Characterization of optical fiber imaging bundles for swept-source optical coherence tomography

Fiber imaging bundles have been investigated for use in endoscopic optical coherence tomography (OCT) systems, to obviate the requirement for scanning components within the endoscope probe section. Images have been acquired using several optical configurations, two of which are common path in design. Configurations have been selected as having potential for miniaturization and inclusion in endoscopic-type systems, since the advantages of employing imaging bundles are most clearly seen in this type of system. The various types of bundle available are described, and the properties of the leached bundles used here are discussed in detail, with reference to their effect upon the performance of OCT systems. Images are displayed from measurements made on a range of samples.     

Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography

We present theoretical analysis and experimental verification of the signal to noise ratio (SNR) of a common-path interferometer-based optical coherence tomography (OCT) system. Based on fully integrated all-fiber implementation of a common-path time-domain OCT system, we derived the SNR of the system including the effect of beat noise, which turns out to be twice as large as the excess noise term. We verified the theoretical SNR through a series of experiments, utilizing both controlled phantom and biological samples such as a rat brain with tumor and a frog retina. The results showed that the source power and the reference reflectivity can be easily controlled to optimize the SNR of OCT imaging. We have also analyzed the effect of the fiber delays and the offset in the fiber autocorrelator of the common-path OCT system on the overall SNR.     

Optimal signal processing of nonlinearity in swept-source and spectral-domain optical coherence tomography

We demonstrate the efficiency of the convolution using an optimized Kaiser-Bessel window to resample nonlinear data in wavenumber for Fourier-domain optical coherence tomography (OCT). We extend our previous experimental demonstration that was performed with a specific swept-source nonlinearity. The method is now applied to swept-source OCT data obtained for various simulated swept-source nonlinearities as well as spectral-domain OCT data obtained from both simulations and experiments. Results show that the new optimized method is the most efficient for handling all the different types of nonlinearities in the wavenumber domain that one can encounter in normal practice. The efficiency of the method is evaluated through comparison with common methods using resampling through interpolation prior to performing a fast-Fourier transform and with the accurate but time-consuming discrete Fourier transform for unequally spaced data, which involves Vandermonde matrices.     

Characterisation of scaffold architecture and tendons using optical coherence tomography and polarisation-sensitive optical coherence tomography

Scaffolds play an important role in the generation of functional tissues using tissue-engineering techniques. To generate highly organised tissue, scaffolds must have specific internal and external architectures. Here, optical coherence tomography (OCT) is exploited to characterise the architectures of various scaffolds, in particular scaffolds which have been fabricated to support the formation of uniaxially orientated collagen bundle for use in tendon tissue engineering. In parallel, a polarisation-sensitive OCT (PSOCT) has been built to assess the collagen fibre organisation in human tendon and monitor the growth of engineering tendon constructs online and non-destructively. The impact of mechanical stimuli on the modulation of tendon tissue formation and organisation was also assessed. It is shown that conventional OCT is capable of characterising scaffold architecture and the pore size, porosity or microchannel dimension can be determined quantitatively and qualitatively. PSOCT generated birefringence images of human tendon and demonstrated that low birefringence images, associated with fewer microstructural variations, correlated to the presence of scar tissue or degenerated tissue; whereas the tissue-engineered tendon exhibited lower degree of birefringence.     

Polarization-sensitive optical coherence tomography for imaging human atherosclerosis

Polarization-sensitive optical coherence tomography (PS-OCT) combines the advantages of OCT with image contrast enhancement, which is based on its ability to detect phase retardation and the fast-axis angle. Both PS-OCT images and histopathology have demonstrated similar features that allowed differentiation of atherosclerotic structures (i.e., plaques) from normal tissue. Moreover, the picrosirius polarization method was used to confirm PS-OCT assessment of collagen in the fibrous cap of atherosclerotic plaques, and high-frequency (40 MHz) ultrasound images were used to identify calcium in the vessel wall. Our preliminary ex vivo investigation of human aortic specimens indicated that PS-OCT might help to identify atherosclerotic lesions.     

Multiscan time-domain optical coherence tomography for retina imaging

A versatile time-domain optical coherence tomography system is presented that can generate cross-sectional images by using either transverse priority or depth priority scanning. This is made possible by using a transmissive scanning delay line compatible with balance detection operating at a speed similar to that of the transverse scanner used to scan the beam across the target. In vivo images from the retina are generated and shown using the same system switched to either transverse or depth priority scanning regime, by using the scanning delay line either in slow or fast scanning modes, respectively. A comparative analysis of different scanning regimes depending on image size to fit different areas to be imaged is presented. Safety thresholds due to the different continuous irradiation time per transverse pixel in different scanning regimes are also considered. We present the maximum exposure level for a variety of scanning procedures, employing either A scanning (depth priority) or T scanning (transverse priority) when generating cross-sectional images, en face images, or collecting 3D volumes.     

3x3 Mach-Zehnder interferometer with unbalanced differential detection for full-range swept-source optical coherence tomography

Quadrature interferometry based on 3x3 fiber couplers could be used to double the effective imaging depth in swept-source optical coherence tomography. This is due to its ability to suppress the complex conjugate artifact naturally. We present theoretical and experimental results for a 3x3 Mach-Zehnder interferometer using a new unbalanced differential optical detection method. The new interferometer provides simultaneous access to complementary phase components of the complex interferometric signal. No calculations by trigonometric relationships are needed. We demonstrate a complex conjugate artifact suppression of 27 dB obtained in swept-source optical coherence tomography using our unbalanced differential detection. We show that our unbalanced differential detection has increased the signal-to-noise ratio by at least 4 dB compared to the commonly used balanced detection technique. This is due to better utilization of optical power.     

Quantitative imaging of red blood cell velocity invivo using optical coherence Doppler tomography

We present particle counting ultrahigh-resolution optical Doppler tomography (pc-μODT) that enables accurate imaging of red blood cell velocities (ν(RBC)) of cerebrovascular networks by detecting the Doppler phase transients induced by the passage of a RBC through a capillary. We apply pc-μODT to image the response of capillary ν(RBC) to mild hypercapnia in mouse cortex. The results show that ν(RBC) in normocapnia (ν(N)?=?0.72?±?0.15?mm/s) increased 36.1%?±?5.3% (ν(H)?=?0.98?±?0.29?mm/s) in response to hypercapnia. Due to uncorrected angle effect and low hematocrit (e.g., ～10%), ν(RBC) directly measured by μODT were markedly underestimated (ν(N) ≈ 0.27?±?0.03?mm/s, ν(H) ≈ 0.37±?0.05?mm/s). Nevertheless, the measured ν(RBC) increase (35.3%) matched that (36.1%?±?5.3%) by pc-μODT.     

Wide-field optical coherence tomography: imaging of biological tissues

We describe a two-dimensional optical coherence tomography technique with which we were able to obtain multiple longitudinal slices of a biological sample directly in a single Z scan. The system is based on a femtosecond Cr4+:forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 5 microm. With this imaging apparatus we were able to investigate human skin and mouse ear samples and to observe the different constitutive tissues.     

Cancellation of coherent artifacts in optical coherence tomography imaging

Coherent artifacts in optical coherence tomography (OCT) images can severely degrade image quality by introducing false targets if no targets are present at the artifact locations. Coherent artifacts can also add constructively or destructively to the targets that are present at the artifact locations. This constructive or destructive interference will result in cancellation of the true targets or in display of incorrect echo amplitudes of the targets. We introduce the use of a nonlinear deconvolution algorithm, CLEAN, to cancel coherent artifacts in OCT images of extracted human teeth. The results show that CLEAN can reduce the coherent artifacts to the noise background, sharpen the air-enamel and enamel-dentin interfaces, and improve the image contrast.     

Primate retina imaging with polarization-sensitive optical coherence tomography.

Polarization-sensitive optical coherence tomography (PSOCT) is applied to determine the depth-resolved polarization state of light backreflected from the eye. The birefringence of the retinal nerve fiber layer (RNFL) was observed and measured from PSOCT images recorded postmortem in a Rhesus monkey. An image-processing algorithm was developed to identify birefringent regions in acquired PSOCT retinal images and automatically determine the thickness of the RNFL. Values of the RNFL thickness determined from histology and PSOCT were compared. PSOCT may provide a new method to determine RNFL thickness and birefringence for glaucoma diagnostics.     

Full-field optical coherence tomography using nematic liquid-crystal phase shifter

We present a polarization Linnik interference microscope with a nematic liquid-crystal (NLC) phase shifter for full-field optical coherence tomography of high-quality images. The rotating half-wave plate in conventional achromatic phase shifters was replaced by three liquid-crystal (LC) half-wave plates for implementing three-step phase-shifting interferometry. Thus, the NLC device generates phase shifts quickly and has no vibrations. In addition, the phase shift can be set to an arbitrary value between 0 and 2π by altering the azimuth angles of the LC cells. A tomographic image is retrieved from three sequential phase-shifted interferograms by using a three-step algorithm. The experimental results confirm the feasibility of the proposed technology.     

Phenotyping transgenic embryonic murine hearts using optical coherence tomography

We used optical coherence tomography (OCT) to characterize the morphological phenotype of embryonic murine hearts discerning hexamethylene-bis-acetamide-inducible protein 1 (HEXIM1) mutants from their wild-type littermates. At E12.5 and E13.5 murine embryos were excised from the mother, the hearts were removed, and 3D OCT data sets were obtained from each heart in the litter. Next, we segmented the morphological borders to obtain cavity volumes and wall thicknesses. The mutant hearts exhibited increased ventricular chamber volume and decreased compact myocardium wall thickness when compared with their wild-type littermates. Also, the E13.5 HEXIM1 -/- embryo was distinguished by morphological asymmetry (underdeveloped left side).     

Monte Carlo modeling of optical coherence tomography imaging through turbid media

We combine a Monte Carlo technique with Mie theory to develop a method for simulating optical coherence tomography (OCT) imaging through homogeneous turbid media. In our model the propagating light is represented by a plane wavelet; its line propagation direction and path length in the turbid medium are determined by the Monte Carlo technique, and the process of scattering by small particles is computed according to Mie theory. Incorporated into the model is the numerical phase function obtained with Mie theory. The effect of phase function on simulation is also illustrated. Based on this improved Monte Carlo technique, OCT imaging is directly simulated and phase information is recorded. Speckles, resolution, and coherence gating are discussed. The simulation results show that axial and transversal resolutions decrease as probing depth increases. Adapting a light source with a low coherence improves the resolution. The selection of an appropriate coherence length involves a trade-off between intensity and resolution.     

Compact multimodal adaptive-optics spectral-domain optical coherence tomography instrument for retinal imaging

We have developed a compact, multimodal instrument for simultaneous acquisition of en face quasi-confocal fundus images and adaptive-optics (AO) spectral-domain optical coherence tomography (SDOCT) cross-sectional images. The optical system including all AO and SDOCT components occupies a 60x60 cm breadboard that can be readily transported for clinical applications. The AO component combines a Hartmann-Shack wavefront sensor and a microelectromechanical systems-based deformable mirror to sense and correct ocular aberrations at 15 Hz with a maximum stroke of 4 microm. A broadband superluminescent diode source provides 4 mum depth resolution for SDOCT imaging. In human volunteer testing, we observed up to an 8 dB increase in OCT signal and a corresponding lateral resolution of <10 microm as a result of AO correction.     

Imaging of oocyte development using ultrahigh-resolution full-field optical coherence tomography

In this paper, a white-light full-field optical coherence tomography is developed to provide three-dimensional imaging of the development of a mouse embryo with ultrahigh-resolution. Spatial resolution of 1.8 μm×1.12 μm (transverse×axial) is achieved owing to the extremely short coherence length of the light source and optimized compensation of dispersion mismatch. A shot-noise limited detection sensitivity of 80 dB is obtained at an acquisition time of 5 seconds per image. To enable in vivo imaging of the mouse embryo development, a homemade incubator is applied to provide appropriate CO2 concentration, temperature, and humidity. An electronic light shutter is used to control the light source in order to avoid unnecessary exposure to the embryo development when the sample is not being scanned. To demonstrate our method, in vivo time series two-dimensional images of the in vitro fertilization process of mouse oocytes at the germinal vesicles stage are presented.     

Interferometer for optical coherence tomography

We describe a new interferometer setup for optical coherence tomography (OCT). The interferometer is based on a fiber arrangement similar to Young's two-pinhole interference experiment with spatial coherent and temporal incoherent light. Depth gating is achieved detection of the interference signal on a linear CCD array. Therefore no reference optical delay scanning is needed. The interference signal, the modulation of the signal, the axial resolution, and the depth range are derived theoretically and compared with experiments. The dynamic range of the setup is compared with OCT sensors in the time domain. To our knowledge, the first images of porcine brain and heart tissue and human skin are presented.     

Ultrahigh-resolution full-field optical coherence tomography

We have developed a white-light interference microscope for ultrahigh-resolution full-field optical coherence tomography of biological media. The experimental setup is based on a Linnik-type interferometer illuminated by a tungsten halogen lamp. En face tomographic images are calculated by a combination of interferometric images recorded by a high-speed CCD camera. Spatial resolution of 1.8 microm x 0.9 microm (transverse x axial) is achieved owing to the extremely short coherence length of the source, the compensation of dispersion mismatch in the interferometer arms, and the use of relatively high-numerical-aperture microscope objectives. A shot-noise-limited detection sensitivity of 90 dB is obtained in an acquisition time per image of 4 s. Subcellular-level images of plant, animal, and human tissues are presented.     

Parallel optical coherence tomography system

We present the design and procedures for implementing a parallel optical coherence tomography (POCT) imaging system that can be adapted to an endoscopic format. The POCT system consists of a single mode fiber (SMF) array with multiple reduced diameter (15 microm) SMFs in the sample arm with 15 microm center spacing between fibers. The size of the array determines the size of the transverse imaging field. Electronic scanning eliminates the need for mechanically scanning in the lateral direction. Experimental image data obtained with this system show the capability for parallel axial scan acquisition with lateral resolution comparable to mechanically scanned optical coherence tomography systems.     

Demonstration of complex-conjugate-resolved harmonic Fourier-domain optical coherence tomography imaging of biological samples

Complex-conjugate-resolved Fourier-domain optical coherence tomography, where the quadrature components of the interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is applied to multisurface test targets and biological samples. The method provides efficient suppression of the complex-conjugate, dc, and autocorrelation artifacts. A complex-conjugate rejection ratio as high as 70 dB is achieved.