Phase estimation for global defocus correction in optical coherence tomography

In this work we investigate three techniques for estimation of the non-linear phase present due to defocus in optical coherence tomography, and apply them with the angular spectrum method. The techniques are: Least squares fitting the of unwrapped phase of the angular spectrum, iterative optimization, and sub-aperture correlations. The estimated phase of a single en-face image is used to extrapolate the non-linear phase at all depths, which in the end can be used to correct the entire 3-D tomogram, and any other tomogram from the same system

Real-time high-resolution mid-infrared optical coherence tomography

The potential for improving the penetration depth of optical coherence tomography systems by using light sources with longer wavelengths has been known since the inception of the technique in the early 1990s. Nevertheless, the development of mid-infrared optical coherence tomography has long been challenged by the maturity and fidelity of optical components in this spectral region, resulting in slow acquisition, low sensitivity, and poor axial resolution. In this work, a mid-infrared spectral-domain optical coherence tomography system operating at a central wavelength of 4?µm and an axial resolution of 8.6?µm is demonstrated. The system produces two-dimensional cross-sectional images in real time enabled by a high-brightness 0.9- to 4.7-µm mid-infrared supercontinuum source with a pulse repetition rate of 1?MHz for illumination and broadband upconversion of more than 1-µm bandwidth from 3.58–4.63?µm to 820–865?nm, where a standard 800-nm spectrometer can be used for fast detection. The images produced by the mid-infrared system are compared with those delivered by a state-of-the-art ultra-high-resolution near-infrared optical coherence tomography system operating at 1.3??m, and the potential applications and samples suited for this technology are discussed. In doing so, the first practical mid-infrared optical coherence tomography system is demonstrated, with immediate applications in real-time non-destructive testing for the inspection of defects and thickness measurements in samples that exhibit strong scattering at shorter wavelengths

Mid-infrared OCT imaging in highly scattering samples using real-time upconversion of broadband supercontinuum covering from 3.6-4.6 μm

We present a mid-infrared spectral-domain optical coherence tomography system operating at 4.1 μm central wavelength with a high axial resolution of 8.6 μm enabled by more than 1 μm bandwidth from 3.58-4.63 μm produced by a mid-infrared supercontinuum laser. The system produces 2D cross-sectional images in real-time enabled the high-brightness of the supercontinuum source in combination with broadband upconversion of the signal to the range 820-865 nm, where a standard 800 nm array spectrometer can be used for fast detection. We discuss the potential applications within nondestructive testing in highly scattering materials and within biomedical imaging for achieving the in-vivo optical biopsy

Supercontinuum applications in high resolution non invasive optical imaging

Progress will be presented in adapting supercontinuum sources to a variety of applications with emphasis on signal processing procedures. These are customised to alleviate noise and take full advantage of the large bandwidth and large power spectral density of modern supercontinuum sources

Supercontinuum sources in optical coherence tomography: how pulse-to-pulse fluctuations affect the noise performance

Supercontinuum sources are increasingly applied to spectral domain optical coherence tomography (OCT) due to their high power across octave-spanning bandwidths from the visible to the mid-infrared, enabling ultra-high resolution imaging with great flexibility in choice of operating wavelength region [1]. However, one of the main drawback of supercontinuum sources in OCT imaging is the large pulse-to-pulse fluctuations which often acts as the limiting factor in terms of sensitivity rather than the shot noise [2]. The theoretical noise description widely used by the OCT community assumes that the light source operation is based on spontaneous emission, which is not the case for supercontinuum laser source [3]. As a result, the optimal operating conditions must be evaluated experimentally without a reliable prediction [2,4]. Without a reliable theoretical noise model, optimization can be challenging. We present a new and simple noise model that allows prediction of the noise performance of an OCT system driven by a supercontinuum sources, without any assumptions regarding the type of light source. We show that the predictions are in excellent agreement with the experimental results obtained by employing a widely used commercial supercontinuum source. We further investigate the shape of the noise floor in an A-scan obtained with a commercial supercontinuum source, which is not flat, as expected for shot-noise limited light sources. We demonstrate that this shape is predicted solely by the spectral correlations of the supercontinuum, which therefore must be taken into account when characterizing the sensitivity of the OCT system driven by a supercontinuum source

Automatic Segmentation of Epidermis and Hair Follicles in Optical Coherence Tomography Images of Normal Skin by Convolutional Neural Networks

Optical coherence tomography (OCT) is a well-established bedside imaging modality that allows analysis of skin structures in a non-invasive way. Automated OCT analysis of skin layers is of great relevance to study dermatological diseases. In this paper, an approach to detect the epidermal layer along with the follicular structures in healthy human OCT images is presented. To the best of the authors' knowledge, the approach presented in this paper is the only epidermis detection algorithm that segments the pilosebaceous unit, which is of importance in the progression of several skin disorders such as folliculitis, acne, lupus erythematosus, and basal cell carcinoma. The proposed approach is composed of two main stages. The first stage is a Convolutional Neural Network based on U-Net architecture. The second stage is a robust post-processing composed by a Savitzky-Golay filter and Fourier Domain Filtering to fully define the borders belonging to the hair follicles. After validation, an average Dice of 0.83 ± 0.06 and a thickness error of 10.25 μm is obtained on 270 human skin OCT images. Based on these results, the proposed method outperforms other state-of-the-art methods for epidermis segmentation. It demonstrates that the proposed image segmentation method successfully detects the epidermal region in a fully automatic way in addition to defining the follicular skin structures as main novelty.</p

High resolution mid-infrared optical coherence tomography with kHz line rate

We report on Mid-infrared (MIR) OCT at 4 $\mu$m based on collinear sum-frequency upconversion and promote the A-scan scan rate to 3 kHz. We demonstrate the increased imaging speed for two spectral realizations, one providing an axial resolution of 8.6 $\mu$m, and one providing a record axial resolution of 5.8 $\mu$m. Image performance is evaluated by sub-surface micro-mapping of a plastic glove and real-time monitoring of CO$_2$ in parallel with OCT imaging