Analytical method for localizing a fluorescent inclusion in a turbid medium

Macroscopic-imaging technique for subsurface quantification of near-infrared markers during surgery

Obtaining accurate quantitative information on the concentration and distribution of fluorescent markers lying at a depth below the surface of optically turbid media, such as tissue, is a significant challenge. Here, we introduce a fluorescence reconstruction technique based on a diffusion light transport model that can be used during surgery, including guiding resection of brain tumors, for depth-resolved quantitative imaging of near-infrared fluorescent markers. Hyperspectral fluorescence images are used to compute a topographic map of the fluorophore distribution, which yields structural and optical constraints for a three-dimensional subsequent hyperspectral diffuse fluorescence reconstruction algorithm. Using the model fluorophore Alexa Fluor 647 and brain-like tissue phantoms, the technique yielded estimates of fluorophore concentration within ±25% of the true value to depths of 5 to 9 mm, depending on the concentration. The approach is practical for integration into a neurosurgical fluorescence microscope and has potential to further extend fluorescence-guided resection using objective and quantified metrics of the presence of residual tumor tissue.

Macroscopic optical imaging technique for wide-field estimation of fluorescence depth in optically turbid media for application in brain tumor surgical guidance

A diffuse imaging method is presented that enables wide-field estimation of the depth of fluorescent molecular markers in turbid media by quantifying the deformation of the detected fluorescence spectra due to the wavelength-dependent light attenuation by overlying tissue. This is achieved by measuring the ratio of the fluorescence at two wavelengths in combination with normalization techniques based on diffuse reflectance measurements to evaluate tissue attenuation variations for different depths. It is demonstrated that fluorescence topography can be achieved up to a 5 mm depth using a near-infrared dye with millimeter depth accuracy in turbid media having optical properties representative of normal brain tissue. Wide-field depth estimates are made using optical technology integrated onto a commercial surgical microscope, making this approach feasible for real-world applications.

Time-domain geometrical localization of point-like fluorescence inclusions in turbid media with early photon arrival times

We introduce a novel approach for localizing a plurality of discrete point-like fluorescent inclusions embedded in a thick turbid medium using time-domain measurements. The approach uses early photon information contained in measured time-of-flight distributions originating from fluorescence emission. Fluorescence time point-spread functions (FTPSFs) are acquired with ultrafast time-correlated single photon counting after short pulse laser excitation. Early photon arrival times are extracted from the FTPSFs obtained from several source-detector positions. Each source-detector measurement allows defining a geometrical locus where an inclusion is to be found. These loci take the form of ovals in 2D or ovoids in 3D. From these loci a map can be built, with the maxima thereof corresponding to positions of inclusions. This geometrical approach is supported by Monte Carlo simulations performed for biological tissue-like media with embedded fluorescent inclusions. To validate the approach, several experiments are conducted with a homogeneous phantom mimicking tissue optical properties. In the experiments, inclusions filled with indocyanine green are embedded in the phantom and the fluorescence response to a short pulse of excitation laser is recorded. With our approach, several inclusions can be localized with low millimeter positional error. Our results support the approach as an accurate, efficient, and fast method for localizing fluorescent inclusions embedded in highly turbid media mimicking biological tissues. Further Monte Carlo simulations on a realistic mouse model show the feasibility of the technique for small animal imaging.

Imaging a photodynamic therapy photosensitizer in vivo with a time-gated fluorescence tomography system

We report the tomographic imaging of a photodynamic therapy (PDT) photosensitizer, 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a (HPPH) in vivo with time-domain fluorescence diffuse optical tomography (TD-FDOT). Simultaneous reconstruction of fluorescence yield and lifetime of HPPH was performed before and after PDT. The methodology was validated in phantom experiments, and depth-resolved in vivo imaging was achieved through simultaneous three-dimensional (3-D) mappings of fluorescence yield and lifetime contrasts. The tomographic images of a human head-and-neck xenograft in a mouse confirmed the preferential uptake and retention of HPPH by the tumor 24-h post-injection. HPPH-mediated PDT induced significant changes in fluorescence yield and lifetime. This pilot study demonstrates that TD-FDOT may be a good imaging modality for assessing photosensitizer distributions in deep tissue during PDT monitoring.

Analytic expression of fluorescence ratio detection correlates with depth in multi-spectral sub-surface imaging

Here we derived analytical solutions to diffuse light transport in biological tissue based on spectral deformation of diffused near-infrared measurements. These solutions provide a closed-form mathematical expression which predicts that the depth of a fluorescent molecule distribution is linearly related to the logarithm of the ratio of fluorescence at two different wavelengths. The slope and intercept values of the equation depend on the intrinsic values of absorption and reduced scattering of tissue. This linear behavior occurs if the following two conditions are satisfied: the depth is beyond a few millimeters and the tissue is relatively homogeneous. We present experimental measurements acquired with a broad-beam non-contact multi-spectral fluorescence imaging system using a hemoglobin-containing diffusive phantom. Preliminary results confirm that a significant correlation exists between the predicted depth of a distribution of protoporphyrin IX molecules and the measured ratio of fluorescence at two different wavelengths. These results suggest that depth assessment of fluorescence contrast can be achieved in fluorescence-guided surgery to allow improved intra-operative delineation of tumor margins.

Cramer-Rao analysis of steady-state and time-domain fluorescence diffuse optical imaging

Using a Cramer-Rao analysis, we study the theoretical performances of a time and spatially resolved fDOT imaging system for jointly estimating the position and the concentration of a point-wide fluorescent volume in a diffusive sample. We show that the fluorescence lifetime is a critical parameter for the precision of the technique. A time resolved fDOT system that does not use spatial information is also considered. In certain cases, a simple steady-state configuration may be as efficient as this time resolved fDOT system.

Fluorescence time-resolved imaging system embedded in an ultrasound prostate probe

Ultrasound imaging (US) of the prostate has a low specificity to distinguish tumors from the surrounding tissues. This limitation leads to systematic biopsies. Fluorescent diffuse optical imaging may represent an innovative approach to guide biopsies to tumors marked with high specificity contrast agents and therefore enable an early detection of prostate cancer. This article describes a time-resolved optical system embedded in a transrectal US probe, as well as the fluorescence reconstruction method and its performance. Optical measurements were performed using a pulsed laser, optical fibers and a time-resolved detection system. A novel fast reconstruction method was derived and used to locate a 45 µL ICG fluorescent inclusion at a concentration of 10 µM, in a liquid prostate phantom. Very high location accuracy (0.15 cm) was achieved after reconstruction, for different positions of the inclusion, in the three directions of space. The repeatability, tested with ten sequential measurements, was of the same order of magnitude. Influence of the input parameters (optical properties and lifetime) is presented. These results confirm the feasibility of using optical imaging for prostate guided biopsies.

Analytical method for the fast time-domain reconstruction of fluorescent inclusions in vitro and in vivo

A novel time-domain optical method to reconstruct the relative concentration, lifetime, and depth of a fluorescent inclusion is described. We establish an analytical method for the estimations of these parameters for a localized fluorescent object directly from the simple evaluations of continuous wave intensity, exponential decay, and temporal position of the maximum of the fluorescence temporal point-spread function. Since the more complex full inversion process is not involved, this method permits a robust and fast processing in exploring the properties of a fluorescent inclusion. This method is confirmed by in vitro and in vivo experiments.

Real time optical method for localization of inclusions embedded in turbid media

A simple and fast time-domain method for localizing inclusions, fluorescent optical probes or absorbers, is presented. The method offers new possibilities for situations where complete tomographic measurements are not permitted by the examined object, for example in endoscopic examination of the human prostate or the oesophagus. Feasibility has been envisioned with a phantom study conducted on a point-like fluorochrome embedded in a diffusing medium mimicking the optical properties of biological tissues.

Optical calibration protocol for an x-ray and optical multimodality tomography system dedicated to small-animal examination

A small-animal multimodality tomography system dedicated to the coregistration of fluorescence optical signal and x-ray measurements has been developed in our laboratory. The purpose of such a system is to offer the possibility of getting in vivo anatomical and functional information simultaneously. Moreover, anatomical measurements can be used as a regularization factor to achieve more accurate reconstructions of the biodistribution of fluorochromes and to speed up treatment. A dedicated acquisition protocol has been established, and the methodology of the reconstruction of the three-dimensional distribution of the biomarkers under cylindrical geometry consistent with classic computed tomography has been implemented. A phantom study was conducted to evaluate and to fix the parameters for the coregistration. These test experiments were reproduced by considering anesthetized mice that had thin glass tubes containing fluorochromes inserted into their esophagus. The instrument is also used for an in vivo biological study conducted on mice with lung tumors, tagged with near-infrared optical probes (targeting probes such as Transferin-AlexaFluor750).

Estimating the depth and lifetime of a fluorescent inclusion in a turbid medium using a simple time-domain optical method

A simple time-domain optical method for estimating the depth (d) and lifetime (tau) of fluorescent inclusions in a turbid medium is described. We demonstrate the method for depth and lifetime estimation of a fluorescent inclusion directly by fitting a monoexponential decay (tau(eff)) of the temporal position of the temporal point-spread function and the measurement of its maximum temporal position (t(max)). Since both of these measurements are intensity independent, this method provides a robust and efficient approach. This method is validated with experimental data.

Use of scaling relations to extract intrinsic fluorescence lifetime of targets embedded in turbid media

We present a novel method for estimating the intrinsic fluorescence lifetime of deeply embedded localized fluorophores. It is based on scaling relations, characteristic for turbid media. The approach is experimentally substantiated by successfully reconstructing lifetimes for targets at depths up to 14.5 mm. A derived correction factor was determined from the product of the transport-corrected scattering coefficient mu(s) (') and the index of refraction n(r). In addition, data from an array of detectors (> or =2) can be used to estimate mu(s) (')n(r). The suggested algorithm is a promising tool for diagnostic fluorescence, since lifetime can be a sensitive indicator of the fluorophore environment.

A simulation-based study of reconstruction in time-resolved fluorescence diffuse optical tomography in cylindrical geometry

The present paper is devoted to fluorescence diffuse optical tomography by using time resolved signals for the reconstruction of the 3D biodistribution of fluorescent probes embedded in biological tissues. The focus is made on the reconstruction of a methodology for an efficient exploitation of the time resolved data. The study is restricted to the examination of the three first moments of signals acquired in cylindrical geometry. The interest of temporal information upon CW information has been shown especially when only a limited number of views is available.