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

AAPM Task Group Report 311: Guidance for performance evaluation of fluorescence-guided surgery systems

The last decade has seen a large growth in fluorescence-guided surgery (FGS) imaging and interventions. With the increasing number of clinical specialties implementing FGS, the range of systems with radically different physical designs, image processing approaches, and performance requirements is expanding. This variety of systems makes it nearly impossible to specify uniform performance goals, yet at the same time, utilization of different devices in new clinical procedures and trials indicates some need for common knowledge bases and a quality assessment paradigm to ensure that effective translation and use occurs. It is feasible to identify key fundamental image quality characteristics and corresponding objective test methods that should be determined such that there are consistent conventions across a variety of FGS devices. This report outlines test methods, tissue simulating phantoms and suggested guidelines, as well as personnel needs and professional knowledge bases that can be established. This report frames the issues with guidance and feedback from related societies and agencies having vested interest in the outcome, coming from an independent scientific group formed from academics and international federal agencies for the establishment of these professional guidelines.

A New Optical Method for Online Monitoring of the Light Dose and Dose Profile in Photodynamic Therapy

BACKGROUND AND OBJECTIVES

Photodynamic therapy (PDT) has gained widespread popularity in the last decades because of its distinctive advantages over the other commonly used cancer treatments. PDT dosimetry is a crucial factor in achieving a good optimization of PDT treatment planning. PDT dosimetry is a complex task since light dose as well as photosensitizer and oxygen concentrations in tissue need to be measured (ideally continuously) to be able to fully characterize the biological response. Light dose in PDT is routinely measured by the optical fibers that provide dose data at a limited number of discrete points and are not able to capture spatial dose profiles. The objective of this study is to propose and develop a new optical method for online monitoring of the dose profile data for PDT.

STUDY DESIGN/MATERIALS AND METHODS

Using the digital holography technique, first, the general sketch of an experimental setup for PDT light dosimetry is provided. The theory behind the proposed method for using the experimental setup in PDT light dosimetry is fully described, and its limits of validity are determined. In a proof of principle study, the ability of the method for online monitoring of the absorbed light dose profile in PDT is evaluated by a simple experimental setup.

RESULTS

The experimental results confirm the usefulness of the proposed method in providing continuous online dose profiles. The absorbed light dose profiles from an infrared light source in a quartz cell containing water are measured and shown. The depth-dose curves are extracted and it is shown that how these dosimetric data can be used for assisting the physicians in determining the appropriate spatiotemporal characteristics for treating the infected tissues and solid tumors with the required light dose amounts. A conversion relation is also derived for transforming the measured light dose with the proposed method to the most frequently used dose values by PDT practitioners, in terms of light power per square area.

CONCLUSIONS

There is no restriction in using the method with other commonly used light sources in PDT, like light-emitting diodes and filtered lamps, with different wavelengths in visible or infrared regions of the spectrum. More complex experimental setups can be used in future studies to study the role of accumulated photosensitizers in malignant tissues. The proposed method in this study can also be used for light dose monitoring in other biomedical applications, where light is used for treating special diseases, and patients must receive sufficient amounts of light dose. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

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.

Correcting for targeted and control agent signal differences in paired-agent molecular imaging of cancer cell-surface receptors

Paired-agent kinetic modeling protocols provide one means of estimating cancer cell-surface receptors with in vivo molecular imaging. The protocols employ the coadministration of a control imaging agent with one or more targeted imaging agent to account for the nonspecific uptake and retention of the targeted agent. These methods require the targeted and control agent data be converted to equivalent units of concentration, typically requiring specialized equipment and calibration, and/or complex algorithms that raise the barrier to adoption. This work evaluates a kinetic model capable of correcting for targeted and control agent signal differences. This approach was compared with an existing simplified paired-agent model (SPAM), and modified SPAM that accounts for signal differences by early time point normalization of targeted and control signals (SPAMPN). The scaling factor model (SPAMSF) outperformed both SPAM and SPAMPN in terms of accuracy and precision when the scale differences between targeted and imaging agent signals (α) were not equal to 1, and it matched the performance of SPAM for α  =  1. This model could have wide-reaching implications for quantitative cancer receptor imaging using any imaging modalities, or combinations of imaging modalities, capable of concurrent detection of at least two distinct imaging agents (e.g., SPECT, optical, and PET/MR).

PDT dose dosimetry for Photofrin-mediated pleural photodynamic therapy (pPDT)

Photosensitizer fluorescence excited by photodynamic therapy (PDT) treatment light can be used to monitor the in vivo concentration of the photosensitizer and its photobleaching. The temporal integral of the product of in vivo photosensitizer concentration and light fluence is called PDT dose, which is an important dosimetry quantity for PDT. However, the detected photosensitizer fluorescence may be distorted by variations in the absorption and scattering of both excitation and fluorescence light in tissue. Therefore, correction of the measured fluorescence for distortion due to variable optical properties is required for absolute quantification of photosensitizer concentration. In this study, we have developed a four-channel PDT dose dosimetry system to simultaneously acquire light dosimetry and photosensitizer fluorescence data. We measured PDT dose at four sites in the pleural cavity during pleural PDT. We have determined an empirical optical property correction function using Monte Carlo simulations of fluorescence for a range of physiologically relevant tissue optical properties. Parameters of the optical property correction function for Photofrin fluorescence were determined experimentally using tissue-simulating phantoms. In vivo measurements of photosensitizer fluorescence showed negligible photobleaching of Photofrin during the PDT treatment, but large intra- and inter-patient heterogeneities of in vivo Photofrin concentration are observed. PDT doses delivered to 22 sites in the pleural cavity of 8 patients were different by 2.9 times intra-patient and 8.3 times inter-patient.

Fluorescence Imaging in Vivo: Raster Scanned Point-Source Imaging Provides More Accurate Quantification than Broad Beam Geometries

Two fluorescence imaging systems were compared for their ability to quantify mean fluorescence intensity from surface-weighted imaging of tissue. A broad beam CCD camera system was compared to a point sampling system that raster scans to create the image. The effects of absorption and scattering in the background tissue volume were shown to be similar in their effect upon the signal, but the effect of the three-dimensional shape of the tissue was shown to be a significant distortion upon the signal. Spherical phantoms with Intralipid and blood for absorber and scatterer were used with a fixed concentration of aluminum phthalocyanine fluorophore to illustrate that the mean intensity observed with the broad beam system increased with size, while the mean intensity observed with the raster scanned system was not as significantly affected. Similar results were observed in vivo with mice injected with the fluorophore and imaged multiple times to observe the pharmacokinetics of the drug. The fluorescence in the tumor observed with the broad beam system was higher than that observed with the raster scanned system. Based upon the phantom and animal observations in this study, it should be concluded that using broad beam fluorescence imaging systems to quantify fluorescence in vivo may be problematic when comparing tissues with different three dimensional characteristics. In particular, the ratio of fluorescence from tumor to normal tissue can yield inaccurate results when the tumor is large. However, similar measurements with a narrow beam system that is raster scanned to create the images are not as significantly affected by the three dimensional shape of the tissue. Raster scanned imaging appears to provide a more uniform and accurate way to quantify fluorescence signals from distributed tissues in vivo.

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.

Correction for tissue optical properties enables quantitative skin fluorescence measurements using multi-diameter single fiber reflectance spectroscopy

BACKGROUND AND OBJECTIVE

Fluorescence measurements in the skin are very much affected by absorption and scattering but existing methods to correct for this are not applicable to superficial skin measurements.

STUDY DESIGN/MATERIALS AND METHODS

The first use of multiple-diameter single fiber reflectance (MDSFR) and single fiber fluorescence (SFF) spectroscopy in human skin was investigated. MDSFR spectroscopy allows a quantification of the full optical properties in superficial skin (μa, μs' and γ), which can next be used to retrieve the corrected - intrinsic - fluorescence of a fluorophore Qμa,x(f). Our goal was to investigate the importance of such correction for individual patients. We studied this in 22 patients undergoing photodynamic therapy (PDT) for actinic keratosis.

RESULTS

The magnitude of correction of fluorescence was around 4 (for both autofluorescence and protoporphyrin IX). Moreover, it was variable between patients, but also within patients over the course of fractionated aminolevulinic acid PDT (range 2.7-7.5). Patients also varied in the amount of protoporphyrin IX synthesis, photobleaching percentages and resynthesis (>100× difference between the lowest and highest PpIX synthesis). The autofluorescence was lower in actinic keratosis than contralateral normal skin (0.0032 versus 0.0052; P<0.0005).

CONCLUSIONS

Our results clearly demonstrate the importance of correcting the measured fluorescence for optical properties, because these vary considerably between individual patients and also during PDT. Protoporphyrin IX synthesis and photobleaching kinetics allow monitoring clinical PDT which facilitates individual-based PDT dosing and improvement of clinical treatment protocols. Furthermore, the skin autofluorescence can be relevant for diagnostic use in the skin, but it may also be interesting because of its association with several internal diseases.

Nano-enabled SERS reporting photosensitizers

To impart effective cellular damage via photodynamic therapy (PDT), it is vital to deliver the appropriate light dose and photosensitizer concentration, and to monitor the PDT dose delivered at the site of interest. In vivo monitoring of photosensitizers has in large part relied on their fluorescence emission. Palladium-containing photosensitizers have shown promising clinical results by demonstrating near full conversion of light to PDT activity at the cost of having undetectable fluorescence. We demonstrate that, through the coupling of plasmonic nanoparticles with palladium-photosensitizers, surface-enhanced Raman scattering (SERS) provides both reporting and monitoring capability to otherwise quiescent molecules. Nano-enabled SERS reporting of photosensitizers allows for the decoupling of the therapeutic and imaging mechanisms so that both phenomena can be optimized independently. Most importantly, the design enables the use of the same laser wavelength to stimulate both the PDT and imaging features, opening the potential for real-time dosimetry of photosensitizer concentration and PDT dose delivery by SERS monitoring.

Photosensitizers in clinical PDT

Photosensitizers in photodynamic therapy allow for the transfer and translation of light energy into a type II chemical reaction. In clinical practice, photosensitizers arise from three families-porphyrins, chlorophylls, and dyes. All clinically successful photosensitizers have the ability to a greater or lesser degree, to target specific tissues or their vasculature to achieve ablation. Each photosensitizer needs to reliably activate at a high enough light wavelength useful for therapy. Their ability to fluoresce and visualize the lesion is a bonus. Photosensitizers developed from each family have unique properties that have so far been minimally clinically exploited. This review looks at the potential benefits and consequences of each major photosensitizer that has been tried in a clinical setting.

Extraction of intrinsic fluorescence from single fiber fluorescence measurements on a turbid medium: experimental validation

The detailed mechanisms associated with the influence of scattering and absorption properties on the fluorescence intensity sampled by a single optical fiber have recently been elucidated based on Monte Carlo simulated data. Here we develop an experimental single fiber fluorescence (SFF) spectroscopy setup and validate the Monte Carlo data and semi-empirical model equation that describes the SFF signal as a function of scattering. We present a calibration procedure that corrects the SFF signal for all system-related, wavelength dependent transmission efficiencies to yield an absolute value of intrinsic fluorescence. The validity of the Monte Carlo data and semi-empirical model is demonstrated using a set of fluorescent phantoms with varying concentrations of Intralipid to vary the scattering properties, yielding a wide range of reduced scattering coefficients (μ's = 0-7 mm (-1)). We also introduce a small modification to the model to account for the case of μ's = 0 mm (-1) and show its relation to the experimental, simulated and theoretically calculated value of SFF intensity in the absence of scattering. Finally, we show that our method is also accurate in the presence of absorbers by performing measurements on phantoms containing red blood cells and correcting for their absorption properties.

Intravascular near-infrared fluorescence catheter with ultrasound guidance and blood attenuation correction

Intravascular near-infrared fluorescence (NIRF) imaging offers a new approach for characterizing atherosclerotic plaque, but random catheter positioning within the vessel lumen results in variable light attenuation and can yield inaccurate measurements. We hypothesized that NIRF measurements could be corrected for variable light attenuation through blood by tracking the location of the NIRF catheter with intravascular ultrasound (IVUS). In this study, a combined NIRF-IVUS catheter was designed to acquire coregistered NIRF and IVUS data, an automated image processing algorithm was developed to measure catheter-to-vessel wall distances, and depth-dependent attenuation of the fluorescent signal was corrected by an analytical light propagation model. Performance of the catheter sensing distance correction method was evaluated in coronary artery phantoms and ex vivo arteries. The correction method produced NIRF estimates of fluorophore concentrations, in coronary artery phantoms, with an average root mean square error of 17.5%. In addition, the correction method resulted in a statistically significant improvement in correlation between spatially resolved NIRF measurements and known fluorophore spatial distributions in ex vivo arteries (from r = 0.24 to 0.69, p < 0.01, n = 6). This work demonstrates that catheter-to-vessel wall distances, measured from IVUS images, can be employed to compensate for inaccuracies caused by variable intravascular NIRF sensing distances.

Photodynamic therapy for cholangiocarcinoma using low dose mTHPC (Foscan(®))

BACKGROUND

Photodynamic therapy (PDT) combined with stenting is an effective treatment modality for palliation of nonresectable cholangiocarcinoma (CC). A drawback of standard PDT using Photofrin(®) as photosensitizer is the long lasting skin photosensitivity of up to 3 months. The aim of this study was to show the outcome of PDT of CC, potential side effects and to determine the best drug light interval (DLI) using mTHPC (Foscan(®)) at a low dose.

METHODS

13 patients with nonresectable CC were treated with stenting and PDT (3mg Foscan(®) per treatment, 0.032-0.063 mg/kg body weight, 652 nm, 50 J/cm). Fluorescence measurements were performed with a single bare fiber for 5/13 patients prior to PDT at the tumor site to determine the fluorescence contrast. For another 7/13 patients, long-term fluorescence-kinetics were measured on the oral mucosa to determine the time of maximal relative fluorescence intensity.

RESULTS

The results so far indicate a median survival time of 13 months. Side effects such as perforations or skin phototoxicity could not be observed. Foscan(®) fluorescence within the tumor site was clearly detectable but a significant fluorescence contrast of tumor to adjacent healthy tissue could not be found. The fluorescence kinetics measured in the oral mucosa showed a maximum at 3.85 days (median) after drug administration.

CONCLUSION

Combined stenting and PDT performed with a low Foscan(®) dose results in equal and potentially longer survival times compared to standard Photofrin(®) PDT, while lowering the risk of side effects strongly. Thus it may improve the quality of life.

Monitoring photosensitizer uptake using two photon fluorescence lifetime imaging microscopy

Photodynamic Therapy (PDT) provides an opportunity for treatment of various invasive tumors by the use of a cancer targeting photosensitizing agent and light of specific wavelengths. However, real-time monitoring of drug localization is desirable because the induction of the phototoxic effect relies on interplay between the dosage of localized drug and light. Fluorescence emission in PDT may be used to monitor the uptake process but fluorescence intensity is subject to variability due to scattering and absorption; the addition of fluorescence lifetime may be beneficial to probe site-specific drug-molecular interactions and cell damage. We investigated the fluorescence lifetime changes of Photofrin(®) at various intracellular components in the Mat-LyLu (MLL) cell line. The fluorescence decays were analyzed using a bi-exponential model, followed by segmentation analysis of lifetime parameters. When Photofrin(®) was localized at the cell membrane, the slow lifetime component was found to be significantly shorter (4.3 ± 0.5 ns) compared to those at other locations (cytoplasm: 7.3 ± 0.3 ns; mitochondria: 7.0 ± 0.2 ns, p < 0.05).

Extraction of intrinsic fluorescence from single fiber fluorescence measurements on a turbid medium

This study utilizes Monte Carlo simulations of single fiber fluorescence to develop an empirical model that corrects for the influence of scattering and absorption on fluorescence intensity (F(SF)). The model expresses F(SF) in terms of the reduced scattering coefficient (μs') and absorption coefficient (μ(a)), each determined independently at excitation and emission wavelengths (λ(x) and λ(m)), and the fiber diameter (d(f)). This model returns accurate descriptions (mean residual <6%) of F(SF) across a biologically relevant range of μs' and μ(a) values and is insensitive to the form of the scattering phase function.

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).

Depth profiling of photothermal compound concentrations using phase sensitive optical coherence tomography

A model that describes the concentration of photothermal (light-to-heat converters) compounds as a function of depth in a turbid medium is developed. The system consists of a pump laser (808 nm modulated at 400 Hz), which heats a photothermal compound, and a phase sensitive spectral domain optical coherence tomography system, which detects the changes in the optical path length of the sample induced by the temperature increase. The model is theoretically derived and the coefficients are empirically determined using solid homogeneous gel phantoms. The model is validated by reconstructing the concentration of a photothermal compound in thick single and double layer solid phantoms.

Estimating the density of fluorescent nanoparticles in the primo vessels in the fourth ventricle and the spinal cord of a rat

The primo vascular system is a novel circulatory system forming a network throughout an animal's body. Primo vessels were recently observed in the fourth ventricle of the brain and in the spinal cord of a rat by using fluorescent nanoparticles. In order to quantify the nanoparticles in the primo vessels, we measured the florescence of the nanoparticles and calibrated the measurements by using a reference suspension. We removed the noise due to autofluorescence with the technique of multispectral imaging. The line densities of nanoparticles and the contrast values of their images were, respectively, 0.5 ± 0.5 ng/mm and 0.7 ± 0.5 for primo vessels in the fourth ventricle, and 1.3 ± 0.6 ng/mm and 1.4 ± 0.2 for primo vessels in the spinal cord. The data obtained from and the procedures used in this work could be useful in evaluating the feasibility of using nanoparticles as a contrast agent during MRI or CT imaging of primo vessels in the brain or the spinal cord.

Measurement of the reduced scattering coefficient of turbid media using single fiber reflectance spectroscopy: fiber diameter and phase function dependence

This paper presents a relationship between the intensity collected by a single fiber reflectance device (R(SF)) and the fiber diameter (d(fib)) and the reduced scattering coefficient ( μs') and phase function (p(θ)) of a turbid medium. Monte Carlo simulations are used to identify and model a relationship between R(SF) and dimensionless scattering ( μs'dfib). For μs'dfib > 10 we find that R(SF) is insensitive to p(θ). A solid optical phantom is constructed with μs' ≈ 220 mm-1 and is used to convert R(SF) of any turbid medium to an absolute scale. This calibrated technique provides accurate estimates of μs' over a wide range ([0.05 - 8] mm(-1)) for a range of d(fib) ([0.2 - 1] mm).

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.

The use of magnetic field effects on photosensitizer luminescence as a novel probe for optical monitoring of oxygen in photodynamic therapy

The effect of a magnetic field on the steady-state and time-resolved optical emission of a custom fullerene-linked photosensitizer (PS) in liposome cell phantoms was studied at various oxygen concentrations (0.19-190 microM). Zeeman splitting of the triplet state and hyperfine coupling, which control intersystem crossing between singlet and triplet states, are altered in the presence of low magnetic fields (B < 320 mT), perturbing the luminescence intensity and lifetime as compared to the triplet state at B = 0. Measurements of the luminescence intensity and lifetime were performed using a time-domain apparatus integrated with a magnet. We propose that by probing magnet-affected optical emissions, one can monitor the state of oxygenation throughout the course of photodynamic therapy. Since the magnetic field effect (MFE) operates primarily by affecting the radical ion pairs related to type I photodynamic action, the enhancement or suppression of the MFE can be used as a measure of the dynamic equilibrium between the type I and II photodynamic pathways. The unique photo-initiated charge-transfer properties of the PS used in this study allow it to serve as both cytotoxic agent and oxygen probe that can provide in situ dosimetric information at close to real time.

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.

The physics, biophysics and technology of photodynamic therapy

Photodynamic therapy (PDT) uses light-activated drugs to treat diseases ranging from cancer to age-related macular degeneration and antibiotic-resistant infections. This paper reviews the current status of PDT with an emphasis on the contributions of physics, biophysics and technology, and the challenges remaining in the optimization and adoption of this treatment modality. A theme of the review is the complexity of PDT dosimetry due to the dynamic nature of the three essential components -- light, photosensitizer and oxygen. Considerable progress has been made in understanding the problem and in developing instruments to measure all three, so that optimization of individual PDT treatments is becoming a feasible target. The final section of the review introduces some new frontiers of research including low dose rate (metronomic) PDT, two-photon PDT, activatable PDT molecular beacons and nanoparticle-based PDT.

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%.

Monte Carlo simulation of time-dependent, transport-limited fluorescent boundary measurements in frequency domain

Recently, we have presented and experimentally validated a unique numerical solver of the coupled radiative transfer equations (RTEs) for rapidly computing time-dependent excitation and fluorescent light propagation in small animal tomography. Herein, we present a time-dependent Monte Carlo algorithm to validate the forward RTE solver and investigate the impact of physical parameters upon transport-limited measurements in order to best direct the development of the RTE solver for optical tomography. Experimentally, the Monte Carlo simulations for both transport-limited and diffusion-limited propagations are validated using frequency domain photon migration measurements for 1.0%, 0.5%, and 0.2% intralipid solutions containing 1 microM indocyanine green in a 49 cm3 cylindrical phantom corresponding to the small volume employed in small animal tomography. The comparisons between Monte Carlo simulations and the numerical solutions result in mean percent error in amplitude and the phase shift less than 5.0% and 0.7 degrees, respectively, at excitation and emission wavelengths for varying anisotropic factors, lifetimes, and modulation frequencies. Monte Carlo simulations indicate that the accuracy of the forward model is enhanced using (i) suitable source models of photon delivery, (ii) accurate anisotropic factors, and (iii) accurate acceptance angles of collected photons. Monte Carlo simulations also show that the accuracy of the diffusion approximation in the small phantom depends upon (i) the ratio d(phantom)/l(tr), where d(phantom) is the phantom diameter and l(tr) is the transport mean free path; and (ii) the anisotropic factor of the medium. The Monte Carlo simulations validates and guides the future development of an appropriate RTE solver for deployment in small animal optical tomography.

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.

Prostate PDT dosimetry

We provide a review of the current state of dosimetry in prostate photodynamic therapy (PDT). PDT of the human prostate has been performed with a number of different photosensitizers and with a variety of dosimetry schemes. The simplest clinical light dose prescription is to quantify the total light energy emitted per length (J/cm) of cylindrical diffusing fibers (CDF) for patients treated with a defined photosensitizer injection per body weight. However, this approach does not take into account the light scattering by tissue and usually underestimates the local light fluence rate, and consequently the fluence. Techniques have been developed to characterize tissue optical properties and light fluence rates in vivo using interstitial measurements during prostate PDT. Optical methods have been developed to characterize tissue absorption and scattering spectra, which in turn provide information about tissue oxygenation and drug concentration. Fluorescence techniques can be used to quantify drug concentrations and photobleaching rates of photosensitizers.

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.

Application of the modified spherical harmonics method to some problems in biomedical optics

It is demonstrated that a semi-analytical method, modified spherical harmonics (MSH), is a fast and rigorous solution for the radiative transport equation in an infinite medium containing an isotropic source. The optical properties of the medium can be determined using an inverse scheme based on fluence data (including phase if a modulated source is used) without the drawbacks inherent in the diffusion approximation. The MSH method can also be used to solve for the fluorescence fluence due to a point excitation source in an infinite medium.

ART AND SCIENCE OF PHOTODYNAMIC THERAPY

1. Photodynamic therapy is an established modality for the treatment of solid tumours and other accessible lesions. Although the concept and practice of combining light with a photosensitizing agent for the treatment of disease states has been around for almost a century, the understanding of the art and science therein has been tremendously enhanced over the past few years. 2. Photosensitized reactions are dependent on the generation of reactive oxygen species, in particular singlet oxygen, which accounts for the damaging effects on biological macromolecules, such as membrane lipids and proteins. Therefore, compounds that give a good yield of (1)O(2) are used as photosensitizers. 3. The main photosensitizers used in the clinical setting belong to the photofrin family; however, newer and more effective sensitizers are being evaluated for their potential clinical effectiveness. 4. Light sources have moved from the use of white light with specific filters in the old days to the more recent use of monochromatic light sources, such as lasers, to more sophisticated light-emitting diodes. However, dosimetry remains a big issue mainly because of difficulties in establishing the optimum treatment conditions for an approach that requires the fine-tuning of several variables, such as sensitizer and light doses and drug-to-light interval, as well as the issues of skin photosensitivity and low selectivity. A newer development to circumvent these and provide a broader application for this concept has been the phenomenon of photo-activation, whereby photo-exposure of chromophores to generate novel, small biologically active compounds has been demonstrated successfully. 5. The aim of the present review was to provide a general overview of the art and science of photodynamic therapy and to highlight some of the issues and recent developments in further advancing this modality of treatment.

Depth-sensitive reflectance measurements using obliquely oriented fiber probes

Computer simulation is used to facilitate the design of fiber-probe geometries that enable enhanced detection of optical signals arising from specific tissue depths. Obtaining understanding of the relationship between fiber-probe design and tissue interrogation is critical when developing strategies for optical detection of epithelial precancers that originate at known depths from the tissue surface. The accuracy of spectroscopic diagnostics may be enhanced by discretely probing the optical properties of epithelium and underlying stroma, within which the morphological and biochemical features vary as a function of depth. While previous studies have investigated controlling tissue-probing depth for fluorescence-based modalities, in this study we focus on the detection of reflected light scattered by tissue. We investigate how the depth of optical interrogation may be controlled through combinations of collection angles, source-detector separations, and numerical apertures. We find that increasing the obliquity of collection fibers at a given source-detector separation can effectively enhance the detection of superficially scattered signals. Fiber numerical aperture provides additional depth selectivity; however, the perturbations in sampling depth achieved through this means are modest relative to the changes generated by modifying the angle of collection and source-detection separation.

Determination of the distribution of light, optical properties, drug concentration, and tissue oxygenation in-vivo in human prostate during motexafin lutetium-mediated photodynamic therapy

It is desirable to quantify the distribution of the light fluence rate, the optical properties, the drug concentration, and the tissue oxygenation for photodynamic therapy (PDT) of prostate cancer. We have developed an integrated system to determine these quantities before and after PDT treatment using motorized probes. The optical properties (absorption (micro(a)), transport scattering (micro(s'), and effective attenuation (micro(eff)) coefficients) of cancerous human prostate were measured in-vivo using interstitial isotropic detectors. Measurements were made at 732 nm before and after motexafin lutetium (MLu) mediated PDT at different locations along each catheter. The light fluence rate distribution was also measured along the catheters during PDT. Diffuse absorption spectroscopy measurement using a white light source allows extrapolation of the distribution of oxygen saturation StO2, total blood volume ([Hb]t), and MLu concentration. The distribution of drug concentration was also studied using fluorescence from a single optical fiber, and was found to be in good agreement with the values determined by absorption spectroscopy. This study shows significant inter- and intra-prostatic variations in the tissue optical properties and MLu drug distribution, suggesting that a real-time dosimetry measurement and feedback system for monitoring these values during treatment should be considered in future PDT studies.

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.

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.

Characterization of single and double fiber-coupled diffusing spheres

A fiber-optic sensor, consisting of an optical fiber with and without a 1.59-mm-diameter spherical ceramic tip, inserted into a 19-mm-diameter Spectralon sphere has been characterized. This sensor is evaluated as a large-area omnidirectional sensor. An optical transport measurement system that rotates the sphere about two axes has been designed. The system measured the UV transport efficiency at 351 nm of light impinging on the sphere with an f-6 cone angle. When a bare fiber was placed at the center of the target sphere, the detection sensitivity was biased in the forward direction. The peak of the sensitivity of the inverse integrating sphere shifted from front to back as the bare fiber was withdrawn from the sphere and the numerical aperture of the fiber viewed more of the scattering volume. The response function with respect to the angle of incidence of the dual sphere was much more uniform than that obtained with a bare fiber in the center of the sphere.

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.