Full-field time-encoded frequency-domain optical coherence tomography

Polarization-resolved analysis of outer retinal bands: investigating ballistic and multiply scattered photons using full-field swept-source optical coherence tomography

Precise interpretation of the anatomical origins of outer retinal optical coherence tomography (OCT) presents technical challenges owing to the delicate nature of the retina. To address this challenge, our study introduces a novel polarization-sensitive full-field swept-source OCT (FF-SS-OCT) that provides parallel-polarization and cross-polarization OCT measurements, predominantly capturing ballistically reflected photons and multiply scattered photons, respectively. Notably, parallel-polarization OCT unveils layer-like structures more effectively, including the inner plexiform layer (IPL) sub-layers, outer plexiform layer (OPL) sub-layers (in rod-dominant regions), and rod/cone outer segment (OS) tips, compared to cross-polarization OCT, where such sub-layers are not visible. Through a comparative analysis of parallel-polarization and cross-polarization OCT images of the outer retina, we discovered that the 2nd outer retinal OCT band results from contributions from both the ellipsoid zone (EZ) and the inner segment/outer segment (IS/OS) junction. Similarly, the 3rd outer retinal OCT band appears to reflect contributions from both the interdigitation zone (IZ) and photoreceptor OS tips. This polarization-sensitive approach advances our understanding of the origins of outer retinal OCT signals and proposes potential new biomarkers for assessing retinal health and diseases.

Optical tomography in a single camera frame using fringe-encoded deep-learning full-field OCT

Optical coherence tomography is a valuable tool for in vivo examination thanks to its superior combination of axial resolution, field-of-view and working distance. OCT images are reconstructed from several phases that are obtained by modulation/multiplexing of light wavelength or optical path. This paper shows that only one phase (and one camera frame) is sufficient for en face tomography. The idea is to encode a high-frequency fringe patterns into the selected layer of the sample using low-coherence interferometry. These patterns can then be efficiently extracted with a high-pass filter enhanced via deep learning networks to create the tomographic full-field OCT view. This brings 10-fold improvement in imaging speed, considerably reducing the phase errors and incoherent light artifacts related to in vivo movements. Moreover, this work opens a path for low-cost tomography with slow consumer cameras. Optically, the device resembles the conventional time-domain full-field OCT without incurring additional costs or a field-of-view/resolution reduction. The approach is validated by imaging in vivo cornea in human subjects. Open-source and easy-to-follow codes for data generation/training/inference with U-Net/Pix2Pix networks are provided to be used in a variety of image-to-image translation tasks.

Diffuse-illumination holographic optical coherence tomography

Holographic optical coherence tomography (OCT) is a powerful imaging technique, but its ability to reveal low-reflectivity features is limited. In this study, we performed holographic OCT by incoherently averaging volumes with changing diffuse illumination of numerical aperture (NA) equal to the detection NA. While the reduction of speckle from singly scattered light is only modest, we discovered that speckle from multiply scattered light can be arbitrarily reduced, resulting in substantial improvements in image quality. This technique also offers the advantage of suppressing noises arising from spatial coherence, and can be implemented with a partially spatially incoherent light source for further mitigation of multiple scattering. Finally, we show that although holographic reconstruction capabilities are increasingly lost with decreasing spatial coherence, they can be retained over an axial range sufficient to standard OCT applications.

Time-domain full-field optical coherence tomography (TD-FF-OCT) in ophthalmic imaging

Ocular imaging plays an irreplaceable role in the evaluation of eye diseases. Developing cellular-resolution ophthalmic imaging technique for more accurate and effective diagnosis and pathogenesis analysis of ocular diseases is a hot topic in the cross-cutting areas of ophthalmology and imaging. Currently, ocular imaging with traditional optical coherence tomography (OCT) is limited in lateral resolution and thus can hardly resolve cellular structures. Conventional OCT technology obtains ultra-high resolution at the expense of a certain imaging range and cannot achieve full field of view imaging. In the early years, Time-domain full-field OCT (TD-FF-OCT) has been mainly used for ex vivo ophthalmic tissue studies, limited by the low speed and low full-well capacity of existing two-dimensional (2D) cameras. The recent improvements in system design opened new imaging possibilities for in vivo applications thanks to its distinctive optical properties of TD-FF-OCT such as a spatial resolution almost insensitive to aberrations, and the possibility to control the curvature of the optical slice. This review also attempts to look at the future directions of TD-FF-OCT evolution, for example, the potential transfer of the functional-imaging dynamic TD-FF-OCT from the ex vivo into in vivo use and its expected benefit in basic and clinical ophthalmic research. Through non-invasive, wide-field, and cellular-resolution imaging, TD-FF-OCT has great potential to be the next-generation imaging modality to improve our understanding of human eye physiology and pathology.

Line-scanning SD-OCT for in-vivo, non-contact, volumetric, cellular resolution imaging of the human cornea and limbus

In-vivo, non-contact, volumetric imaging of the cellular and sub-cellular structure of the human cornea and limbus with optical coherence tomography (OCT) is challenging due to involuntary eye motion that introduces both motion artifacts and blur in the OCT images. Here we present the design of a line-scanning (LS) spectral-domain (SD) optical coherence tomography system that combines 2 × 3 × 1.7 µm (x, y, z) resolution in biological tissue with an image acquisition rate of ∼2,500 fps, and demonstrate its ability to image in-vivo and without contact with the tissue surface, the cellular structure of the human anterior segment tissues. Volumetric LS-SD-OCT images acquired over a field-of-view (FOV) of 0.7 mm × 1.4 mm reveal fine morphological details in the healthy human cornea, such as epithelial and endothelial cells, sub-basal nerves, as well as the cellular structure of the limbal crypts, the palisades of Vogt (POVs) and the blood microvasculature of the human limbus. LS-SD-OCT is a promising technology that can assist ophthalmologists with the early diagnostics and optimal treatment planning of ocular diseases affecting the human anterior eye.

Ultra-broadband wavelength-swept Ti:sapphire crystal fiber laser

An ultra-broadband wavelength-swept laser (WSL) was generated using glass-clad Ti:sapphire crystal fiber as the gain media. Due to the low signal propagation loss of the crystal fiber, the swept laser has a tuning bandwidth of 250 nm (i.e., 683 nm to 933 nm) at a repetition rate of 1200 Hz. The steady-state and pulsed dynamics of the WSL were analyzed. The 0.018-nm instantaneous linewidth corresponds to a 3-dB coherence roll-off of 7 mm. When using the laser for swept-source optical coherence tomography, an estimated axial resolution of 1.8 µm can be achieved.

Retinal blood flow imaging with combined full-field swept-source optical coherence tomography and laser Doppler holography

Full-field swept-source optical coherence tomography (FF-SS-OCT) and laser Doppler holography (LDH) are two holographic imaging techniques presenting unique capabilities for ophthalmology. We report on interlaced FF-SS-OCT and LDH imaging with a single instrument. Effectively, retinal blood flow and pulsation could be quasi-simultaneously monitored. This instrument holds potential for a wide scope of ophthalmic applications.

Multimode fiber as a tool to reduce cross talk in Fourier-domain full-field optical coherence tomography

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is an emerging tool for high-speed eye imaging. However, cross-talk formation in images limits the imaging depth. To this end, we have recently shown that reducing spatial coherence with a fast deformable membrane can suppress the noise but over a limited axial range and with substantial data processing. Here, we demonstrate that a multimode fiber with carefully chosen parameters enables cross-talk-free imaging over a long axial range and without significant artifacts. We also show that it can be used to image the human retina and choroid in vivo with exceptional contrast.

Optical phase modulation by natural eye movements: application to time-domain FF-OCT image retrieval

Eye movements are commonly seen as an obstacle to high-resolution ophthalmic imaging. In this context we study the natural axial movements of the in vivo human eye and show that they can be used to modulate the optical phase and retrieve tomographic images via time-domain full-field optical coherence tomography (TD-FF-OCT). This approach opens a path to a simplified ophthalmic TD-FF-OCT device, operating without the usual piezo motor-camera synchronization. The device demonstrates in vivo human corneal images under the different image retrieval schemes (2-phase and 4-phase) and different exposure times (3.5 ms, 10 ms, 20 ms). Data on eye movements, acquired with a spectral-domain OCT with axial eye tracking (180 B-scans/s), are used to study the influence of ocular motion on the probability of capturing high-signal tomographic images without phase washout. The optimal combinations of camera acquisition speed and amplitude of piezo modulation are proposed and discussed.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Simultaneous directional full-field OCT using path-length and carrier multiplexing

Full-field swept-source optical coherence tomography (FF-SS-OCT) is an emerging technology with potential applications in ophthalmic imaging, microscopy, metrology, and other domains. Here we demonstrate a novel method of multiplexing FF-SS-OCT signals using carrier modulation (CM). The principle of CM could be used to inspect various properties of the scattered light, e.g. its spectrum, polarization, Doppler shift, or distribution in the pupil. The last of these will be explored in this work, where CM was used to acquire images passing through two different optical pupils. The two pupils contained semicircular optical windows with perpendicular orientations, with each window permitting measurement of scattering anisotropy in one dimension by inducing an optical delay between the images formed by the two halves of the pupil. Together, the two forms of multiplexing permit measurement of differential scattering anisotropy in the x and y dimensions simultaneously. To demonstrate the feasibility of this technique our carrier multiplexed directional FF-OCT (CM-D-FF-OCT) system was used to acquire images of a microlens array, human hair, onion skin and in vivo human retina. The results of these studies are presented and briefly discussed in the context of future development and application of this technique.

Fourier-domain full-field optical coherence tomography with real-time axial imaging

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is a fast interferometric imaging technique capable of volumetric sample imaging. However, half of the backscattered light from a sample is lost as it passes through a 50/50 beam splitter, which is at the heart of almost every interferometer. Here, it is demonstrated that this light could be extracted by spatially splitting the illumination pupil plane and detecting it with a separate camera. When a line camera is used to detect the recovered signal, it enables real-time axial imaging of the human cornea in vivo, which serves as a useful visual feedback for aligning a patient for imaging.

Time-encoded mid-infrared Fourier-domain optical coherence tomography

We report on a technically simple approach to achieve high-resolution and high-sensitivity Fourier-domain optical coherence tomography (OCT) imaging in the mid-infrared (mid-IR) range. The proposed OCT system employs an InF₃ supercontinuum source. A specially designed dispersive scanning spectrometer based on a single InAsSb point detector is employed for detection. The spectrometer enables structural OCT imaging in the spectral range from 3140 nm to 4190 nm with a characteristic sensitivity of over 80 dB and an axial resolution below 8µm. The capabilities of the system are demonstrated for imaging of porous ceramic samples and transition-stage green parts fabricated using an emerging method of lithography-based ceramic manufacturing. Additionally, we demonstrate the performance and flexibility of the system by OCT imaging using an inexpensive low-power (average power of 16 mW above 3µm wavelength) mid-IR supercontinuum source.

Manifestation of aberrations in full-field optical coherence tomography

We report on a theoretical model for image formation in full-field optical coherence tomography (FFOCT). Because the spatial incoherence of the illumination acts as a virtual confocal pinhole in FFOCT, its imaging performance is equivalent to a scanning time-gated coherent confocal microscope. In agreement with optical experiments enabling a precise control of aberrations, FFOCT is shown to have nearly twice the resolution of standard imaging at moderate aberration level. Beyond a rigorous study on the sensitivity of FFOCT with respect to aberrations, this theoretical model paves the way towards an optimized design of adaptive optics and computational tools for high-resolution and deep imaging of biological tissues.

Multimode fiber enables control of spatial coherence in Fourier-domain full-field optical coherence tomography for in vivo corneal imaging

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) has recently emerged as a fast alternative to point-scanning confocal OCT in eye imaging. However, when imaging the cornea with FD-FF-OCT, a spatially coherent laser can focus down on the retina to a spot that exceeds the maximum permissible exposure level. Here we demonstrate that a long multimode fiber with a small core can be used to reduce the spatial coherence of the laser and, thus, enable ultrafast in vivo volumetric imaging of the human cornea without causing risk to the retina.

Kilohertz retinal FF-SS-OCT and flood imaging with hardware-based adaptive optics

A retinal imaging system was designed for full-field (FF) swept-source (SS) optical coherence tomography (OCT) with cellular resolution. The system incorporates a real-time adaptive optics (AO) subsystem and a very high-speed CMOS sensor, and is capable of acquiring volumetric images of the retina at rates up to 1 kHz. While digital aberration correction (DAC) is an attractive potential alternative to AO, it has not yet been shown to provide resolution allowing visualization of cones in the fovea, where early detection of functional deficits is most critical. Here we demonstrate that FF-SS-OCT with hardware AO permits resolution of foveal cones, imaged at eccentricities of 1° and 2°, with volume rates adequate to measure light-evoked changes in photoreceptors. With the reference arm blocked, the system can operate as a kilohertz AO flood illumination fundus camera with adjustable temporal coherence and is expected to allow measurement of light-evoked changes caused by common path interference in photoreceptor outer segments (OS). In this paper, we describe the system's optical design, characterize its performance, and demonstrate its ability to produce images of the human photoreceptor mosaic.

In vivo imaging of the human cornea with high-speed and high-resolution Fourier-domain full-field optical coherence tomography

Corneal evaluation in ophthalmology necessitates cellular-resolution and fast imaging techniques that allow for accurate diagnoses. Currently, the fastest volumetric imaging technique is Fourier-domain full-field optical coherence tomography (FD-FF-OCT), which uses a fast camera and a rapidly tunable laser source. Here, we demonstrate high-resolution, high-speed, non-contact corneal volumetric imaging in vivo with FD-FF-OCT that can acquire a single 3D volume with a voxel rate of 7.8 GHz. The spatial coherence of the laser source was suppressed to prevent it from focusing on a spot on the retina, and therefore, exceeding the maximum permissible exposure (MPE). The inherently volumetric nature of FD-FF-OCT data enabled flattening of curved corneal layers. The acquired FD-FF-OCT images revealed corneal cellular structures, such as epithelium, stroma and endothelium, as well as subbasal and mid-stromal nerves.

Crosstalk-free volumetric in vivo imaging of a human retina with Fourier-domain full-field optical coherence tomography

Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is currently the fastest volumetric imaging technique that is able to generate a single 3-D volume of retina in less than 9 ms, corresponding to a voxel rate of 7.8 GHz. FD-FF-OCT is based on a fast camera, a rapidly tunable laser source, and Fourier-domain signal detection. However, crosstalk appearing due to multiply scattered light corrupts images with the speckle pattern, and therefore, lowers image quality. Here, for the first time, we report on a system that can acquire essentially crosstalk-free volumes of the retina by using a fast deformable membrane. It enables the visualization of choroids and a clear delineation of the retinal layers that is not possible with conventional FD-FF-OCT.

En face optical coherence tomography: a technology review [Invited]

A review on the technological development of en face optical coherence tomography (OCT) and optical coherence microscopy (OCM) is provided. The terminology originally referred to time domain OCT, where the preferential scanning was performed in the en face plane. Potentially the fastest realization of en face image recording is full-field OCT, where the full en face plane is illuminated and recorded simultaneously. The term has nowadays been adopted for high-speed Fourier domain approaches, where the en face image is reconstructed from full 3D volumes either by direct slicing or through axial projection in post processing. The success of modern en face OCT lies in its immediate and easy image interpretation, which is in particular of advantage for OCM or OCT angiography. Applications of en face OCT with a focus on ophthalmology are presented. The review concludes by outlining exciting technological prospects of en face OCT based both on time as well as on Fourier domain OCT.

Spatiotemporal optical coherence (STOC) manipulation suppresses coherent cross-talk in full-field swept-source optical coherence tomography

Full-field swept-source optical coherence tomography (FF-SS-OCT) provides high-resolution depth-resolved images of the sample by parallel Fourier-domain interferometric detection. Although FF-SS-OCT implements high-speed volumetric imaging, it suffers from the cross-talk-generated noise from spatially coherent lasers. This noise reduces the transversal image resolution, which in turn, limits the wide adaptation of FF-SS-OCT for practical and clinical applications. Here, we introduce the novel spatiotemporal optical coherence (STOC) manipulation. In STOC the time-varying inhomogeneous phase masks are used to modulate the light incident on the sample. By properly adjusting these phase masks, the spatial coherence can be reduced. Consequently, the cross-talk-generated noise is suppressed, the transversal image resolution is improved by the factor of2 , and sample features become visible. STOC approach is validated by imaging 1951 USAF resolution test chart covered by the diffuser, scattering phantom and the rat skin ex vivo. In all these cases STOC suppresses the cross-talk-generated noise, and importantly, do not compromise the transversal resolution. Thus, our method provides an enhancement of FF-SS-OCT that can be beneficial for imaging biological samples.

Single-exposure full-field multi-depth imaging using low-coherence holographic multiplexing

We present a new interferometric imaging approach that allows for multiple-depth imaging in a single acquisition, using off-axis low-coherence holographic multiplexing. This technique enables sectioned imaging of multiple slices within a thick sample, in a single image acquisition. Each slice has a distinct off-axis interference fringe orientation indicative of its axial location, and the camera acquires the multiplexed hologram containing the different slices at once. We demonstrate the proposed technique for amplitude and phase imaging of optically thick samples.

Off-axis reference beam for full-field swept-source OCT and holoscopy

In numerous applications, Fourier-domain optical coherence tomography (FD-OCT) suffers from a limited imaging depth due to signal roll-off, a limited focal range, and autocorrelation noise. Here, we propose a parallel full-field FD-OCT imaging method that uses a swept laser source and an area camera in combination with an off-axis reference, which is incident on the camera at a small angle. As in digital off-axis holography, this angle separates autocorrelation signals and the complex conjugated mirror image from the actual signal in Fourier space. We demonstrate that by reconstructing the signal term only, this approach enables full-range imaging, i.e., it increases the imaging depth by a factor of two, and removes autocorrelation artifacts. The previously demonstrated techniques of inverse scattering and holoscopy can then numerically extend the focal range without loss of lateral resolution or imaging sensitivity. The resulting, significantly enhanced measurement depth is demonstrated by imaging a porcine eye over its entire depth, including cornea, lens, and retina. Finally, the feasibility of in vivo measurements is demonstrated by imaging the living human retina.

Twenty-five years of optical coherence tomography: the paradigm shift in sensitivity and speed provided by Fourier domain OCT [Invited]

Optical coherence tomography (OCT) has become one of the most successful optical technologies implemented in medicine and clinical practice mostly due to the possibility of non-invasive and non-contact imaging by detecting back-scattered light. OCT has gone through a tremendous development over the past 25 years. From its initial inception in 1991 [Science254, 1178 (1991)] it has become an indispensable medical imaging technology in ophthalmology. Also in fields like cardiology and gastro-enterology the technology is envisioned to become a standard of care. A key contributor to the success of OCT has been the sensitivity and speed advantage offered by Fourier domain OCT. In this review paper the development of FD-OCT will be revisited, providing a single comprehensive framework to derive the sensitivity advantage of both SD- and SS-OCT. We point out the key aspects of the physics and the technology that has enabled a more than 2 orders of magnitude increase in sensitivity, and as a consequence an increase in the imaging speed without loss of image quality. This speed increase provided a paradigm shift from point sampling to comprehensive 3D in vivo imaging, whose clinical impact is still actively explored by a large number of researchers worldwide.

Optical Coherence Microscopy

The present chapter aims at demonstrating the capabilities of optical coherence microscopy (OCM) for applications in biomedical imaging. We furthermore review the functional imaging capabilities of OCM focusing on lable-free optical angiography. We conclude with a section on digital wavefront control and a short outlook on future developments, in particular for contrast enhancement techniques.

Reduction of frame rate in full-field swept-source optical coherence tomography by numerical motion correction [Invited]

Full-field swept-source optical coherence tomography (FF-SS-OCT) was recently shown to allow new and exciting applications for imaging the human eye that were previously not possible using current scanning OCT systems. However, especially when using cameras that do not acquire data with hundreds of kHz frame rate, uncorrected phase errors due to axial motion of the eye lead to a drastic loss in image quality of the reconstructed volumes. Here we first give a short overview of recent advances in techniques and applications of parallelized OCT and finally present an iterative and statistical algorithm that estimates and corrects motion-induced phase errors in the FF-SS-OCT data. The presented algorithm is in many aspects adopted from the phase gradient autofocus (PGA) method, which is frequently used in synthetic aperture radar (SAR). Following this approach, the available phase errors can be estimated based on the image information that remains in the data, and no parametrization with few degrees of freedom is required. Consequently, the algorithm is capable of compensating even strong motion artifacts. Efficacy of the algorithm was tested on simulated data with motion containing varying frequency components. We show that even in strongly blurred data, the actual image information remains intact, and the algorithm can identify the phase error and correct it. Furthermore, we use the algorithm to compensate real phase error in FF-SS-OCT imaging of the human retina. Acquisition rates can be reduced by a factor of three (from 60 to 20 kHz frame rate) with an image quality that is even higher compared to uncorrected volumes recorded at the maximum acquisition rate. The presented algorithm for axial motion correction decreases the high requirements on the camera frame rate and thus brings FF-SS-OCT closer to clinical applications.

High-speed OCT light sources and systems [Invited]

Imaging speed is one of the most important parameters that define the performance of optical coherence tomography (OCT) systems. During the last two decades, OCT speed has increased by over three orders of magnitude. New developments in wavelength-swept lasers have repeatedly been crucial for this development. In this review, we discuss the historical evolution and current state of the art of high-speed OCT systems, with focus on wavelength swept light sources and swept source OCT systems.

Full-Field Optical Coherence Tomography Using Galvo Filter-Based Wavelength Swept Laser

We report a wavelength swept laser-based full-field optical coherence tomography for measuring the surfaces and thicknesses of refractive and reflective samples. The system consists of a galvo filter–based wavelength swept laser and a simple Michelson interferometer. Combinations of the reflective and refractive samples are used to demonstrate the performance of the system. By synchronizing the camera with the source, the cross-sectional information of the samples can be seen after each sweep of the swept source. This system can be effective for the thickness measurement of optical thin films as well as for the depth investigation of samples in industrial applications. A resolution target with a glass cover slip and a step height standard target are imaged, showing the cross-sectional and topographical information of the samples.

Aberration-free volumetric high-speed imaging of in vivo retina

Certain topics in research and advancements in medical diagnostics may benefit from improved temporal and spatial resolution during non-invasive optical imaging of living tissue. However, so far no imaging technique can generate entirely diffraction-limited tomographic volumes with a single data acquisition, if the target moves or changes rapidly, such as the human retina. Additionally, the presence of aberrations may represent further difficulties. We show that a simple interferometric setup–based on parallelized optical coherence tomography–acquires volumetric data with 10 billion voxels per second, exceeding previous imaging speeds by an order of magnitude. This allows us to computationally obtain and correct defocus and aberrations resulting in entirely diffraction-limited volumes. As demonstration, we imaged living human retina with clearly visible nerve fiber layer, small capillary networks, and photoreceptor cells. Furthermore, the technique can also obtain phase-sensitive volumes of other scattering structures at unprecedented acquisition speeds.

In vivo optical imaging of physiological responses to photostimulation in human photoreceptors

Noninvasive functional imaging of molecular and cellular processes of vision may have immense impact on research and clinical diagnostics. Although suitable intrinsic optical signals (IOSs) have been observed ex vivo and in immobilized animals in vivo, detecting IOSs of photoreceptor activity in living humans was cumbersome and time consuming. Here, we observed clear spatially and temporally resolved changes in the optical path length of the photoreceptor outer segment as a response to an optical stimulus in the living human eye. To witness these changes, we evaluated phase data obtained with a parallelized and computationally aberration-corrected optical coherence tomography system. The noninvasive detection of optical path length changes shows neuronal photoreceptor activity of single cones in living human retina, and therefore, it may provide diagnostic options in ophthalmology and neurology and could provide insights into visual phototransduction in humans.

High-resolution full-field optical coherence microscopy using a broadband light-emitting diode

High-resolution full-field optical coherence microscopy (FF-OCM) is demonstrated using a single broadband light-emitting diode (LED). The characteristics of the LED-illumination FF-OCM system are measured and compared to those obtained using a halogen lamp, the light source of reference in FF-OCM. Both light sources yield identical performance in terms of spatial resolution and detection sensitivity, using the same setup and camera. In particular, an axial resolution of 0.7 μm (in water) is reached. A Xenopus laevis tadpole and ex-vivo human skin have been imaged using both sources, resulting in similar images, showing for the first time that LEDs could favorably replace halogen lamps in high-resolution FF-OCM for biomedical imaging.

Acousto-optical tunable filter for combined wideband, spectral, and optical coherence microscopy

A multimodal technique for inspection of microscopic objects by means of wideband optical microscopy, spectral microscopy, and optical coherence microscopy is described, implemented, and tested. The key feature is the spectral selection of light in the output arm of an interferometer with use of the specialized imaging acousto-optical tunable filter. In this filter, two interfering optical beams are diffracted via the same ultrasound wave without destruction of interference image structure. The basic requirements for the acousto-optical tunable filter are defined, and mathematical formulas for calculation of its parameters are derived. Theoretical estimation of the achievable accuracy of the 3D image reconstruction is presented and experimental proofs are given. It is demonstrated that spectral imaging can also be accompanied by measurement of the quantitative reflectance spectra. Examples of inspection of optically transparent and nontransparent samples demonstrate the applicability of the technique.

Line-field parallel swept source interferometric imaging at up to 1 MHz

We present a novel medical imaging modality based on optical coherence tomography (OCT) that enables in vivo 3D tomography at acquisition rates up to 1 MHz. Line field parallel swept source interferometric imaging (LPSI) combines line-field swept source OCT with modulation of the interferometric signal in spatial direction for full range imaging. This method enables high speed imaging with cost-effective and commercially available technology. We explain the realization of the LPSI setup, acquisition, and postprocessing and finally demonstrate 3D in vivo imaging of human nail fold. To the best of our knowledge, sensitivity and depth penetration are competitive with respective point scanning OCT methods at a comparable wavelength. Measured maximum sensitivity is 98.5 dB for 100 kHz and 90 dB for 1 MHz. Together with the significantly relaxed technological requirements regarding detection and swept source technology, LPSI might be a promising concept for future diagnostic OCT imaging.

Numerically focused full-field swept-source optical coherence microscopy with low spatial coherence illumination

We propose a 3D imaging technique based on the combination of full-field swept-source optical coherence microscopy (FF-SSOCM) with low spatial coherence illumination and a special numerical processing that allows for numerically focused coherent-noise-free imaging without mechanical scanning in longitudinal or transversal directions. We show, both theoretically and experimentally, that the blurring effects arising in FF-SSOCM due to defocus can be corrected by appropriate numerical processing even when low spatial coherence illumination is used. A FF-SSOCM system was built for testing the performance of this technique. Coherent-noise-free imaging of a sample with longitudinal extent exceeding the optical depth of field is demonstrated without displacement of the sample or any optical element.

Efficient holoscopy image reconstruction

Holoscopy is a tomographic imaging technique that combines digital holography and Fourier-domain optical coherence tomography (OCT) to gain tomograms with diffraction limited resolution and uniform sensitivity over several Rayleigh lengths. The lateral image information is calculated from the spatial interference pattern formed by light scattered from the sample and a reference beam. The depth information is obtained from the spectral dependence of the recorded digital holograms. Numerous digital holograms are acquired at different wavelengths and then reconstructed for a common plane in the sample. Afterwards standard Fourier-domain OCT signal processing achieves depth discrimination. Here we describe and demonstrate an optimized data reconstruction algorithm for holoscopy which is related to the inverse scattering reconstruction of wavelength-scanned full-field optical coherence tomography data. Instead of calculating a regularized pseudoinverse of the forward operator, the recorded optical fields are propagated back into the sample volume. In one processing step the high frequency components of the scattering potential are reconstructed on a non-equidistant grid in three-dimensional spatial frequency space. A Fourier transform yields an OCT equivalent image of the object structure. In contrast to the original holoscopy reconstruction with backpropagation and Fourier transform with respect to the wavenumber, the required processing time does neither depend on the confocal parameter nor on the depth of the volume. For an imaging NA of 0.14, the processing time was decreased by a factor of 15, at higher NA the gain in reconstruction speed may reach two orders of magnitude.

Full-Field and Single-Shot Full-Field Optical Coherence Tomography: A Novel Technique for Biomedical Imaging Applications

Since its introduction, optical coherence tomography (OCT) technology has advanced dramatically in various field of both clinical and fundamental research. Full-field and Single-shot full-field OCT (FF-OCT and SS-FF-OCT) are alternative OCT concepts, which aims to improve the image acquisition speed and to simplify the optical setup of conventional point-scan OCT by realizing direct line field or full-field sample imaging onto an array or line detector such as CCD or CMOS camera. FF-OCT and SS-FF-OCT are based on bulk optics Linnik-type Michelson interferometer with relatively high numerical aperture (NA) microscopic objectives. This paper will give you an overview of the principle of various types of FF-OCT and SS-FF-OCT techniques and its associated system design concept and image reconstruction algorithms.

Common approach for compensation of axial motion artifacts in swept-source OCT and dispersion in Fourier-domain OCT

Swept-source optical coherence tomography (SS-OCT) is sensitive to sample motion during the wavelength sweep, which leads to image blurring and image artifacts. In line-field and full-field SS-OCT parallelization is achieved by using a line or area detector, respectively. Thus, approximately 1000 lines or images at different wavenumbers are acquired. The sweep duration is identically with the acquisition time of a complete B-scan or volume, rendering parallel SS-OCT more sensitive to motion artifacts than scanning OCT. The effect of axial motion on the measured spectra is similar to the effect of non-balanced group velocity dispersion (GVD) in the interferometer arms. It causes the apparent optical path lengths in the sample arm to vary with the wavenumber. Here we propose the cross-correlation of sub-bandwidth reconstructions (CCSBR) as a new algorithm that is capable of detecting and correcting the artifacts induced by axial motion in line-field or full-field SS-OCT as well as GVD mismatch in any Fourier-domain OCT (FD-OCT) setup. By cross-correlating images which were reconstructed from a limited spectral range of the interference signal, a phase error is determined which is used to correct the spectral modulation prior to the calculation of the A-scans. Performance of the algorithm is demonstrated on in vivo full-field SS-OCT images of skin and scanning FD-OCT of skin and retina.

Holoscopy--holographic optical coherence tomography

Scanning optical coherence tomography (OCT) is limited in sensitivity and resolution by the restricted focal depth of the confocal detection scheme. Holoscopy, a combination of holography and Fourier-domain full-field OCT, is proposed as a way to detect photons from all depths of a sample volume simultaneously with uniform sensitivity and lateral resolution, even at high NAs. By using the scalar diffraction theory, as frequently applied in digital holographic imaging, we fully reconstruct the object field with depth-invariant imaging quality. In vivo imaging of human skin is demonstrated with an image quality comparable to conventionally scanned OCT.

In vivo Fourier-domain full-field OCT of the human retina with 1.5 million A-lines/s

In vivo full-field (FF) optical coherence tomography (OCT) images of human retina are presented by using a rapidly tunable laser source in combination with an ultra-high-speed camera. Fourier-domain FF-OCT provided a way to increase the speed of retinal imaging by parallel acquisition of A-scans. Reduced contrast caused by cross talk was observed only below the retinal pigment epithelium. With a 100Hz sweep rate, FF-OCT was fast enough to acquire OCT images with acceptable motion artifacts. FF-OCT allows ultrafast retinal imaging, boosting image speed by a lack of moving parts and a considerably higher irradiation power.

Gabor-based fusion technique for Optical Coherence Microscopy

We recently reported on an Optical Coherence Microscopy technique, whose innovation intrinsically builds on a recently reported - 2 microm invariant lateral resolution by design throughout a 2 mm cubic full-field of view - liquid-lens-based dynamic focusing optical probe [Murali et al., Optics Letters 34, 145-147, 2009]. We shall report in this paper on the image acquisition enabled by this optical probe when combined with an automatic data fusion method developed and described here to produce an in-focus high resolution image throughout the imaging depth of the sample. An African frog tadpole (Xenopus laevis) was imaged with the novel probe and the Gabor-based fusion technique, demonstrating subcellular resolution in a 0.5 mm (lateral) x 0.5 mm (axial) without the need, for the first time, for x-y translation stages, depth scanning, high-cost adaptive optics, or manual intervention. In vivo images of human skin are also presented.

Partially coherent illumination in full-field interferometric synthetic aperture microscopy

A model is developed for optical coherence tomography and interferometric synthetic aperture microscopy (ISAM) systems employing full-field frequency-scanned illumination with partial spatial coherence. This model is used to derive efficient ISAM inverse scattering algorithms that give diffraction-limited resolution in regions typically regarded as out of focus. Partial spatial coherence of the source is shown to have the advantage of mitigating multiple-scattering effects that can otherwise produce significant artifacts in full-field coherent imaging.

Interferometric Synthetic Aperture Microscopy: Computed Imaging for Scanned Coherent Microscopy

Three-dimensional image formation in microscopy is greatly enhanced by the use of computed imaging techniques. In particular, Interferometric Synthetic Aperture Microscopy (ISAM) allows the removal of out-of-focus blur in broadband, coherent microscopy. Earlier methods, such as optical coherence tomography (OCT), utilize interferometric ranging, but do not apply computed imaging methods and therefore must scan the focal depth to acquire extended volumetric images. ISAM removes the need to scan the focus by allowing volumetric image reconstruction from data collected at a single focal depth. ISAM signal processing techniques are similar to the Fourier migration methods of seismology and the Fourier reconstruction methods of Synthetic Aperture Radar (SAR). In this article ISAM is described and the close ties between ISAM and SAR are explored. ISAM and a simple strip-map SAR system are placed in a common mathematical framework and compared to OCT and radar respectively. This article is intended to serve as a review of ISAM, and will be especially useful to readers with a background in SAR.

Quasi-single shot axial-lateral parallel time domain optical coherence tomography with Hilbert transformation

We developed axial-lateral parallel time-domain optical coherence tomography (ALP TD-OCT) from a single interference image. A two-dimensional camera can produce a depth-resolved interference image using diffracted light as the reference beam and a linear illumination beam without any mechanical scan. An OCT image of biological tissues with sufficient sensitivity requires extraction of interference signals by subtracting the DC image, which contains the intensity of noninterference light and the electrical noise of the camera, from a single interference image and subsequent application of the Hilbert transformation for each axial direction. We measured 300 interference images of a moving human finger in vivo using an indium gallium arsenide (InGaAs) camera (320 x 250 pixels) operating at 60 frames per second and then obtained OCT images with an imaging range of 5.0 x 1.7-mm(2) (lateral x axial) using a DC image based on averaged interference images. The system sensitivity was 90.5 dB with a 1.05-ms exposure. As the OCT image depends on the interference signals in a single interference image, the OCT signals were stable compared with OCT images based on the phase-shift method.

Inverse scattering for frequency-scanned full-field optical coherence tomography

Full-field optical coherence tomography (OCT) is able to image an entire en face plane of scatterers simultaneously, but typically the focus is scanned through the volume to acquire three-dimensional structure. By solving the inverse scattering problem for full-field OCT, we show it is possible to computationally reconstruct a three-dimensional volume while the focus is fixed at one plane inside the sample. While a low-numerical-aperture (NA) OCT system can tolerate defocus because the depth of field is large, for high NA it is critical to correct for defocus. By deriving a solution to the inverse scattering problem for full-field OCT, we propose and simulate an algorithm that recovers object structure both inside and outside the depth of field, so that even for high NA the focus can be fixed at a particular plane within the sample without compromising resolution away from the focal plane.