Vadim Y. Soloviev

Fluorescence lifetime imaging by using time-gated data acquisition

The use of the time gating technique for lifetime reconstruction in the Fourier domain is a novel technique. Time gating provides sufficient data points in the time domain for reliable application of the Fourier transform, which is essential for the time deconvolution of the system of the integral equations employed in the reconstruction. The Fourier domain telegraph equation is employed to model the light transport, which allows a sufficiently broad interval of frequencies to be covered. Reconstructed images contain enough information needed for recovering the lifetime distribution in a sample for any given frequency within the megahertz-gigahertz band. The use of this technique is essential for recovering time-dependent information in fluorescence imaging. This technique was applied in reconstruction of the lifetime distribution of four tubes filled with Rhodamine 6G embedded inside a highly scattering slab. Relatively accurate fluorescence lifetime reconstruction demonstrates the effectiveness and the potential of the proposed technique.

In vivo fluorescence lifetime tomography of a FRET probe expressed in mouse

Förster resonance energy transfer (FRET) is a powerful biological tool for reading out cell signaling processes. In vivo use of FRET is challenging because of the scattering properties of bulk tissue. By combining diffuse fluorescence tomography with fluorescence lifetime imaging (FLIM), implemented using wide-field time-gated detection of fluorescence excited by ultrashort laser pulses in a tomographic imaging system and applying inverse scattering algorithms, we can reconstruct the three dimensional spatial localization of fluorescence quantum efficiency and lifetime. We demonstrate in vivo spatial mapping of FRET between genetically expressed fluorescent proteins in live mice read out using FLIM. Following transfection by electroporation, mouse hind leg muscles were imaged in vivo and the emission of free donor (eGFP) in the presence of free acceptor (mCherry) could be clearly distinguished from the fluorescence of the donor when directly linked to the acceptor in a tandem (eGFP-mCherry) FRET construct.

Fast image reconstruction in fluoresence optical tomography using data compression

We present a method for fast reconstruction in fluorescence optical tomography with very large data sets. In recent reports, CCD cameras at multiple positions have been used to collect optical measurements, producing more than 10(7) data samples. This makes storage of the full system Jacobian infeasible, and so data are usually subsampled. The method reported here allows use of the full data set, via image compression methods, and explicit construction of the (small) Jacobian, meaning optimal inversion methods can be applied, and thus leading to very fast reconstruction.

Fluorescence lifetime tomography of live cells expressing enhanced green fluorescent protein embedded in a scattering medium exhibiting background autofluorescence

We present a novel fluorescence lifetime tomography system applied to a highly scattering autofluorescent phantom containing live cells expressing the fluorophore enhanced green fluorescent protein (EGFP). The fluorescence signal was excited using a fiber-laser-pumped supercontinuum source and detected using wide-field time gating imaging. To facilitate rapid 3D reconstruction of the fluorescence lifetime distribution, the time-resolved data were Fourier-transformed in time to give complex functions that formed a data set for the Fourier domain reconstruction. Initially the presence of an unspecified background autofluorescence signal impeded reconstruction of the lifetime distribution, but we show that this problem can be addressed using a simple iterative technique.

Combined reconstruction of fluorescent and optical parameters using time-resolved data

We present an algorithm for simultaneous reconstruction of optical parameters, quantum yield, and lifetime in turbid media with embedded fluorescent inclusions. This algorithm is designed in the Fourier domain as an iterative solution of a system of differential equations of the Helmholtz type and does not involve full ill-conditioned matrix computations. The approach is based on allowing the unknown optical parameters, quantum yield, and lifetime to depend on the Fourier spectral parameter. The algorithm was applied to a time-gated experimental data set acquired by imaging a highly scattering cylindrical phantom concealing small fluorescent tubes. Relatively accurate reconstruction demonstrates the potential of the method.

Tomographic bioluminescence imaging with varying boundary conditions.

Three-dimensional bioluminescence imaging is an emerging technique that can be used to monitor molecular events in intact living systems. The inverse problem of 3D bioluminescence imaging does not have a unique solution because it requires reconstruction of a 3D source function from a 2D one. A novel approach that addresses this problem with the aid of a simple experimental setup and solves the uniqueness problem of the solution for a monochromatic measurement set is suggested here. The approach is verified numerically by reconstructing bioluminescent objects of various shapes embedded inside highly scattering media, such as biologiçal tissue.

Mesh adaptation technique for Fourier-domain fluorescence lifetime imaging

A novel adaptive mesh technique in the Fourier domain is introduced for problems in fluorescence lifetime imaging. A dynamical adaptation of the three-dimensional scheme based on the finite volume formulation reduces computational time and balances the ill-posed nature of the inverse problem. Light propagation in the medium is modeled by the telegraph equation, while the lifetime reconstruction algorithm is derived from the Fredholm integral equation of the first kind. Stability and computational efficiency of the method are demonstrated by image reconstruction of two spherical fluorescent objects embedded in a tissue phantom.

Optical Tomography in weakly scattering media in the presence of highly scattering inclusions

We consider the problem of optical tomographic imaging in a weakly scattering medium in the presence of highly scattering inclusions. The approach is based on the assumption that the transport coefficient of the scattering media differs by an order of magnitude for weakly and highly scattering regions. This situation is common for optical imaging of live objects such an embryo. We present an approximation to the radiative transfer equation, which can be applied to this type of scattering case. Our approach was verified by reconstruction of two optical parameters from numerically simulated datasets.

Adjoint time domain method for fluorescent imaging in turbid media

Application of adjoint time domain methods to the inverse problem in 3D fluorescence imaging is a novel approach. We demonstrate the feasibility of this approach experimentally on the basis of a time gating technique completely in the time domain by using a small number of time windows. The evolution of the fluorescence energy density function inside a highly scattering cylinder was reconstructed together with optical parameters. Reconstructed energy density was used in localizing two fluorescent tubes. Relatively accurate reconstruction demonstrates the effectiveness and the potential of the proposed technique.

Tomographic imaging with polarized light

We report three-dimensional tomographic reconstruction of optical parameters for the mesoscopic light scattering regime from experimentally obtained datasets by using polarized light. We present a numerically inexpensive approximation to the radiative transfer equation governing the polarized light transport. This approximation is employed in the reconstruction algorithm, which computes two optical parameters by using parallel and perpendicular polarizations of transmitted light. Datasets were obtained by imaging a scattering phantom embedding highly absorbing inclusions. Reconstruction results are presented and discussed.