Diffuse Optical Spectroscopy and Imaging of Turbid Media

Sammanfattning: This thesis deals with optical spectroscopy and imaging of complex and strongly scattering turbid media. Focus is mainly placed on three topics, being i) diffuse optical spectroscopy of choroidal tumors, ii) gas spectroscopy in scattering materials using the GASMAS (gas in scattering media absorption spectroscopy) technique, and iii) luminescence diffuse optical imaging and tomography of tissues. The first topic, which is diffuse optical spectroscopy of choroidal tumors, aims to develop a system which can be used for tissue diagnostics and to quantify relevant chromophores in an in-vivo setting through a transscleral measurement geometry. The present primary non-invasive diagnostic techniques are ophthalmoscopy, ultrasonography, and magnetic resonance imaging (MRI). These techniques are excellent for mapping inhomogeneities in the choroid, however, they cannot in general provide any detailed chemical information on the tissue of interest to identify a tumor. Instead, intraocular biopsies are employed for chemical analysis which are always associated with risks. Thus, there is a strong need for non-invasive techniques which are able to extract chemical information from the choroid. In the second topic of this thesis, where gas spectroscopy in scattering materials is studied, the GASMAS technique is employed to interrogate gas embedded inside of strongly scattering and highly porous ceramic tablets. Porous materials are of high importance in many applications and are, for example, used in catalysts, isolation, storage of gases, solar cells, and pharmaceutical tablets. One of the most important characterization techniques for porous materials is mercury intrusion porosimetry. In this technique, the sample is submerged in mercury and high pressure is used to force the mercury into the pores of the sample. Mercury intrusion porosimetry is destructive, hazardous and in addition inappropriate for certain material structures due to the employed models. This thesis explores a fast, inexpensive and non-invasive optical technique for characterization of porous materials. The principle of the technique is based on the fact that the lineshape of gas absorption profiles is highly sensitive to its surroundings. Thus, the absorption lineshape of the gas molecules will be influenced by the pore sizes due to wall collisions. Under standard conditions, pressure broadening is the dominating perturbation factor and the contribution from wall collisions is often negligible for most porous materials with micron-sized pores. However, by reducing the pressure, the contribution from wall collisions will be more prominent and even materials with large pores exhibit clear absorption lineshape deviations which can be used to assess the pore sizes within porous materials. In addition, initial steps toward a theoretical model have been taken. Such development will be of fundamental importance to be able to accurately quantify pore geometries within porous materials. Finally, the strongly scattering ceramic tablets are shown to also be promising for developing compact, robust and inexpensive gas sensors owing to the relaxed alignment requirements and the large path length enhancements of the injected photons. The final topic of this thesis, luminescence diffuse optical imaging and tomography of tissues, deals with the development of a system to image a new class of contrast agents known as upconverting nanoparticles (UCNPs) and of models to improve diffuse imaging and tomography. Optical bioimaging techniques have become highly interesting during the past few decades. Compared to the conventional imaging techniques such as MRI, CT, and positron emission tomography, optical imaging techniques can be significantly more cost and time effective. In particular, it has been shown that fluorescence and luminescence imaging are very attractive imaging modalities for small animal imaging in, for example, the development and evaluation of new drugs. The conventional fluorescent and luminescent contrast agents often have high quantum efficiencies, however, all of them suffer from the ambiguous tissue background autofluorescence. The use of UCNPs for bioimaging is initially motivated in the present thesis by their intrinsic emission properties yielding a completely autofluorescence-free environment. The reason is that the luminescence emission from UCNPs is anti-Stokes shifted in contrast to the Stokes-shifted emission from endogenous luminescent compounds found in biological tissues. The upconversion process, as expected, has a nonlinear power dependence on the excitation intensity. This nonlinearity is exploited to demonstrate that the present spatial resolution limits can be breached both for planar luminescence diffuse optical imaging and luminescence diffuse optical tomography. In addition, other applications of the nonlinearity are explored which include, for example, the topic of multi-beam excitation schemes. These schemes can through the nonlinearity of UCNPs enable the possibility to extract an unprecedented amount of information to be used in optical tomographic reconstructions. Finally, the thesis work also involves optical characterization of UCNPs. In particular, the importance of proper optical characterization is highlighted which involves the fundamental quantum yield of UCNPs as well as the lifetimes of the relevant energy states. Based on the characterization work, an initial model is developed to describe the quantum yield of UCNPs as a function of the excitation intensity and it is shown that knowledge of the optical properties of UCNPs is of paramount importance to realize optimal imaging systems.