Quantitative birefringence measurements using a single-arm polarization-sensitive OCT system

This Letter presents a spectral-domain, polarization-sensitive optical coherence tomography (PS-OCT) system, where the light collection from the two arms of the interferometer is performed exclusively using single-mode fibers and couplers, and the two orthogonal polarization components are sequentially detected by a single line camera. Retardance measurements can be affected by polarimetric effects because of fiber birefringence and diattenuation in fiber couplers. This configuration bypasses such issues by performing polarization selection before the collection fiber through the combination of a polarization rotator and a linear polarizer. Retardance calibration is achieved with a Berek compensator. Similar net retardance maps of a birefringent phantom are obtained for two different settings of induced fiber birefringence, effectively demonstrating the tolerance of the configuration to fiber-based disturbances.

Conventional optical coherence tomography (OCT) systems generally have a limited imaging range within a depth of only 1-2 mm and suffers from unwanted noise such as speckle, ghost or mirror noise. To overcome these limitations, we developed a motorized-stage-based OCT system with an extended imaging range, using a common-path Fourier-domain optical coherence tomography (CP-FD-OCT) configuration. Using this OCT systems, OCT image was obtained from an onion, and their subsurface structure were observed. The result showed that the OCT images obtained using our motorized-stage-based system had a significantly extended imaging range due to its real-time accurate depth tracking. Consequently, the devised CP-FD-OCT systems and algorithms have good potential for the further development of endoscopic OCT for microsurgery.

Zebrafish is an important animal model, which is used to study development, pathology, and genetic research. The zebrafish skin model is widely used in cutaneous research, and angiogenesis is critical for cutaneous wound healing. However, limited by the penetration depth, the available optical methods are difficult to describe the internal skin structure and the connection of blood vessels between the skin and subcutaneous tissue. By a homemade high-resolution polarization-sensitive optical coherence tomography (PS-OCT) system, we imaged the polarization contrast of zebrafish skin and the zebrafish skin vasculature with optical coherence tomography angiography (OCTA). Based on these OCT images, the spatial distribution of the zebrafish skin vasculature was described. Furthermore, we monitored the healing process of zebrafish cutaneous wounds. We think the high-resolution PS-OCT system will be a promising tool in studying cutaneous models of zebrafish.

Osteoarthritis is a prevalent medical condition that presents a diagnostic and therapeutic challenge to physicians today because of the inability to assess the integrity of the articular cartilage early in the disease. Polarization sensitive optical coherence tomography (PS-OCT) is a high resolution, non-contact imaging modality that provides cross-sectional images with additional information regarding the integrity of the collagen matrix. Using PS-OCT to image provides information regarding thickness of the articular cartilage and gives an index of biochemical changes based on alterations in optical properties (i.e. birefringence) of the tissue. We demonstrate initial experiments performed on specimens collected following total knee replacement surgery. Articular cartilage was imaged using a 1310 nm PS-OCT system where both intensity and phase images were acquired. PS-OCT images were compared with histology, and the changes in tissue optical properties were characterized. Analysis of the intensity images demonstrates differences between healthy and diseased cartilage surface and thickness. Phase maps of the tissue demonstrated distinct differences between healthy and diseased tissue. PS-OCT was able to image a gradual loss of birefringence as the tissue became more diseased. In this way, determining the rate of change of the phase provides a quantitative measure of pathology. Thus, imaging and evaluation of osteoarthritis using PS-OCT can be a useful means of quantitative assessment of the disease.

We demonstrate cross-sectional birefringence- and polarization-independent backscatter imaging of laser-induced thermal damage in porcine myocardium in vitro, using a polarization-sensitive optical coherence tomography system. We compare the generated images with histological sections of the tissue and demonstrate that birefringence is a more sensitive indicator of thermal damage than is backscattered light. Loss of birefringence in thermally damaged regions is quantified and shown to have significant contrast with undamaged sections of the tissue. A detailed theoretical analysis of the birefringence measurements is provided, including a calculation of the systematic errors associated with background noise, system imperfections, and tissue dichroism.

We demonstrate theoretically and experimentally that the phase retardance and relative optic-axis orientation of a sample can be calculated without prior knowledge of the actual value of the phase modulation amplitude when using a polarization-sensitive optical coherence tomography system based on continuous polarization modulation (CPM-PS-OCT). We also demonstrate that the sample Jones matrix can be calculated at any values of the phase modulation amplitude in a reasonable range depending on the system effective signal-to-noise ratio. This has fundamental importance for the development of clinical systems by simplifying the polarization modulator drive instrumentation and eliminating its calibration procedure. This was validated on measurements of a three-quarter waveplate and an equine tendon sample by a fiber-based swept-source CPM-PS-OCT system.

A new method based on polarization-sensitive optical coherence tomography (PS-OCT) is introduced to determine the polarization properties of human retinal vessel walls, in vivo. Measurements were obtained near the optic nerve head of three healthy human subjects. The double pass phase retardation per unit depth (DPPR/UD), which is proportional to the birefringence, is higher in artery walls, presumably because of the presence of muscle tissue. Measurements in surrounding retinal nerve fiber layer tissue yielded lower DPPR/UD values, suggesting that the retinal vessel wall tissue near the optic nerve is not covered by retinal nerve fiber layer tissue (0.43°/µm vs. 0.77°/µm, respectively). Measurements were obtained from multiple artery-vein pairs, to quantify the different polarization properties. Measurements were taken along a section of the vessel wall, with changes in DPPR/UD up to 15%, while the vessel wall thickness remained relatively constant. A stationary scan pattern was applied to determine the influence of involuntary eye motion on the measurement, which was significant. Measurements were also analyzed by two examiners, with high inter-observer agreement. The measurement repeatability was determined with measurements that were acquired during multiple visits. An improvement in accuracy can be achieved with an ultra-broad-bandwidth PS-OCT system since it will provide more data points in-depth, which reduces the influence of discretization and helps to facilitate better fitting of the birefringence data.

In swept source polarization depth encoding polarization sensitive optical coherence tomography (PS-OCT), the laser jitter induces additional noise to the polarization sensitive measurement. In this Letter, we developed a numerical algorithm to correct the jitter phases based on the image data using the Mueller matrix calculus. The algorithm was demonstrated on in vivo retina imaging of a guinea pig with a custom-built PS-OCT system. The performance of the proposed algorithm was almost comparable to the conventional method of using a physical calibration signal. By not requiring a hardware generated calibration signal and k-clock, the proposed algorithm is useful to reduce the complexity and the cost of a polarization depth encoding PS-OCT system.

Radiofrequency ablation (RFA) is an important standard therapy for cardiac arrhythmias, but direct monitoring of tissue treatment is currently lacking. We demonstrate an RFA catheter integrated with polarization sensitive optical coherence tomography (PSOCT) for directly monitoring the RFA process in real time. The integrated RFA/OCT catheter was modified from a standard clinical RFA catheter and includes a miniature forward-viewing cone-scanning OCT probe. The PSOCT system was validated with a quarter-wave plate while the RFA function of the integrated catheter was validated by comparing lesion sizes with those made with an unmodified RFA catheter. Additionally, the integrated catheter guided catheter-tissue apposition and monitored RFA lesion formation in cardiac tissue in real time. The results show that catheter-tissue contact can be characterized by observing the features of the blood and tissue in the acquired OCT images and that RFA lesion formation can be confirmed by monitoring the change in phase retardance in the acquired PSOCT images. This system demonstrates the feasibility of an integrated RFA/OCT catheter to deliver RF energy and image the cardiac wall simultaneously and justifies further research into use of this technology to aid RFA therapy for cardiac arrhythmias.

Accurate wavelength assignment of each spectral element for spectral-domain optical coherence tomography (SD-OCT) and optical frequency domain imaging (OFDI) is required for proper construction of biological tissue cross-sectional images. This becomes more critical for functional extensions of these techniques, especially in polarization-sensitive optical coherence tomography (PS-OCT), where incorrect wavelength assignment between the two orthogonal polarization channels leads to polarization artifacts. We present an autocalibration method for wavelength assignment that does not require separate calibration measurements and that can be applied directly on actual data. Removal of the birefringence artifact is demonstrated in a PS-OCT system with picometer accuracy in the relative wavelength assignment, resulting in a residual phase error of 0.25 deg/100 microm. We also demonstrate, for the first time, a quantitative birefringence map of an in vivo human retinal nerve fiber layer.

Modern optical volumetric imaging modalities, such as optical coherence tomography (OCT), provide enormous information about the structure, function, and physiology of living tissue. Although optical imaging achieves lateral resolution on the order of the wavelength of light used, and OCT achieves axial resolution on a similar micron scale, tissue optical properties, particularly high scattering and absorption, limit light penetration to only a few millimeters. In addition, in vivo imaging modalities are susceptible to significant motion artifacts due to cardiac and respiratory function. These effects limit access to artifact-free optical measurements during peripheral neurosurgery to only a portion of the exposed nerve without further modification to the procedure. We aim to improve in vivo OCT imaging during peripheral neurosurgery in small and large animals by increasing the amount of visualized nerve volume as well as suppressing motion of the imaged area. We designed a nerve holder with embedded mirror prisms for peripheral nerve volumetric imaging as well as a specific beam steering strategy to acquire prism and direct view volumes in one session with minimal motion artifacts. The axially imaged volumes from mirror prisms increased the OCT signal intensity by over a 1.25-mm imaging depth in tissue-mimicking phantoms. We then demonstrated the new imaging capabilities in visualizing peripheral nerves from direct and side views in living rats and minipigs using a polarization-sensitive OCT system. Prism views have shown nerve fascicles and vasculature from the bottom half of the imaged nerve which was not visible in direct view. We demonstrated improved OCT imaging during neurosurgery in small and large animals by combining the use of a prism nerve holder with a specifically designed beam scanning protocol. Our strategy can be applied to existing OCT imaging systems with minimal hardware modification, increasing the nerve tissue volume visualized. Enhanced imaging depth techniques may lead to a greater adoption of structural and functional optical biomarkers in preclinical and clinical medicine.

Retardation induced by media can be used as an image contrast to depict the cumulative birefringent features and local variations of the sample, respectively. It is commonly assumed that the retardation is induced by the light propagation; however, the light scattering would generate the retardation as well. In our work, the scattering-induced retardation as a high-sensitivity image contrast for revealing collagen fibers is presented. First, it is shown that the retardation induced by fiber scattering is equal to π when modeled as cylinders. Using the data for the chicken breast and the palm measured by the polarization-sensitive optical coherence tomography system as an example, the scattering-induced retardation is calculated. The measured value of π is in complete agreement with the theory, and the corresponding retardation per unit distance is two orders of magnitude greater than the light-propagation-induced retardation, demonstrating its predominant role on the overall retardation and providing a possibility for highly sensitive displays. Compared with the accumulated retardation image and the differential retardation image, the scattering-induced retardation images could exhibit sharper fiber structures even in deeper regions. This work might be helpful for the early diagnosis of collagen-related diseases.

Intraoperative delineation of breast cancer is a major challenge. An effective breast tissue screening technique may reduce the risk of re-excision during surgery by specifically identifying positive margins. In this study, a high-resolution automated full-field polarization-sensitive optical coherence tomography (FF-PS-OCT) system was developed to classify healthy and malignant human breast tissue from quantitative phase retardation information of the tissues in ex vivo. Twelve breast tissue samples [four healthy, eight malignant (cancerous)] were imaged with the FF-PS-OCT system and the different phase features were extracted from the acquired OCT images (106), based on the differences in the optical signatures of the healthy and malignant tissues. A linear support vector model classifier was trained using 75 images, with a sensitivity of 92.10% and specificity of 89.18% was achieved. Thirty-one images were used to test the model, with a sensitivity of 90.90% and specificity of 85.0% achieved.

A fiber based polarization-sensitive Fourier domain optical coherence tomography system was demonstrated. The complex fringe signal containing amplitude and phase information was acquired by cancellation of DC and autocorrelation terms as well as the mirror image using an electro-optic phase modulator. The Stokes vectors of the reflected and scattered light from sample were determined by processing the analytical complex fringe signals from two perpendicular polarization-detection channels. From the Stokes vectors for four different input polarization states, the birefringence image and the polarization diversity intensity image were obtained.

Intravascular polarization-sensitive optical coherence tomography (IV-PSOCT) provides depth-resolved tissue birefringence which can be used to evaluate the mechanical stability of a plaque. Here, we demonstrate a very simple method of constructing an intravascular polarization-sensitive optical coherence tomography (IV-PSOCT) system. A conventional intravascular OCT system can be modified for IV-PSOCT by applying a 12-m polarization-maintaining fiber based imaging probe. Its two polarization modes separately produce OCT images of polarization detection channels spatially distinguished by an image separation of 2.7 mm. We experimentally validated our IV-PSOCT with chicken tendon, chicken breast, and coronary artery for the image samples. We found that the birefringent properties can be successfully visualized by our endoscopic imaging tool.