Optical coherence tomography (OCT) and speckle imaging are two coherence-based imaging modalities with burgeoning applications in rapid non-invasive measurement of the skin. Both modalities are sensitive to tissue morphology and have polarization-sensitive augmentations. This study compares Polarization Sensitive OCT (PS-OCT) and Polarization Speckle measurements to better understand the relationship between polarization and coherence properties of skin. Volunteers of Fitzpatrick skin type I through VI were recruited and healthy skin was measured at four body sites (palm, inner forearm, forehead, and eye corner). Preliminary results indicate a strong similarity between the skin surface roughness measurements of PS-OCT and polarization speckle. In regards to tissue depolarization, PS-OCT measurements appear minimally affected by skin color, whereas polarization speckle was strongly affected due to differing measurement wavelengths. Among body sites, the palm and face were found to be generally smoother than the forearm; however the epidermis layer of the palm demonstrated notably greater polarization scrambling.
Polarization-Sensitive Optical Coherence Tomography (PS-OCT) is a real-time 3D imaging technique providing structural and functional contrasts in tissue. Previously, we have quantified multiple contrasts such as back-scattered intensity, accumulated phase retardation, local birefringence, and degree of polarization uniformity in the epidermis and dermis of the skin. Here, we add the Attenuation Coefficient (AC) contrast to quantify light attenuation from absorption and scattering in skin layers. Two techniques are utilized to obtain AC. One involves slope fitting to the logarithm of intensity A-lines, and the other uses a depth-resolved model-based reconstruction of AC. To investigate the effect of skin tone on AC, we first used skin phantoms with various absorption and scattering coefficients. It is found that the color of the phantom directly correlates with the absorption coefficient, while AC correlates with the sum of the absorption and reduced scattering coefficient. Darker objects, indicative of a higher concentration of melanin in the skin tissue, show a higher absorption coefficient. AC is further analyzed on in vivo skin imaging with different skin tones. However, no clear correlation between AC and skin tone is observed. This lack of correlation is likely due to the absorption coefficient being much smaller than the scattering coefficient in skin tissue at the OCT imaging wavelength of 1060nm.
Polarization-sensitive optical coherence tomography (PS-OCT) is a promising tool for non-invasive skin imaging, with its capability for depth-resolved, high-resolution, fast, and polarization-sensitive imaging. Thirty volunteers are recruited for skin imaging on the palm, arm, forehead, and eye corner. A segmentation algorithm based on intensity images will segment the epidermis and dermis layers and also stratum corneum if applicable. Multi-contrast images, including phase retardation, local birefringence, and degree of polarization uniformity, will be obtained from different skin layers. Possible relationships between the optical properties and skin features including layered structure, collagen organization, melanin concertation, and skin roughness will be investigated.
Jones matrix optical coherence tomography (JM-OCT) is a form of polarization-sensitive OCT (PS-OCT) that allows for the simultaneous and high-quality in vivo capture of multiple polarization-based imaging contrasts. Our system allows for the capture of high-sensitivity structural OCT, degree of polarization uniformity (DOPU), and birefringence images. Segmenting the epidermal-dermal junction is a topic of high interest in OCT and dermatology. While others have explored this with traditional OCT, no other groups have used this type of segmentation with JM-OCT. We believe that combining a reliable segmentation procedure with the robust and localized characterization provided by JM-OCT can help fully utilize the strengths of both techniques and allow for a better characterization of the skin layers. Here, we utilize JM-OCT to assess the skin properties of healthy volunteers. Using high-sensitivity OCT, we quantified the epidermal thickness of various locations in vivo and were able to segment the epidermis, dermis, and stratum corneum in thick skin. Polarization imaging is sensitive to specific structures in the skin, such as collagen and melanin, and we were able to quantify the depolarization, and birefringence caused by these structures in different skin layers. In thick skin, localized polarization results showed an average DOPU of 0.89 in the dermis compared to an average of 0.79 in the stratum corneum. However, in both thick and thin skin, DOPU was lower overall in the epidermis compared to the dermis, indicating that most observed depolarization occurs due to the structure of the stratum corneum. Birefringence was found to be higher in the dermis than the epidermis in both cases.
Skin cancer is one of the most prevalent types of cancer in the world, with a steadily increasing incidence rate and associated health burden [1, 2]. While it is generally treatable when detected, survival rates decrease dramatically as the disease progresses – highlighting the importance of early detection [3]. Unfortunately, the current gold standard in diagnosing skin cancers involves taking biopsies followed by histopathology, which is invasive and time consuming. Some studies have shown that the majority of biopsies ordered by primary care providers were found to be benign, meaning that biopsies are often performed when there is no cancer present [4]. Given this, there is a great interest in developing noninvasive diagnostic tools for skin imaging. Optical coherence tomography (OCT) is an imaging modality that is particularly well suited for this area, owing to its ability to provide high-resolution (3-15 μm) volumetric data at a penetration depth of up to 1.5 mm [5]. In a manner analogous to ultrasound, it provides cross-sectional images which can be comparable to histology slides [6]. However, conventional intensity-based OCT only provides structural information with no functional contrast, and as a result it has encountered difficulty in diagnosing specific cancers such as melanoma [7]. Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT which can characterize polarization properties such as the birefringence of tissue samples – birefringence specifically occurs in fibrous structures such as collagen [8]. Several groups have investigated birefringence in skin tissue using PS-OCT, but little work has been done with the more recently defined degree of polarization uniformity (DOPU) contrast. The functional contrast provided by DOPU PS-OCT imaging can provide localized, depth-resolved information on the polarization scrambling properties of samples. A specific example of this is in ophthalmic imaging, where DOPU contrast in PS-OCT imaging demonstrated the ability to segment the retinal pigment epithelium – a layer otherwise hard to differentiate in intensity-based OCT imaging [9]. To our knowledge, very few if any groups have investigated PS-OCT imaging with DOPU contrast in understanding the layered-structure of skin. We have recently investigated the DOPU in skin tissue phantoms and reported that DOPU is sensitive to surface roughness – an important factor in differentiating skin cancers from benign lesions. Our group has a previously reported PS-OCT system that can simultaneously acquire reflectance, phase retardation (birefringence), and DOPU images that we aim to use in this study to expand on our previous work and further investigate polarization properties in skin in vivo [10].
Skin cancer is the most common form of cancer in North America, and melanoma is the most deadly form of skin cancer. Roughness assessment of epidermis has been shown to be valuable in detecting potential skin neoplasia. However, the existing roughness assessment techniques cannot also provide volumetric information. For greater insight, we propose polarization sensitive optical coherence tomography (PS-OCT) for skin assessment. The intensity channel of OCT visualizes the layered structure and surface roughness profile of skin in 3D. Furthermore, PS-OCT can simultaneously conduct polarization related measurements such as the degree of polarization uniformity (DOPU) in a separate imaging channel. Skin phantoms of different surface roughness ranging from 1 to 68 μm have been studied. It was observed that for rougher surfaces, the roughness can be quantified from the surface profile visible in the intensity channel. In smoother surfaces for which the profile is not sensitive, the DOPU decreases with roughness in a quantifiable correlation. The contrast in the DOPU channel is sensitive to polarization and phase fluctuations. Smoother surfaces tend to maintain the polarization state, whereas the height differences in a rougher surface contribute to larger phase shifts between light waves within the coherence volume, leading to greater depolarization. PS-OCT was also applied to in vivo imaging of human skin. The skin at the palm edge shows lower DOPU compared to the skin on the back of the hand, an indication of greater polarization state modification caused by skin roughness. PS-OCT can provide a comprehensive evaluation of skin, which has great potential for detecting melanoma.
A segmentation method based on phase retardation measurements from polarization-sensitive optical coherence tomography (PS-OCT) is developed to differentiate the structural zones of articular cartilage. The organization of collagen matrix in articular cartilage varies over the different structural zones, generating different tissue birefringence. Analyzing the slope of the accumulated phase retardation at different depths can detect the variation in tissue birefringence and be used to segment the structural zones. The method is validated on phantoms composed of layers of different materials. Articular cartilage samples from adult swine are segmented with the method. The characteristics in each segmented zone are also examined by histology and high-resolution second-harmonic generation imaging, showing distinctive properties that match with the anatomical structure of articular cartilage. The segmentation algorithm is also applied on PS-OCT images acquired at multiple illumination angles, where the angular dependence of tissue birefringence in the deep zone is detected. This method offers a noninvasive imaging approach to differentiating the structural zones of articular cartilage, as well as a quantification approach based on the phase retardation measurements of PS-OCT. This method has great potential in studying depth-related progression of cartilage degeneration.
Osteoarthritis (OA) is the most common form of arthritis, where the protective cartilage on the ends of bones wears down over time, causing pain, tenderness, stiffness, loss of flexibility and bone spurs. Degenerative alterations start before cartilage loss happens, which include surface swelling, cartilage fibrillation, and calcification. Detecting the early degenerative alterations can assist the diagnosis of early-stage OA. In this study, two imaging modalities are applied on human hip-joint specimens in ex vivo imaging, including polarization-sensitive optical coherence tomography (PS-OCT) and multiphoton microscopy (MPM). OCT detects the layered tissue structure of cartilage and bone using backscattered light and PS-OCT is a functional extension of OCT. PS-OCT measures tissue birefringence which is sensitive to the orderly organization of collagen in cartilage. MPM can visualize collagen fibers with sub-cellular resolution. Complementary information about cartilage on the cellular and tissue level can be obtained by the multimodal imaging. Using the multimodal system, the variation of the thickness of the cartilage structural zones, abnormal birefringence caused by collagen alterations and fibrillation, and uneven structure resulted from calcification are imaged and quantified. The imaging results show distinctive features of degenerative alterations in the OA specimen, such as uneven tissue surface, fibrillation, and reduced birefringence. It is shown that PS-OCT has great potential in detecting early stage OA.
Frequency-doubled femtosecond Er-doped fiber laser is a low-cost and portable excitation source suitable for multiphoton endoscopy. The frequency-doubled wavelength at 780 nm is used to excite the intrinsic fluorescence signal. The frequency-doubling with a periodically poled MgO : LiNbO3 (PPLN) is integrated in the distal end of the imaging head to achieve fiber connection. The imaging speed is further improved by optimizing the excitation laser source. A 0.3-mm length of PPLN crystal is selected and the Er-doped fiber laser is manipulated to match its bandwidth with the acceptance bandwidth of the PPLN. Through this optimization, a reduced pulsewidth of 80 fs of the frequency-doubled pulse is achieved. All-fiber dispersion compensation and pulse compression by single mode fiber is conducted, which makes the fiber laser directly fiber-coupled to the imaging head. An imaging speed of 4 frames / s is demonstrated on ex vivo imaging of unstained biological tissues, which is 10 times faster than our previous study using a 1-mm-long PPLN. The results show that miniature multiphoton endoscopy using frequency-doubled Er-doped fiber laser has great potential for clinical applications.
Polarization-sensitive optical coherence tomography (PS-OCT) is capable of measuring tissue birefringence. It has been widely applied to access the birefringence in tissues such as skin and cartilage. The vocal cord tissue consists of three anatomical layers from the surface to deep inside, the epithelium that contains almost no collagen, the lamina propria that is composed with abundant collagen, and the vocalis muscle layer. Due to the variation in the organization of collagen fibers, the different tissue layers show different tissue birefringence, which can be evaluated by PS-OCT phase retardation measurement. Furthermore, collagen fibers in healthy connective tissues are usually well organized, which provides relatively high birefringence. When the collagen organization is destroyed by diseases such as tumor, the birefringence of the tissue will decrease. In this study, a rabbit laryngeal tumor model with different stages of tumor progression is investigated ex-vivo by PS-OCT. The PS-OCT images show a gradual decrease in birefringence from normal tissue to severe tumor tissue. A phase retardation slope-based analysis is conducted to distinguish the epithelium, lamina propria, and muscle layers, respectively. The phase retardation slope quantifies the birefringence in different layers. The quantitative study provides a more detailed comparison among different stages of the rabbit laryngeal tumor model. The PS-OCT result is validated by the corresponding histology images of the same samples.
Polarization-sensitive optical coherence tomography (PS-OCT) and second harmonic generation (SHG) microscopy are
two imaging modalities with different resolutions, field-of-views (FOV), and contrasts, while they both have the
capability of imaging collagen fibers in biological tissues. PS-OCT can measure the tissue birefringence which is
induced by highly organized fibers while SHG can image the collagen fiber organization with high resolution. Articular
cartilage, with abundant structural collagen fibers, is a suitable sample to study the correlation between PS-OCT and
SHG microscopy. Qualitative conjecture has been made that the phase retardation measured by PS-OCT is affected by
the relationship between the collagen fiber orientation and the illumination direction. Anatomical studies show that the
multilayered architecture of articular cartilage can be divided into four zones from its natural surface to the subchondral
bone: the superficial zone, the middle zone, the deep zone, and the calcified zone. The different zones have different
collagen fiber orientations, which can be studied by the different slopes in the cumulative phase retardation in PS-OCT.
An algorithm is developed based on the quantitative analysis of PS-OCT phase retardation images to analyze the
microstructural features in swine articular cartilage tissues. This algorithm utilizes the depth-dependent slope changing
of phase retardation A-lines to segment structural layers. The results show good consistency with the knowledge of
cartilage morphology and correlation with the SHG images measured at selected depth locations. The correlation
between PS-OCT and SHG microscopy shows that PS-OCT has the potential to analyze both the macro and micro
characteristics of biological tissues with abundant collagen fibers and other materials that may cause birefringence.
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