Full-field Optical Coherence Tomography Research Articles

Fibre imaging bundles for full-field optical coherence tomography

An imaging fibre bundle is incorporated into a full-field imaging optical coherence tomography system, with the aim of eliminating the mechanical scanning currently required at the probe tip in endoscopic systems. Each fibre within the imaging bundle addresses a Fizeau interferometer formed between the bundle end and the sample, a configuration which ensures down-lead insensitivity of the probe fibres, preventing variations in sensitivity due to polarization changes in the many thousand constituent fibres. The technique allows acquisition of information across a planar region with single-shot measurement, in the form of a 2D image detected using a digital CCD camera. Depth scanning components are now confined within a processing interferometer external to the completely passive endoscopic probe. The technique has been evaluated in our laboratory for test samples, and images acquired using the bundle-based system are presented. Data are displayed either as en-face scans, parallel to the sample surface, or as slices through the depth of the sample, with a spatial resolution of about 30 µm. The minimum detectable reflectivity at present is estimated to be about 10−3, which is satisfactory for many inorganic samples. Methods of improving the signal-to-noise ratio for imaging of lower reflectivity samples are discussed.

Polarization-sensitive full-field optical coherence tomography

We present a polarization-sensitive full-field optical coherence tomography system that can produce high-resolution images of the linear retardance and reflectivity properties of biological media. En face images can be delivered at a frame rate of 3.5 Hz by combination of interferometric images acquired by two CCD cameras in an interference microscope illuminated with a tungsten halogen lamp. Isotropic spatial resolution of approximately 1.0 microm is achieved. The technique is demonstrated on ex vivo muscle tissues.

Realization of 3-D Topographic and Tomograpic Images with Ultrahigh-resolution Full-field Optical Coherence Tomography

We present an ultrahigh-resolution full-field optical coherence tomography (FF-OCT) implemented with a white-light interference microscope and a detector array as an alternative OCT technique. The use of detector array allows the capture of two-dimensional en-face images in parallel without taking any lateral scanning process. The phase shifting interferometric technique with the sinusoidal phase modulation (SPM) is utilized to get the demodulated OCT images. The configuration of the system and the resolution of the obtained image are presented. The topographic images, taken with the implemented system, of a coin, an integrated circuit chip, and the tomographic images of an onion epithelium are demonstrated also. Axial and lateral spatial resolution of ${\sim}1.0{\mu}m$ and ${\sim}2.0{\mu}m$ are achieved with the system respectively.

Realization of 3-D Topographic and Tomograpic Images with Ultrahigh-resolution Full-field Optical Coherence Tomography

Realization of 3-D Topographic and Tomograpic Images with Ultrahigh-resolution Full-field Optical Coherence Tomography

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.

Ultrahigh-resolution imaging of human donor cornea using full-field optical coherence tomography

A feasibility study of ultrahigh-resolution full-field optical coherence tomography (FF-OCT) for a subcellular-level imaging of human donor corneas is presented. The FF-OCT system employed in this experiment is based on a white light interference microscope, where the sample is illuminated by a thermal light source and a horizontal cross-sectional (en face) image is detected using a charge coupled device (CCD) camera. A conventional four-frame phase-shift detection technique is employed to extract the interferometric image from the CCD output. A 95-nm-broadband full-field illumination yields an axial resolution of 2.0 microm, and the system covers an area of 850 microm x 850 microm with a transverse resolution of 2.4 microm using a 0.3-NA microscope objective and a CCD camera with 512 x 512 pixels. Starting a measurement from the epithelial to the endothelial side, a series of en face images was obtained. From detected en face images, the epithelial cells, Bowman's layer, stromal keratocyte, nerve fiber, Descemet's membrane, and endothelial cell were clearly observed. Keratocyte cytoplasm, its nuclei, and its processes were also separately detected. Two-dimensional interconnectivity of the keratocytes is visualized, and the keratocytes existing between collagen lamellaes are separately extracted by exploiting a high axial resolution ability of FF-OCT.

Full-Field Optical Coherence Tomography System with Controllable Longitudinal Resolution

A full-field optical coherence tomography (OCT) system that can control longitudinal resolution is presented. The system is based on the synthesis of the optical coherence function from the wavelength scanning of laser light within a frame time of an imaging device. On the basis of the results of our experiments, it is confirmed that our system can select any longitudinal resolution between 6–120 µm and that this performance is available for practical use such as for the tomographic imaging of various sizes of targets in a scattering medium.

L’OCT plein champ

Optical coherence tomography (OCT) is an emerging technique for imaging of biological media with micrometer-scale resolution, whose most significant impact concerns ophthalmology. Since its introduction in the early 1990's, OCT has known a lot of improvements and sophistications. Full-field OCT is our original approach of OCT, based on white-light interference microscopy. Tomographic images are obtained by combination of interferometric images recorded in parallel by a detector array such as a CCD camera. Whereas conventional OCT produces B-mode (axially-oriented) images like ultrasound imaging, full-field OCT acquires tomographic images in the en face (transverse) orientation. Full-field OCT is an alternative method to conventional OCT to provide ultrahigh resolution images (approximately 1 microm), using a simple halogen lamp instead of a complex laser-based source. Various studies have been carried, demonstrating the performances of this technology for three-dimensional imaging of ex vivo specimens. Full-field OCT can be used for non-invasive histological studies without sample preparation. In vivo imaging is still difficult because of the object motions. A lot of efforts are currently devoted to overcome this limitation. Ultra-fast full-field OCT was recently demonstrated with unprecedented image acquisition speed, but the detection sensitivity has still to be improved. Other research directions include the increase of the imaging penetration depth in highly scattering biological tissues such as skin, and the exploitation of new contrasts such as optical birefringence to provide additional information on the tissue morphology and composition.

Competence between spatial and temporal coherence in full field optical coherence tomography and interference microscopy

Distinction between spatial and temporal coherence characteristics of the interferogram from low-coherence microscopic systems is investigated. It is shown that the coherence region is determined by the temporal coherence only when the path length difference is scanned in a region where the reference and sample beams are collimated, that is, containing a single spatial frequency. Spatial coherence has an effect when the path length scan is performed with a non-collimated beam, such as a defocus scan of a lens where the focal depth of the lens mainly determines the coherence region. In the latter case, the fringe size for a monochromatic light is smaller than half the wavelength by a factor determined by the spatial coherence. A new analytic relation is derived for the fringe size showing that it has better validity than the Ingelstam relation in the high NA region.

Image enhancement for multilayer information retrieval by using full-field optical coherence tomography

When a full-field optical coherence tomography (OCT) system is used to extract tomographic images from a multilayer information carrier, the resulting images may suffer from interlayer modulations and parasitic patterns derived from interference fringes. We describe and analyze these negative influences that degrade the quality of extracted tomographic images and propose practical algorithms and methods to minimize them. The emphasis of the discussion will be the removal of the parasitic fringes produced by the imperfection of a CCD camera. The simulative and experimental results of image enhancement for multilayer tomography extraction using full-field OCT are provided.

Thermal-light full-field optical coherence tomography in the 1.2 μm wavelength region

Full-field optical coherence tomography with dynamic focusing correction is demonstrated in the 1.2μm wavelength region using an Indium Gallium Arsenide area camera as image sensor and a halogen lamp as illumination source. An axial image resolution of 1.6μm is measured in air (∼1.1μm in tissue) which is the best resolution ever achieved in OCT in this wavelength region. En face tomographic images are produced using a phase-shifting method requiring the acquisition of only two phase-opposed interferometric images. In spite of a relatively moderate detection sensitivity of 80dB within 1.0s image acquisition time, ultrahigh resolution imaging in highly scattering biological tissues is possible without immersion medium.

Study of Reconstruction Algorithms for Phase-Stepped Full-Field Optical Coherence Tomography

We evaluated several amplitude reconstruction algorithms for a phase-stepped full-field optical coherence tomography (OCT), including three-, four-, and five-step algorithms, as well as a derived amplitude-based version of the Carré method, a four-step algorithm that requires equal but arbitrary step sizes that need not be known. Using a planar mirror as the sample and different phase stepping protocols, the sensitivities of the four reconstruction algorithms were measured at 85, 87, 95, and 90 dB.

Full-field Optical Coherence Tomography using Parallel Detection

Full-field optical coherence tomography (FF-OCT) is a newly developed technique capable of acquiring a horizontal cross-sectional (en-face) image without lateral scanning. In this paper two configurations of FFOCT system are described. The first one employs a frequency-synchronous detection method that operates a pair of CCD cameras as the heterodyne detector arrays. A series of en-face images are thus acquired at the camera frame rate in a single longitudinal scan, making the fast acquisition of three-dimensional information possible. The second system is based on a Linnik microscope for optical coherence microcopy. Using a tungsten halogen lamp as the low-cost ultra-broadband source, sub-cellular imaging is demonstrated in biological objects.

Full-field optical coherence tomography with a complimentary metal-oxide semiconductor digital signal processor camera

Full-field optical coherence tomography OCT using a complementary metal-oxide semiconductor CMOS camera with an in- tegrated a digital signal processor DSP is demonstrated. The CMOS- DSP camera employed is typically used in machine vision applications and is based on an array of 10241024 direct readout pixels that are randomly addressable in space and time. These characteristics enable the camera to be used as a fast full-field detector in carrier-based optical metrology systems. The integrated DSP facilitates basic signal process- ing including real-time filtering and undersampling. The optical setup used to implement this OCT method is composed of a free-space Mich- elson interferometer and a superluminescent diode SLD light source, with an electromechanical shaker for depth scanning. Unlike classical OCT approaches, however, the setup does not require any electrome- chanical device for lateral scanning. A 6430 pixel region of interest was imaged at 235 frames/s and sampled in depth, corresponding to a volumetric measurement of 875410150 m. Measurements carried out on a simple calibration specimen indicated lateral and axial resolu- tions of 14 and 22 m, respectively. The presented approach offers an inexpensive and versatile alternative to traditional OCT systems and pro- vides the basis for a functional machine vision system suitable for indus- trial applications.

Quadarature fringes full-field OCT

Quadarature fringes full-field OCT

Signal-to-noise ratio study of full-field Fourier-domain optical coherence tomography

We report a new approach in optical coherence tomography (OCT) called full-field Fourier-domain OCT (3F-OCT). A three-dimensional image of a sample is obtained by digital reconstruction of a three-dimensional data cube, acquired with a Fourier holography recording system, illuminated with a swept source. We present a theoretical and experimental study of the signal-to-noise ratio of the 3F-OCT approach versus serial image acquisition (flying-spot OCT) approach.

Theory, developments and applications of optical coherence tomography

In this paper, we review the developments in optical coherence tomography (OCT) for three-dimensional non-invasive imaging. A number of different OCT techniques are discussed in some detail including time-domain, frequency-domain, full-field, quantum and Doppler OCT. A theoretical treatment is given and some relevant comparisons made between various implementations. The current and potential applications of OCT are discussed, with close attention paid to biomedical imaging and its metrological issues.

Ultrahigh resolutionex vivoocular imaging using ultrashort acquisition timeen faceoptical coherence tomography

We explore the potential and the limitations of a new ultrashort acquisition time en face optical coherence tomography (OCT) technique in application to ocular imaging. The instrument is based on the Linnik interferometer, illuminated with a xenon flash lamp. Transverse resolution is determined by the numerical aperture of the microscope objectives, whilst axial resolution is governed by the source coherence length. An isotropic resolution of ~1 µm is achieved. The acquisition speed is determined by the flash pulse duration, i.e. 10 µs. A full two-dimensional en face tomographic image is captured in this time period. Preliminary results of imaging in ex vivo ocular tissues of the rat are presented and compared with results obtained using our original full-field OCT system. The possibility of extension to in vivo imaging is discussed.

Image reconstruction in full-field Fourier-domain optical coherence tomography

Full-field Fourier-domain optical coherence tomography (3F-OCT) is a full-field version of spectral domain/swept source optical coherence tomography. A set of two-dimensional Fourier holograms is recorded at discrete wavenumbers spanning the swept source tuning range. The resultant three-dimensional data cube contains comprehensive information on the three-dimensional spatial properties of the sample, including its morphological layout and optical scatter. The morphological layout can be reconstructed in software via three-dimensional discrete Fourier transformation. The spatial resolution of the 3F-OCT reconstructed image, however, is degraded due to the presence of a phase cross-term, whose origin and effects are addressed in this paper. We present a theoretical and experimental study of the imaging performance of 3F-OCT, with particular emphasis on elimination of the deleterious effects of the phase cross-term.

Stroboscopic ultrahigh-resolution full-field optical coherence tomography

We present a new technique that produces en face tomographic images with a 10-micros acquisition time per image. The setup consists of an interference microscope with stroboscopic illumination provided by a xenon arc flash lamp (10-micros flashes at 15 Hz). The tomographic images are obtained from two phase-opposed interferometric images recorded simultaneously by two synchronized CCD cameras. Transverse resolution better than 1.0 microm is achieved by use of high-numerical-aperture microscope objectives. The short coherence length of the source yields an axial resolution of 0.9 microm. 3 x 3 pixel binning leads to a detection sensitivity of 71 dB. Our system is suitable for various applications, particularly in biology for in vivo cellular-level imaging.