Browsing by Subject "Super resolution"

Advancements in retinal imaging technologies have drastically improved the quality of eye care in the past couple decades. Scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT) are two examples of critical imaging modalities for the diagnosis of retinal pathologies. However current-generation SLO and OCT systems have limitations in diagnostic capability due to the following factors: the use of bulky tabletop systems, monochromatic imaging, and resolution degradation due to ocular aberrations and diffraction.

Bulky tabletop SLO and OCT systems are incapable of imaging patients that are supine, under anesthesia, or otherwise unable to maintain the required posture and fixation. Monochromatic SLO and OCT imaging prevents the identification of various color-specific diagnostic markers visible with color fundus photography like those of neovascular age-related macular degeneration. Resolution degradation due to ocular aberrations and diffraction has prevented the imaging of photoreceptors close to the fovea without the use of adaptive optics (AO), which require bulky and expensive components that limit the potential for widespread clinical use.

In this dissertation, techniques for extending the diagnostic capability of SLO and OCT systems are developed. These techniques include design strategies for miniaturizing and combining SLO and OCT to permit multi-modal, lightweight handheld probes to extend high quality retinal imaging to pediatric eye care. In addition, a method for extending true color retinal imaging to SLO to enable high-contrast, depth-resolved, high-fidelity color fundus imaging is demonstrated using a supercontinuum light source. Finally, the development and combination of SLO with a super-resolution confocal microscopy technique known as optical photon reassignment (OPRA) is demonstrated to enable high-resolution imaging of retinal photoreceptors without the use of adaptive optics.

The human retina is essential to quality of life and therefore a topic of intense clinical and research interest. The combination of this interest with modern biophotonics has yielded a number of technological and medical developments now in various stages of adoption.

Optical coherence tomography (OCT) is a noninvasive optical imaging technique that utilizes coherent light to produce 3-D images with resolutions as fine as a micrometer. Since its invention in 1990, it has become part of the standard of care in opthalmology, shedding new light on the progression of diseases, therapeutic efficacy, childhood development, and real-time surgery in the retina. OCT has also found applications in microscopy, cardiology, pulmonology, and many other fields.

OCT has become valuable for the standard of care primarily due to its abilities to visualize the structural and functional layers of the retina. The thicknesses and volumes of certain can be used as diagnostic criteria and thus there is a high demand of OCT image assessment. In response, many researchers have developed software algorithms to automatically identify and mark, or segment, each layer.

Scanning light ophthalmoscopy or scanning laser ophthalmoscopy (SLO) is similar to OCT but uses confocal gating to produce high-contrast high-speed en face images of the retina. Although SLO has not become as prevalent as OCT in the clinic, it is frequently combined with adaptive optics (AO) to produce extremely high-resolution images of rod and cone photoreceptors, ganglion cells, and moving blood cells in the living retina.

AO is a technique to eliminate image blurring due to monochromatic aberrations in optical systems. By using a spatial light modulator, such as a deformable mirror or liquid crystal array, the wavefront of a beam sent into the eye can be engineered to compensate for the eye's aberrations. AO-SLO was initially developed in 2002 and has continued to be a field of research growth and interest. However, the majority of AOSLO systems require a dedicated room and staff, hindering their clinical adoption

The objective of the work presented herein was to explore the limits of the above imaging modalities. First, we explored the limits of OCT segmentation and demonstrated that the field of automated segmentation is far from its accuracy limit. Second, we explored the limits on SLO portability and developed both the world's smallest SLO probe and the first handheld AOSLO probe. Finally, we explored the limits of SLO resolution, developing the first super-resolution human retinal imaging system through the use of optical reassignment (OR) SLO.

The National Research Council’s Planetary Science 2013-2022 Decadal Survey underscores three interrelated themes pivotal to planetary science research: understanding solar system beginnings, searching for the requirements for life, and understanding the workings of solar systems. In situ mass spectrometry (MS) is the primary technique for the analysis of planetary substances, directly addressing the critical inquiries associated with these themes. The quintessential mass analyzer engineered for space exploration is envisioned to embody a suite of features: a mass range extending from 1 u to at least 500 u, capability for high-precision measurement of stable isotope ratios within a tolerance of ±1‰, and the ability to resolve distinct isobaric species at a low mass below 60 u, all with low power requirements. Incorporation of these capabilities within a single instrument is crucial for facilitating the exploration of the necessities of life and for advancing our understanding of solar system genesis and planetary development. Nevertheless, state-of-the-art existing spaceflight mass spectrometers do not fully integrate all these capabilities.In this research, three technologies are investigated to close this gap; spatial aperture coding, super-resolution, and field emission electron sources . The development of these three technologies as presented in this dissertation represent a significant step towards a mass spectrometer having all of the characteristics described above. First, Spatial aperture coding is a technique used to improve throughput without sacrificing resolution, historically in optical spectroscopy, and more recently as demonstrated by our laboratory at Duke University, in sector mass spectrometry (MS). Previously we demonstrated that aperture coding combined with a position-sensitive array detector in a miniature cycloidal mass spectrometer was successful in providing high-throughput, high-resolution measurements. However, due to poor alignment and field non-uniformities, reconstruction artifacts were present. In this dissertation, two methods were implemented to significantly reduce the presence of artifacts in reconstructed spectra. First, I employed a variable system response function across the mass range (10 – 110 u) instead of using a fixed function. Second, I modified the design by shifting the coded aperture slits relative to the center of the ionization volume to enable even illumination of the coded aperture slits. Both methods were successful in significantly reducing artifacts at low mass from above 35% of the peak height to less than 6% of the peak height. Second, higher resolution in fieldable mass spectrometers (MS) is desirable in space flight applications to enable resolving isobaric interferences at m/z < 60 u. Resolution in portable cycloidal MS coupled with array detectors could be improved by reducing the slit width and/or by reducing the width of the detector pixels. However, these solutions are expensive and can result in reduced sensitivity. In this dissertation, I demonstrate high-resolution spectral reconstruction in a cycloidal coded aperture miniature mass spectrometer (C-CAMMS) without changing the slit or detector pixel sizes using a class of signal processing techniques called super resolution (SR). I developed an SR reconstruction algorithm using a sampling SR approach whereby a set of spatially shifted low-resolution measurements are reconstructed into a higher-resolution spectrum. This algorithm was applied to experimental data collected using the C-CAMMS prototype. It was then applied to synthetic data with additive noise, system response variation, and spatial shift nonuniformity to investigate the source of reconstruction artifacts in the experimental data. Experimental results using two 1/2 pixel shifted spectra resulted in a resolution of 3/4 pixel full width at half maximum (FWHM) at m/z = 28 u. This resolution is equivalent to 0.013 u, six times better than the resolution previously published at m/z = 28 for N2+ using C-CAMMS. However, the reconstructed spectra exhibited some artifacts. The results of the synthetic data study indicate that the artifacts are most likely caused by the system response variation. Despite these artifacts, it was shown that the super-resolution algorithm is capable of resolving the isobaric interference between N2 and CO at m/z = 28. Third, Field emission electron sources for MS electron ionization have been of interest to spaceflight applications due to their low power compared to thermionic sources. However, state-of-the-art devices suffer from limitations such as high turn-on macroscopic field, low macroscopic current density, poor emission stability, and short lifetime. Field emitter arrays with a high spatial density of uniform emitters have the potential to address these problems. In this work, process development, fabrication, and testing of two novel field emission based devices are presented, including CNT array emitters and metallic nanowires. Instability in CNT emission was investigated using noise analysis and a polymer encapsulation process to reduce the effect of adsorbates on the tips of CNTs. This treatment was not successful in reducing emission noise in CNTs. Thus, electron beam lithography and templated electrodeposition were used to fabricate a high spatial density array of metallic nanowires, resulting in electron field emission with high macroscopic current density (2 A/cm2) and low turn-on macroscopic field (4.35 V/μm). Results indicate that templated electrodeposition of metallic nanowire arrays is a promising method for producing high-performance field emitters.

Image super resolution is defined as recovering a high-resolution image given a low-resolution image input. It has a wide area of applications in modern digital image processing, producing better results in areas including satellite image processing, medical image processing, microscopy image processing, astrological studies and surveillance area. However, image super resolution is an ill-posed question since there exists non-deterministic answer in the high resolution image space, making it difficult to find the optimal solution.

In this work, various research directions in the area of single image super resolution are thoroughly studied. Each of the proposed methods' achievements as well as limitations including computational efficiency, perceptual performance limits are compared. The main contribution in this work including implementing a perceptual score predictor and integrating as part of the objective function in the upsampler algorithm. Apart from that, a test-time optimization algorithm is proposed, aiming at further enhance the image quality for the obtained super-resolution image from any upsampler. The proposed methods are implemented and tested using Pytorch. Results are compared on baseline applied datasets including Set5, Set14, Urban100 and DIV2K.

Results from perceptual score predictor was evaluated on both PSNR precision index and perceptual index, which is a combination of perceptual evaluation Ma score and NIQE score. With new objective function, the upsampler achieved to move along the trade-off curve of precision and perception. The test-time optimization algorithm achieved slightly improvements in both precision and perception index. Note that the proposed test time optimization does not require training of new neural network, thus, is computationally efficient.