We present a novel (to our knowledge) approach for measurement of the three-dimensional point spread function (PSF) of optical coherence tomography (OCT) systems using a nanoparticle-embedded phantom (NEP), toward development of standardized test methods for biophotonic imaging. The NEP comprises highly reflective plasmonic nanoparticles, homogeneously distributed in a transparent silicone matrix. OCT image volumes were analyzed to characterize PSFs in axial and lateral directions at a variety of locations in the NEP. Results indicate submicrometer agreement with conventional approaches to measure dimensions of the PSF. The NEP offers a robust approach for validating and comparing imaging performance of OCT devices.
"Recently, our group  and others  have developed phantoms for three-dimensional characterization of the OCT point spread function (PSF) using a sparse distribution of nanoparticles or microparticles embedded in a transparent polymer. This type of phantom permits rapid and detailed measurements of spatial resolution in both lateral and axial dimensions, and indicates how the resolution varies over the imaged volume. "
[Show abstract][Hide abstract] ABSTRACT: In optical coherence tomography (OCT), axial resolution is one of the most critical parameters impacting image quality. It is commonly measured by determining the point spread function (PSF) based on a specular surface reflection. The contrast transfer function (CTF) provides more insights into an imaging system's resolving characteristics and can be readily generated in a system-independent manner, without consideration for image pixel size. In this study, we developed a test method for determination of CTF based on multi-layer, thin-film phantoms, evaluated using spectral- and time-domain OCT platforms with different axial resolution values. Phantoms representing six spatial frequencies were fabricated and imaged. The fabrication process involved spin coating silicone films with precise thicknesses in the 8-40 μm range. Alternating layers were doped with a specified concentration of scattering particles. Validation of layer optical properties and thicknesses were achieved with spectrophotometry and stylus profilometry, respectively. OCT B-scans were used to calculate CTFs and results were compared with convetional PSF measurements based on specular reflections. Testing of these phantoms indicated that our approach can provide direct access to axial resolution characteristics highly relevant to image quality. Furthermore, tissue phantoms based on our thin-film fabrication approach may have a wide range of additional applications in optical imaging and spectroscopy.
"Our group and others have designed and fabricated phantoms incorporating high-contrast, near- or sub-resolution particles to enable characterization of axial and lateral PSFs across the entire three-dimensional OCT image field. In our prior work we introduced a nanoparticle-embedded phantom (NEP) comprising gold nanoshells embedded in silicone , as these plasmonic nanoparticles have been shown to generate strong backscattering signals at 1300 nm . Other approaches have included resin-based phantoms with silica microspheres  and submicron iron oxide particles [13,14]. "
[Show abstract][Hide abstract] ABSTRACT: We have designed, fabricated, and tested a nanoparticle-embedded phantom (NEP) incorporated into a model eye in order to characterize the point spread function (PSF) of retinal optical coherence tomography (OCT) devices in three dimensions under realistic imaging conditions. The NEP comprises a sparse distribution of highly backscattering silica-gold nanoshells embedded in a transparent UV-curing epoxy. The commercially-available model eye replicates the key optical structures and focusing power of the human eye. We imaged the model eye-NEP combination with a research-grade spectral domain OCT system designed for in vivo retinal imaging and quantified the lateral and axial PSF dimensions across the field of view in the OCT images. We also imaged the model eye-NEP in a clinical OCT system. Subtle features in the PSF and its dimensions were consistent with independent measurements of lateral and axial resolution. This model eye-based phantom can provide retinal OCT device developers and users a means to rapidly, objectively, and consistently assess the PSF, a fundamental imaging performance metric.
"The method  and  were generalized by Woolliams et al.  to estimate 2D PSF at multiple locations across the B-scans to account for the variation in PSF as a function of both depth and lateral position. Agrawal et al.  present a PSF measurement approach using a nanoparticle-embedded phantom, towards the development of standardized test methods. However, the optical parameters or the PSF of OCT system needed to be calculated by performing a separate experiment on a specialized phantom to calibrate the parameters and the parameters may not be valid for complex heterogeneous structures. "
[Show abstract][Hide abstract] ABSTRACT: This paper proposes an automatic point spread function (PSF) estimation method to de-blur out-of-focus optical coherence tomography (OCT) images. The method utilizes Richardson-Lucy deconvolution algorithm to deconvolve noisy defocused images with a family of Gaussian PSFs with different beam spot sizes. Then, the best beam spot size is automatically estimated based on the discontinuity of information entropy of recovered images. Therefore, it is not required a prior knowledge of the parameters or PSF of OCT system for de-convoluting image. The model does not account for the diffraction and the coherent scattering of light by the sample. A series of experiments are performed on digital phantoms, a custom-built phantom doped with microspheres, fresh onion as well as the human fingertip in vivo to show the performance of the proposed method. The method may also be useful in combining with other deconvolution algorithms for PSF estimation and image recovery.
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