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Sensory adaptation to blur improves visual acuity under defocused conditions. This phenomenon has been successfully demonstrated using subjective measures of acuity and is known as blur adaptation. This study investigates aspects of the mechanism of blur adaptation in human vision using subjective and objective methods. Parafoveal visual acuity measured under defocused conditions demonstrates that blur adaptation is not limited to the fovea. The presence of the adaptive mechanism in the parafovea also suggests that the neural compensation that takes places under defocused conditions acts across a spatial range and is not limited to specific frequency bands. An evaluation of the contrast sensitivity function under defocus provides further evidence. Electrophysiological methods measure the effect of blur adaptation at the retina and at the visual cortex to provide objective evidence for the presence of the blur adaptation mechanism. Finally enhanced-depth imaging optical coherence tomography examines whether a period of prolonged defocus triggers any short-term changes in choroidal thickness in a similar manner to that reported in animal emmetropisation.

Two applications of second harmonic generation (SHG), a nonlinear optical technique, are studied. First, Fresnel factors are used with a bond model to describe SHG from vicinal silicon at five incidence angles: 7.5°, 22°, 30°, 45°, and 52°. Second, a prototype apparatus for applying SHG to enhance imaging capabilities of optical coherence tomography, a microscopy technique used in many biological fields, is briefly described. / text

Confocal microendoscopy provides real-time high resolution cellular level images via a minimally invasive procedure. Results from an ongoing clinical study to detect ovarian cancer with a novel confocal fluorescent microendoscope are presented. As an imaging modality, confocal fluorescence microendoscopy typically requires exogenous fluorophores, has a relatively limited penetration depth (100μm), and often employs specialized aperture configurations to achieve real-time imaging in vivo. Two primary research directions designed to overcome these limitations and improve diagnostic capability are presented. Ideal confocal imaging performance is obtained with a scanning point illumination and confocal aperture, but this approach is often unsuitable for real-time, in vivo biomedical imaging. By scanning a slit aperture in one direction, image acquisition speeds are greatly increased, but at the cost of a reduction in image quality. The design, implementation, and experimental verification of a custom multi-point-scanning modification to a slit-scanning multi-spectral confocal microendoscope is presented. This new design improves the axial resolution while maintaining real-time imaging rates. In addition, the multi-point aperture geometry greatly reduces the effects of tissue scatter on imaging performance. Optical coherence tomography (OCT) has seen wide acceptance and FDA approval as a technique for ophthalmic retinal imaging, and has been adapted for endoscopic use. As a minimally invasive imaging technique, it provides morphological characteristics of tissues at a cellular level without requiring the use of exogenous fluorophores. OCT is capable of imaging deeper into biological tissue (~1-2 mm) than confocal fluorescence microscopy. A theoretical analysis of the use of a fiber-bundle in spectral-domain OCT systems is presented. The fiber-bundle enables a flexible endoscopic design and provides fast, parallelized acquisition of the optical coherence tomography data. However, the multi-mode characteristic of the fibers in the fiber-bundle affects the depth sensitivity of the imaging system. A description of light interference in a multi-mode fiber is presented along with numerical simulations and experimental studies to illustrate the theoretical analysis.

Optical coherence tomography (OCT) is a technology that enables 2D cross-sectional images of tissue microstructure. This interferometric technique provides resolutions of approximately 10-20 um with a penetration depth of 1-2 mm in highly scattering tissues. With the use of fiber optics, OCT systems have been developed for intravascular imaging with a demonstrated improvement in both resolution and dynamic range compared to commercial intravascular ultrasound systems. OCT studies of normal, atherosclerotic, and stented arteries indicate the ability of OCT to visualize arterial structures. These results suggest OCT may be a valuable tool for studying luminal structures in tissue engineered constructs.In the present study, new endoscopic OCT systems and analysis techniques were developed to visualize the growth and response of the cellular lining within a tissue engineered blood vessel mimic (BVM). The BVM consists of two primary components. A biocompatible polymeric scaffold is used to form the tubular structure. Human microvessel cells from adipose tissue are sodded on to the inner surface of the scaffold. These constructs are then developed and imaged within a sterile bioreactor.Three specific aims were defined for the present study. First, an OCT longitudinal scanning endoscope was developed. With this endoscope, a study of 16 BVMs was performed comparing images from OCT and corresponding histological sections. The study demonstrated that endoscopic imaging did not visually damage the mimic cellular lining. OCT images showed excellent correlation with corresponding histologicalsections. Second, a concentric three element endoscope was developed to provide radial cross-sections of the BVM. OCT images using this endoscope monitored lining development on three types of polymeric scaffolds. In the third specific aim, automated algorithms were developed to assess the percent cellular coverage of a stent using volumetric OCT images.The results of the present study suggest that OCT endoscopic systems may be a valuable tool for assessing and optimizing the development of tissue engineered constructs. Conversely, the BVMs modeled the arterial response to deployed stents allowing the development of automated OCT analysis software. These results suggest that blood vessel mimics may be used to advance OCT technology and techniques.

Advanced optical imaging techniques are emerging as useful ways to screen tissues for the presence of cancer. Plasmon resonant nanoparticles have unique optical properties that make them ideal for use as optical contrast agents. The capacity of these particles to serve a multifunctional role dependent on their composition and the intensity of incident light enables them to serve as diagnostic tools and to provide the therapeutic capability of photo-thermal energy conversion or the controlled release of an encapsulated agent. Likewise, the ability to degrade into components of a clearable size may enable the clinical translation of these types of particles.These properties were demonstrated by means of experiments in the support of three specific aims. The first specific aim was to determine whether the unique and tunable optical properties of nanorods lend them to generate signal in advanced optical imaging techniques, and that nanorods can facilitate photo-thermal conversion. The second specific aim was to show that liposomes can serve as a scaffold for the support of an array of gold nanodots to generate a structure that exhibit tunable plasmon resonant characteristics and a resultant ability to generate signal in optical imaging techniques while having the capability to degrade into inert particles of a size that can be readily cleared from the body via the kidney. The final specific aim was to determine whether the gold-coated liposomes of the second specific aim can serve as system for light-based delivery of an encapsulated agent in addition to its role as an optical contrast agent and its biodegradation capacity.Plasmon resonant nanorods and plasmon resonant gold-coated liposomes were generated by reducing free gold from solution onto surfactant coated seed particles and phospholipid liposomes, respectively. Both structures demonstrated the ability to generate signal in optical coherence tomography and in multi-photon confocal microscopy images. Nanorods in high intensity light demonstrate a capacity to mediate photo-thermal energy conversion. While, in similar conditions, gold-coated liposomes are shown to release their contents. Gold-coated liposomes are also shown to degrade to bioinert components of a size reasonable for rapid renal clearance using either surfactant or enzyme.

This thesis presents the first work, to our knowledge, to evaluate the potential of Optical Coherence Tomography (OCT) as an intravascular imaging modality to characterize and guide interventions on chronic total occlusions (CTOs) in arteries. An ex vivo imaging study using OCT is presented that characterizes various pathologies associated with peripheral CTOs and illustrates the ability to differentiate between the vessel wall and the occluded lumen. We also found that, while OCT could image approximately 1mm through tissue, it was effective for imaging deeper through clarified microchannels seen within the occluded lumen. While others had reported observing such microchannels within the lumen before, little was known about the global architecture of these channels. This motivated a study of the global morphology of microchannels in occlusions using micro computed tomography (microCT). In this microCT study, we found that microchannels within the occluded lumen of the artery appeared to be continuous over several millimeters. However, these channels also exited the artery frequently, suggesting the need for some form of imaging guidance. As a potential intravascular imaging set-up, a forward-viewing OCT catheter was built. This catheter uses a novel scanning mechanism that combines high voltage and a dissipative polymer to achieve fast compact actuation. Doppler OCT results are presented using this catheter to image flow in the forward direction. Doppler OCT imaging of microchannels in vivo is also shown in a surgically exposed occluded artery in situ.

This thesis presents the first work, to our knowledge, to evaluate the potential of Optical Coherence Tomography (OCT) as an intravascular imaging modality to characterize and guide interventions on chronic total occlusions (CTOs) in arteries. An ex vivo imaging study using OCT is presented that characterizes various pathologies associated with peripheral CTOs and illustrates the ability to differentiate between the vessel wall and the occluded lumen. We also found that, while OCT could image approximately 1mm through tissue, it was effective for imaging deeper through clarified microchannels seen within the occluded lumen. While others had reported observing such microchannels within the lumen before, little was known about the global architecture of these channels. This motivated a study of the global morphology of microchannels in occlusions using micro computed tomography (microCT). In this microCT study, we found that microchannels within the occluded lumen of the artery appeared to be continuous over several millimeters. However, these channels also exited the artery frequently, suggesting the need for some form of imaging guidance. As a potential intravascular imaging set-up, a forward-viewing OCT catheter was built. This catheter uses a novel scanning mechanism that combines high voltage and a dissipative polymer to achieve fast compact actuation. Doppler OCT results are presented using this catheter to image flow in the forward direction. Doppler OCT imaging of microchannels in vivo is also shown in a surgically exposed occluded artery in situ.

Purpose: The aim of the present study was to compare and investigate morphometric characteristics of limbal vasculature within the superior and inferior limbal regions using ultra-high resolution optical coherence tomography. Method: Cross-sectional images of the human corneo-scleral limbus were acquired with a research grade ultra-high resolution optical coherence tomographer (UHR-OCT) from 14 healthy subjects after manual retraction of the upper and lower eyelid. The UHR-OCT provides an axial and lateral resolution in biological tissue of ~3μm and ~18μm, respectively. 3D stacks of OCT images (1000 x 1024 x 256) were acquired of the transition from cornea to bulbar conjunctiva at the superior and inferior limbal region. All visible vessels within the limbal region were measured using an Image J circle or ellipse tool. Vessel depth and size measurements were repeated for the same vessel and the concordance correlation coefficient was computed. Quantitative differences in vessel size and depth in the limbal region were analyzed using repeated measured ANOVA. R and SPSS were used for all data analysis procedures. Results: The average vessel size for superior and inferior limbus were 29.28µm ± 17.649 µm (SD) and 23.68µm ± 18.118µm (SD) respectively. The average vessel depth for superior and inferior limbus were 176.76µm ± 108.698µm (SD) and 205.62µm ± 131.991µm (SD) respectively. The concordance correlation coefficient for superior and inferior limbal vessel size were 0.95 (95% CI, 0.935 to 0.964) and 0.994 (95% CI, 0.991 to 0.995) respectively, while that of vessel depth were 0.998 (95% CI, 0.997 to 0.999) and 0.998 (95% CI, 0.997 to 0.998) respectively. The vessels within the superior limbus were larger than the vessels found in the inferior limbus (RM-ANOVA POS p = 0.004), and the vessels within the inferior limbus were on average deeper than the vessels found within the superior limbus (RM-ANOVA POS p = 0.042). There was a positive linear relationship between limbal vessel depth and size within the superior and inferior limbus with a Pearson correlation coefficient of 0.803 and 0.754, respectively. There were on average 9 vessels per subject within the superior limbal region as compared to 13 vessels per subject with the inferior limbal region. Conclusion: This study provided evidence that the UHR-OCT was capable of imaging (and therefore measuring) morphometric characteristics such as the size and depth of vessels within the limbus in a repeatable manner. A positive linear association between vessel depth and size was identified in the superior and inferior limbal regions. The results of this study suggest a definite difference in the size and depth of vessels across different positions of the limbus, and this may be indicative of adaptations to chronic hypoxia caused by the constant covering of the superior limbus by the upper eyelid.

Precise measurements of corneal layer thickness are required to treat, evaluate risk of, and determine the progression of pathologies within the eye. The thickness measurements are typically acquired as 2d images, known as tomograms, from an optical coherence tomography (OCT) system. With the creation of ultra-high resolution OCT (UHROCT), there is active research in precisely measuring, in vivo, previously unresolvable corneal structures at arbitrary locations within the cornea to determine their relationship with corneal health. In order to obtain arbitrary corneal thickness measurements, existing reconstruction techniques require the cornea to be densely sampled so that a 3d representation can be interpolated from a stack of tomograms. Unfortunately, tomogram alignment relies solely on image properties such as pixel intensity, and does not constrain the reconstruction to corneal anatomy. Further, the reconstruction method cannot properly compensate for eye-motion. The deficiencies due to eye-motion are exacerbated due to the amount of time required in a single imaging session to acquire a sufficient number of tomograms in the region of interest. The proposed methodology is the first to incorporate models of the anatomy and the imaging system to address the limitations of existing corneal reconstruction methods. By constructing the model in such a way as to decouple anatomy from the imaging system, it becomes less computationally expensive to estimate model parameters. The decoupling provides an iterative methodology that can allow additional constraints to be introduced in the future. By combining sparsely sampled UHROCT measurements with a properly designed corneal model, reconstruction allows researchers to determine corneal layer thicknesses at arbitrary positions in both sampled and unsampled regions. The proposed methodology demonstrates an approach to decouple anatomy and physiology from measurements of a cornea, allowing for characterization of pathologies through corneal thickness measurements. Another significant contribution resulting from the corneal model allows five of the corneal layer boundaries to be automatically located and has already been used to process thousands of UHROCT tomograms. Recent studies using this method have also been used to correlate contact-lens wear to hypoxia and corneal layer swelling. While corneal reconstruction represents the main application of this work, the reconstruction methodology can be extended to other medical imaging domains and can even represent temporal changes in tissue with minor modifications to the framework.