SignificanceAlthough the depth detection limit of fluorescence objects in tissue has been studied, reports with a model including noise statistics for designing the optimum measurement configuration are missing. We demonstrate a variance analysis of the depth detection limit toward clinical applications such as noninvasively assessing the risk of aspiration.AimIt is essential to analyze how the depth detection limit of the fluorescence object in a strong scattering medium depends on the measurement configuration to optimize the configuration. We aim to evaluate the depth detection limit from theoretical analysis and phantom experiments and discuss the source–detector distance that maximizes this limit.ApproachExperiments for detecting a fluorescent object in a biological tissue-mimicking phantom of ground beef with background emission were conducted using continuous wave fluorescence measurements with a point source–detector scheme. The results were analyzed using a model based on the photon diffusion equations. Then, variance analysis of the signal fluctuation was introduced.ResultsThe model explained the measured fluorescence intensities and their fluctuations well. The variance analysis showed that the depth detection limit in the presence of ambient light increased with the decrease in the source–detector distance, and the optimum distance was in the range of 10 to 15 mm. The depth detection limit was found to be ∼30 mm with this optimum distance for the phantom.ConclusionsThe presented analysis provides a guide for the optimum design of the measurement configuration for detecting fluorescence objects in clinical applications.
We investigated the feasibility of a two-step scheme for reconstruction of a fluorophore target embedded in a semi-infinite medium. In this scheme, we neglected the presence of the fluorophore target for the excitation light and used an analytical solution of the time-dependent radiative transfer equation (RTE) for the excitation light in a homogeneous semi-infinite media instead of solving the RTE numerically in the forward calculation. In the first step of this reconstruction scheme, we implemented a pixel-based reconstruction using the Landweber method with adjoint fields. The second step uses this result as an initial guess for solving the shape and contrast value reconstruction problem using the level set method. Numerical experiments using Monte Carlo data measurements, show that the proposed scheme provides reconstructions of shape, location and contrast value of the target with rather good accuracy. The computation times of the solution of the forward problem and the whole reconstruction process were reduced by about forty and fifteen percent, respectively.
KEYWORDS: Luminescence, Scattering, Absorption, Light scattering, Optical properties, Scatter measurement, Monte Carlo methods, In vivo imaging, Contamination, Quantum efficiency
Fluorescence lifetime in heterogeneous multiple light scattering systems is analyzed by an algorithm without solving the diffusion or radiative transfer equations. The algorithm assumes that the optical properties of medium are constant in the excitation and emission wavelength regions. If the assumption is correct and the fluorophore is a single species, the fluorescence lifetime can be determined by a set of measurements of temporal point-spread function of the excitation light and fluorescence at two different concentrations of the fluorophore. This method is not dependent on the heterogeneity of the optical properties of the medium as well as the geometry of the excitation–detection on an arbitrary shape of the sample. The algorithm was validated by an indocyanine green fluorescence in phantom measurements and demonstrated by an in vivo measurement.
A non-contact type time-domain system for the fluorescence diffuse optical tomography was designed. The system is evaluated by a phantom with a fluorescence target. The contamination of the non-specific scattering superimposed on the excitation profile but it could be reduced with closely locating the detection fiber to the surface (~1 mm). Next, we analyzed the contamination in the temporal profiles with an Intralipid solution phantom with a fluorescent target. The contamination to the excitation profile is not clearly observed but that to the fluorescence is strong with a short distance between the excitation source and detection. Finally, we have concluded that a larger distance of source and detector yields better fluorescence sensitivity because the background is limiting the fluorescence detection. On the other hand, the signal quality depends on the statistics and thus the optimum range of the distance comes around 30 mm. Finally, this research gives the idea for the design of the source and detection configuration.
We are reporting the first trial image reconstruction of a implanted fluorescent target into a live rat abdomen. We
use a simplified algorithm for fluorescence diffuse optical tomography (FDOT), so-called the Total-light algorithm
to obtain the absorption image of the target from the measured mean-transit time (MTT). We reconstructed two
absorption images with and without a fluorescence target. It is difficult to identify something in the absorption
images. However, the difference image between the two images highlights the target. This suggests that our
algorithm is robust to the artifacts in the images in the real situation of in vivo measurements.
We report on a simple correlation method for lifetime measurements using a random modulated excitation light source. We use an intensity correlation function of emission for lifetime analyses. In this method, no reference timing of the excitation is required. We apply the correlation method to measure phosphorescence decays and successfully demonstrate in the analysis of the phosphorescence decay from Pd(II) porphine in HeLa cells under aerobic and anaerobic conditions to understand the oxygen dynamics in individual cells. The method is applicable to faster decay time measurements down to a nanosecond range when the detection system is improved. Current fluorescence correlation setups can easily be modified for lifetime measurements, expanding the applicability in biological problems.
Analysis of time-of-flight (TOF) data is sometimes limited by the instrumental response function, and optical parameters are extracted from the observed response curve by several mathematical methods, such as deconvolution. In contrast to this, we demonstrate that a method using shifts of the peak time of the response curve with different source-detector separations can yield the average path length of the light traveling in a tissue-like sample without deconvolution. In addition, combining the intensity information allows us to separate the scattering and absorption coefficients. This simple method is more robust in signal-to-noise ratio than the moment analysis, which also does not require the deconvolution procedure, because the peak position is not significantly dependent on the baseline fluctuation and the contamination of the scattering. The analysis is demonstrated by TOF measurements of an Intralipid solution at 800 nm, and is applied to the measurements at 1.29 µm, where the temporal response of photomultiplier tubes is not sufficiently good.
The non-invasive determination of the depth of severe burns is an important problem whose solution would offer medical practitioners a valuable tool for diagnosing and treating severe burns. Burned tissue is essentially a turbid medium with spatial varying dynamics: light is multiply scattered by the tissue and layers of burned tissue are distinguished by the degree of blood flow. The dynamical properties of turbid media can be probed by monitoring the temporal fluctuations of scattered light speckles. Information on a system's dynamics is obtained from the temporal autocorrelation function of these intensity fluctuations. We have recently shown that the correlation diffusion equation (CDE) accurately predicts the temporal correlation function for turbid systems with spatially varying dynamics and that the dynamical properties of such systems can be imaged using standard reconstruction algorithms. In this contribution, we demonstrate the sensitivity of temporal field correlation measurements to variations of 100 micrometers in burn thickness and the potential applicability of the CDE for quantitation of burn thickness. Results are presented from burn phantoms and pig models. The combination of diffusing temporal light correlation with diffuse reflectometry for enhanced burn diagnosis is investigated.
Time-resolved absorption spectra were measured in the near-IR wavelength range from 700 to 900 nm with rat heads in vivo under various respiratory conditions, based on our time- resolved microscopic Beer-Lambert law. The spectra were expressed in absolute absorption per unit optical path (cm-1). Applying a regression analysis to the spectra, we obtained absolute concentrations of HbO2 and Hb in the rat head. These values agreed with those measured in direct assay analysis, and the feasibility of our time-resolved photometry for quantitation of chromophores in living tissue was demonstrated. This fact supports our assumption that light absorption in a scattering media such as living tissues in independent of the scattering. Based on this study, we constructed a portable system for clinical applications. The system employs three diode lasers of different wavelengths for generating picosecond light pulses and high speed PMT for a time-correlated single photon counting method. The system performance shows its applicability to quantitative monitoring of oxygen metabolism in living tissues.
In this paper, we describe the experimental evaluation of the temporally extrapolated absorbance method (TEAM) in turbid media. The TEAM is the method utilizing the extrapolated absorbance, obtained by temporally extrapolating the time-resolved absorbance difference between an object and a reference to the shortest photon flight time. Using the TEAM, we reconstructed the CT image of an absorber-containing cylinder in a strong scattering media and compared it with those obtained with the time gating and cw methods based on the same data. Among the three methods, the TEAM presented best spatial resolution in the reconstructed CT images.
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