Dimethyl sulfoxide (DMSO) is one of the most commonly used pharmaceutical drugs in life sciences. It has a wide spectrum of pharmacological effects, including anti-inflammatory effects, local analgesia, weak bacteriostasis and most importantly membrane penetration. We recently developed novel hyperspectral excitation-resolved near-infrared fluorescence imaging system (HER-NIRF) based on a continuous-wave wavelength-swept laser. In this study, this technique is applied for measuring the distribution of the therapeutic agent dimethyl sulfoxide (DMSO) by utilizing solvatochromic shift in the spectral profile of albumin-bound Indocyanine green (ICG). Phantom experiments are conducted to evaluate the performance of the HER-NIRF system. The results show that the distribution of DMSO can be visualized in the wide-field reflection geometry.
Fluorescence diffuse optical tomography (FDOT) has been widely used for in vivo small animal studies and the illposed problem in reconstruction can be eased by utilizing structural a priori obtained from an anatomic imaging modality. In this study, a multispectral fluorescence tomography (FT) is used, which has shown the ability to detect subtle shifts in the ICG absorption spectrum in our previous study. The imaging system is in trans-illumination mode with a swept-wavelength laser and a CCD on a rotation gantry and the structural image from the X-ray computed tomography is used to guide and constrain the FT reconstruction algorithm. In this work, a phantom with two inclusions filled with different fluorophores is utilized to evaluate whether the spectral information obtained using sweptwavelength laser can distinguish these two inclusions. The images are captured from 8 different views with three different wavelengths.
Near-infrared fluorescence imaging (NIRF) is a powerful wide-field optical imaging tool that has a potential to visualize molecular-specific exogenous fluorescence agents, such as FDA approved Indocyanine Green (ICG), in thick tissue. Indeed, ICG is sensitive to biochemical environment such that it can be used to detect micro- or macroscopic environmental changes in tissue by solvatochromic shift that is defined by the dependence of absorption and emission spectra with the solvent polarity. For example, dimethyl sulfoxide (DMSO) is a very powerful drug carrier that can penetrate biological barriers such as the skin, the membranes, and the blood-brain-barrier. In presence of DMSO, ICG in tissue shows the excitation blue shift. However, NIRF imaging of microenvironment dependent changes of ICG has been challenging for the following reasons. First, the Stoke’s shift of ICG is too small to separate the excitation and emission spectra easily. Second, the solvatochromic shift of ICG is too small to be detected by conventional NIRF techniques. Last but not least, the multiple scattering in tissue degrades not only the spatial information but also the spectral contents by the red-shift. We developed a wavelength-swept laser-based NIRF system that can resolve the excitation shift of ICG in tissue such that DMSO can be indirectly visualized. We plan to conduct an in-vivo lymph-node drug-delivery study in a mouse model to show feasibility of the indirect imaging of the drug-carrier with the wavelength-swept-laser based NIRF system.
The strong scattering and absorption of light in biological tissue makes it challenging to model the propagation of light, especially in deep tissue. This is especially true in fluorescent tomography, which aims to recover the internal fluorescence source distribution from the measured light intensities on the surface of the tissue. The inherently ill-posed and underdetermined nature of the inverse problem along with strong tissue scattering makes Fluorescence Tomography (FT) extremely challenging. Previously, multispectral detection fluorescent tomography (FT) has been shown to improve the image quality of FT by incorporating the spectral filtering of biological tissue to provide depth information to overcome the inherent absorption and scattering limitations. We investigate whether multi-wavelength fluorescent tomography can be used to distinguish the signals from multiple fluorophores with overlapping fluorescence spectrums using a unique near-infrared (NIR) swept laser. In this work, a small feasibility study was performed to see whether multi-wavelength FT can be used to detect subtle shifts in the absorption spectrum due to differences in fluorophore microenvironment.
Breast density is an independent risk factor for breast cancer, where women with denser breasts are more likely to
develop cancer. By identifying women at higher risk, healthcare providers can suggest screening at a younger age to
effectively diagnose and treat breast cancer in its earlier stages. Clinical risk assessment models currently do not
incorporate breast density, despite its strong correlation with breast cancer. Current methods to measure breast density rely
on mammography and MRI, both of which may be difficult to use as a routine risk assessment tool. We propose to use
diffuse optical tomography with structured-light to measure the dense, fibroglandular (FGT) tissue volume, which has a
different chromophore signature than the surrounding adipose tissue. To test the ability of this technique, we performed
simulations by creating numerical breast phantoms from segmented breast MR images. We looked at two different cases,
one with a centralized FGT distribution and one with a dispersed distribution. As expected, the water and lipid volumes
segmented at half-maximum were overestimated for the dispersed case. However, it was noticed that the recovered water
and lipid concentrations were lower and higher, respectively, than the centralized case. This information may provide
insight into the morphological distribution of the FGT and can be a correction in estimating the breast density.
We are developing a ballistic-photon based approach for improving the spatial resolution of fluorescence tomography
using time-domain measurements. This approach uses early photon information contained in measured time-of-fight
distributions originating from fluorescence emission. The time point spread functions (TPSF) from both excitation light
and emission light are acquired with gated single photon Avalanche detector (SPAD) and time-correlated single photon
counting after a short laser pulse. To determine the ballistic photons for reconstruction, the lifetime of the fluorophore
and the time gate from the excitation profiles will be used for calibration, and then the time gate of the fluorescence
profile can be defined by a simple time convolution. By mimicking first generation CT data acquisition, the sourcedetector
pair will translate across and also rotate around the subject. The measurement from each source-detector
position will be reshaped into a histogram that can be used by a simple back-projection algorithm in order to reconstruct
high resolution fluorescence images. Finally, from these 2D sectioning slides, a 3D inclusion can be reconstructed
accurately. To validate the approach, simulation of light transport is performed for biological tissue-like media with
embedded fluorescent inclusion by solving the diffusion equation with Finite Element Method using COMSOL
Multiphysics simulation. The reconstruction results from simulation studies have confirmed that this approach
drastically improves the spatial resolution of fluorescence tomography. Moreover, all the results have shown the
feasibility of this technique for high resolution small animal imaging up to several centimeters.
Fluorescence tomography is a non invasive, non ionizing imaging technique able to provide a 3D distribution of fluorescent
agents within thick highly scattering mediums, using low cost instrumentation. However, its low spatial resolution due to
undetermined and ill-posed nature of its inverse problem has delayed its integration into the clinical settings. In addition,
the quality of the fluorescence tomography images is degraded due to the excitation light leakage contaminating the
fluorescence measurements. This excitation light leakage results from the excitation photons that cannot be blocked by the
fluorescence filters. In this contribution, we present a new method to remove this excitation light leakage noise based on
the use of a temperature sensitive fluorescence agents. By performing different sets of measurements using this temperature
sensitive agents at multiple temperatures, the excitation light leakage can be estimated and then removed from the
measured fluorescence signals . The results obtained using this technique demonstrate its potential for use in in-vivo small
animal imaging.
Diffuse optical imaging with structured-light illumination and detection can provide rapid, wide-field anatomical and functional imaging of the breast with an application for breast cancer screening. Our aims for this study were to test the feasibility of structured-light, test our pattern set, and develop and optimize our image reconstruction algorithm. For our phantom studies, we created an agar phantom with dimensions similar to a compressed breast. A cubic inclusion of 30mm by 30mm by 25mm with twice the amount of absorption contrast than the background was placed at the center. Near-infrared light of eleven patterns including a full illumination and single stripes was illuminated onto the breast phantom and detected with a CCD camera, with integration of the signals according to the patterns performed post-data acquisition, with a total of 121 measurements. These measurements were then used in our reconstruction algorithm that iteratively minimized the difference between the collected data and the estimation from our FEM-based forward model of photon diffusion to calculate the absorption values. Reconstructions of the 3D absorption maps detect an inclusion at the center and indicate that our selected set of patterns may be sufficient for structured-light imaging. We are currently improving our instrumentation and testing with additional phantom studies, while also performing simulations of numerical breast phantoms created from MR images to test structured-light’s ability to image complex and realistic breast tissue composition. We hope to use this technique as optical method to image molecular markers, such as hemoglobin, water and lipid, within the breast.
We developed a spectrally-resolved fluorescence tomography (FT) system using a new source and detection unit. On the source side, we utilized a near-infrared (NIR) swept laser-based technology and on the detection side, we developed a digital micromirror device (DMD) based spectrally-resolved detection unit. We demonstrated the development of a NIR swept laser centered at 800 nm for FT, which covers the maximum absorption wavelength of a NIR fluorescence dye, indo-cyanine green (ICG) in plasma. Two different ICG samples whose absorption characteristics were slightly different were used to demonstrate the performance of the NIR swept laser-based FT system, and this FT system was able to show the difference of absorption between the ICG samples. In addition, we also developed a prototype spectrally-resolved detection unit based on the DMD. This detection system provided a spectral resolution of 15 nm and the possibility of simultaneous detection of multiple fluorescence spectra.
The design and implementation of a diffuse optical tomography system using wavelength-swept laser is described. Rapid and continuous wavelength change is utilized for high speed spectral scanning from 775 nm to 875 nm optical wavelength. Maximum speed of wavelength repetition is 1 kHz and averaged output power of the wavelength-swept laser is 20 mW. A fiber-optic Sagnac interferometer is incorporated to conduct passive amplitude modulation of the wavelength-swept laser. It is shown that the wavelength-swept laser can be successfully incorporated to the DOT system, and then reduces wavelength-shifting time and hardware complexity in multi-wavelength DOT implementation.
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