolus-injected indocyanine green dye and to apply the dye-dilution method for estimating cardiac output and blood volume. Blood ICG concentration can be derived from a measurement of the ICG fluorescence through the dialysis tubing if a simple and unique calibration relationship can be established between transmural fluorescence intensity and blood ICG concentration. We investigated this relationship using Monte Carlo simulations of light transport in blood with varying hematocrit and ICG concentrations and performed empiric measurements of optical absorption and ICG fluorescence emission to confirm our findings. The ICG fluorescence intensity measured at the blood surface, as well as the light intensity remitted by the blood, varied as hematocrit changes modified the absorption and scattering characteristics of the blood. Calibration relationships were developed between fluorescence intensity and ICG concentration that accounted for hematocrit changes. Combining the backreflected fluorescence and the reflected light measured near the point of illumination provided optimal signal intensity, linearity, and robustness to hematocrit changes. These results provide a basis for developing a noninvasive approach to derive optically circulating blood ICG concentration in hemodialysis circuits.

In vivo quantification of chromophore concentration using fluorescence differential path length spectroscopy

We present an optical method based on fluorescence spectroscopy for measuring chromophore concentrations in vivo. Fluorescence differential path length spectroscopy (FPDS) determines chromophore concentration based on the fluorescence intensity corrected for absorption. The concentration of the photosensitizer m-THPC (Foscan) was studied in vivo in normal rat liver, which is highly vascularized and therefore highly absorbing. Concentration estimates of m-THPC measured by FDPS on the liver are compared with chemical extraction. Twenty-five rats were injected with 0.3 mg kg m-THPC. In vivo optical concentration measurements were performed on tissue 3, 24, 48, and 96 h after m-THPC administration to yield a 10-fold variation in tissue concentration. After the optical measurements, the liver was harvested for chemical extraction. FDPS showed good correlation with chemical extraction. FDPS also showed a correlation between m-THPC fluorescence and blood volume fraction at the two shortest drug-light intervals. This suggests different compartmental localization of m-THPC for different drug-light intervals that can be resolved using fluorescence spectroscopy. Differences in measured m-THPC concentration between FDPS and chemical extraction are related to the interrogation volume of each technique; approximately 0.2 mm(3) and approximately 10(2) mm(3), respectively. This indicates intra-animal variation in m-THPC distribution in the liver on the scale of the FDPS sampling volume.

Quantitative imaging of scattering changes associated with epithelial proliferation, necrosis, and fibrosis in tumors using microsampling reflectance spectroscopy

Highly localized reflectance measurements can be used to directly quantify scatter changes in tissues. We present a microsampling approach that is used to raster scan tumors to extract parameters believed to be related to the tissue ultrastructure. A confocal reflectance imager was developed to examine scatter changes across pathologically distinct regions within tumor tissues. Tissue sections from two murine tumors, AsPC-1 pancreas tumor and the Mat-LyLu Dunning prostate tumor, were imaged. After imaging, histopathology-guided region-of-interest studies of the images allowed analysis of the variations in scattering resulting from differences in tissue ultra-structure. On average, the median scatter power of tumor cells with high proliferation index (HPI) was about 26% less compared to tumor cells with low proliferation index (LPI). Necrosis exhibited the lowest scatter power signature across all the tissue types considered, with about 55% lower median scatter power than LPI tumor cells. Additionally, the level and maturity of the tumor's fibroplastic response was found to influence the scatter signal. This approach to scatter visualization of tissue ultrastructure in situ could provide a unique tool for guiding surgical resection, but this kind of interpretation into what the signal means relative to the pathology is required before proceeding to clinical studies.

A Ratiometric Fluorescence Imaging System for Surgical Guidance

A 3-chip CCD imaging system has been developed for quantitative in vivo fluorescence imaging. This incorporates a ratiometric algorithm to correct for the effects of tissue optical absorption and scattering, imaging “geometry” and tissue autofluorescence background. The performance was characterized, and the algorithm was validated in tissue-simulating optical phantoms for quantitative measurement of the fluorescent molecule protoporphyrin IX (PpIX). The technical feasibility to use this system for fluorescence-guided surgical resection of malignant brain tumor tissue was assessed in an animal model in which PpIX was induced exogenously in the tumor cells by systemic administration of aminolevulinic acid (ALA).

Quantitative fluorescence spectroscopy in turbid media using fluorescence differential path length spectroscopy

We have developed a new technique, fluorescence differential path length spectroscopy (FDPS), that enables the quantitative investigation of fluorophores in turbid media. FDPS measurements are made with the same probe geometry as differential path length spectroscopy (DPS) measurements. Phantom measurements are performed for two fiber diameters (400 microm and 800 microm) and for a wide range of optical properties (mu(s)': 0 to 10 mm(-1); mu(a): 0 to 2 mm(-1)) to investigate the influence of the optical properties on the measured differential fluorescence signal. The differential fluorescence signal varies by a factor of 1.4 and 2.2 over the biologically relevant scattering range (0.5 to 5 mm(-1)) for a given fluorophore concentration for 400 microm and 800 microm fibers, respectively. The differential fluorescence signal is attenuated due to absorption at the excitation wavelength following Lambert-Beer's law with a path length equal to the differential path length.

Intrinsic Raman spectroscopy for quantitative biological spectroscopy part I: theory and simulations

We present a novel technique, intrinsic Raman spectroscopy (IRS), to correct turbidity-induced Raman spectral distortions, resulting in the intrinsic Raman spectrum that would be observed in the absence of scattering and absorption. We develop an expression relating the observed and intrinsic Raman spectra through diffuse reflectance using the photon migration depiction of light transport. Numerical simulations are employed to validate the theoretical results and study the dependence of this expression on sample size and elastic scattering anisotropy.

Monte Carlo algorithm for efficient simulation of time-resolved fluorescence in layered turbid media

We present an efficient Monte Carlo algorithm for simulation of time-resolved fluorescence in a layered turbid medium. It is based on the propagation of excitation and fluorescence photon bundles and the assumption of equal reduced scattering coefficients at the excitation and emission wavelengths. In addition to distributions of times of arrival of fluorescence photons at the detector, 3-D spatial generation probabilities were calculated. The algorithm was validated by comparison with the analytical solution of the diffusion equation for time-resolved fluorescence from a homogeneous semi-infinite turbid medium. It was applied to a two-layered model mimicking intra- and extracerebral compartments of the adult human head.

Monte-Carlo-based model for the extraction of intrinsic fluorescence from turbid media

A Monte-Carlo-based model of fluorescence is developed that is capable of extracting the intrinsic fluorescence properties of tissue, which are independent of the absorption and scattering properties of tissue. This model is flexible in its applicability to different illumination-collection geometries and is also valid for a wide range of optical properties, representative of tissue in the UV-visible spectrum. This is potentially useful in a variety of biomedical applications, including cancer diagnostics and monitoring the physiological response to therapy. The model is validated using phantoms composed of hemoglobin (absorber), polystyrene spheres (scatterer), and furan-2 (fluorophore). It is found that this model is able to retrieve the intrinsic fluorescence spectra of the tissue phantoms and recover the intrinsic fluorescence intensity of furan within the phantoms to within a mean error of less than 10%.

Integration of optical fiber light guide, fluorescence detection system, and multichannel disposable microfluidic chip

A combination of fluorescence detection and microfluidic technology provides promising applications in life sciences. A prototype of an integrated fluorescence detection system and optical fiber light guide on a laminate-based multichannel microfluidic chip has been developed and tested. A blue LED, plastic optical fiber, photodiode, Mylar and PMMA, and fluorescein and BSA-FITC were used as an excitation source, light coupler and guide, detector, microfluidic substrate and sample, respectively. The results show that the system is capable of detecting weak fluorescence emission from a fluorescein solution at concentration down to 0.01 ng/ml, and gives linear response. The results were also reproducible, and no cross-talk between adjacent channels was observed. The test using BSA as a model analyte demonstrates its feasibility for on-chip immunosensor applications. The performance and applications can be developed further. This prototype can be used as a platform to develop a simple and compact bio-fluorescence detection system integrated with an inexpensive and disposable multichannel microfluidic chip for biomedical devices.

Interstitial fluorescence spectroscopy in the human prostate during motexafin lutetium-mediated photodynamic therapy

The in vivo fluorescence emission from human prostates was measured before and after motexafin lutetium (MLu)-mediated photodynamic therapy (PDT). A single side-firing optical fiber was used for both the delivery of 465 nm light-emitting diode excitation light and the collection of emitted fluorescence. It was placed interstitially within the prostate via a closed transparent plastic catheter. Fitting of the collected fluorescence emission spectra using the known fluorescence spectrum of 1 mg/kg MLu in an intralipid phantom yields a quantitative measure of the local MLu concentration. We found that an additional correction factor is needed to account for the reduction of the MLu fluorescence intensity measured in vivo due to strong optical absorption in the prostate. We have adopted an empirical correction formula given by C = (3.1 cm(-1)/micro's) exp (microeff x 0.97 cm), which ranges from approximately 3 to 16, with a mean of 9.3 +/-4.8. Using a computer-controlled step motor to move the probe incrementally along parallel tracks within the prostate we can determine one-dimensional profiles of the MLu concentration. The absolute MLu concentration and the shape of its distribution are confirmed by ex vivo assay and by diffuse absorption measurements, respectively. We find significant heterogeneity in photosensitizer concentration within and among five patients. These variations occur over large enough spatial scales compared with the sampling volume of the fluorescence emission that mapping the distribution in three dimensions is possible.

A review of attenuation correction techniques for tissue fluorescence

Fluorescence intensity measurements have the potential to facilitate the diagnoses of many pathological conditions. However, accurate interpretation of the measurements is complicated by the distorting effects of tissue scattering and absorption. Consequently, different techniques have been developed to attempt to compensate for these effects. This paper reviews currently available correction techniques with emphasis on clinical application and consideration given to the intrinsic accuracy and limitations of each technique.

Non-invasive detection of fluorescence from exogenous chromophores in the adult human brain

This is the first report on results proving that fluorescence of exogenous dyes inside the human brain can be excited and detected non-invasively at the surface of the adult head. Boli of indocyanine green (ICG) were intravenously applied to healthy volunteers, and the passage of the contrast agent in the brain was monitored by detecting the corresponding fluorescence signal following pulsed laser excitation at 780 nm. Our hypothesis that the observed fluorescence signal contains a considerable cortical fraction was corroborated by performing measurements with picosecond temporal resolution and analyzing distributions of times of arrival of photons, hence taking advantage of the well-known depth selectivity of that method. Our experimental findings are explained by Monte Carlo simulations modeling the head as a layered medium and taking into account realistic bolus kinetics within the extra- and intracerebral compartment. Although a particular non-specific dye (ICG) was used, the results clearly demonstrate that fluorescence-mediated imaging of the adult human brain is generally feasible. In particular, we will discuss how these results serve as proof of concept for non-invasive fluorescence brain imaging and may thus open the door towards optical molecular imaging of the human brain.

Cross-Talk Problem on a Fluorescence Multi-Channel Microfluidic Chip System

Development of a compact fluorescence-based detection system for use in a micro-analytical system, such as a point-of-care diagnostic system, often requires a multi-channel microfluidic chip system. Since the materials used for microfluidic chips usually are transparent in the visible region and have a refractive indices higher than that of air or the surrounding environment, the fluorescence emission and scattered excitation light can propagate through the chip. We observed that such propagation can cause cross-talk between adjacent channels, and may become the major source of noise in the system and/or photo bleach the fluorescent samples in the adjacent channels, particularly for the small distances between the channels found in microfluidic chips, usually in order of several micro m. We monitored this cross-talk using fluorescein as a fluorescent sample and Mylar sheeting as a microfluidic chip material. We then discuss how this cross-talk can be avoided using a simple, inexpensive and effective method.

In vivo measurement of fluorescence emission in the human prostate during photodynamic therapy

Among the challenges to the clinical implementation of photodynamic therapy (PDT) is the delivery of a uniform photodynamic dose to induce uniform damage to the target tissue. As the photodynamic dose depends on both the local sensitizer concentration and the local fluence rate of treatment light, knowledge of both of these factors is essential to the delivery of uniform dose. In this paper, we investigate the distribution and kinetics of the photosensitizer motexafin lutetium (MLu, Lutrin®) as revealed by its fluorescence emission. Our current prostate treatment protocol involves interstitial illumination of the organ via cylindrical diffusing fibers (CDF's) inserted into the prostate though clear catheters. For planning and treatment purposes, the prostate is divided into 4 quadrants. We use one catheter in each quadrant to place an optical fiber-based fluorescence probe into the prostate. This fiber is terminated in a beveled tip, allowing it to deliver and collect light perpendicular to the fiber axis. Excitation light is provided by a 465 nm light emitting diode (LED) source coupled to a dichroic beamsplitter, which passes the collected fluorescence emission to a CCD spectrograph. Spectra are obtained before and after PDT treatment in each quadrant of the prostate and are analyzed via a linear fitting algorithm to separate the MLu fluorescence from the background fluorescence originating in the plastic catheter. A computer-controlled step motor allows the excitation/detection fiber to be moved along the catheter, building up a linear profile of the fluorescence emission spectrum of the tissue as a function of position. We have analyzed spectral fluorescence profiles obtained in 4 patients before and after MLu-mediated PDT. We find significant variation both within individual prostates and among patients. Within a single quadrant, we have observed the fluorescence signal to change by as much as a factor of 3 over a distance of 2 cm. Comparisons of pre- and post-PDT spectra allow a quantification treatment-induced photobleaching. Like the drug distribution, the extent of photobleaching varies widely among patients. In two cases, we observed bleaching of approximately 50% of the drug, while others exhibited negligible photobleaching.

Recovery of hemoglobin oxygen saturation and intrinsic fluorescence with a forward-adjoint model

We present two forward-adjoint models for recovering intrinsic fluorescence spectra and hemoglobin oxygen saturation of turbid samples. The first fits measured diffuse reflectance spectra to obtain the absorption and scattering spectra of the medium, and these are then used to correct distortions imposed on the fluorescence spectrum by absorption and scattering. The second fits only the measured fluorescence spectrum to determine simultaneously the amplitudes of absorption and fluorescence basis spectra and scattering parameters. Both methods are validated with Monte Carlo simulations and experimentally in scattering phantoms containing nicotinamide adenine dinucleotide and human erythrocytes. Preliminary measurements from murine tumors in vivo are presented.

Quantification of fluorophore concentration in vivo using two simple fluorescence-based measurement techniques

The effect of photodynamic therapy treatments depends on the concentration of photosensitizer at the treatment site; thus a simple method to quantify concentration is desirable. This study compares the concentration of a fluorophore and sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4), measured by two simple fluorescence-based techniques in vivo to post mortem chemical extraction and fluorometric assay of those tissues: skin, muscle, fascia, liver, and kidney (cortex and medulla). Fluorescence was excited and detected by a single optical fiber, or by an instrument that measured the ratio of the fluorescence and excitation reflectance. The in vivo measurements were compared to calibration measurements made in tissue-simulating phantoms to estimate the tissue concentrations. Reasonable agreement was observed between the concentration estimates of the two instruments in the lighter colored tissues (skin, muscle, and fascia). The in vivo measurements also agreed with the chemical extractions at low (< 0.6 microg/g) tissue concentrations, but underestimated higher tissue concentrations. Measurements of fluorescence lifetime in vivo demonstrated that AlPcS4 retains its mono-exponential decay in skin, muscle, and fascia tissues with a lifetime similar to that measured in aqueous tissue-simulating phantoms. In liver and kidney an additional short lifetime component was evident.

Effects of fiber-optic probe design and probe-to-target distance on diffuse reflectance measurements of turbid media: an experimental and computational study at 337 nm

Fiber-optic probes are widely used in optical spectroscopy of biological tissues and other turbid media. Only limited information exists, however, on the ways in which the illumination-collection geometry and the overall probe design influence the interrogation of media. We have investigated both experimentally and computationally the effect of probe-to-target distance (PTD) on the diffuse reflectance collected from an isotropically (Lambertian) scattering target and an agar-based tissue phantom. Studies were conducted with three probes characterized by either common (single-fiber) or separate (two bifurcated multifiber probes) illumination and collection channels. This study demonstrates that PTD, probe design, and tissue scattering anisotropy influence the extent of the transport of light into the medium, the light-collection efficiency, and the sampling volume of collected light. The findings can be applied toward optimization of fiber-optic probe designs for quantitative optical spectroscopy of turbid media including biological tissues.

Measurement of fluorophore concentrations and fluorescence quantum yield in tissue-simulating phantoms using three diffusion models of steady-state spatially resolved fluorescence

Steady-state diffusion theory models of fluorescence in tissue have been investigated for recovering fluorophore concentrations and fluorescence quantum yield. Spatially resolved fluorescence, excitation and emission reflectance Carlo simulations, and measured using a multi-fibre probe on tissue-simulating phantoms containing either aluminium phthalocyanine tetrasulfonate (AlPcS4), Photofrin meso-tetra-(4-sulfonatophenyl)-porphine dihydrochloride The accuracy of the fluorophore concentration and fluorescence quantum yield recovered by three different models of spatially resolved fluorescence were compared. The models were based on: (a) weighted difference of the excitation and emission reflectance, (b) fluorescence due to a point excitation source or (c) fluorescence due to a pencil beam excitation source. When literature values for the fluorescence quantum yield were used for each of the fluorophores, the fluorophore absorption coefficient (and hence concentration) at the excitation wavelength (mu(a,x,f)) was recovered with a root-mean-square accuracy of 11.4% using the point source model of fluorescence and 8.0% using the more complicated pencil beam excitation model. The accuracy was calculated over a broad range of optical properties and fluorophore concentrations. The weighted difference of reflectance model performed poorly, with a root-mean-square error in concentration of about 50%. Monte Carlo simulations suggest that there are some situations where the weighted difference of reflectance is as accurate as the other two models, although this was not confirmed experimentally. Estimates of the fluorescence quantum yield in multiple scattering media were also made by determining mu(a,x,f) independently from the fitted absorption spectrum and applying the various diffusion theory models. The fluorescence quantum yields for AlPcS4 and TPPS4 were calculated to be 0.59 +/- 0.03 and 0.121 +/- 0.001 respectively using the point source model, and 0.63 +/- 0.03 and 0.129 +/- 0.002 using the pencil beam excitation model. These results are consistent with published values.

Quantification of fluorophore concentration in tissue-simulating media by fluorescence measurements with a single optical fiber

Quantifying fluorescent compounds in turbid media such as tissue is made difficult by the effects of multiple scattering and absorption of the excitation and emission light. The approach that we used was to measure fluorescence using a single 200-microm optical fiber as both the illumination source and the detector. Fluorescence of aluminum phthalocyanine tetrasulfonate (AlPcS4) was measured over a wide range of fluorophore concentrations and optical properties in tissue-simulating phantoms. A root-mean-square accuracy of 10.6% in AlPcS4 concentration was attainable when fluorescence was measured either interstitially or at the phantom surface. The individual effects of scattering, absorption, and the scattering phase function on the fluorescence signal were also studied by experiments and Monte Carlo simulations.