Dark-field polarization-sensitive optical coherence tomography Yeoreum Yoon,1 Qingyun Li,1 Viet Hoan Le,2 Won Hyuk Jang,2 Taejun Wang,2 Bumju Kim,1 Sihyung Son,1 Wan Kyun Chung,1 Chulmin Joo,3 and Ki Hean Kim1,2,* 1
Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Namgu, Pohang, Gyeongbuk 790-784, South Korea 2 Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, South Korea 3 School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, South Korea * [email protected]
Abstract: Polarization-sensitive optical coherence tomography (PS-OCT) is a functional OCT providing both structural and birefringent information of the sample, and it has been applied to the studies of various organs having polarization properties. Fiber-based PS-OCT is sensitive to specular reflection from the sample surface, because signal saturation due to the strong specular reflection can make the polarization measurement difficult. We developed a dark-field PS-OCT which can avoid the specular reflection problem. Dark-field PS-OCT was implemented by adapting a hybrid method of Bessel-beam illumination and Gaussian-beam detection, and a PS-OCT method based on passive delay unit (PDU). The new system was characterized in comparison with the conventional Gaussian-beam based method in both polarization components and various samples including the human skin. Dark-field PS-OCT performed as good as the conventional PSOCT without the specular reflection artifact. Dark-field PS-OCT may be useful in practical situations where the specular reflection is unavoidable. ©2015 Optical Society of America OCIS codes: (170.4500) Optical coherence tomography; (260.5430) Polarization; (230.5440) Polarization-selective devices; (170.4580) Optical diagnostics for medicine.
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1. Introduction Optical coherence tomography (OCT) is a 3D imaging modality based on light back reflection within the sample and has become indispensable in biomedical applications with many advantages such as non-invasiveness, high resolution, high imaging speed, and relatively low cost [1]. Polarization-sensitive OCT (PS-OCT) is a functional OCT that provides both structural and polarization information. This additional contrast is useful for distinguishing lesions from normal tissues, if the normal ones have polarization properties. PS-OCT has been applied to the pre-clinical and clinical studies of various organs which have distinct polarization properties. These organs include the cornea [2] and retina [3–7] of eye, skin [8– 13], and coronary artery [14, 15] etc. PS-OCT has advanced in both the imaging system and data analysis method. PS-OCT systems were initially developed by using bulk optical elements, since they could control the polarization of illumination beam onto the sample for robust polarization measurement [16, 17]. Later, PS-OCT systems using optical fibers in the sample arm were developed for endoscopic applications and became popular due to the simplicity [9, 10, 18–22]. Since fiberbased PS-OCT systems did not have control on the polarization of illumination beam, these
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12876
systems typically probed the sample with two different polarizations for reliable measurement. Fiber-based PS-OCT systems were developed by using active optical elements such as polarization modulators [19] and frequency shifters [20, 22] for the polarization modulation and detection. Recently fiber-based PS-OCT systems based on passive optical elements were developed by using new light sources which had long depth ranges [23, 24]. Various analysis methods of PS-OCT data have been developed along with the development of imaging systems, and detailed and robust polarization properties could be obtained [9, 17, 18, 25–31]. Recently new analysis methods, which could correct polarization artifacts due to polarization mode dispersion (PMD) of both the optical fibers and fiber based elements in the system, were developed for more precise polarization measurement [32–37]. In practical applications of PS-OCT, specular reflection from the sample surface causes problems in the polarization measurement. Fiber-based PS-OCT systems are sensitive to the specular reflection, because they use the polarization of sample surface as a reference in order to remove the effects of polarization changes in the system and to calculate the sample polarization properties [28]. Strong specular reflection from the sample surface can cause signal saturation and makes the calculation of surface polarization difficult. In order to avoid such problems, samples are typically slanted from the normal of illumination beam either by using an optical window or other methods. As an example, a previous PS-OCT study of human burn patients used a hand-held imaging probe [11]. The imaging probe had an optical window at the distal end to position the sample within the OCT imaging depth range by touching the sample. This window was slanted by 8° to the normal of the illumination beam in order to reduce the specular reflection from both the window and sample surfaces. However, PS-OCT imaging of burn patients with this probe caused mild pain to the patients and posed the risk of infection. An OCT method, which can reduce the specular reflection, will be beneficial for such practical PS-OCT applications. There have been such methods developed recently. These methods separated the illumination and detection paths by using Bessel-beam and Gaussian-beam respectively and could obtain the dark-field property. The use of Bessel-beam and Gaussian-beams combination was advantageous by getting both the dark-field property and the extended focal depth in the imaging, although there was some decrease in the sensitivity [38–45]. The extended focal depth came from the Bessel-beam property: preservation of the transverse resolution in the longer depth range compared to Gaussianbeam [46, 47]. Several methods of Bessel-beam based OCT and optical coherence microscopy (OCM) were developed and applied to various applications [39, 40, 48, 49]. Adaptation of a dark-field method to PS-OCT will be beneficial in the practical applications of PS-OCT. In this paper, we developed a dark-field PS-OCT which can avoid the specular reflection problem. Dark-field effect was obtained by adapting the hybrid method using Bessel-beam illumination and Gaussian-beam detection. System setup and characterization were described, and its performance was characterized in both polarization components and various samples including the human skin in comparison to conventional PS-OCT using Gaussian-beam illumination and detection. 2. Materials and methods 2.1 Instrumentation A schematic of dark-field PS-OCT is shown in Fig. 1(a). Dark-field PS-OCT was implemented by adapting the axicon lens-based Bessel-beam illumination and Gaussian-beam detection method [39] and a PS-OCT method based on passive delay unit (PDU) [23, 24]. This PDU based PS-OCT took the advantage of a light source having a large imaging depth range. The light source was a wavelength swept source (SSOCT-1310, AXSUN Technologies) with a center wavelength of 1310 nm, bandwidth of 107 nm, sweeping speed of 50 kHz, and imaging depth range of 6 mm in the air. The illumination beam was a combination of two orthogonal polarization states having their optical path lengths separated
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12877
by 2.25 mm in the air, and this illumination beam was generated by PDU. Reflection of the two polarization states could be acquired simultaneously by using the large imaging depth range. Light from the source was split with 95:5 ratio: 95% of light went into the interferometry setup, and the remaining 5% was directed to a fiber Bragg grating (FBG, λ0 = 1307.8 nm, Reflectivity = 97%, Δλ = 0.08 nm, OE Land) to generate an external trigger signal for data acquisition [20, 22]. In the interferometry, light was split into the sample and reference arms by using an 80:20 coupler. Light in the sample arm passed through the PDU. A detail schematic of PDU is shown in Fig. 1(b). Light was collimated and split into two pure linear polarization states, horizontal and vertical by a polarizing beam splitter (PBS, PBS104, Thorlabs). The 45° tilted PBS was used to generate 45° linearly polarized lights for applying equal power distribution to each polarization states. Separated individual polarized light passed a quarter wave plate (QWP, WPQ05M-1310, Thorlabs), reflected by a mirror (M), reverse passed the QWP, recombined at a PBS, and coupled by a collimator (CM). The path length difference (Δz) between two polarized lights was adjusted by using a translation stage. After the PDU, the light was collimated to 1.8 mm in diameter with a fiber collimator (HPUCO-13A-1300/1550-S-11AS, OZ optics), and changed its field distribution from Gaussian to radial zero-order Bessel via an axicon lens (AX255-C, Thorlabs) with 170° apex angle. The focal plane of Bessel-beam was relayed to the object plane by three pairs of lens combinations (L1-L6) which were in 4-f configuration. L1-L5 were standard achromatic lenses (NT45-806, F = 100 mm, Edmund Optics) and L6 was a 5x OCT scan lens (LSM03, EF = 36 mm, Thorlabs). A 2D galvano scanning mirror (GVS012, Thorlabs) was placed in between L3 and L4. The illumination field following Bessel distribution had 0.11 NA, and the size of central lobe (1/e2) was 9.2 μm with 2.9 mm DOF. Detection field following Gaussian distribution had 0.025 NA for dark-field effect, and the full width half maximum (FWHM) intensity of focal spot was 20 μm with 1.5 mm DOF. Detection path was separated from the illumination path by a right angled prism (aluminum (Al) coated and 5 mm leg, Edmund optics), placed in between L1 and L2 [40]. Conventional PS-OCT was realized by using this Gaussian detection field of the dark-field setup for both the illumination and detection, and a polarization insensitive optical circulator (PIOC313P2111-0.05PMD, AC photonics). The optical path for conventional PS-OCT was depicted by a broken line in Fig. 1(a). These two configurations were easily switchable and were used for comparison. Reflected light from both the sample and reference arms was combined and collected by a polarization diverse detection (PDD) setup. Interference signals was split into horizontal and vertical polarization states by a non-polarizing beam splitter (BS, 05BC17MB.3, Newport) and two polarizing beam splitter (PBS), and collected by two balanced detectors (PDB410C, Thorlabs). The linear polarizer (LP, 5526, Newport) was used to generate 45° linearly polarized lights for applying equal reference power distribution to each polarization states. Signals from the detectors were digitized by a two-channel data acquisition board (Alazartech) after passing through low pass filters (81 MHz cut-off frequency, Mini-circuits). 1280 samples were acquired per each depth-scan. Data was processed for display and stored by a custom software in real-time. Stored data was post-processed to get both intensity and polarization sensitive (PS) images. Dispersion difference between the reference and sample arms was compensated numerically by using pre-calibration data with a mirror sample [50, 51]. Simultaneous sample illumination with two polarization states enabled the determination of depth-resolved Jones matrices of the sample. The polarization of sample surface acts as a reference in order to remove the effects of polarization changes in the system and to calculate the sample polarization properties. Polarization properties of the sample were obtained by analyzing the sample Jones matrices through eigenvector decomposition [22, 28], and phase retardation was calculated as a function of depth. PS-OCT images were obtained in 3D by acquiring multiple cross-sectional images in the x-z plane with step-wise increment in the y direction. Imaging field was typically 5 mm x 5 mm x 2.25 mm in the x, y, and z directions, consisting of 500 pixels x 500 pixels x 320 pixels. The x-y plane images were generated from 3D reconstructed data set by compiling cross-sectional images using visualization software.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12878
Fig. 1. (a) System configuration of dark-field PS-OCT. Bessel-beam illumination and Gaussian-beam detection path for dark-field PS-OCT are depicted in gray and red respectively. Illumination and detection path of conventional PS-OCT used the detection path of dark-field PS-OCT. Optical path of conventional PS-OCT is represented by a broken line. (b) Scheme of the passive delay unit (Δz = 2.25 mm in air). PC: polarization controller, FBG: fiber Bragg grating, CM: collimator, CIR: fiber circulator, FC: fiber coupler, QWP: quarter wave plate, LP: linear polarizer, BS: non-polarizing beam splitter, PBS: polarizing beam splitter, PD: photodetector, PDD: polarization diverse detection, M: mirror, SM: scanning mirror, P: prism, L1-L5: Achromatic lens (f = 100 mm), L6: 5x OCT scan lens (EF = 36 mm).
2.2 Characterization Performance of dark-field PS-OCT was characterized by imaging microspheres (15716-5, 6 μm, Polysciences Inc.) embedded in 2% agarose gel in comparison with the conventional configuration, and their results are shown in Fig. 2. For each setup, images in both the x-z and x-y planes are presented. Cross-sections of the x-y plane images were depicted by blue broken lines in the x-z plane images. PS-OCT intensity images showed that individual microspheres were clearly distinguished in both configurations. However, the shapes of microsphere images were slightly different. The cross-sectional image of conventional configuration (Fig. 2(a)) showed relatively round microspheres, while the one of dark-field configuration (Fig. 2(c)) showed microspheres elongated and shortened in the transverse and axial directions respectively. The x-y plane image of dark-field configuration (Fig. 2(d)) showed irregular outlines of microspheres, while the one of conventional configuration (Fig. 2(b)) showed smooth and round outlines. This different shape and irregular outline of microspheres in the images of dark-field configuration is due to the side lobes of the Bessel-beam. Intensity images of dark-field configuration had lower background intensities compared to the ones of the conventional configuration, which may be due to the dark-field effect suppressing specular reflection within the sample. Axial resolutions of conventional OCT and dark-field OCT were measured by a mirror, and full width half maximum (FWHM) intensities were calculated as 10.8 ± 2.41 μm and 10.9 ± 2.16 μm in the air, respectively.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12879
Fig. 2. Images of microspheres (6 μm in diameter) by conventional setup (a, b) and dark-field setup (c, d) in the x-z and x-y planes respectively. Blue broken lines in the x-z plane images indicate the location of the x-y plane images. All scale bars are 300 μm.
3. Results 3.1 Sensitivity Illumination power onto the sample was 6.3 mW and 6.9 mW in the conventional PS-OCT and dark-field PS-OCT respectively. The conventional setup had a theoretical sensitivity of 112 dB by considering transmission efficiency in the sample and detection arms, and the PDD [52]. Measured sensitivity of the conventional setup was 103.2 dB by using a mirror as the sample and an attenuator, and the sensitivity roll-off was −0.83 dB/mm. Since the sensitivity of dark-field setup could not be measured directly by the same method used in the conventional setup, it was estimated by imaging tissue samples with both methods and comparing their images. The back of a healthy volunteer hand was imaged by both the conventional and dark-field PS-OCTs, and their intensity images in the x-z plane and average intensity profiles with respect to depth are shown in Fig. 3. Skin intensity images of both configurations showed similar skin structures: the superficial epidermis and underlying dermis were resolved and vascular structures in the dermis were visualized. The skin intensity image of dark-field PS-OCT showed lower intensities compared to the one of conventional PS-OCT due to reduction of specular reflection, especially in the superficial epidermis. However, both images showed similar imaging depths. Average intensity profiles with depth were calculated from the two images by averaging intensities in Fig. 3(a) and 3(b) in the transverse direction, and they are shown in Fig. 3(c). Linear fits were applied to obtain the slopes of intensity decays with depth, and the calculated slopes were −0.034 ± 0.006 dB/μm and −0.022 ± 0.004 dB/μm in the images of the conventional and dark-field setup respectively. Dark-field PS-OCT had a lower intensity peak than a conventional PS-OCT by 5 - 10 dB, and this result was consistent with a previous report [40]. However, the intensity decay slope of dark-field OCT was lower than the one of conventional OCT, and the average
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12880
intensity profile of dark-field PS-OCT reached the background noise level at the similar imaging depth to the one of the conventional PS-OCT.
Fig. 3. Cross-sectional OCT images of back of a human hand by using (a) conventional setup and (b) dark-field setup. (c) Mean intensity profiles of images (a) and (b) as a function of depth. All scale bars are 1 mm.
3.2 Dark-field property The advantage of dark-field configuration was demonstrated in the imaging of a plant leaf. The plant leaf has layered structures of the epidermis and mesophyll from superficial to deep. The epidermis consists of tightly arranged epidermal cells for protection from outside. The mesophyll consists of parenchyma cells containing chloroplasts for photosynthesis, and is divided into two layers: the palisade layer consisting of regularly arranged cells, and the spongy layer consisting of more branched and less tightly packed cells and air spaces. Crosssectional intensity images of the plant leaf in the x-z plane by the conventional and dark-field configurations are shown in Fig. 4(a) and 4(b) respectively. Their en-face images in the x-y plane at 90 μm deep from the surface are shown in Fig. 4(c) and 4(d) respectively. Crosssection lines of the en-face images are marked by broken lines in the cross-sectional images. Both cross-sectional images showed layered structures of the plant leaf: thin lines with high intensities on the surface were the epidermis, and thick bands with low intensities and some structures with high intensities were the palisade layer and spongy layer respectively [53]. Individual cells in the mesophyll were not visible due to the limitation of image resolution. The palisade layer appeared with low intensities due to the regular arrangement of vertically elongated cells, and the underlying spongy layer had high intensities due to irregular distribution of cells and air spaces. Cross-sectional image of dark-field configuration had much lower intensities in the palisade layer than the one of conventional configuration by reducing specular reflection. This effect was clearer in the en-face images. Lower background intensities helped to visualize some features in the palisade layer.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12881
Fig. 4. Images of a plant leaf by conventional setup (a, c) and dark-field setup (b, d) in the xz and x-y planes respectively. (Video clips of the x-z plane images are presented in Media 1.) All scale bars are 1 mm.
3.3. Measurements of a quarter-wave plate and a polarizing film Performance of dark-field PS-OCT in polarimetric measurement was tested by using a quarter wave plate (QWP, WPQ05M-1310, Thorlabs) and a polarizing film (PF) in comparison with the conventional PS-OCT. These polarization components were imaged without beam scanning, and polarization changes between their top and bottom surfaces were analyzed by averaging 460 depths scan data. Double-pass phase retardation (DPPR) of the QWP and the relative optic axis of the PF were measured, and these measurements were repeated with the stepwise rotation of the samples by 10° until 180° rotation was reached. Measurement results are shown in Fig. 5. DPPR of the QWP and the optic axis of the PF as a function of the orientation angle is shown in Fig. 5(a) and 5(b) respectively. Measured DPPR values were 171.8 ± 3.2° and 171.8 ± 2.9° on average in the conventional and dark-field PS-OCT, respectively. The manufacturer specification was 173.45°. Measurement by both the conventional and dark-field PS-OCTs agreed well with the manufacturer specification. The measured optic axes of the PF by both the conventional and dark-field PS-OCT showed good correlations with the set orientation angle.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12882
Fig. 5. Polarimetric measurement of the conventional and dark-field PS-OCT systems (a) Double-pass phase retardation (DPPR) of the quarter wave plate (QWP) and (b) the optic axis of the polarizing film (PF) as a function of the sample rotation
3.4 Skin imaging 3.4.1 Rat skin, ex-vivo Excised rat skin specimens were imaged by both conventional and dark-field PS-OCT. The same regions were imaged for comparison and results are shown in Fig. 6. Intensity and PS images of conventional and dark-field PS-OCTs are shown in Fig. 6(a, b) and Fig. 6(c, d) respectively. Intensity image by conventional PS-OCT showed vertical lines in some regions (red arrows in Fig. 6(a)). These lines were artifacts generated due to signal saturation by specular reflection, and these artifacts appeared in PS image by conventional PS-OCT (red arrows in Fig. 6(b)). PS image displayed the accumulated phase retardation with depth from the tissue surface in a gray scale where black and white colors corresponded to 0° and 180° phase retardations respectively. PS image by conventional PS-OCT showed black-white banding patterns indicating birefringence in the skin due to highly ordered collagen composition. These banding patterns were distorted in the vertical saturation lines due to phase miscalculation by signal saturation. Intensity and PS images of the same skin specimen by dark-field PS-OCT in Fig. 6(c, d) did not show such saturation lines by suppressing specular reflection. Although intensity image by dark-field PS-OCT had lower intensities in the superficial region than the one by conventional PS-OCT, PS image by dark-field PS-OCT showed similar banding patterns to the one by conventional PS-OCT without artifacts.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12883
Fig. 6. Rat skin images in the x-z plane by conventional and dark-field PS-OCTs. (a, b) and (c, d) are intensity and PS images by conventional PS-OCT (Media 2) and dark-field PS-OCT (Media 3) respectively. Red arrows indicate signal artifacts. All scale bars are 1 mm.
3.4.2 Human skin The back of a human hand was imaged by both conventional and dark-field PS-OCT for comparison, and results are shown in Fig. 7. Intensity and PS images of the same section by conventional and dark-field PS-OCT are shown in Fig. 7(a, b) and 7(c, d) respectively. Intensity image by conventional PS-OCT had vertical saturation lines due to strong specular reflection, and PS image showed distortion in the banding pattern in those vertical lines (red arrows in Fig. 7(a, b)). In contrast, intensity and PS images by dark-field PS-OCT showed no vertical saturation lines and no distortion in the black-white banding pattern. PS image by dark-field PS-OCT showed a thin white band just beneath the skin surface, which did not appear in the PS image by the conventional PS-OCT. It might be due to polarizationscrambling effect (i.e. depolarizing) by stratum corneum in the epidermis [54], and further analysis will be needed. Even though dark-field PS-OCT had lower intensity levels, depth range of the banding pattern in PS image was similar to that of conventional PS-OCT. Therefore, dark-field PS-OCT produced skin images with equivalent qualities as the conventional PS-OCT without the specular reflection artifact.
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12884
Fig. 7. Human skin images in the x-z plane by conventional and dark-field PS-OCTs. (a, b) and (c, d) are intensity and PS images by conventional PS-OCT (Media 4) and dark-field PSOCT (Media 5) respectively. Red arrows indicate artifacts. All scale bars are 1 mm.
In addition to qualitative comparison, the human skin PS images were compared quantitatively by calculating the average accumulated phase retardation and results are shown in Fig. 8. The accumulated phase retardations were calculated from 3D human skin images in Fig. 7 by selecting some region with no surface signal saturation. Approximately 10 B-scan images were averaged. The average accumulated phased retardation curves showed linear regions at 100 – 400 μm deep from the surface, corresponding the superficial dermis of the skin after the epidermis. These linear regions showed the increase of accumulated phase retardation with depth [10, 11]. A linear fit was applied to the initial linear regions and the birefringence values were 0.782 ± 0.024 °/μm, and 0.784 ± 0.023 °/μm in conventional and dark-field PS-OCTs respectively. After the linear region, the curves did not showed the banding pattern due to the low signal to noise ratio in the deeper region. This is due to both low signal intensity and randomized polarization state of the light [10]. Dark-field PS-OCT showed the comparable birefringence measurement in the human skin with the conventional PS-OCT.
Fig. 8. Average accumulated phase retardation with depth by conventional and dark-field PSOCT setup
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12885
4. Discussion and conclusion Dark-field PS-OCT was implemented by adapting the hybrid method of Bessel-beam illumination and Gaussian-beam detection, and its performance was characterized in comparison with the conventional PS-OCT in both polarization components and various samples including the human skin. Dark-field PS-OCT showed much less specular reflection on the sample surfaces in intensity images, and provided PS images without surface saturation artifacts. Although dark-field PS-OCT had lower peak intensities by 5 - 10 dB than the conventional PS-OCT in the skin specimens, it had equivalent imaging depths with the slower intensity decay with depth. Dark-field PS-OCT will be useful in practical situations where strong specular reflection on sample surfaces is unavoidable. As an application, this dark-field PS-OCT system will be applied to the human burn study by using a new imaging probe which can image without touching the burn damaged skin. The current dark-field PS-OCT has a complex sample arm configuration. For the application of imaging external organs such as the skin, this sample arm will be mounted on an articulated arm for the flexible positioning. However, the current configuration has a limitation for the endoscopic applications and miniaturization methods need to be developed [48, 55]. The current system was designed to have a large illumination/detection NA ratio by using 0.11 NA for illumination and 0.025 NA for detection, which was sub-optimal. Increasing detection NA, while maintaining dark-field, will help to collect more scattered photons and to increase signal. Also, the effect of side lobe can be reduced if the NA of detection path is increased. Acknowledgments This research was supported in part by the Engineering Research Center grant (No. 20110030075), the Bio and Medical Technology Development Program (No. 2011-0019633) and the Korea-Sweden Research Cooperation Programme (No. NRF-2014R1A2A1A12067510) of the National Research Foundation (NRF) funded by the Korean government (MEST), National R&D Program for Cancer Control (No. 1320220) by National Cancer Center Korea, and the Industrial Technology Innovation Program (No. 10048358) funded by the Ministry Of Trade, Industry & Energy (MI, Korea).
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Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12886
Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Namgu, Pohang, Gyeongbuk 790-784, South Korea 2 Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, South Korea 3 School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, South Korea * [email protected]
Abstract: Polarization-sensitive optical coherence tomography (PS-OCT) is a functional OCT providing both structural and birefringent information of the sample, and it has been applied to the studies of various organs having polarization properties. Fiber-based PS-OCT is sensitive to specular reflection from the sample surface, because signal saturation due to the strong specular reflection can make the polarization measurement difficult. We developed a dark-field PS-OCT which can avoid the specular reflection problem. Dark-field PS-OCT was implemented by adapting a hybrid method of Bessel-beam illumination and Gaussian-beam detection, and a PS-OCT method based on passive delay unit (PDU). The new system was characterized in comparison with the conventional Gaussian-beam based method in both polarization components and various samples including the human skin. Dark-field PS-OCT performed as good as the conventional PSOCT without the specular reflection artifact. Dark-field PS-OCT may be useful in practical situations where the specular reflection is unavoidable. ©2015 Optical Society of America OCIS codes: (170.4500) Optical coherence tomography; (260.5430) Polarization; (230.5440) Polarization-selective devices; (170.4580) Optical diagnostics for medicine.
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1. Introduction Optical coherence tomography (OCT) is a 3D imaging modality based on light back reflection within the sample and has become indispensable in biomedical applications with many advantages such as non-invasiveness, high resolution, high imaging speed, and relatively low cost [1]. Polarization-sensitive OCT (PS-OCT) is a functional OCT that provides both structural and polarization information. This additional contrast is useful for distinguishing lesions from normal tissues, if the normal ones have polarization properties. PS-OCT has been applied to the pre-clinical and clinical studies of various organs which have distinct polarization properties. These organs include the cornea [2] and retina [3–7] of eye, skin [8– 13], and coronary artery [14, 15] etc. PS-OCT has advanced in both the imaging system and data analysis method. PS-OCT systems were initially developed by using bulk optical elements, since they could control the polarization of illumination beam onto the sample for robust polarization measurement [16, 17]. Later, PS-OCT systems using optical fibers in the sample arm were developed for endoscopic applications and became popular due to the simplicity [9, 10, 18–22]. Since fiberbased PS-OCT systems did not have control on the polarization of illumination beam, these
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12876
systems typically probed the sample with two different polarizations for reliable measurement. Fiber-based PS-OCT systems were developed by using active optical elements such as polarization modulators [19] and frequency shifters [20, 22] for the polarization modulation and detection. Recently fiber-based PS-OCT systems based on passive optical elements were developed by using new light sources which had long depth ranges [23, 24]. Various analysis methods of PS-OCT data have been developed along with the development of imaging systems, and detailed and robust polarization properties could be obtained [9, 17, 18, 25–31]. Recently new analysis methods, which could correct polarization artifacts due to polarization mode dispersion (PMD) of both the optical fibers and fiber based elements in the system, were developed for more precise polarization measurement [32–37]. In practical applications of PS-OCT, specular reflection from the sample surface causes problems in the polarization measurement. Fiber-based PS-OCT systems are sensitive to the specular reflection, because they use the polarization of sample surface as a reference in order to remove the effects of polarization changes in the system and to calculate the sample polarization properties [28]. Strong specular reflection from the sample surface can cause signal saturation and makes the calculation of surface polarization difficult. In order to avoid such problems, samples are typically slanted from the normal of illumination beam either by using an optical window or other methods. As an example, a previous PS-OCT study of human burn patients used a hand-held imaging probe [11]. The imaging probe had an optical window at the distal end to position the sample within the OCT imaging depth range by touching the sample. This window was slanted by 8° to the normal of the illumination beam in order to reduce the specular reflection from both the window and sample surfaces. However, PS-OCT imaging of burn patients with this probe caused mild pain to the patients and posed the risk of infection. An OCT method, which can reduce the specular reflection, will be beneficial for such practical PS-OCT applications. There have been such methods developed recently. These methods separated the illumination and detection paths by using Bessel-beam and Gaussian-beam respectively and could obtain the dark-field property. The use of Bessel-beam and Gaussian-beams combination was advantageous by getting both the dark-field property and the extended focal depth in the imaging, although there was some decrease in the sensitivity [38–45]. The extended focal depth came from the Bessel-beam property: preservation of the transverse resolution in the longer depth range compared to Gaussianbeam [46, 47]. Several methods of Bessel-beam based OCT and optical coherence microscopy (OCM) were developed and applied to various applications [39, 40, 48, 49]. Adaptation of a dark-field method to PS-OCT will be beneficial in the practical applications of PS-OCT. In this paper, we developed a dark-field PS-OCT which can avoid the specular reflection problem. Dark-field effect was obtained by adapting the hybrid method using Bessel-beam illumination and Gaussian-beam detection. System setup and characterization were described, and its performance was characterized in both polarization components and various samples including the human skin in comparison to conventional PS-OCT using Gaussian-beam illumination and detection. 2. Materials and methods 2.1 Instrumentation A schematic of dark-field PS-OCT is shown in Fig. 1(a). Dark-field PS-OCT was implemented by adapting the axicon lens-based Bessel-beam illumination and Gaussian-beam detection method [39] and a PS-OCT method based on passive delay unit (PDU) [23, 24]. This PDU based PS-OCT took the advantage of a light source having a large imaging depth range. The light source was a wavelength swept source (SSOCT-1310, AXSUN Technologies) with a center wavelength of 1310 nm, bandwidth of 107 nm, sweeping speed of 50 kHz, and imaging depth range of 6 mm in the air. The illumination beam was a combination of two orthogonal polarization states having their optical path lengths separated
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12877
by 2.25 mm in the air, and this illumination beam was generated by PDU. Reflection of the two polarization states could be acquired simultaneously by using the large imaging depth range. Light from the source was split with 95:5 ratio: 95% of light went into the interferometry setup, and the remaining 5% was directed to a fiber Bragg grating (FBG, λ0 = 1307.8 nm, Reflectivity = 97%, Δλ = 0.08 nm, OE Land) to generate an external trigger signal for data acquisition [20, 22]. In the interferometry, light was split into the sample and reference arms by using an 80:20 coupler. Light in the sample arm passed through the PDU. A detail schematic of PDU is shown in Fig. 1(b). Light was collimated and split into two pure linear polarization states, horizontal and vertical by a polarizing beam splitter (PBS, PBS104, Thorlabs). The 45° tilted PBS was used to generate 45° linearly polarized lights for applying equal power distribution to each polarization states. Separated individual polarized light passed a quarter wave plate (QWP, WPQ05M-1310, Thorlabs), reflected by a mirror (M), reverse passed the QWP, recombined at a PBS, and coupled by a collimator (CM). The path length difference (Δz) between two polarized lights was adjusted by using a translation stage. After the PDU, the light was collimated to 1.8 mm in diameter with a fiber collimator (HPUCO-13A-1300/1550-S-11AS, OZ optics), and changed its field distribution from Gaussian to radial zero-order Bessel via an axicon lens (AX255-C, Thorlabs) with 170° apex angle. The focal plane of Bessel-beam was relayed to the object plane by three pairs of lens combinations (L1-L6) which were in 4-f configuration. L1-L5 were standard achromatic lenses (NT45-806, F = 100 mm, Edmund Optics) and L6 was a 5x OCT scan lens (LSM03, EF = 36 mm, Thorlabs). A 2D galvano scanning mirror (GVS012, Thorlabs) was placed in between L3 and L4. The illumination field following Bessel distribution had 0.11 NA, and the size of central lobe (1/e2) was 9.2 μm with 2.9 mm DOF. Detection field following Gaussian distribution had 0.025 NA for dark-field effect, and the full width half maximum (FWHM) intensity of focal spot was 20 μm with 1.5 mm DOF. Detection path was separated from the illumination path by a right angled prism (aluminum (Al) coated and 5 mm leg, Edmund optics), placed in between L1 and L2 [40]. Conventional PS-OCT was realized by using this Gaussian detection field of the dark-field setup for both the illumination and detection, and a polarization insensitive optical circulator (PIOC313P2111-0.05PMD, AC photonics). The optical path for conventional PS-OCT was depicted by a broken line in Fig. 1(a). These two configurations were easily switchable and were used for comparison. Reflected light from both the sample and reference arms was combined and collected by a polarization diverse detection (PDD) setup. Interference signals was split into horizontal and vertical polarization states by a non-polarizing beam splitter (BS, 05BC17MB.3, Newport) and two polarizing beam splitter (PBS), and collected by two balanced detectors (PDB410C, Thorlabs). The linear polarizer (LP, 5526, Newport) was used to generate 45° linearly polarized lights for applying equal reference power distribution to each polarization states. Signals from the detectors were digitized by a two-channel data acquisition board (Alazartech) after passing through low pass filters (81 MHz cut-off frequency, Mini-circuits). 1280 samples were acquired per each depth-scan. Data was processed for display and stored by a custom software in real-time. Stored data was post-processed to get both intensity and polarization sensitive (PS) images. Dispersion difference between the reference and sample arms was compensated numerically by using pre-calibration data with a mirror sample [50, 51]. Simultaneous sample illumination with two polarization states enabled the determination of depth-resolved Jones matrices of the sample. The polarization of sample surface acts as a reference in order to remove the effects of polarization changes in the system and to calculate the sample polarization properties. Polarization properties of the sample were obtained by analyzing the sample Jones matrices through eigenvector decomposition [22, 28], and phase retardation was calculated as a function of depth. PS-OCT images were obtained in 3D by acquiring multiple cross-sectional images in the x-z plane with step-wise increment in the y direction. Imaging field was typically 5 mm x 5 mm x 2.25 mm in the x, y, and z directions, consisting of 500 pixels x 500 pixels x 320 pixels. The x-y plane images were generated from 3D reconstructed data set by compiling cross-sectional images using visualization software.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12878
Fig. 1. (a) System configuration of dark-field PS-OCT. Bessel-beam illumination and Gaussian-beam detection path for dark-field PS-OCT are depicted in gray and red respectively. Illumination and detection path of conventional PS-OCT used the detection path of dark-field PS-OCT. Optical path of conventional PS-OCT is represented by a broken line. (b) Scheme of the passive delay unit (Δz = 2.25 mm in air). PC: polarization controller, FBG: fiber Bragg grating, CM: collimator, CIR: fiber circulator, FC: fiber coupler, QWP: quarter wave plate, LP: linear polarizer, BS: non-polarizing beam splitter, PBS: polarizing beam splitter, PD: photodetector, PDD: polarization diverse detection, M: mirror, SM: scanning mirror, P: prism, L1-L5: Achromatic lens (f = 100 mm), L6: 5x OCT scan lens (EF = 36 mm).
2.2 Characterization Performance of dark-field PS-OCT was characterized by imaging microspheres (15716-5, 6 μm, Polysciences Inc.) embedded in 2% agarose gel in comparison with the conventional configuration, and their results are shown in Fig. 2. For each setup, images in both the x-z and x-y planes are presented. Cross-sections of the x-y plane images were depicted by blue broken lines in the x-z plane images. PS-OCT intensity images showed that individual microspheres were clearly distinguished in both configurations. However, the shapes of microsphere images were slightly different. The cross-sectional image of conventional configuration (Fig. 2(a)) showed relatively round microspheres, while the one of dark-field configuration (Fig. 2(c)) showed microspheres elongated and shortened in the transverse and axial directions respectively. The x-y plane image of dark-field configuration (Fig. 2(d)) showed irregular outlines of microspheres, while the one of conventional configuration (Fig. 2(b)) showed smooth and round outlines. This different shape and irregular outline of microspheres in the images of dark-field configuration is due to the side lobes of the Bessel-beam. Intensity images of dark-field configuration had lower background intensities compared to the ones of the conventional configuration, which may be due to the dark-field effect suppressing specular reflection within the sample. Axial resolutions of conventional OCT and dark-field OCT were measured by a mirror, and full width half maximum (FWHM) intensities were calculated as 10.8 ± 2.41 μm and 10.9 ± 2.16 μm in the air, respectively.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12879
Fig. 2. Images of microspheres (6 μm in diameter) by conventional setup (a, b) and dark-field setup (c, d) in the x-z and x-y planes respectively. Blue broken lines in the x-z plane images indicate the location of the x-y plane images. All scale bars are 300 μm.
3. Results 3.1 Sensitivity Illumination power onto the sample was 6.3 mW and 6.9 mW in the conventional PS-OCT and dark-field PS-OCT respectively. The conventional setup had a theoretical sensitivity of 112 dB by considering transmission efficiency in the sample and detection arms, and the PDD [52]. Measured sensitivity of the conventional setup was 103.2 dB by using a mirror as the sample and an attenuator, and the sensitivity roll-off was −0.83 dB/mm. Since the sensitivity of dark-field setup could not be measured directly by the same method used in the conventional setup, it was estimated by imaging tissue samples with both methods and comparing their images. The back of a healthy volunteer hand was imaged by both the conventional and dark-field PS-OCTs, and their intensity images in the x-z plane and average intensity profiles with respect to depth are shown in Fig. 3. Skin intensity images of both configurations showed similar skin structures: the superficial epidermis and underlying dermis were resolved and vascular structures in the dermis were visualized. The skin intensity image of dark-field PS-OCT showed lower intensities compared to the one of conventional PS-OCT due to reduction of specular reflection, especially in the superficial epidermis. However, both images showed similar imaging depths. Average intensity profiles with depth were calculated from the two images by averaging intensities in Fig. 3(a) and 3(b) in the transverse direction, and they are shown in Fig. 3(c). Linear fits were applied to obtain the slopes of intensity decays with depth, and the calculated slopes were −0.034 ± 0.006 dB/μm and −0.022 ± 0.004 dB/μm in the images of the conventional and dark-field setup respectively. Dark-field PS-OCT had a lower intensity peak than a conventional PS-OCT by 5 - 10 dB, and this result was consistent with a previous report [40]. However, the intensity decay slope of dark-field OCT was lower than the one of conventional OCT, and the average
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12880
intensity profile of dark-field PS-OCT reached the background noise level at the similar imaging depth to the one of the conventional PS-OCT.
Fig. 3. Cross-sectional OCT images of back of a human hand by using (a) conventional setup and (b) dark-field setup. (c) Mean intensity profiles of images (a) and (b) as a function of depth. All scale bars are 1 mm.
3.2 Dark-field property The advantage of dark-field configuration was demonstrated in the imaging of a plant leaf. The plant leaf has layered structures of the epidermis and mesophyll from superficial to deep. The epidermis consists of tightly arranged epidermal cells for protection from outside. The mesophyll consists of parenchyma cells containing chloroplasts for photosynthesis, and is divided into two layers: the palisade layer consisting of regularly arranged cells, and the spongy layer consisting of more branched and less tightly packed cells and air spaces. Crosssectional intensity images of the plant leaf in the x-z plane by the conventional and dark-field configurations are shown in Fig. 4(a) and 4(b) respectively. Their en-face images in the x-y plane at 90 μm deep from the surface are shown in Fig. 4(c) and 4(d) respectively. Crosssection lines of the en-face images are marked by broken lines in the cross-sectional images. Both cross-sectional images showed layered structures of the plant leaf: thin lines with high intensities on the surface were the epidermis, and thick bands with low intensities and some structures with high intensities were the palisade layer and spongy layer respectively [53]. Individual cells in the mesophyll were not visible due to the limitation of image resolution. The palisade layer appeared with low intensities due to the regular arrangement of vertically elongated cells, and the underlying spongy layer had high intensities due to irregular distribution of cells and air spaces. Cross-sectional image of dark-field configuration had much lower intensities in the palisade layer than the one of conventional configuration by reducing specular reflection. This effect was clearer in the en-face images. Lower background intensities helped to visualize some features in the palisade layer.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12881
Fig. 4. Images of a plant leaf by conventional setup (a, c) and dark-field setup (b, d) in the xz and x-y planes respectively. (Video clips of the x-z plane images are presented in Media 1.) All scale bars are 1 mm.
3.3. Measurements of a quarter-wave plate and a polarizing film Performance of dark-field PS-OCT in polarimetric measurement was tested by using a quarter wave plate (QWP, WPQ05M-1310, Thorlabs) and a polarizing film (PF) in comparison with the conventional PS-OCT. These polarization components were imaged without beam scanning, and polarization changes between their top and bottom surfaces were analyzed by averaging 460 depths scan data. Double-pass phase retardation (DPPR) of the QWP and the relative optic axis of the PF were measured, and these measurements were repeated with the stepwise rotation of the samples by 10° until 180° rotation was reached. Measurement results are shown in Fig. 5. DPPR of the QWP and the optic axis of the PF as a function of the orientation angle is shown in Fig. 5(a) and 5(b) respectively. Measured DPPR values were 171.8 ± 3.2° and 171.8 ± 2.9° on average in the conventional and dark-field PS-OCT, respectively. The manufacturer specification was 173.45°. Measurement by both the conventional and dark-field PS-OCTs agreed well with the manufacturer specification. The measured optic axes of the PF by both the conventional and dark-field PS-OCT showed good correlations with the set orientation angle.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12882
Fig. 5. Polarimetric measurement of the conventional and dark-field PS-OCT systems (a) Double-pass phase retardation (DPPR) of the quarter wave plate (QWP) and (b) the optic axis of the polarizing film (PF) as a function of the sample rotation
3.4 Skin imaging 3.4.1 Rat skin, ex-vivo Excised rat skin specimens were imaged by both conventional and dark-field PS-OCT. The same regions were imaged for comparison and results are shown in Fig. 6. Intensity and PS images of conventional and dark-field PS-OCTs are shown in Fig. 6(a, b) and Fig. 6(c, d) respectively. Intensity image by conventional PS-OCT showed vertical lines in some regions (red arrows in Fig. 6(a)). These lines were artifacts generated due to signal saturation by specular reflection, and these artifacts appeared in PS image by conventional PS-OCT (red arrows in Fig. 6(b)). PS image displayed the accumulated phase retardation with depth from the tissue surface in a gray scale where black and white colors corresponded to 0° and 180° phase retardations respectively. PS image by conventional PS-OCT showed black-white banding patterns indicating birefringence in the skin due to highly ordered collagen composition. These banding patterns were distorted in the vertical saturation lines due to phase miscalculation by signal saturation. Intensity and PS images of the same skin specimen by dark-field PS-OCT in Fig. 6(c, d) did not show such saturation lines by suppressing specular reflection. Although intensity image by dark-field PS-OCT had lower intensities in the superficial region than the one by conventional PS-OCT, PS image by dark-field PS-OCT showed similar banding patterns to the one by conventional PS-OCT without artifacts.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12883
Fig. 6. Rat skin images in the x-z plane by conventional and dark-field PS-OCTs. (a, b) and (c, d) are intensity and PS images by conventional PS-OCT (Media 2) and dark-field PS-OCT (Media 3) respectively. Red arrows indicate signal artifacts. All scale bars are 1 mm.
3.4.2 Human skin The back of a human hand was imaged by both conventional and dark-field PS-OCT for comparison, and results are shown in Fig. 7. Intensity and PS images of the same section by conventional and dark-field PS-OCT are shown in Fig. 7(a, b) and 7(c, d) respectively. Intensity image by conventional PS-OCT had vertical saturation lines due to strong specular reflection, and PS image showed distortion in the banding pattern in those vertical lines (red arrows in Fig. 7(a, b)). In contrast, intensity and PS images by dark-field PS-OCT showed no vertical saturation lines and no distortion in the black-white banding pattern. PS image by dark-field PS-OCT showed a thin white band just beneath the skin surface, which did not appear in the PS image by the conventional PS-OCT. It might be due to polarizationscrambling effect (i.e. depolarizing) by stratum corneum in the epidermis [54], and further analysis will be needed. Even though dark-field PS-OCT had lower intensity levels, depth range of the banding pattern in PS image was similar to that of conventional PS-OCT. Therefore, dark-field PS-OCT produced skin images with equivalent qualities as the conventional PS-OCT without the specular reflection artifact.
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12884
Fig. 7. Human skin images in the x-z plane by conventional and dark-field PS-OCTs. (a, b) and (c, d) are intensity and PS images by conventional PS-OCT (Media 4) and dark-field PSOCT (Media 5) respectively. Red arrows indicate artifacts. All scale bars are 1 mm.
In addition to qualitative comparison, the human skin PS images were compared quantitatively by calculating the average accumulated phase retardation and results are shown in Fig. 8. The accumulated phase retardations were calculated from 3D human skin images in Fig. 7 by selecting some region with no surface signal saturation. Approximately 10 B-scan images were averaged. The average accumulated phased retardation curves showed linear regions at 100 – 400 μm deep from the surface, corresponding the superficial dermis of the skin after the epidermis. These linear regions showed the increase of accumulated phase retardation with depth [10, 11]. A linear fit was applied to the initial linear regions and the birefringence values were 0.782 ± 0.024 °/μm, and 0.784 ± 0.023 °/μm in conventional and dark-field PS-OCTs respectively. After the linear region, the curves did not showed the banding pattern due to the low signal to noise ratio in the deeper region. This is due to both low signal intensity and randomized polarization state of the light [10]. Dark-field PS-OCT showed the comparable birefringence measurement in the human skin with the conventional PS-OCT.
Fig. 8. Average accumulated phase retardation with depth by conventional and dark-field PSOCT setup
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12885
4. Discussion and conclusion Dark-field PS-OCT was implemented by adapting the hybrid method of Bessel-beam illumination and Gaussian-beam detection, and its performance was characterized in comparison with the conventional PS-OCT in both polarization components and various samples including the human skin. Dark-field PS-OCT showed much less specular reflection on the sample surfaces in intensity images, and provided PS images without surface saturation artifacts. Although dark-field PS-OCT had lower peak intensities by 5 - 10 dB than the conventional PS-OCT in the skin specimens, it had equivalent imaging depths with the slower intensity decay with depth. Dark-field PS-OCT will be useful in practical situations where strong specular reflection on sample surfaces is unavoidable. As an application, this dark-field PS-OCT system will be applied to the human burn study by using a new imaging probe which can image without touching the burn damaged skin. The current dark-field PS-OCT has a complex sample arm configuration. For the application of imaging external organs such as the skin, this sample arm will be mounted on an articulated arm for the flexible positioning. However, the current configuration has a limitation for the endoscopic applications and miniaturization methods need to be developed [48, 55]. The current system was designed to have a large illumination/detection NA ratio by using 0.11 NA for illumination and 0.025 NA for detection, which was sub-optimal. Increasing detection NA, while maintaining dark-field, will help to collect more scattered photons and to increase signal. Also, the effect of side lobe can be reduced if the NA of detection path is increased. Acknowledgments This research was supported in part by the Engineering Research Center grant (No. 20110030075), the Bio and Medical Technology Development Program (No. 2011-0019633) and the Korea-Sweden Research Cooperation Programme (No. NRF-2014R1A2A1A12067510) of the National Research Foundation (NRF) funded by the Korean government (MEST), National R&D Program for Cancer Control (No. 1320220) by National Cancer Center Korea, and the Industrial Technology Innovation Program (No. 10048358) funded by the Ministry Of Trade, Industry & Energy (MI, Korea).
#235654 - $15.00 USD (C) 2015 OSA
Received 4 Mar 2015; revised 2 May 2015; accepted 3 May 2015; published 7 May 2015 18 May 2015 | Vol. 23, No. 10 | DOI:10.1364/OE.23.012874 | OPTICS EXPRESS 12886
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