Comment on "Optical-fiber-based Mueller optical coherence tomography"

We comment on the recent Letter by Jiao et al. [Opt. Lett. 28, 1206 (2003)] in which a polarization-sensitive optical coherence tomography system was presented. Interrogating a sample with two orthogonal incident polarization states cannot always recover birefringence correctly. A previously presented fiber-based polarization-sensitive system was inaccurately characterized, and its method of eliminating the polarization distortion caused by single-mode optical fiber was presented earlier by Saxer et al. [Opt. Lett. 25, 1355 (2000)].     

Two-dimensional depth-resolved Mueller matrix characterization of biological tissue by optical coherence tomography

We built a polarization-sensitive optical coherence tomographic system and measured the two-dimensional depth-resolved full 4 x 4 Mueller matrix of biological tissue for what is believed to be the first time. The Mueller matrix measurements, which we made by varying the polarization states of the light source and the detector, yielded a complete characterization of the polarization property of the tissue sample. The initial experimental results indicated that this new approach reveals some tissue structures that are not perceptible in standard optical coherence tomography.     

Two-dimensional depth-resolved Mueller matrix of biological tissue measured with double-beam polarization-sensitive optical coherence tomography

A double-beam polarization-sensitive system based on optical coherence tomography was built to measure the Mueller matrix of scattering biological tissue with high spatial resolution. The Jones matrix of a sample can be determined with a single scan and subsequently converted into an equivalent nondepolarizing Mueller matrix. As a result, the system can be used to measure the Mueller matrix of an unstable sample, such as soft tissue. The polarization parameters of a porcine tendon, including magnitude and orientation of birefringence and diattenuation, were extracted by decomposition of the measured Mueller matrix.     

Spectroscopic optical coherence tomography

Spectroscopic optical coherence tomography (OCT), an extension of conventional OCT, is demonstrated for performing cross-sectional tomographic and spectroscopic imaging. Information on the spectral content of backscattered light is obtained by detection and processing of the interferometric OCT signal. This method allows the spectrum of backscattered light to be measured over the entire available optical bandwidth simultaneously in a single measurement. Specific spectral features can be extracted by use of digital signal processing without changing the measurement apparatus. An ultrabroadband femtosecond Ti:Al(2)O(3) laser was used to achieve spectroscopic imaging over the wavelength range from 650 to 1000 nm in a simple model as well as in vivo in the Xenopus laevis (African frog) tadpole. Multidimensional spectroscopic data are displayed by use of a novel hue-saturation false-color mapping.     

Second-harmonic optical coherence tomography

Second-harmonic optical coherence tomography, which uses coherence gating of second-order nonlinear optical responses of biological tissues for imaging, is described and demonstrated. Femtosecond laser pulses were used to excite second-harmonic waves from collagen harvested from rat tail tendon and a reference non-linear crystal. Second-harmonic interference fringe signals were detected and used for image construction. Because of the strong dependence of second-harmonic generation on molecular and tissue structures, this technique imparts contrast and resolution enhancement to conventional optical coherence tomography.     

Fiber-optic-bundle-based optical coherence tomography

A fiber-optic-bundle-based optical coherence tomography (OCT) probe method is presented. The experimental results demonstrate this multimode optical fiber-bundle-based OCT system can achieve a lateral resolution of 12 microm and an axial resolution of 10 microm with a superluminescent diode source. This novel OCT imaging approach eliminates any moving parts in the probe and has a primary advantage for use in extremely compact and safe OCT endoscopes for imaging internal organs and great potential to be combined with confocal endoscopic microscopy.     

Tracking optical coherence tomography

An experimental tracking optical coherence tomography (OCT) system has been clinically tested. The prototype instrument uses a secondary sensing beam and steering mirrors to compensate for eye motion with a closed-loop bandwidth of 1 kHz and tracking accuracy, to within less than the OCT beam diameter. The retinal tracker improved image registration accuracy to <1 transverse pixel (<60 microm). Composite OCT images averaged over multiple scans and visits show a sharp fine structure limited only by transverse pixel size. As the resolution of clinical OCT systems improves, the capability to reproducibly map complex structures in the living eye at high resolution will lead to improved understanding of disease processes and improved sensitivity and specificity of diagnostic procedures.     

Two-wavelength optical coherence tomography

We present the results of studies of the basic principles and describe the design of a low-coherence two-wavelength interferometer based on polarization-maintaining fiber. The interferometer was developed for optical coherence tomography (OCT) imaging of the internal structure of living biological tissue simultaneously at two wavelengths, 830 and 1300 nm. Images of several sites of living biological tissue are presented and analyzed.     

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.     

Time-gated Fourier-domain optical coherence tomography

A novel optical coherence tomography (OCT) system is presented that combines Fourier-domain OCT with incoherent nonlinear time gating. By processing backscattered light in the optical domain, the user can select a restricted depth field of view for improved contrast and acquisition speed. This technique has the additional advantage that imaging is done in the infrared (approximately 1280 nm) but is detected in the visible(approximately 504 nm).     

Self-referenced Doppler optical coherence tomography

Doppler optical coherence tomography (DOCT) allows simultaneous micrometer-scale resolution cross-sectional imaging of tissue structure and blood flow. We demonstrate a fiber-optic polarization-diversity-based differential phase contrast DOCT system as a method to perform self-referenced velocimetry in highly scattering media. Using this strategy, we reduced common-mode interferometer noise to <1 Hz and improved Doppler estimates in a scattering flow phantom by a factor of 5.     

Molecularly sensitive optical coherence tomography

Molecular contrast in optical coherence tomography (OCT) is demonstrated by use of coherent anti-Stokes Raman scattering (CARS) for molecular sensitivity. Femtosecond laser pulses are focused into a sample by use of a low-numerical-aperture lens to generate CARS photons, and the backreflected CARS signal is interferometrically measured. With the chemical selectivity provided by CARS and the advanced imaging capabilities of OCT, this technique may be useful for molecular contrast imaging in biological tissues. CARS can be generated and interferometrically measured over at least 600 microm of the depth of field of a low-numerical-aperture objective.     

Thermal-light full-field optical coherence tomography

We have built a high-resolution optical coherence tomography (OCT) system, based on a Linnik-type interference microscope, illuminated by a white-light thermal lamp. The extremely short coherence length of the illumination source and the large aperture of the objectives permit resolution close to 1 microm in three dimensions. A parallel detection scheme with a CCD camera provides cross-section (x-y) image acquisition without scanning at a rate of up to 50 Hz. To our knowledge, our system has the highest resolution demonstrated to date for OCT imaging. With identical resolution in three dimensions, realistic volume rendering of structures inside biological tissues is possible.     

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.     

Structured interference optical coherence tomography

We developed a structured interference optical coherence tomography (SIOCT) to enhance the lateral resolution beyond the diffraction limit. A sinusoidal pattern is created on the interferometric beam with the reference intensity temporally modulated. In the Fourier domain, the high spatial frequencies are shifted into the detectable range, which enhances the lateral resolution beyond the diffraction limit by a factor of 2. The lateral resolution of SIOCT was characterized in our study as ～5.5 μm, surpassing the diffraction limit ～9.6 μm as in conventional Fourier-domain optical coherence tomography. SIOCT was demonstrated on phantoms and ex vivo adipose tissues.     

Adaptive-optics ultrahigh-resolution optical coherence tomography

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.     

Interstitial Doppler optical coherence tomography

Doppler optical coherence tomography (OCT) can image tissue structure and blood flow at micrometer-scale resolution but has limited imaging depth. We report a novel, linear-scanning, needle-based Doppler OCT system using angle-polished gradient-index or ball-lensed fibers. A prototype system with a 19-guage (diameter of approximately 0.9 mm) echogenic needle is constructed and demonstrates in vivo imaging of bidirectional blood flow in rat leg and abdominal cavity. To our knowledge, this is the first demonstration of Doppler OCT through a needle probe in interstitial applications to visualize deeply situated microcirculation.     

相关多普勒光学层析成像

提出了基于互相关的多普勒OCT(correlated Doppler optical coherence tomography, CD-OCT)方法, 能够有效的抑制噪声, 实现低信噪比条件下的流速探测. 对CD-OCT算法进行了详细的推导, 分析了噪声的相关性对该算法结果的影响, 最后基于谱域和时域联合探测方法(joint spectral and time domain optical coherence tomography, STD-OCT)以及CD-OCT算法的对比实验证明了该算法能够进一步实现信噪比的提高, 使测量的结果更为稳定.     

Homodyne en face optical coherence tomography

We demonstrate, for what we believe to be the first time, the use of a 3 x 3 fiber-optic coupler to realize a homodyne optical coherence tomography (OCT) system for en face imaging of highly scattering tissues and turbid media. The homodyne OCT setup exploits the inherent phase shifts between different output ports of a 3 x 3 fiber-optic coupler to extract amplitude information of a sample. Our homodyne en face OCT system features a measured resolution of 14 microm axially and 9.4 microm laterally with a 90 dB signal-to-noise ratio at 10 micros integration time. En face OCT imaging of a stage 52 Xenopus laevis was successfully demonstrated at a depth of 600 microm within the sample.     

A theory of optical coherence tomography

Analytical models of optical coherence tomography (OCT) of strongly turbid media of the type of biological tissues are developed on the basis of the theory of wave scattering in random inhomogeneous media. Similarity relations for signals of coherent and pulsed sounding are established, and general expressions for random realizations and statistical characteristics of tomograms are obtained. It is shown that after the appropriate modification the theory of image transfer in turbid media can be used for analysis of their informative properties. Simple formulas for estimating the visibility depth into the internal structure of biological tissues are proposed. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41. No. 10, pp. 1258–1289, October 1998.