Assessment of structure and composition of tissue using time-resolved fluorescence and ultrasound techniques

Multimodal imaging techniques allow simultaneous acquisition of complementary data from tissue during a single examination. Such techniques have become increasingly popular in a variety of biomedical applications related to disease diagnosis, prognosis and theragnosis. In this work, time-resolved autofluorescence combined with ultrasound techniques are evaluated as multimodal imaging methods to study biological tissues for applications related to cancer, atherosclerosis and tissue engineering. Time-resolved spectroscopic fluorescence measurements are performed using two methods namely -- (1) Time-resolved fluorescence spectroscopy (TRFS) and (2) Rotational or planar multi-spectral fluorescence lifetime imaging (FLIm). Ultrasound techniques include (1) high frequency (40 MHz) focused ultrasound backscatter microscopy (UBM), (2) photoacoustic imaging (PAI) and (3) rotational unfocused intravascular ultrasound (IVUS).;The first original contribution of this work explores the autofluorescence of pure biological compounds using 337 nm and 355 nm excitation wavelengths. TRFS measurements from common tissue fluorophores such as various collagen types, elastin, co-enzymes -- NADH and FAD, lipids and other metabolic by-products are documented that serve as a summary for interpreting tissue autofluorescence for exploring the potential of time-resolved fluorescence imaging for tissue characterization.;The second part of the thesis demonstrates for the first time, the ability of rotational FLIm to complement IVUS for improved assessment of atherosclerotic plaques in ex vivo human coronary arteries. Results show that FLIm allows detection of macrophages in fibrous caps (sensitivity, 86%) and distinguishing between relatively stable thick-cap fibroatheromas and rupture-prone thin-cap fibroatheromas (sensitivity, 80%) amongst other features. These results establish FLIm-IVUS as a potentially new intravascular method for evaluating coronary plaques that may subsequently aid in guiding coronary intervention and understanding atherosclerosis in animals as well as humans.;The third part of the thesis demonstrates the ability of FLIm, UBM and PAI for detection of oral carcinoma in a hamster model in vivo. Results establish the ability of FLIm to assess changes in collagen and NADH autofluorescence to differentiate between normal, precancerous and carcinoma tissue. UBM and related radiofrequency parameters are shown to identify disruptions in the tissue microarchitecture, and PAI to map vascularization.;The final part of the thesis explores the potential of TRFS, FLIm and UBM as non-destructive methods to assess the development of engineered tissues. Results from ex vivo vascular grafts explanted from a porcine carotid arteriotomy model are presented. TRFS and FLIm are able to assess alterations in luminal composition namely elastin, collagen and cellular (hyperplasia) content via changes in fluorescence lifetime values between normal and grafted tissue. These observations are complemented by structural changes observed in UBM pertaining to graft integration and neo-intimal thickness over the grafted region. These results encourage the future application of a catheter-based technique that combines these imaging modalities for non-destructive characterization of vascular grafts in vivo..;The entirety of this work enables a better understanding of the potential of time-resolved fluorescence and ultrasound as multimodal imaging techniques for the characterization of biological tissues.