Analytical model of fluorescence intensity for the estimation of fluorophore localisation in biotissue with dual-wavelength fluorescence imaging

Abstract

Analytical expression for the fluorescence response of a photosensitiser uniformly distributed in the superficial layer of biotissue is obtained in the diffusion approximation of radiative transfer theory, and the approach for estimating the fluorescent layer thickness based on dual-wavelength excitation of fluorescence is proposed. It is shown that the error in estimation of the fluorescent layer thickness employing the ratio of the fluorescence signals obtained at different excitation wavelengths does not exceed 30 % for the thickness range of 0.1 – 2 mm in the case of 30 %-variation of biotissue optical properties.

Dual-Wavelength Fluorescence Monitoring of Photodynamic Therapy: From Analytical Models to Clinical Studies

“…Theoretical evaluation of fluorescence signal from scattering and absorbing media with distributed fluorophore is arranged similarly to [ 22 ]. Biotissue is considered to be a semi-infinite volume of uniform turbid medium illuminated from the top by a plane-wave excitation source with the intensity I ex at the wavelength λ ex .…”

“…Assuming a unidirectional incidence of excitation light and an isotropic angular emission of fluorescence, the propagation of excitation light was derived in accordance with the semi-empirical model developed by Jacques [ 25 ], while the fluorescence signal was derived within the frames of the diffusion approximation of RTT with the account of the refractive index mismatch at the biotissue–air boundary [ 22 , 26 , 27 ]. As the result, the outgoing flux of fluorescence radiation F at the biotissue boundary z = 0 normal to its surface can be derived from the expression [ 22 ]: where µ ex and µ em are the values of the diffusion attenuation coefficient at the corresponding wavelengths λ ex and λ em ; , where m is the factor accounting for the total internal reflectance of diffusive fluorescence light determined by formula (2.4.1) in [ 26 ]; and m ≅ 2.76 for the predefined refractive index n = 1.37. The value is the backscattering factor evaluated in the following form [ 25 ]: …”

“…Dual-wavelength excitation of fluorescence with single-wavelength detection was proposed by Miller et al [ 19 ] and allowed for non-invasive depth estimation of a fluorescent inclusion in a mouse. This approach was further developed for monitoring the chlorin-based PSs accumulation [ 20 , 21 , 22 ]. This class of PSs features two pronounced peaks in red and blue bands of the absorption spectrum, allowing for the efficient dual-wavelength excitation of fluorescence emitted in far-red range.…”

“…In our previous studies, we have demonstrated the perspectives of the dual-wavelength-excited fluorescence approach by numerical simulations and theoretical analysis, and have revealed the difference in the fluorescence response ratio in the cases of topical PS application and intravenous injection in in vivo experiments in mice bearing subcutaneous tumors [ 22 , 24 ]. It has been shown that systemic and topical methods of PS administration yield a principal difference in the outcome of dual-wavelength fluorescence imaging.…”

“…In this paper we have generalized the concept of fluorescence imaging based on dual-wavelength fluorophore excitation applied to monitor PDT with chlorin e6-based photosensitizers. The study includes the extension of a previously developed analytical model of fluorescence signal formation [ 22 ] to account for a more realistic distribution of PS within biotissues, as well as its verification in numerical and phantom experiments, the analysis of PS monitoring in animal studies, observations from human volunteers, and a retrospective clinical study including both non-tumor and tumor pathologies. All of the PSs considered in this study are based on chlorin e6, and despite a difference in the particular formulations they are equivalent from the point of dual-wavelength imaging.…”