| dc.description.abstract | Ophthalmic imaging has long played an important role in the understanding, diagnosis, and treatment of a wide variety of ocular disorders. Currently, available clinical ophthalmic imaging instruments are primarily optical-based, including slit-lamp microscopy, fundus photography, confocal microscopy, scanning laser ophthalmoscopy, and optical coherence tomography (OCT). The development of these imaging instruments has greatly extended our ability to evaluate the ocular environment. Studies have shown that at least 40% of blinding disorders in the United States are either preventable or treatable with timely diagnosis and intervention.
OCT is a state-of-the-art imaging technique extensively used in preclinical and clinical applications for imaging both anterior and posterior parts of the eye. OCT has become a standard of care for the assessment and treatment of most ocular conditions. The technology enables non-contact, high-speed, cross-sectional imaging over a large field of view with submicron resolutions.
In eye imaging applications, functional extensions of OCT such as spectroscopic OCT and Doppler OCT have been applied to provide a better understanding of tissue activity. Spectroscopic OCT is usually achieved through OCT systems in the visible spectral range, and it enables the amount of light absorption inside the ocular environment to be measured. This indirect optical absorption measurement is used to estimate the amount of ocular oxygen saturation (SO2) which is a well-known biomarker in prevalent eye diseases including diabetic retinopathy, glaucoma, and retinal vein occlusions. Despite all the advancements in functional spectroscopic OCT methods, they still rely primarily on measuring the backscattered photons to quantify the absorption of chromophores inside the tissue. Therefore, they are sensitive to local geometrical parameters, such as retinal thickness, vessel diameters, and retinal pigmentation, and may result in biased estimations.
Of the various optical imaging modalities, photoacoustic imaging (PAI) offers unique imaging contrast of optical absorption because PAI can image any target that absorbs light energy. This unique imaging ability makes PAI a favorable candidate for various functional and molecular imaging applications as well as for measuring chromophore concentration.
Over the past decade, photoacoustic ophthalmoscopy has been applied for visualizing hemoglobin and melanin content in ocular tissue, quantifying ocular SO2, and measuring the metabolic rate of oxygen consumption (MRO2). Despite all these advantages offered by PAI devices, a major limitation arises from their need to be in contact with the ocular tissues. This physical contact may increase the risk of infection and cause patient discomfort. Furthermore, this contact-based imaging approach applies pressure to the eye and introduces barriers to oxygen diffusion. Thus, it has a crucial influence on the physiological and pathophysiological balance of ocular vasculature function, and it is not capable of studying dynamic processes under normal conditions. To overcome these limitations and to benefit from the numerous advantages offered by photoacoustic ophthalmoscopy, non-contact detection of photoacoustic signals has been a long-lasting goal in the field of ocular imaging.
In 2017 Haji Reza et al. developed photoacoustic remote sensing (PARS) for non-contact, non-interferometric detection of photoacoustic signals. PARS is the non-contact, all-optical version of optical-resolution photoacoustic microscopy (OR-PAM), where the acoustically coupled ultrasound transducer is replaced with a co-focused probe beam. This all-optical detection scheme allows the system to measure the photoacoustic pressure waves at the subsurface origin where the pressure is at a maximum. In a very short time, PARS technology has proven its potential for various biomedical applications, including label-free histological imaging, SO2 mapping, and angiogenesis imaging. PARS is an ideal companion for OCT in ophthalmic applications, where the depth-resolved, detailed scattering information of OCT is well complemented by rich absorption information of PARS. This combined multimodal imaging technology has the potential to provide chromophore selective absorption contrast in concert with depth-resolved scattering contrast in the ocular environment.
The main goals of this PhD project are to:
• Develop a photoacoustic remote sensing microscopy system for in-vivo, non-contact ophthalmic imaging. This is the first time a non-contact photoacoustic imaging has been used for in-vivo imaging of the eye.
• Develop a robust and temporally stable multiwavelength light source for functional photoacoustic imaging applications.
• Develop a multimodal PARS-OCT imaging system that can image in-vivo and record, simultaneously, functional, and structural information in the anterior segment of a rodent eye. This is the first time a multiwavelength non-contact photoacoustic system is used for in-vivo measurement of oxygen saturation in the ocular environment.
• Develop and modify the multimodal PARS-OCT imaging system for non-contact, in-vivo, functional, and structural imaging of the posterior part of the rodent eye. | en |
