The modulation transfer function of an optical coherence tomography imaging system in turbid media

Nano and Multifunctional Materials Group, National Physical Laboratory, Teddington, TW11 0LW, UK.
Physics in Medicine and Biology (Impact Factor: 2.76). 05/2011; 56(9):2855-71. DOI: 10.1088/0031-9155/56/9/014
Source: PubMed


In this paper we describe measurements of the contrast transfer function, modulation transfer function and point-spread function of an optical coherence tomography (OCT) imaging system through scattering layers having a dimension-less scattering depth over the range 0.2-6.9. The results were found to be insensitive to scattering density, indicating that these measurement parameters alone do not well characterize the practical imaging ability of an OCT instrument. Attenuation and increased noise floor due to optical scattering were found to be the primary imaging limit and the effect of multiple scattering on OCT resolution was negligible.

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    • "By varying the dimensions of the embedded microspheres and the thickness of intervening polymer layers, different spatial frequencies are replicated in the axial dimension of the phantom. Furthermore, these frequencies can provide a standardized approach to determine the axial contrast transfer function for the quantitative application in an OCT imaging system [6]. For precise spatial calibration, bulk phantom dimensions were independently measured using a surface interferometric technique. "
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    ABSTRACT: We report on a novel fabrication approach to build multilayered optical tissue phantoms that serve as independently validated test targets for axial resolution and contrast in scattering measurements by depth-resolving optical coherent tomography (OCT) with general applicability to a variety of three-dimensional optical sectioning platforms. We implement a combinatorial bottom-up approach to prepare monolayers of light-scattering microspheres with interspersed layers of transparent polymer. A dense monolayer assembly of monodispersed microspheres is achieved via a combined methodology of polyelectrolyte multilayers (PEMs) for particle-substrate binding and convective particle flux for two-dimensional crystal array formation on a glass substrate. Modifications of key parameters in the layer-by-layer polyelectrolyte deposition approach are applied to optimize particle monolayer transfer from a glass substrate into an elastomer while preserving the relative axial positioning in the particle monolayer. Varying the dimensions of the scattering microspheres and the thickness of the intervening transparent polymer layers enables different spatial frequencies to be realized in the transverse dimension of the solid phantoms. Step-wise determination of the phantom dimensions is performed independently of the optical system under test to enable precise spatial calibration, independent validation, and quantitative dimensional measurements.
    Biomedical Optics Express 06/2012; 3(6):1326-39. DOI:10.1364/BOE.3.001326 · 3.65 Impact Factor
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    • "Standard approaches for assessing OCT system axial resolution have typically involved imaging a specular surface such as a mirror [7]. Lateral resolution is typically calculated based on the profile of the OCT illumination beam [8] or measured with a resolution target [9]. These approaches are limited in that they do not characterize variations in resolution throughout the 3D imaging volume, and they are difficult to implement in a geometry that simulates the human eye. "
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    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.
    Biomedical Optics Express 05/2012; 3(5):1116-26. DOI:10.1364/BOE.3.0011163 · 3.65 Impact Factor
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    ABSTRACT: Selecting the most representative site for biopsy is crucial in establishing a definitive diagnosis of oral epithelial dysplasia. The current process involves clinical examination that can be subjective and prone to sampling errors. The aim of this study was therefore to investigate the use of optical coherence tomography (OCT) for differentiation of normal and dysplastic oral epithelial samples, with a view to developing an objective and reproducible approach for biopsy site selection. Biopsy samples from patients with fibro-epithelial polyps (n = 13), mild dysplasia (n = 2), and moderate/severe dysplasia (n = 4) were scanned at 5-μm intervals using an OCT microscope and subsequently processed and stained with hematoxylin and eosin (H&E). Epithelial differentiation was measured from the rate of change (gradient) of the backscattered light intensity in the OCT signal as a function of depth. This parameter is directly related to the density of optical scattering from the cell nuclei. OCT images of normal oral epithelium showed a clear delineation of the mucosal layers observed in the matching histology. However, OCT images of oral dysplasia did not clearly identify the individual mucosal layers because of the increased density of abnormal cell nuclei, which impeded light penetration. Quantitative analysis on 2D-OCT and histology images differentiated dysplasia from normal control samples. Similar analysis on 3D-OCT datasets resulted in the reclassification of biopsy samples into the normal/mild and moderate/severe groups. Quantitative differentiation of normal and dysplastic lesions using OCT offers a non-invasive objective approach for localizing the most representative site to biopsy, particularly in oral lesions with similar clinical features.
    Lasers in Medical Science 08/2011; 27(4):795-804. DOI:10.1007/s10103-011-0975-1 · 2.49 Impact Factor
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