Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography

Combined Rotating Ultra-High-Resolution Spectral Domain OCT and Scheimpflug Imaging for In Vivo Corneal Optical Biopsy

PURPOSE

This article introduces the Pentacam® Cornea OCT (optical coherence tomography). This advanced corneal imaging system combines rotating ultra-high-resolution spectral domain OCT with sub- 2-micron axial resolution and Scheimpflug photography. The purpose of this study is to present the first experience with the instrument and its potential for corneal diagnostics, including optical biopsy.

METHODS

In this prospective study, the Pentacam® Cornea OCT was used to image the corneas of seven patients. The novel wide-angle pericentric scan system enables optimal OCT imaging performance for the corneal layer structure over the entire width of the cornea, including the limbal regions. A detailed analysis of the resulting images assessed the synergism between the OCT and Scheimpflug photography.

RESULTS

The Pentacam® Cornea OCT demonstrated significantly improved image resolution and ability to individualize corneal layers with high quality. There is a synergism between the OCT high-definition signal to individualize details on the cornea and Scheimpflug photography to detect and quantify corneal scattering. The noncontact exam was proven safe, user-friendly, and effective for enabling optical biopsy.

CONCLUSIONS

Pentacam® Cornea OCT is an advancement in corneal imaging technology. The ultra-high-resolution spectral domain OCT and Scheimpflug photography provide unprecedented detail and resolution, enabling optical biopsy and improving the understanding of corneal pathology. Further studies are necessary to compare and analyze the tomographic reconstructions of the cornea with the different wavelengths, which may provide helpful information for diagnosing and managing corneal diseases.

Suppression of the conjugate signal for broadband computed imaging via synthetic phase modulation

We present synthetic-phase-modulated interferometric synthetic aperture microscopy (SPM-ISAM), a method to perform 3D object reconstructions from data acquired with confocal broadband interferometric microscopy (BIM) that reconstructs images virtually free of coherent and depth-dependent defocus artifacts. This is achieved by implementing a sinusoidal SPM method in combination with an ISAM reconstruction algorithm that uses relatively low-modulation frequencies compared with acquisition frequencies. A theoretical framework and numerical results are provided here.

Calibration procedure for enhanced mirror artifact removal in full-range optical coherence tomography using passive quadrature demultiplexing

SIGNIFICANCE

Passive quadrature demultiplexing allows full-range optical coherence tomography (FR-OCT). However, imperfections in the wavelength- and frequency-response of the demodulation circuits can cause residual mirror artifacts, which hinder high-quality imaging on both sides of zero delay.

AIM

We aim at achieving high mirror artifact extinction by calibrated postprocessing of the FR-OCT signal.

APPROACH

We propose a mathematical framework for the origin of the residual mirror peaks as well as a protocol allowing the precise measurement and correction of the associated errors directly from mirror measurements.

RESULTS

We demonstrate high extinction of the mirror artifact over the entire imaging range, as well as an assessment of the method's robustness to time and experimental conditions. We also provide a detailed description of the practical implementation of the method to ensure optimal reproducibility.

CONCLUSION

The proposed method is simple to implement and produces high mirror artifact extinction. This may encourage the adoption of FR-OCT in clinical and industrial systems or loosen the performance requirements on the optical demodulation circuit, as the imperfections can be handled in postprocessing.

Low-Cost In Vivo Full-Range Optical Coherence Tomography Using a Voice Coil Motor

In this work, we demonstrated a novel and low-cost full-range optical coherence tomography (FROCT) method. In comparison with the off-pivot approach, which needs precise control of the deflecting distance and should be adjusted for different situations, our proposed method is more flexible without regulating the system itself. Different from the previous systems reported in the literature, which used a high-cost piezo-driven stage to introduce the phase modulation, our system utilizes a cost-effective voice coil motor for retrieving the complex-valued spectral signal. The complex-valued data, with a twofold increase in the accessible depth range, can be calculated using an algorithm based on the Hilbert transform and Dirac delta function. To confirm the effectivity of our method, both simulation and experiments were performed. In particular, for the in vivo experiment, we presented the FROCT result of a fingernail fold, demonstrating the availability of in vivo imaging. Since the key element of our system is a low-cost voice coil motor, which is flexible and more accessible for most of the clinics, we believe that it has great potential to be a clinical modality in the future.

Numerical method for axial motion artifact correction in retinal spectral-domain optical coherence tomography

A numerical method that compensates image distortions caused by random fluctuations of the distance to an object in spectral-domain optical coherence tomography (SD OCT) has been proposed and verified experimentally. The proposed method is based on the analysis of the phase shifts between adjacent scans that are caused by micrometer-scale displacements and the subsequent compensation for the displacements through phase-frequency correction in the spectral space. The efficiency of the method is demonstrated in model experiments with harmonic and random movements of a scattering object as well as during in vivo imaging of the retina of the human eye.

High-Speed, Ultrahigh-Resolution Spectral-Domain OCT with Extended Imaging Range Using Reference Arm Length Matching

Purpose

To develop high-speed, extended-range, ultrahigh-resolution spectral-domain optical coherence tomography (UHR SD-OCT) and demonstrate scan protocols for clinical retinal imaging.

Methods

A UHR SD-OCT operating at 840-nm with 150-nm bandwidths was developed. The axial imaging range was extended by dynamically matching reference arm length to the retinal contour during acquisition. Two scan protocols were demonstrated for imaging healthy participants and patients with dry age-related macular degeneration. A high-definition raster protocol with intra–B-scan reference arm length matching (ReALM) was used for high-quality cross-sectional imaging. A cube volume scan using horizontal and vertical rasters with inter–B-scan ReALM and software motion correction was used for en face and cross-sectional imaging. Linear OCT signal display enhanced visualization of outer retinal features.

Results

UHR SD-OCT was demonstrated at 128- and 250-kHz A-scan rates with 2.7 µm axial resolution and a 1.2-mm, 6-dB imaging range in the eye. Dynamic ReALM was used to maintain the retina within the 6-dB imaging range over wider fields of view. Outer retinal features, including the rod and cone interdigitation zones, retinal pigment epithelium, and Bruch's membrane were visualized and alterations observed in age-related macular degeneration eyes.

Conclusions

Technological advances and dynamic ReALM improve the imaging performance and clinical usability of UHR SD-OCT.

Translational Relevance

These advances should simplify clinical imaging workflow, reduce imaging session times, and improve yield of high quality images. Improved visualization of photoreceptors, retinal pigment epithelium, and Bruch's membrane may facilitate diagnosis and monitoring of age-related macular degeneration and other retinal diseases.

Spectral Domain Optical Coherence Tomography Imaging Performance Improvement Based on Field Curvature Aberration-Corrected Spectrometer

We designed and fabricated a telecentric f-theta imaging lens (TFL) to improve the imaging performance of spectral domain optical coherence tomography (SD-OCT). By tailoring the field curvature aberration of the TFL, the flattened focal surface was well matched to the detector plane. Simulation results showed that the spot in the focal plane fitted well within a single pixel and the modulation transfer function at high spatial frequencies showed higher values compared with those of an achromatic doublet imaging lens, which are commonly used in SD-OCT spectrometers. The spectrometer using the TFL had an axial resolution of 7.8 μm, which was similar to the theoretical value of 6.2 μm. The spectrometer was constructed so that the achromatic doublet lens was replaced by the TFL. As a result, the SD-OCT imaging depth was improved by 13% (1.85 mm) on a 10 dB basis in the roll-off curve and showed better sensitivity at the same depth. The SD-OCT images of a multi-layered tape and a human palm proved that the TFL was able to achieve deeper imaging depth and better contrast. This feature was seen very clearly in the depth profile of the image. SD-OCT imaging performance can be improved simply by changing the spectrometer’s imaging lens. By optimizing the imaging lens, deeper SD-OCT imaging can be achieved with improved sensitivity.

Stable multi-megahertz circular-ranging optical coherence tomography at 1.3 µm

In Fourier-domain optical coherence tomography (OCT), the finite bandwidth of the acquisition electronics constrains the depth range and speed of the system. Circular-ranging (CR) OCT methods use optical-domain compression to surpass this limit. However, the CR-OCT system architectures of prior reports were limited by poor stability and were confined to the 1.55 µm wavelength range. In this work, we describe a novel CR-OCT architecture that is free from these limitations. To ensure stable operation, temperature sensitive optical modules within the system were replaced; the kilometer-length fiber spools used in the stretched-pulse mode-locked (SPML) laser was eliminated in favor of a single 10 meter, continuously chirped fiber Bragg grating, and the interferometer's passive optical quadrature demodulation circuit was replaced by an active technique using a lithium niobate phase modulator. For improved imaging penetration in biological tissues, the system operating wavelength was shifted to a center wavelength of 1.29 µm by leveraging the wavelength flexibility intrinsic to CFBG-based dispersive fibers. These improvements were achieved while maintaining a broad (100 nm) optical bandwidth, a long 4 cm imaging range, and a high 7.6 MHz A-line rate. By enhancing stability, simplifying overall system design, and operating at 1.3 µm, this CR-OCT architecture will allow a broader exploration of CR-OCT in both medical and non-medical applications.

Two dimensional non-scanning transform-free spatial-domain optical coherence tomography

A novel optical coherence tomography system that can perform scanless two dimensional imaging without Fourier transform is proposed and demonstrated. In the system, a convex cylindrical mirror generates an extended spatial distribution of optical delay in the reference arm and a cylindrical lens is used to form a focused line beam in the sample arm. A charge-coupled device camera captures the two dimensional tomographic image of a sample in a snap-shot manner. Due to its simple configuration and operation, the system is suitable for developing a compact device for tomographic imaging and measurement.

Double Interferometer Design for Independent Wavefront Manipulation in Spectral Domain Optical Coherence Tomography

Spectral domain optical coherence tomography (SD-OCT) is a highly versatile method which allows for three dimensional optical imaging in scattering media. A number of recent publications demonstrated the technique to benefit from structured illumination and beam shaping approaches, e.g. to enhance the signal-to-noise ratio or the penetration depth with samples such as biological tissue. We present a compact and easy to implement design for independent wavefront manipulation and beam shaping at the reference and sample arm of the interferometric OCT device. The design requires a single spatial light modulator and can be integrated to existing free space SD-OCT systems by modifying the source arm only. We provide analytical and numerical discussion of the presented design as well as experimental data confirming the theoretical analysis. The system is highly versatile and lends itself for applications where independent phase or wavefront control is required. We demonstrate the system to be used for wavefront sensorless adaptive optics as well as for iterative optical wavefront shaping for OCT signal enhancement in strongly scattering media.

Micromachined phase-shifted array-type Mirau interferometer for swept-source OCT imaging: design, microfabrication and experimental validation

OCT instruments permit fast and non-invasive 3D optical biopsies of biological tissues. However, they are bulky and expensive, making them only affordable at the hospital and thus, not sufficiently used as an early diagnostic tool. Significant reduction of system cost and size is achieved by implementation of MOEMS technologies. We propose an active array of 4x4 Mirau microinterferometers where the reference micro-mirrors are carried by a vertical comb-drive microactuator, enabling the implementation of the phase-shifting technique that improves the sensitivity and eliminates unwanted interferometric terms. We focus on the design of the imaging system, the microfabrication and the assembly of the Mirau microinterferometer, and the swept-source OCT imaging.

Full-depth spectral domain optical coherence tomography technology insensitive to phase disturbance

To achieve full-depth spectral domain optical coherence tomography in the case of strong environmental disturbance, the iterative phase-shifting (IPS) method and modified dispersion-coded (MDC) method are proposed in this work. In IPS, the precise amount of phase shift is retrieved by iteration, and the direction of the phase shift is determined by dispersion compensation. Conjugate mirror items and noise can be simultaneously eliminated by two captured interferograms, whereas only one of them can be removed in the traditional phase-shift method with two interferograms. In MDC, they are removed through dispersion compensation and signal extraction with a single interferogram. Full-depth images of a glass slide, an onion, and a live fish eye are obtained by the two methods. The advantages and disadvantages of each method are analyzed and compared. IPS is found to be more effective for removing conjugate artifacts, whereas MDC is more conducive to real-time imaging. For a 2 mm × 3.6 mm image of a fish eye (200 depth scans and 1200 spectral sampling points per depth scan), the mirror image artifact is reduced by 28.55 dB in MDC and 41.53 dB in IPS. Processing times are 5.1 seconds (20 iterations) for the IPS method and 0.91 seconds for MDC.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

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.

Spatial convolution for mirror image suppression in Fourier domain optical coherence tomography

We developed a spatial convolution approach for mirror image suppression in phase-modulated Fourier domain optical coherence tomography, and demonstrated it in vivo for small animal imaging. Utilizing the correlation among neighboring A-scans, the mirror image suppression process was simplified to a three-parameter convolution. By adjusting the three parameters, we can implement different Fourier domain sideband windows, which is important but complicated in existing approaches. By properly selecting the window size, we validated the spatial convolution approach on both simulated and experimental data, and showed that it is versatile, fast, and effective. The new approach reduced the computational cost by 32% and improved the mirror image suppression by 10%. We adapted the spatial convolution approach to a GPU accelerated system for ultrahigh-speed processing in 0.1 ms. The advantage of the ultrahigh speed was demonstrated in vivo for small animal imaging in a mouse model. The fast scanning and processing speed removed respiratory motion artifacts in the in vivo imaging.

Compact high-resolution Littrow conical diffraction spectrometer

This paper presents a compact high-resolution Littrow conical diffraction spectrometer (LCDS) that includes an echelle grating for horizontally dispersing the incident light beam into several high diffraction orders, a prism for vertically separating the overlapping diffraction orders, and a shared focusing lens used for both the incident and dispersed beams. The unique design of the optics enables the LCDS to give high dispersion on the detector without requiring a large field of view and, therefore, to achieve the benefits of high spectral resolution and compactness. The use of the Littrow conical diffraction coupled with the shared focusing lens makes the LCDS more compact. The formulas of the footprint of the dispersed spectra are derived, and the numerical simulation is given. The design calculations for application of the LCDS to an optical coherence tomography system are illustrated by an example.

Polarization-based balanced detection for spectral-domain optical coherence tomography

We present a new design for spectral-domain optical coherence tomography that allows balanced detection using a single camera. The design uses polarization optics to encode the light in reference and sample arms. Two parallel and highly aligned spectra, which carry out-of-phase interference signals, in-phase common noise, and auto-interference terms, are focused on the camera, which performs the digital balanced detection for each wavelength. The optical system is characterized and tested for tissue imaging. Results demonstrate consistent signal gains in depth and suppression of DC and sample auto-interference. The design could be further amended for polarization-sensitive imaging and might demonstrate a market for manufacturing dual-line cameras with analog-balanced detection capability.

Compensation of spectral and RF errors in swept-source OCT for high extinction complex demodulation

We provide a framework for compensating errors within passive optical quadrature demodulation circuits used in swept-source optical coherence tomography (OCT). Quadrature demodulation allows for detection of both the real and imaginary components of an interference fringe, and this information separates signals from positive and negative depth spaces. To achieve a high extinction (∼60 dB) between these positive and negative signals, the demodulation error must be less than 0.1% in amplitude and phase. It is difficult to construct a system that achieves this low error across the wide spectral and RF bandwidths of high-speed swept-source systems. In a prior work, post-processing methods for removing residual spectral errors were described. Here, we identify the importance of a second class of errors originating in the RF domain, and present a comprehensive framework for compensating both spectral and RF errors. Using this framework, extinctions >60 dB are demonstrated. A stability analysis shows that calibration parameters associated with RF errors are accurate for many days, while those associated with spectral errors must be updated prior to each imaging session. Empirical procedures to derive both RF and spectral calibration parameters simultaneously and to update spectral calibration parameters are presented. These algorithms provide the basis for using passive optical quadrature demodulation circuits with high speed and wide-bandwidth swept-source OCT systems.

Line-field parallel swept source MHz OCT for structural and functional retinal imaging

We demonstrate three-dimensional structural and functional retinal imaging with line-field parallel swept source imaging (LPSI) at acquisition speeds of up to 1 MHz equivalent A-scan rate with sensitivity better than 93.5 dB at a central wavelength of 840 nm. The results demonstrate competitive sensitivity, speed, image contrast and penetration depth when compared to conventional point scanning OCT. LPSI allows high-speed retinal imaging of function and morphology with commercially available components. We further demonstrate a method that mitigates the effect of the lateral Gaussian intensity distribution across the line focus and demonstrate and discuss the feasibility of high-speed optical angiography for visualization of the retinal microcirculation.

Background elimination and interferometric capability in optical coherence tomography by a nonlinear optical gating based on type-II second-harmonic generation

A novel method that uses nonlinear optical gating to control background illumination in optical coherence tomography is presented. With this method, the user can adjust the amount of undesired backscattering or eliminate it completely, which enables dark-field measurements. The interferometric capability of the method in a nonlinear optical regime is demonstrated with the coupling of three overlapping input waves to yield Fizeau fringes. The measurement of a 265 nm step is performed to validate this method, which was originally conceived for 3D MEMS characterization.

Single-step method for fiber-optic probe-based full-range spectral domain optical coherence tomography

We propose a single-step method appropriated for a fiber-optic probe-based full-range spectral domain optical coherence tomography (OCT). The fiber-optic probe was scanned over a sample with a magnetically driven actuator. In the reference arm, a phase shift of π/2 was applied during two neighbor axial scanning, from which the complex spectral interferogram was directly reconstructed. Since the complex-conjugate-free OCT image is obtained by doing just one Fourier transform on the complex interferogram, obtaining the full-range image is simple in algorithm and effective in computation time. Some full-range images of biological samples created with the proposed method are presented and the processing time is analyzed.

Frequency multiplexed long range swept source optical coherence tomography

We present a novel swept source optical coherence tomography configuration, equipped with acousto-optic deflectors that can be used to simultaneously acquire multiple B-scans originating from different depths. The sensitivity range of the configuration is evaluated while acquiring five simultaneous B-scans. Then the configuration is employed to demonstrate long range B-scan imaging by combining two simultaneous B-scans from a mouse head sample.

Experimental demonstration of high-speed full-range Fourier domain optical coherence tomography imaging using orthogonally polarized light and a phase-shifting algorithm

This study describes a phase-shifting method based on orthogonal polarized light by using complex Fourier domain optical coherence tomography (FD-OCT) to increase the speed of image scanning and to resist vibration and other environmental disturbances. Two FD-OCT interferograms corresponding to orthogonal polarization components can be obtained simultaneously. After using a π/2 phase-shifting algorithm, removing unwanted components becomes possible, including dc and autocorrelation terms, from the interferogram. This method doubles the measurement range. In other words, this approach enables one-shot and full-range FD-OCT. Experimental results show that the reconstruction parameters of the sample are close to the conventional time-domain optical coherence tomography.

Two-reference swept-source optical coherence tomography of high operation flexibility

The significantly less stringent operation of a two-reference swept-source optical coherence tomography (OCT) system for suppressing the mirror image is demonstrated based on the spatially localized image processing method. With this method, the phase difference between the two reference signals is not limited to 90 degrees. Based on the current experimental operation, the mirror image can be effectively suppressed as long as the phase difference is larger than 20 degrees. In other words, the adjustment of the beam splitter orientation for controlling the phase difference becomes much more flexible. Also, based on a phantom experiment, the combination the spatially localized mirror image suppression method with the two-reference OCT operation leads to the implementation of full-range optical Doppler tomography.

Two-step phase demodulation algorithm based on the extreme value of interference

Using the maximum and the minimum of interference, a novel two-step phase demodulation algorithm is proposed to perform the phase extraction in phase-shifting interferometry. By means of the simulation calculation and the experimental research, it is proved that both the measured phase and the phase shift with high precision can be obtained in the proposed algorithm.

Swept source based fourier domain functional optical coherence tomography

Swept source based Fourier domain functional optical coherence tomography, which allows simultaneous imaging of tissue structure and physiology with high speed and high sensitivity, is described and demonstrated.

Balanced detection spectral domain optical coherence tomography with a multiline single camera for signal-to-noise ratio enhancement

In this study, the use and advantages of balanced detection (BD) in spectral domain optical coherence tomography (SD-OCT) are demonstrated. A-scans are calculated as a combination of two phase-opposed interferometric spectra acquired simultaneously by using a multiline single camera spectrometer. Not only does this system suppress artifacts due to autocorrelation, but also the signal of interest is increased by a factor of 2 as experimentally verified. Our BD-based SD-OCT gives a signal-to-noise ratio improvement of 8-14 dB for the peak within 1 mm compared to standard SD-OCT using a single detection scheme. This method is validated by experimental measurement of a glass plate.

Full range spectral domain optical coherence tomography using a fiber-optic probe as a self-phase shifter

We present a full range handheld probe type spectral domain optical coherence tomography (SD-OCT) method. Here, the sample arm is composed of a tilted fiber-optic cantilever scanner; thus, the phase shift concurrently occurs while sample scanning. With the phase shift, we could achieve a full range complex-conjugate-free OCT image with no additional phase shifters in the reference arm. To realize this technique, a magnetically actuated probe was adopted. Full range SD-OCT images of a pearl, human fingernail, and human tooth were subsequently obtained using this suggested probe. The scanning range and acquisition speed were 3 mm and 20 frames/s, respectively.

Instantaneous quadrature components or Jones vector retrieval using the Pancharatnam-Berry phase in frequency domain low-coherence interferometry

We use the Pancharatnam-Berry phase as a multifunctional tool for low-coherence interferometry. This geometric phase shift enables instantaneous retrieval of the quadrature components of the complex interferometric signal. The phase shift is independent of wavelength and allows for a complex conjugate suppression of 40 dB for an optical bandwidth of 115 nm. Furthermore, this paper investigates the versatility of the geometric phase to perform polarization sensitive measurements. The Jones vector of the sample was obtained numerically, allowing sample birefringence and optical axis calculation.

Fourier domain optical coherence tomography system with balance detection

A Fourier domain optical coherence tomography system with two spectrometers in balance detection is assembled using each an InGaAs linear camera. Conditions and adjustments of spectrometer parameters are presented to ensure anti-phase channeled spectrum modulation across the two cameras for a majority of wavelengths within the optical source spectrum. By blocking the signal to one of the spectrometers, the setup was used to compare the conditions of operation of a single camera with that of a balanced configuration. Using multiple layer samples, balanced detection technique is compared with techniques applied to conventional single camera setups, based on sequential deduction of averaged spectra collected with different on/off settings for the sample or reference beams. In terms of reducing the autocorrelation terms and fixed pattern noise, it is concluded that balance detection performs better than single camera techniques, is more tolerant to movement, exhibits longer term stability and can operate dynamically in real time. The cameras used exhibit larger saturation power than the power threshold where excess photon noise exceeds shot noise. Therefore, conditions to adjust the two cameras to reduce the noise when used in a balanced configuration are presented. It is shown that balance detection can reduce the noise in real time operation, in comparison with single camera configurations. However, simple deduction of an average spectrum in single camera configurations delivers less noise than the balance detection.

Graphics processing unit-based dispersion encoded full-range frequency-domain optical coherence tomography

Dispersion encoded full-range (DEFR) frequency-domain optical coherence tomography (FD-OCT) and its enhanced version, fast DEFR, utilize dispersion mismatch between sample and reference arm to eliminate the ambiguity in OCT signals caused by non-complex valued spectral measurement, thereby numerically doubling the usable information content. By iteratively suppressing asymmetrically dispersed complex conjugate artifacts of OCT-signal pulses the complex valued signal can be recovered without additional measurements, thus doubling the spatial signal range to cover the full positive and negative sampling range. Previously the computational complexity and low processing speed limited application of DEFR to smaller amounts of data and did not allow for interactive operation at high resolution. We report a graphics processing unit (GPU)-based implementation of fast DEFR, which significantly improves reconstruction speed by a factor of more than 90 in respect to CPU-based processing and thereby overcomes these limitations. Implemented on a commercial low-cost GPU, a display line rate of ∼21,000 depth scans/s for 2048 samples/depth scan using 10 iterations of the fast DEFR algorithm has been achieved, sufficient for real-time visualization in situ.

Computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography

The theory and experimental results of a computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography, which utilizes the property of reversed system phase shift between the real and mirror images, for differentiating one from the other are demonstrated. By solving a set of two equations based on a reasonable approximation, the real image signal can be obtained. The theoretical backgrounds and the improved real image quality of the average and iteration procedures in this method are particularly illustrated. Also, the mirror image suppression ratios under various process conditions, including different process iteration numbers and different system phase shifts between two neighboring A-mode scans, are evaluated. Meanwhile, the mirror image suppression results based on our method are compared with those obtained from the widely used BM-scan technique. It is found that when a process procedure of two iterations is used, the mirror image suppression quality based on our method can be higher than that obtained from the BM-scan technique. The computation time of our method is significantly shorter than that of the BM-scan technique.

Optical coherence tomography for whole eye segment imaging

We proposed a dual focus dual channel spectral domain optical coherence tomography (SD-OCT) for simultaneous imaging of the whole eye segments from cornea to the retina. By using dual channels the system solved the problem of limited imaging depth of SD-OCT. By using dual focus the system solved the problem of simultaneous light focusing on the anterior segment of the eye and the retina. Dual focusing was achieved by adjusting the collimating lenses so the divergence of the two probing beams was tuned to make them focused at different depth in the eye. We further achieved full range complex (FRC) SD-OCT in one channel to increase the depth range for anterior segment imaging. The system was successfully tested by imaging a human eye in vivo.

Optical coherence tomography-based freeze-drying microscopy

A new type of freeze-drying microscope based upon time-domain optical coherence tomography is presented here (OCT-FDM). The microscope allows for real-time, in situ 3D imaging of pharmaceutical formulations in vials relevant for manufacturing processes with a lateral resolution of <7 μm and an axial resolution of <5 μm. Correlation of volumetric structural imaging with product temperature measured during the freeze-drying cycle allowed investigation of structural changes in the product and determination of the temperature at which the freeze-dried cake collapses. This critical temperature is the most important parameter in designing freeze-drying processes of pharmaceutical products.

Method for suppressing the mirror image in Fourier-domain optical coherence tomography

A method, novel to our knowledge, for effective mirror image suppression in Fourier-domain optical coherence tomography based on a phase shift between neighboring A-mode scans is demonstrated. By realizing that the phase shifts of the real and mirror images are mutually reversed and assuming that the real image intensities of the two successive A-mode scans are the same, we can solve a set of two coupled equations to obtain the real image signals. The images based on the scanning of a high-resolution spectral-domain optical coherence tomography system are processed to show effective mirror image suppression results. Compared with a similar method of broad application, our approach has the advantages of shorter process time and higher flexibility in selecting the concerned image portions for processing.

Optical coherence tomography-current technology and applications in clinical and biomedical research

Optical coherence tomography (OCT) is a noninvasive imaging technique that provides real-time two- and three-dimensional images of scattering samples with micrometer resolution. By mapping the local reflectivity, OCT visualizes the morphology of the sample. In addition, functional properties such as birefringence, motion, or the distributions of certain substances can be detected with high spatial resolution. Its main field of application is biomedical imaging and diagnostics. In ophthalmology, OCT is accepted as a clinical standard for diagnosing and monitoring the treatment of a number of retinal diseases, and OCT is becoming an important instrument for clinical cardiology. New applications are emerging in various medical fields, such as early-stage cancer detection, surgical guidance, and the early diagnosis of musculoskeletal diseases. OCT has also proven its value as a tool for developmental biology. The number of companies involved in manufacturing OCT systems has increased substantially during the last few years (especially due to its success in opthalmology), and this technology can be expected to continue to spread into various fields of application.

Practical aspects of OCT imaging in tissue engineering

Optical coherence tomography (OCT) is a non-destructive, non-invasive imaging modality conceptually similar to ultrasound imaging but uses near-infrared radiation rather than sound. It is attracting interest throughout the medical community as a tool for ophthalmic scanning (especially of the retina) and potentially for the diagnosis of many other illnesses such as epithelial cancer, connective tissue disorders, and atherosclerosis, as well as for surgical guidance. More recently, it has begun to be explored as a tool for the real-time monitoring of the growth and development of tissue-engineered products. OCT has certain unique advantages over traditional confocal microscopy; in particular, it can image to depths measured in hundreds of microns rather than tens of microns in intact biological tissues and with working distances in excess of 1 cm. Also it possesses label-free contrast for imaging ordered collagen (via birefringence), flow velocity and local shear-rate (via Doppler shifts), and sub-cellular structure (via coherent speckle contrast). The purpose of this short review is to introduce OCT technology and also give guidelines on its practical implementation to the interested researcher.

Sample motion-insensitive, full-range, complex, spectral-domain optical-coherence tomography

We present a full-range, complex, spectral-domain optical-coherence-tomography (SD-OCT) system that is based on a double-beam scanning approach. The sample beams of two identical SD-OCT setups are combined collinearly by a bulk optic beam splitter before illuminating the object. The required phase shift for the complex signal reconstruction comes from the phase difference between both interferometers. Because of the double-beam scanning approach, our system is completely insensitive to sample motion. To demonstrate the performance of our setup, we present images of the human optic nerve head in vivo and of a human tooth.

Doppler imaging with dual-detection full-range frequency domain optical coherence tomography

Most of full-range techniques for Frequency Domain Optical Coherence Tomography (FD-OCT) reported to date utilize the phase relation between consecutive axial lines to reconstruct a complex interference signal and hence may exhibit degradation in either mirror image suppression performance or detectable velocity dynamic range or both when monitoring a moving sample such as flow activity. We have previously reported a technique of mirror image removal by simultaneous detection of the quadrature components of a complex spectral interference called a Dual-Detection Frequency Domain OCT (DD-FD-OCT) [Opt. Lett. 35, 1058-1060 (2010)]. The technique enables full range imaging without any loss of acquisition speed and is intrinsically less sensitive to phase errors generated by involuntary movements of the subject. In this paper, we demonstrate the application of the DD-FD-OCT to a phase-resolved Doppler imaging without degradation in either mirror image suppression performance or detectable velocity dynamic range that were observed in other full-range Doppler methods. In order to accommodate for Doppler imaging, we have developed a fiber-based DD-FD-OCT that more efficiently utilizes the source power compared with the previous free-space DD-FD-OCT. In addition, the velocity sensitivity of the phase-resolved DD-FD-OCT was investigated, and the relation between the measured Doppler phase shift and set flow velocity of a flow phantom was verified. Finally, we demonstrate the Doppler imaging using the DD-FD-OCT in a biological sample.

Full Range Complex Spectral Domain Optical Coherence Tomography for Volumetric Imaging at 47, 000 A Scans per Second

In this paper, we demonstrate a high speed spectral domain optical coherence tomography (SDOCT) system capable of achieving full range complex imaging at 47 kHz line scan rate. By applying beam-offset method, a constant modulation frequency is introduced into each B-scan that enables reconstruction of the full range complex SDOCT images of in vivo tissue samples. To make use of the full capacity of detection camera used in the system, system control software is developed that streams the raw spectral fringe data directly into the computer memory. In order to assess performance of the high speed full range SDOCT system for imaging biological specimen, we present results imaged from the cuticle of fingernail of a human volunteer in vivo, and from the chicken embryos ex vivo. We also show the high sensitivity advantages of full range complex imaging as compared to the conventional SDOCT. To the best of our knowledge, 47,000 A-scan imaging rate is the highest imaging rate ever been reported for full range complex imaging. Notwithstanding, the method reported here has no limitations on the imaging speed, thus offers a useful tool to achieve volumetric imaging of living samples where the high sensitivity region around zero-delay line in the system can be utilized for imaging.

Clinical assessment of mirror artifacts in spectral-domain optical coherence tomography

PURPOSE. To investigate the characteristics of a spectral-domain optical coherence tomography (SD-OCT) image phenomenon known as the mirror artifact, calculate its prevalence, analyze potential risk factors, measure severity, and correlate it to spherical equivalent and central visual acuity (VA). METHODS. OCT macular cube 512 x 128 scans taken between January 2008 and February 2009 at the New England Eye Center were analyzed for the presence of mirror artifacts. Artifact severity was determined by the degree of segmentation breakdown that it caused on the macular map. A retrospective review was conducted of the medical records of patients with artifacts and of a random control group without artifacts. RESULTS. Of 1592 patients, 9.3% (148 patients, 200 eyes) had scans that contained mirror artifacts. A significantly more myopic spherical equivalent (P < 0.001), worse VA (P < 0.001), longer axial lengths (P = 0.004), and higher proportions of moderate to high myopia (P < 0.001) were found in patients with mirror artifacts than in patients without artifacts. Worse VA was associated with increased artifact severity (P = 0.04). CONCLUSIONS. In all scans analyzed, a high prevalence of mirror artifacts was found. This image artifact was often associated with patients with moderate to high myopia. Improvements in instrumentation may be necessary to resolve this problem in moderately and highly myopic eyes. Operators should be advised to properly position the retina when scanning eyes. In cases in which peripheral abnormalities in topographic measurements of retinal thickness are found, corresponding OCT scans should be examined for the presence of mirror artifacts.

Dual detection full range frequency domain optical coherence tomography

It has been shown that frequency domain optical coherence tomography (FD-OCT) systems achieve higher sensitivities compared to time domain optical coherence tomography (OCT) systems. However, the obscure object structure due to the mirror image generated by the Fourier transform is one of the remaining issues in the FD-OCT. We designed and developed what we believe to be a novel full range FD-OCT system that we refer to as the dual detection full range frequency domain optical coherence tomography (DD-FDOCT) that enables the instantaneous retrieval of quadrature components of the complex interferometric signal. The DD-FDOCT system enables full range imaging without loss of speed, and it may be less sensitive to phase error generated by involuntary movements of the subject compared to the other established full range OCT systems, because it uses two signals with a phase difference of pi/2 obtained simultaneously from two detection arms to remove mirror images at all depths.

Fast dispersion encoded full range optical coherence tomography for retinal imaging at 800 nm and 1060 nm

The dispersion mismatch between sample and reference arm in frequency-domain optical coherence tomography (OCT) can be used to iteratively suppress complex conjugate artifacts and thereby increase the imaging range. In this paper, we propose a fast dispersion encoded full range (DEFR) algorithm that detects multiple signal components per iteration. The influence of different dispersion levels on the reconstruction quality is analyzed experimentally using a multilayered scattering phantom and in vivo retinal tomograms at 800 nm. Best results have been achieved with 30 mm SF11, with neglectable resolution decrease due to finite resolution of the spectrometer. Our fast DEFR algorithm achieves an average suppression ratio of 55 dB and typically converges within 5 to 10 iterations. The processing time on non-dedicated hardware was 5 to 10 seconds for tomograms with 512 depth scans and 4096 sampling points per depth scan. Application of DEFR to the more challenging 1060 nm wavelength region is also demonstrated by introducing an additional optical fibre in the sample arm.

Parabolic BM-scan technique for full range Doppler spectral domain optical coherence tomography

A full range spectral domain optical coherence tomography (SD-OCT) technique that relies on the linear phase modulation of one of the interferometer arms has been widely utilized. Although this method is useful, the mirror image elimination is not perfect for samples in which regions with high axial motion exist. In this paper, we introduce a new modulation pattern to overcome this mirror image elimination failure. This new modulation is a parabolic phase modulation in the transverse scanning direction, and is applied to the SD-OCT reference beam by an electro-optic modulator. Flow phantom and in vivo experiments demonstrate that for moving structures with large velocities, this parabolic phase modulation technique presents better mirror image elimination than a standard linear phase modulation method. A direct consequence of this enhanced mirror image removal is an improved velocity range obtained with phase-resolved Doppler imaging. Consequently, applying the proposed technique in retinal blood flow measurements may be useful for ophthalmologic diagnosis.

Optical coherence tomography - development, principles, applications

This paper presents a review of the development of optical coherence tomography (OCT), its principles and important applications. Basic OCT systems are described and the physical foundations of OCT signal properties and signal recording systems are reviewed. Recent examples of OCT applications in ophthalmology, cardiology, gastroenterology and dermatology outline the relevance of this advanced imaging modality in the medical field.

Sinusoidal B-M method based spectral domain optical coherence tomography for the elimination of complex-conjugate artifact

A high speed spectral domain optical coherence tomography based on the spatial sinusoidal phase modulation for the elimination of complex-conjugate artifact is presented, where sinusoidal phase modulation of reference arm (M scan) and transverse scanning of sample arm (B scan) are performed simultaneously (sinusoidal B-M method). Herein, the linear phase modulation of the reference arm in conventional linear B-M method is modified to sinusoidal phase modulation. The proposed sinusoidal B-M method relaxes the requirements on the phase-shifting mechanical system and avoids sensitivity fall-off along the transverse direction in contrast to the linear B-M method. A criterion for the relation between transverse over-sampling factor and modulation frequency for optimal complex conjugate rejection is deduced and verified by experiments. Under this criterion, the complex spectral interferogram is reconstructed by harmonic analysis and digital synchronous demodulation. Double imaging depth range on fresh shrimp at A-scan rate of 10 kHz with complex conjugate rejection ratio up to 45 dB is achieved.

Extended in vivo anterior eye-segment imaging with full-range complex spectral domain optical coherence tomography

We demonstrate the capability of full-range complex (FRC) spectral domain optical coherence tomography (SD-OCT) to image the anterior eye segment from the cornea to the posterior surface of the lens. With an adapted spectrometer design, we developed a SD-OCT system with an extended normal (single half-space) depth range of 7 mm (in air). This OCT-intrinsic depth range was doubled with a FRC technique. We demonstrate the performance of our OCT system by imaging the whole anterior segment of a healthy human eye in vivo.

Flow velocity estimation by complex ambiguity free joint Spectral and Time domain Optical Coherence Tomography

We show that recently introduced joint Spectral and Time domain Optical Coherence Tomography (STdOCT) can be used for simultaneous complex ambiguity removal and functional Spectral OCT images. This permits to take advantage of higher sensitivity achievable near the zero-path delay. The technique can be used with all Spectral OCT systems that are equipped with an optical delay line (ODL) and provide oversampled scanning patterns. High sensitivity provided by STdOCT allows this technique to be used in Spectral OCT setups with acquisition speed of 100,000 lines/s. We show that different imaging ranges and velocity ranges can be achieved by switching on/off the ODL and a small modification in the processing algorithm. Additionally, the relatively small computational burden of the technique allows for fast computations in the range of less than 5 minutes for 3D data set. We present application of proposed technique to full-range two- and three-dimensional imaging. Morphological and Doppler tomograms of human retina in-vivo are shown. Finally, we identify and discuss artifacts of the technique.

Dispersion encoded full range frequency domain optical coherence tomography

We propose an iterative algorithm that exploits the dispersion mismatch between reference and sample arm in frequency-domain optical coherence tomography (FD-OCT) to effectively cancel complex conjugate mirror terms in individual A-scans and thereby generate full range tomograms. The resulting scheme, termed dispersion encoded full range (DEFR) OCT, allows distinguishing real structures from complex conjugate mirror artifacts. Even though DEFR-OCT has higher post-processing complexity than conventional FD-OCT, acquisition speed is not compromised since no additional A-scans need to be measured, thereby rendering this technique robust against phase fluctuations. The algorithm uses numerical dispersion compensation and exhibits similar resolution as standard processing. The residual leakage of mirror terms is further reduced by incorporating additional knowledge such as the power spectrum of the light source. The suppression ratio of mirror signals is more than 50 dB and thus comparable to complex FD-OCT techniques which use multiple A-scans.