Spectral distortion in diffuse molecular luminescence tomography in turbid media
ABSTRACT The influence of tissue optical properties on the shape of near-infrared (NIR) fluorescence emission spectra propagating through multiple centimeters of tissue-like media was investigated. Fluorescence emission spectra measured from 6 cm homogeneous tissue-simulating phantoms show dramatic spectral distortion which results in emission peak shifts of up to 60 nm in wavelength. Measured spectral shapes are highly dependent on the photon path length and the scattered photon field in the NIR amplifies the wavelength-dependent absorption of the fluorescence spectra. Simulations of the peak propagation using diffusion modeling describe the experimental observations and confirm the path length dependence of fluorescence emission spectra. Spectral changes are largest for long path length measurements and thus will be most important in human tomography studies in the NIR. Spectrally resolved detection strategies are required to detect and interpret these effects which may otherwise produce erroneous intensity measurements. This observed phenomenon is analogous to beam hardening in x-ray tomography, which can lead to image artifacts without appropriate compensation. The peak shift toward longer wavelengths, and therefore lower energy photons, observed for NIR luminescent signals propagating through tissue may readily be described as a beam softening phenomenon.
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ABSTRACT: Simultaneous detection of several biological processes in vivo is a common requirement in biomedical and biological applications, and in order to address this issue the use of multiple fluorophores is usually the method of choice. Existing methodologies however, do not provide quantitative feedback of multiple fluorophore concentrations in small animals in vivo when their spectra overlap, especially when imaging the whole body in 3D. Here we present an approach where a spectroscopic module has been implemented into a custom-built Fluorescence Molecular Tomography (FMT) system. In contrast with other multispectral approaches, this multimodal imaging system is capable of recording the fluorescence spectra from each illumination point during a tomographic measurement. In situ spectral information can thus be extracted and used to improve the separation of overlapping signals associated with different fluorophores. The results of this new approach tested on both in vitro and in vivo experiments are presented, proving that accurate recovery of fluorophore concentrations can be obtained from multispectral tomography data even in the presence of high autofluorescence.Biomedical Optics Express 01/2011; 2(3):431-9. · 3.18 Impact Factor
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ABSTRACT: Photoluminescence is one of the processes by which photons are emitted after the absorption of incoming photons at a higher energy. But the yield and spectral band shape of the emission can be altered by the optical properties of the luminophore environment through scattering and absorption. To understand these effects on a photoluminescent turbid layer, the Kubelka-Munk model, which is a two-flux approximation of the radiative transfer equation, can be used. Compared to previous works, this translucent layer can be applied on a colored opaque background. The model takes into account the absorption, scattering, and luminescent properties of the layer and the reflection by the background, for both the light excitation and the light emission. The competition between these different optical interactions is studied; e.g., the model can predict the presence of an emission maximum by increasing the thickness of the luminescent layer on a light background. Moreover, the model is extended to two important cases: the presence of a photoluminescent background and the effect of a refractive index discontinuity.Journal of the Optical Society of America A 07/2011; 28(7):1349-57. · 1.67 Impact Factor