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- W1738963404 abstract "Tissue optics is a field devoted to study the interaction of light with tissue. Over the last decades, much thanks to the field of optical spectroscopy, the knowledge of tissue optics has been steadily increasing. This has catalyzed the interest in applying tissue optics as a clinical tool. This thesis studies an area within tissue optics dealing with fluorescence molecular imaging and tomography. For most visible wavelengths, light does not penetrate more than a few millimeters into tissue. But in the diagnostic window (∼ 600 to 1600 nm), penetration up to several centimeters is possible. This opens up the possibility of imaging fluorescent contrast agents deep in tissue. Fluorescent imaging has a notable importance in biomedical applications. Shimomura, Chalfie and Tsien were recently rewarded with the Nobel prize for discovering and developing the green fluorescent protein, which has become a very important fluorescent marker. Fluorescent imaging can, for example, be used to study biological responses from drugs in small animals over a period of time, without the need to sacrifice them. Currently, considerable amounts of research are being performed to enable three-dimensional reconstructions of contrast agent distributions inside animals, so called fluorescent tomography. The area of fluorescent imaging and tomography have long been adversely affected by the ever-present endogenous tissue autofluorescence. The autofluorescence conceals the signal from the contrast agents when using Stokes-shifted fluorophores, effectively limiting the signal-tobackground sensitivity. In this thesis, it is shown that by replacing the traditional Stokes-shifted fluorophores with upconverting nanocrystals, it is possible to avoid the nuisance of autofluorescence. The nanoparticles emit light of a shorter wavelength than their excitation wavelength, effectively shifting the signal to a wavelength region where no autofluorescence is present. Experiments on tissue phantoms, with realistic optical properties, were performed, and it was shown that it is possible to detect an autofluorescence-free signal. Also a theoretical framework for using the nanocrystals for three-dimensional tomographic reconstruction was derived. Simulations were performed based on this framework, showing promising results. Based on the results presented in this thesis, we believe that upconverting nanocrystals may very well be envisaged as important biological markers for tissue imaging purposes. Table of Physical Quantities Symbol Physical quantity Definition Units φ(r, ŝ) Radiance W/msr Φ(r) Fluence rate Φ(r) = ∫ 4π φ(r, ŝ) dΩ W/m n Refractive index c Speed of light in tissue c = c0/n m/s μa Absorption coefficient 1/m μs Scattering coefficient 1/m μtr Transport attenuation coefficient μtr = μa + μs 1/m g Scattering asymmetry parameter – μs Reduced scattering coefficient μ ′ s = (1− g)μs 1/m κ Diffusion coefficient κ = 1/3(μs + μa) m μeff Effective attenuation coefficient μeff = √ μa/κ 1/m η Upconversion efficiency η = I(ωf)/I(ωe) η2p Upconversion two-photon efficiency η2p = I(ωf)/I(ωe) 2 m/W" @default.
- W1738963404 created "2016-06-24" @default.
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- W1738963404 date "2008-01-01" @default.
- W1738963404 modified "2023-09-27" @default.
- W1738963404 title "Upconverting Luminescence Imaging and Tomography for Biomedical Applications" @default.
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