Precision of measurement of tissue optical properties with optical coherence tomography

Accurate and noninvasive measurement of tissue optical properties can be used for biomedical diagnostics and monitoring of tissue analytes. Noninvasive measurement of tissue optical properties (total attenuation and scattering coefficients, optical thickness, etc.) can be performed with the optical coherence tomography (OCT) technique. However, speckle noise substantially deteriorates the accuracy of the measurements with this technique. We studied suppression of speckle noise for accurate measurement of backscattering signal and scattering coefficient with the OCT technique. Our results demonstrate that the precision of measurement of backscattering signals with the OCT technique can be 0.2% for homogeneously scattering media and 0.7% for skin, if spatial averaging of speckle noise is applied. This averaging allows us to achieve the precision of tissue scattering coefficient measurements of approximately +/-0.8%. This precision can be further improved by a factor of 2-3, upon optimization of OCT operating parameters.     

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.     

Inverse scattering for optical coherence tomography

Inverse scattering theory for optical coherence tomography (OCT) is developed. The results are used to produce algorithms to resolve three-dimensional object structure, taking into account the finite beam width, diffraction, and defocusing effects. The resolution normally achieved only in the focal plane of the OCT system is shown to be available for all illuminated depths in the object without moving the focal plane. Spatially invariant resolution is verified with numerical simulations and indicates an improvement of the high-resolution cross-sectional imaging capabilities of OCT.     

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.     

Long-optical-path scanning mechanism for optical coherence tomography

A new scanning mechanism for changing long optical paths is proposed. This mechanism consists of corner reflectors arranged equally upon a disk and an outer mirror. Rotating the 120-mm disk causes a long-optical-path change in each reflector with a near linearity of more than 40 mm. An optical coherence tomography system is described that confirms the usefulness of the proposed mechanism. Its operating characteristics and accuracy are evaluated by analysis and experiment. The deviation of the optical-path change is less than 1.52% at a reflector rotation angle of +/-10 degrees. A high-speed lock-in amplifier is utilized for fundamental measurements of glass samples.     

Common-path interferometer for frequency-domain optical coherence tomography

A Michelson-type spectral interferometer that uses a common beam path for the reference and the sample arms is described. This optical arrangement is more compact and stable than the more commonly used dual-arm interferometer and is well suited for frequency-domain optical coherence tomography of biological samples. With a 16-bit CCD camera, the instrument has sufficient dynamic range and resolution for imaging to depths of 2 mm in scattering biological materials. Images obtained with this spectral interferometer are presented, including cross-sectional images in a Xenopus laevis tadpole.     

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.     

Miniature all-fiber Fabry-Perot sensor for simultaneous measurement of pressure and temperature

This article presents a miniature, high-sensitivity, all-silica Fabry-Perot fiber-optic sensor suitable for simultaneous measurements of pressure and temperature. The proposed sensor diameter does not exceed 125 μm and consists of two low-finesse Fabry-Perot resonators created at the tip of an optical fiber. The first resonator is embodied in the form of a short air cavity positioned at the tip of the fiber. This resonator utilizes a thin silica diaphragm to achieve the sensor's pressure response. The second resonator exploits the refractive index dependence of silica fiber in order to provide the proposed sensor's temperature measurement function. Both resonators have substantially different lengths that permit straightforward spectrally resolved signal processing and unambiguous determination of the applied pressure and temperature.     

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.     

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.     

Inverse scattering for rotationally scanned optical coherence tomography

Optical coherence tomography of luminal structures, such as for intravascular or gastrointestinal imaging, is performed by using a fiber-optic catheter as a beam-delivery probe. The interrogating beam is scanned angularly by rotating the fiber around a fixed central axis. Because the beam is focused only at a fixed distance from the center of the fiber, only scatterers near this distance are resolved. We present a solution of the inverse scattering problem that provides an estimate of the susceptibility of the sample for an angularly scanned Gaussian beam focused at a fixed distance from the origin. This solution provides quantitatively meaningful reconstructions while also extending the volume of the sample that is resolvable by the instrument.     

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.     

Temporal coherence and time-frequency distributions in spectroscopic optical coherence tomography

Traditional analysis of spectroscopic optical coherence tomography (SOCT) signals is limited by an uncertainty relationship between time (depth) and frequency (wavelength). The use of a bilinear time-frequency distribution for analysis, such as those that compose Cohen's class of functions, may provide a way to avoid this limitation. Here we present the relationship between traditional SOCT analysis and the relevant Cohen class functions: the Wigner and Choi-Williams distributions. While cross terms that arise in these bilinear time-frequency distributions have been viewed as an artifact, here we identify these terms with temporal coherence, which contains significant information about the signal through phase relationships. The utility of time-frequency distributions is illustrated through analysis of calculated signals.     

线聚焦光学相干层析术的研究

提出了一种高速光学相干层析(OCT)成像技术方案。利用柱面镜的成像特性将传统OCT的点聚焦成像模式改变为线聚焦成像模式,从而降低二维OCT图像的扫描维数,达到提高成像速度的目的。利用ZEMAX光学软件对系统进行光线追迹获得光束经过柱面镜后的聚焦情况。随后采用635nm的激光光源和柱面镜构建了实验系统,实验结果很好地验证了光线追迹仿真结果。     

High-resolution frequency-domain second-harmonic optical coherence tomography

We used continuum generated in an 8.5 cm long fiber by a femtosecond Yb fiber laser to improve threefold the axial resolution of frequency domain second-harmonic optical coherence tomography (SH-OCT) to 12 microm. The acquisition time was shortened by more than 2 orders of magnitude compared to the time-domain SH-OCT. The system was applied to image biological tissue of fish scales, pig leg tendon, and rabbit eye sclera. Highly organized collagen fibrils can be visualized in the recorded images. Polarization dependence on the SH has been used to obtain polarization resolved images.     

Dispersion in a grating-based optical delay line for optical coherence tomography

Optical coherence tomography (OCT) is a high-resolution imaging technology based on low-coherence interferometry. When OCT imaging is performed in biological tissue, dispersion almost inevitably occurs. We quantify the group-velocity dispersion that a grating-based optical delay line may induce and its contribution to the axial point-spread function of OCT. Among the practical reasons for modeling the dispersion in grating-based optical delay line is that, at maximum compensation, it can provide insight into the dispersive properties of tissues.     

Speckle statistics in optical coherence tomography

The two previously reported calculations of the amplitude distribution of speckles in optical coherence tomography, each based on a different mathematical formulation, yield different results. We show that a modification of an initial assumption in one of the formulations leads to equivalent results.     

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.     

Autofocus algorithm for dispersion correction in optical coherence tomography

Practical clinical optical coherence tomography (OCT) systems require automatic tools for identifying and correcting flaws in OCT images. One type of flaw is the loss of image detail owing to the dispersion of the medium, which in most cases is unknown. We present an autofocus algorithm for estimating the delay line and material dispersion from OCT reflectance data, integrating a previously presented dispersion compensation algorithm to correct the data. The algorithm is based on minimizing the Renyi entropy of the corrected axial-scan image, which is a contrast-enhancement criterion. This autofocus algorithm can be used in conjunction with a high-speed, digital-signal-processor-based OCT acquisition system for rapid image correction.