Dynamic beam steering for wireless optical power transfer in IoT applications.

The alignment of a receiver with a pencil beam in a wireless optical power transfer (WOPT) system employing a resonance beam charging (RBC) technology limits the establishment of a resonance cavity. Accurate tracking necessitates precise and dependable monitoring, which requires the exact placement of transmitting and receiving devices. Herein, we present a concept of a two-dimensional (2D) beam steering mechanism for RBC-based WOPT systems utilizing dispersed laser beams. The proposed approach allows a significant improvement, including reduction of scanning times and minimization of errors, in relation to conventional pencil-beam-based systems. Experimental results reveal 14% faster acquisition time efficiency, an 18% improvement in pointing accuracy, and a 24% enhancement in tracking accuracy. These results establish the prerequisites for the implementation of dispersed beam steering in the RBC-based WOPT system. This capability empowers the system to charge movable devices and Internet of Things devices consistently in smart factories.

Assessment of fractional flow reserve in intermediate coronary stenosis using optical coherence tomography-based machine learning.

Objectives: This study aimed to evaluate and compare the diagnostic accuracy of machine learning (ML)- fractional flow reserve (FFR) based on optical coherence tomography (OCT) with wire-based FFR irrespective of the coronary territory. Background: ML techniques for assessing hemodynamics features including FFR in coronary artery disease have been developed based on various imaging modalities. However, there is no study using OCT-based ML models for all coronary artery territories. Methods: OCT and FFR data were obtained for 356 individual coronary lesions in 130 patients. The training and testing groups were divided in a ratio of 4:1. The ML-FFR was derived for the testing group and compared with the wire-based FFR in terms of the diagnosis of ischemia (FFR ≤ 0.80). Results: The mean age of the subjects was 62.6 years. The numbers of the left anterior descending, left circumflex, and right coronary arteries were 130 (36.5%), 110 (30.9%), and 116 (32.6%), respectively. Using seven major features, the ML-FFR showed strong correlation (r = 0.8782, P < 0.001) with the wire-based FFR. The ML-FFR predicted wire-based FFR ≤ 0.80 in the test set with sensitivity of 98.3%, specificity of 61.5%, and overall accuracy of 91.7% (area under the curve: 0.948). External validation showed good correlation (r = 0.7884, P < 0.001) and accuracy of 83.2% (area under the curve: 0.912). Conclusion: OCT-based ML-FFR showed good diagnostic performance in predicting FFR irrespective of the coronary territory. Because the study was a small-size study, the results should be warranted the performance in further large-scale research.

Long-range wireless optical power transfer system using an EDFA.

A wireless optical power transfer (WOPT) system using an erbium-doped fiber amplifier as an optical power source is proposed to achieve long range, high power, and hazard-free power delivery in the air. The transmitter generates a wide band of amplified spontaneous emission around the central wavelength of 1550 nm. A wavelength division multiplexing (WDM) filter (λ=1552.25 nm) is deployed to obtain a safe narrowband beam illuminating the receiver units. A ball lens retroreflector reflects a small portion of the incident beam back to the transmitter, establishing a closed ring resonance loop. An improved safety mechanism is proposed to terminate the resonance when an obstacle blocks the transmitter-receiver line of sight. The measured incident power of 1 W decreases to 0.79 mW after the WDM filter is deployed which is well within defined maximum permissible exposure standards. For the demonstration of free-space transmission, transmitter-receiver separation is extended to 30 m. The experimental results show that a single-channel WOPT system provides an optical power of 400 mW with a channel linewidth of 1.027 nm over 30 m and an electrical power of 85 mW is acquired using a gallium antimonide photovoltaic.

Computational Fractional Flow Reserve From Coronary Computed Tomography Angiography-Optical Coherence Tomography Fusion Images in Assessing Functionally Significant Coronary Stenosis.

Background: Coronary computed tomography angiography (CTA) and optical coherence tomography (OCT) provide additional functional information beyond the anatomy by applying computational fluid dynamics (CFD). This study sought to evaluate a novel approach for estimating computational fractional flow reserve (FFR) from coronary CTA-OCT fusion images. Methods: Among patients who underwent coronary CTA, 148 patients who underwent both pressure wire-based FFR measurement and OCT during angiography to evaluate intermediate stenosis in the left anterior descending artery were included from the prospective registry. Coronary CTA-OCT fusion images were created, and CFD was applied to estimate computational FFR. Based on pressure wire-based FFR as a reference, the diagnostic performance of Fusion-FFR was compared with that of CT-FFR and OCT-FFR. Results: Fusion-FFR was strongly correlated with FFR (r = 0.836, P < 0.001). Correlation between FFR and Fusion-FFR was stronger than that between FFR and CT-FFR (r = 0.682, P < 0.001; z statistic, 5.42, P < 0.001) and between FFR and OCT-FFR (r = 0.705, P < 0.001; z statistic, 4.38, P < 0.001). Area under the receiver operating characteristics curve to assess functionally significant stenosis was higher for Fusion-FFR than for CT-FFR (0.90 vs. 0.83, P = 0.024) and OCT-FFR (0.90 vs. 0.83, P = 0.043). Fusion-FFR exhibited 84.5% accuracy, 84.6% sensitivity, 84.3% specificity, 80.9% positive predictive value, and 87.5% negative predictive value. Especially accuracy, specificity, and positive predictive value were superior for Fusion-FFR than for CT-FFR (73.0%, P = 0.007; 61.4%, P < 0.001; 64.0%, P < 0.001) and OCT-FFR (75.7%, P = 0.021; 73.5%, P = 0.020; 69.9%, P = 0.012). Conclusion: CFD-based computational FFR from coronary CTA-OCT fusion images provided more accurate functional information than coronary CTA or OCT alone. Clinical Trial Registration: [www.ClinicalTrials.gov], identifier [NCT03298282].

Optical coherence tomography-based machine learning for predicting fractional flow reserve in intermediate coronary stenosis: a feasibility study.

Machine learning approaches using intravascular optical coherence tomography (OCT) to predict fractional flow reserve (FFR) have not been investigated. Both OCT and FFR data were obtained for left anterior descending artery lesions in 125 patients. Training and testing groups were partitioned in the ratio of 5:1. The OCT-based machine learning-FFR was derived for the testing group and compared with wire-based FFR in terms of ischemia diagnosis (FFR ≤ 0.8). The OCT-based machine learning-FFR showed good correlation (r = 0.853, P < 0.001) with the wire-based FFR. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of the OCT-based machine learning-FFR for the testing group were 100%, 92.9%, 87.5%, 100%, and 95.2%, respectively. The OCT-based machine learning-FFR can be used to simultaneously acquire information on both image and functional modalities using one procedure, suggesting that it may provide optimized treatments for intermediate coronary artery stenosis.

Coronary artery decision algorithm trained by two-step machine learning algorithm.

A two-step machine learning (ML) algorithm for estimating both fractional flow reserve (FFR) and decision (DEC) for the coronary artery is introduced in this study. The primary purpose of this model is to suggest the possibility of ML-based FFR to be more accurate than the FFR calculation technique based on a computational fluid dynamics (CFD) method. For this purpose, a two-step ML algorithm that considers the flow characteristics and biometric features as input features of the ML model is designed. The first step of the algorithm is based on the Gaussian progress regression model and is trained by a synthetic model using CFD analysis. The second step of the algorithm is based on a support vector machine with patient data, including flow characteristics and biometric features. Consequently, the accuracy of the FFR estimated from the first step of the algorithm was similar to that of the CFD-based method, while the accuracy of DEC in the second step was improved. This improvement in accuracy was analyzed using flow characteristics and biometric features.

Automated accurate lumen segmentation using L-mode interpolation for three-dimensional intravascular optical coherence tomography.

Intravascular optical coherence tomography (IVOCT) lumen-based computational flow dynamics (CFD) enables physiologic evaluations such as of the fractional flow reserve (FFR) and wall sheer stress. In this study, we developed an accurate, time-efficient method for extracting lumen contours of the coronary artery. The contours of cross-sectional images containing wide intimal discontinuities due to guide wire shadowing and large bifurcations were delineated by utilizing the natural longitudinal lumen continuity of the arteries. Our algorithm was applied to 5931 pre-intervention OCT images acquired from 40 patients. For a quantitative comparison, the images were also processed through manual segmentation (the reference standard) and automated ones utilizing cross-sectional and longitudinal continuities. The results showed that the proposed algorithm outperforms other schemes, exhibiting a strong correlation (R = 0.988) and overlapping and non-overlapping area ratios of 0.931 and 0.101, respectively. To examine the accuracy of the OCT-derived FFR calculated using the proposed scheme, a CFD simulation of a three-dimensional coronary geometry was performed. The strong correlation with a manual lumen-derived FFR (R = 0.978) further demonstrated the reliability and accuracy of our algorithm with potential applications in clinical settings.

Wireless optical power transfer system by spatial wavelength division and distributed laser cavity resonance.

A novel wireless optical power transfer (WOPT) system using diverging angular dispersion and spatially distributed laser cavity resonance is proposed. In the transmitter, a diffraction grating spatially disperses the broadband light from a semiconductor optical amplifier. Receiving units spread across a wide field of view are embedded with retroreflecting beam splitters that reflect the incident beam back to the transmitter, thereby completing multiple resonant cavities. Retroreflectors enable a user-friendly alignment and tap power from the resonating cavity, supplying optical power. We demonstrate an automatic safety mechanism that instantly ceases the cavity resonance should any vulnerable organ break the transmitter-receiver line of sight. The results indicate that a single-channel WOPT system can provide a resonating average power of 17.2 mW (receiving power of 1.7 mW to the photodetector) over a distance of 1 m with a channel linewidth of 0.035 nm. For a proof-of-principle experiment, seven receiver units were successfully demonstrated to supply optical power. With careful retroreflector design and field-of-view optimization, the potential of our scheme can be further exploited toward commercial deployment.

Diversification of mitochondrial genome of Daphnia galeata (Cladocera, Crustacea): Comparison with phylogenetic consideration of the complete sequences of clones isolated from five lakes in Japan.

To characterize genetic diversity and gene flow among Daphnia galeata populations, the complete nucleotide (nt) sequences of the mitochondrial (mt) DNAs of D. galeata clones isolated from five lakes in Japan (Lakes Shirakaba, Suwa, Kizaki, Kasumigaura, and Biwa) were determined. Comparison of non-synonymous (amino acid altering) substitution rates with synonymous substitution rates of D. galeata mt protein-coding genes demonstrated that ATPase8 and COI genes were the most and least susceptible, respectively, to the evolutional forces selecting the aa substitutions. Several non-synonymous substitutions were found in ATPase8 and ATPase6 even in the comparison that no synonymous substitution was found. Comparison of the total number of nt variations among the mt DNAs suggested the phylogenetic relationship ((((Shirakaba/Suwa, Kizaki), Kasumigaura), Biwa), D. pulex). Maximum-likelihood analysis using the total nt sequences of mt protein-coding genes confirmed this relationship with bootstrap values higher than 98%. All the mtDNAs of the analyzed Japanese D. galeata clones contained a control region of essentially the same structure that is distinct from those of the previously reported European Daphnia species of the D. longispina complex. The two control regions of different structures spread among mtDNAs of the Japanese and European Daphnia species, respectively, probably after the divergence of the Japanese D. galeata under different selection pressures associated with their habitats.

Assessing Computational Fractional Flow Reserve From Optical Coherence Tomography in Patients With Intermediate Coronary Stenosis in the Left Anterior Descending Artery.

BACKGROUND: Intravascular optical coherence tomography (OCT) imaging provides limited information on the functional assessment of coronary stenosis. We evaluated a new approach to OCT image-based computation modeling, which can be used to estimate the fractional flow reserve (FFR) in patients with intermediate coronary stenosis. METHODS AND RESULTS: Ninety-two patients with intermediate diameter stenosis in the left anterior descending artery underwent both FFR measurement with pressure wires and OCT examination. Using the OCT data, a computational fluid dynamics algorithm was used to calculate the computational FFR (FFROCT). The diagnostic performance of the FFROCT was assessed based on the pressure wire-based FFR. The median FFR and FFROCT values were 0.86 (0.79-0.89) and 0.89 (0.82-0.94), respectively. The average diameter stenosis in quantitative coronary angiography and area stenosis in OCT were 58.1±13.4% and 67.5±13.5%, respectively. The FFROCT was better correlated to the FFR than were the anatomic variables (r=0.72; P<0.001 versus r=0.46; P<0.001 for minimal luminal diameter on quantitative coronary angiography or r=0.57; P<0.001 for minimal lumen area on OCT). When functionally significant stenosis was defined as an FFR cutoff value of ≤0.8, FFROCT resulted in 88.0% accuracy, 68.7% sensitivity, and 95.6% specificity. The positive and negative predictive values were 84.2% and 89.0%, respectively. CONCLUSIONS: The computation of FFROCT enables assessment not only of anatomic information, but also of the functional significance of intermediate stenosis. This measurement may be a useful approach for the simultaneous evaluation of the functional and anatomic severity of coronary stenosis.

Three-Dimensional Optical Coherence Tomographic Analysis of Eccentric Morphology of the Jailed Side-Branch Ostium in Coronary Bifurcation Lesions.

BACKGROUND: The severity of angiographic diameter stenosis of the jailed side-branch ostium is usually overestimated over the 3-D optical coherence tomography (OCT)-measured actual stenosis. Using 3-D OCT, morphologic changes in the jailed side-branch ostium were evaluated before and after a single stent crossover at coronary bifurcation lesions. METHODS: In 109 patients who received a single stent crossover at coronary bifurcation lesions, the minimal lumen area (MLA) before and after intervention and the eccentricity of the jailed side-branch ostium were measured with 3-D OCT. The eccentricity index was defined as the ratio of maximal diameter/minimal diameter of the jailed side-branch ostium. When the eccentricity index was < 1.5, the shape of the jailed side-branch ostium was defined as circular. The MLA of the jailed side-branch ostium was also calculated from the quantitative coronary angiography (QCA) minimal lumen diameter assuming a circular lumen. RESULTS: The 3-D OCT-measured MLA of the jailed side-branch ostium decreased from 2.91 ± 1.65 mm(2) before intervention to 2.37 ± 1.48 mm(2) after intervention (P < 0.01). The QCA-derived MLA also decreased from 2.35 ± 1.71 mm(2) before intervention to 1.68 ± 1.56 mm(2) after intervention (P < 0.01). However, the 3-D OCT-measured MLA was significantly larger than the QCA-derived MLA (P < 0.01). The shape of the jailed side-branch ostium changed from circular to elliptical after the stent implantation (eccentricity index: 1.40 ± 0.33 before intervention; 1.71 ± 0.60 after intervention; P < 0.01). CONCLUSIONS: Compared with QCA measurements, 3-D OCT analysis could be useful to guide decisions regarding additional intervention after stent implantation across coronary bifurcation lesions.

Eccentric morphology of jailed side-branch ostium after stent crossover in coronary bifurcation lesions: a three-dimensional optical coherence tomographic analysis.

BACKGROUND: Angiographic stenosis of a jailed side-branch ostium is usually observed after a single-stent crossover at coronary bifurcation lesions. However, the stenosis severity is typically overestimated due to the limited information obtained from two-dimensional morphology by angiography. We evaluated the actual stenosis of jailed side-branch ostium using three-dimensional (3D) optical coherence tomography (OCT). METHODS: Using 3D reconstructions of OCT data, we analyzed minimal lumen area (MLA) and eccentricity of the jailed side-branch ostium in 41 patients who were treated with single stent crossover at coronary bifurcation lesions and subsequently underwent serial OCT follow-up. RESULTS: The MLA of jailed side-branch ostium calculated from quantitative coronary angiography (QCA) assuming a circular lumen markedly decreased after stent implantation (1.73±1.22mm(2) pre-intervention to 0.84±0.91mm(2) post-intervention, p<0.001). However, the MLA of jailed side-branch ostium measured at post-intervention by 3D-OCT (2.67±1.75mm(2)) was significantly larger than that measured by QCA (p<0.001). There were no statistically significant changes in MLA of jailed side-branch ostium based on 3D-OCT measurements during the follow-up (2.35±1.50mm(2) at 3-6 months post-intervention; 2.44±1.27mm(2) at 1-2 years post-intervention, p=0.098). The shapes of the jailed side-branch ostium were nearly elliptical (mean eccentricity index: 2.97±1.27 post-intervention; 2.79±1.17 at 3-6 months post-intervention; 2.59±1.02 at 1-2 years post-intervention). CONCLUSIONS: Compared to 3D-OCT measurements, QCA measurements overestimated the jailed side-branch ostial stenosis after single stent crossover due to eccentric morphology from orthogonal projection in coronary angiography. Significant changes in the MLA of jailed side-branch ostium by 3D-OCT were not observed during the follow-up.

Free silver ion as the main cause of acute and chronic toxicity of silver nanoparticles to cladocerans.

We investigated the interspecific variation of silver nanoparticle (SNP) sensitivity in common cladocerans (Daphnia magna, D. galeata, and Bosmina longirostris) and the exact cause of both acute and chronic toxicity focusing on the form of silver (NPs and ions). Materials tested were non-surface-coated silver nanocolloids (SNCs) and AgNO3. The results of the acute toxicity tests support the theory that the effects of SNPs on aquatic organisms is mainly due to Ag(+) released from SNPs. Among the three cladocerans, D. galeata was more sensitive to silver (as Ag(+)) than both D. magna and B. longirostris. Moreover, the chronic toxicity of SNCs was also derived from dissolved silver (especially Ag(+)). SNCs (as total silver concentration) showed far lower chronic compared with acute toxicity to daphnids because the amount of dissolved silver decreased in the presence of prey algae. The chronic end-point values (EC10 values for net reproductive rate and the probability of survival to maturation) did not differ largely from acute ones (48-h EC50 obtained from acute toxicity tests and 48-h LC50 estimated by the biotic ligand model) when the values were calculated based on Ag(+) concentration. The α value (concentration at which intrinsic population growth rate is decreased to zero) estimated by a power function model was a reliable parameter for assessing the chronic toxicity of silver.

A usage of CO2 hydrate: convenient method to increase CO2 concentration in culturing algae.

The addition of CO2 to algal culture systems can increase algal biomass effectively. Generally, gas bubbling is used to increase CO2 levels in culture systems; however, it is difficult to quantitatively operate to control the concentration using this method. In this study, we tested the usability of CO2 hydrate for phytoplankton culture. Specifically, green algae Pseudokirchneriella subcapitata were cultured in COMBO medium that contained dissolved CO2 hydrate, after which its effects were evaluated. The experiment was conducted according to a general bioassay procedure (OECD TG201). CO2 promoted algae growth effectively (about 2-fold relative to the control), and the decrease in pH due to dissolution of the CO2 in water recovered soon because of photosynthesis. Since the CO2 hydrate method can control a CO2 concentration easily and quantitatively, it is expected to be useful in future applications.

The relationship between post-stent strut apposition and follow-up strut coverage assessed by a contour plot optical coherence tomography analysis.

OBJECTIVES: This study sought to evaluate the relationship between post-stent strut apposition and follow-up strut coverage using contour plot optical coherence tomographic analysis. BACKGROUND: Tracking the fate of interested regions of struts at different time points has not been investigated. METHODS: Post-intervention and 6-month follow-up optical coherence tomographic evaluations were performed in 82 patients treated with biolimus- (n = 37) or sirolimus-eluting stents (n = 45). Post-stent apposition was classified as embedded, apposed, or malapposed. For volumetric stent evaluation, the post-intervention strut-artery distance and the neointimal thickness at follow-up were measured as a function of the circumferential arc length and longitudinal stent length. Computer-generated contour plots of the strut-artery distance and neointimal thickness were compared. RESULTS: The percentages of embedded and malapposed struts after intervention were 1.8% (Interquartile range [IQR]: 0.6% to 6.2%) and 2.3% (IQR: 0.5% to 5.2%), respectively. The percentages of uncovered and malapposed struts at 6 months were 16.0% (IQR: 7.4% to 33.3%) and 0% (IQR: 0% to 0.7%), respectively. The percentage of uncovered struts at 6 months varied significantly with post-stent strut apposition (0% [IQR: 0% to 11.4%] in embedded, 16.3% [IQR: 8.1% to 31.3%] in apposed, and 26.8% [IQR: 0% to 56.3%] in malapposed, p < 0.001 for all pairwise comparisons). In lesions without tissue prolapse, embedded struts were all covered (100% covered struts) compared with those with tissue prolapse (76.8% covered, p < 0.001). CONCLUSIONS: The optical coherence tomography-guided optimization of stent strut apposition enhances strut coverage at follow-up. This comprehensive method for evaluating strut apposition may provide more useful information to understanding the serial changes in strut coverage. (Neointimal Coverage After Implantation of Biolimus Eluting Stent With Biodegradable Polymer: Optical Coherence Tomographic Assessment According to the Treatment of Dyslipidemia and Hypertension and the Types of Implanted Drug-Eluting Stents; NCT01502904).

Randomised comparison of strut coverage between Nobori biolimus-eluting and sirolimus-eluting stents: an optical coherence tomography analysis.

AIMS: The aims of this study were to compare strut tissue coverage at six-month follow-up after Nobori biolimus-eluting stent (N-BES) versus sirolimus-eluting stent (SES) implantation. METHODS AND RESULTS: A total of 120 patients with a single stenotic lesion requiring revascularisation were randomly assigned to either N-BES (n=60) or SES (n=60). Baseline optical coherence tomography (OCT) was performed post stent implantation, and follow-up OCT was scheduled at six months. Post-intervention and follow-up OCT analyses were available in 51 (85.0%) and 52 (86.7%) patients, respectively. The primary endpoint was the percentage of uncovered struts at follow-up OCT in all cross-sections with analysis of 0.2 mm longitudinal intervals. In addition, an OCT contour plot analysis was constructed to investigate the distribution patterns of uncovered struts. The percentage of uncovered struts of N-BES was significantly lower versus SES (15.9±12.2% versus 25.1±18.3%, respectively, p=0.003). N-BES showed a greater NIH thickness than SES (median [interquartile range] 52.4 µm [41.9-74.9] versus 42.1 µm [30.6-56.0], p=0.005). Contour plot analyses showed a lower incidence of a diffuse distribution pattern of uncovered struts (46.8% versus 75.0%, respectively, p=0.022). CONCLUSIONS: This randomised OCT study showed that N-BES had a significantly lower percentage of uncovered struts, and less common diffuse distribution pattern of uncovered struts versus SES.

Neointimal patterns obtained by optical coherence tomography correlate with specific histological components and neointimal proliferation in a swine model of restenosis.

AIMS: Although optical coherence tomography (OCT) is capable to detect microscopic peri-strut changes that seem to be related to neointimal inhibition and healing, its ability to characterize these components is still limited. In this study, we aimed to compare different OCT morphological characteristics with different in-stent neointimal tissue types analysed by histology. METHODS: A total of 69 stents (39 drug eluting and 30 bare metal stents) were implanted in coronary arteries of 27 swine. By OCT, neointimal type was classified as homogeneous, heterogeneous, or layered according to its pattern of backscatter and optical intensity. The resulting optical patterns were correlated with several histological findings [external elastic lamina (EEL) disruption, fibrin deposition, circumferential rim of peri-strut inflammatory cell infiltration, and fibrous connective deposition] in every single cross-section (CS) analysed. RESULTS: A total of 197 matched OCT and histological CS were analysed. The heterogeneous (0.44 ± 0.21 mm) and layered (0.65 ± 0.16 mm) patterns had a significantly higher degree of neointimal thickness compared with the homogeneous pattern (0.25 ± 0.16 mm, P < 0.001). Fibrous connective tissue deposition was more frequently present in the homogeneous pattern (71.6%, P < 0.001), whereas significant fibrin deposits were more commonly seen in the heterogeneous pattern (56.9%, P = 0.007). Peri-strut inflammation was less frequently found in the homogeneous pattern (19.8%, P < 0.001) in comparison with the layered (73.9%) or heterogeneous patterns (43.1%). The presence of EEL rupture was also more commonly seen in layered (73.9%) and heterogeneous (46.6%) patterns than in the homogeneous pattern (22.4%, P < 0.001). CONCLUSION: The optical characteristics of neointimal formation seen in OCT properly correlated with the presence of several histological findings involved in stent healing. The biological implications of these findings in clinical outcomes require further investigation.

Complex wavefront shaping for optimal depth-selective focusing in optical coherence tomography.

We report on an approach to exploit multiple light scattering by shaping the incident wavefront in optical coherence tomography (OCT). Most of the reflected signal from biological tissue consists of multiply scattered light, which is regarded as noise in OCT. A digital mirror device (DMD) is utilized to shape the incident wavefront such that the maximal energy is focused at a specific depth in a highly scattering sample using a coherence-gated reflectance signal as feedback. The proof-of-concept experiment demonstrates that this approach enhances depth-selective focusing in the presence of optical inhomogeneity, and thus extends the penetration depth in spectral domain-OCT (SD-OCT).