We discuss and demonstrate the dependence of noise on the signal in time-domain optical coherence tomography (TDOCT). We then derive a depth-dependent matched filter to maximize the signal-to-noise ratio at every pixel in a depth scan (A-scan). We use an empirical estimate of the second order statistics of the noise in OCT images of vascular tissue to implement a depth-dependent filter that is matched to these images. The application of our filter results in an average increase of signal-to-noise ratio of about 7 dB compared to a simple averaging operation. Our filter is not specific to time-domain OCT, but it is applicable to other types of OCT systems.
Optical coherence tomography images of arterial samples harvested from asymptomatic pigs and from lipid-rich
Watanabe heritable hyperlipidemic rabbits were acquired using a fiber catheter-based swept-source optical coherence
tomography system (OCT). A quadrature Mach-Zehnder interferometer based on multi-port fiber couplers and a
semiconductor optical amplifier (SOA) were employed in the swept-source optical coherence tomography system. The
improvement of signal to noise ratio as a result of incorporating the SOA into the configuration translated in an increase
of the penetration depth. A fiber probe ending in a fiber ball lens was developed for the arterial imaging. The images
acquired by this system offer the possibility to investigate anatomical details located under the surface of the artery such
as the intima, media, and adventitia layers (from lumen side) of the blood vessel wall , as well as morphological features
specific to artherosclerotic plaques such as lipid pools, fibrous caps, macrophage accumulations and calcified. This
report indicates that our improved catheter-based swept source OCT is a potential tool for in vivo intravascular imaging.
Two methods for analyzing OCT images of arterial tissues are tested. These methods are applied toward two types of
samples: segments of arteries collected from atherosclerosis-prone Watanabe heritable hyper-lipidemic rabbits and
pieces of porcine left descending coronary arteries without atherosclerosis. The first method is based on finding the
attenuation coefficients for the OCT signal that propagates through various regions of the tissue. The second method
involves calculating the fractal dimensions of the OCT signal textures in the regions of interest identified within the
acquired images. A box-counting algorithm is used for calculating the fractal dimensions. Both parameters, the
attenuation coefficient as well as the fractal dimension correlate very well with the anatomical features of both types of samples.
Tissue morphology, light attenuation and texture are analyzed from images
acquired by swept-source Fourier-domain optical coherence tomography from arterial
samples. The data were corrected for the effect of the confocal point spread function and
were analyzed using the single scattering model.
In-vitro analysis of flowing blood-saline mixtures is performed by time-domain optical coherence tomography
imaging. The mixtures contain blood in concentrations ranging from 100% to 20%. For each image, a
corresponding compounded profile is obtained by adding one thousand adjacent A-scans. The compounded
profiles are used for characterizing the optical coherence tomography signal as it propagates within the studied
blood-saline mixtures. The results obtained point toward the possibility of acquiring intra-vascular images of
arterial tissue that is located behind slabs of flowing blood-saline. A threshold in the propagation distance
beyond which the recorded signal becomes dominated by its multiple scattered components is established along
the compounded profiles. The threshold location, at a depth of ~0.6 mm, is independent of blood concentration.
Further investigation of the compounded profiles reveals that the region extending to a maximum depth of
about 200 μm from the point where the probing beam enters blood-saline mixtures could reveal information
about the flow regime. This opens the possibility of another application for time-domain optical coherence
tomography in intravascular imaging: assessing the flow regime, i.e. laminar or turbulent.
Optical coherence tomography imaging is used to improve the detection of incipient carious lesions in dental enamel. Measurements of signal attenuation in images acquired with an 850-nm light source were performed on 21 extracted molars from eight human volunteers. Stronger attenuation was observed for the optical coherence tomography (OCT) signal in healthy enamel than in carious lesions. The measured attenuation coefficients from the two groups form distinct statistical populations. The coefficients obtained from sound enamel fall within the range of 0.70 to 2.14 mm−1 with a mean value of 1.35 mm−1, while those in carious regions range from 0.47 to 1.88 mm−1, with a mean value of 0.77 mm−1. Three values are selected as the lower threshold for signal attenuation in sound enamel: 0.99, 0.94, and 0.88 mm−1. These thresholds were selected to provide detection of sound enamel with fixed specificities of 90%, 95%, and 97.5%, respectively. The corresponding sensitivities for the detection of carious lesions are 92.8%, 90.4%, and 87%, respectively, for the sample population used in this study. These findings suggest that attenuation of OCT signal at 850 nm could be an indicator of tooth demineralization and could be used as a marker for early caries detection.
KEYWORDS: Optical coherence tomography, Blood, Signal detection, Light scattering, Scattering, Signal attenuation, Monte Carlo methods, Speckle, Interfaces, Tissue optics
Establishing when the amount of recorded multiple scattered signal becomes dominant is important for various clinical
applications that require optical coherence tomography imaging through a turbid environment such as blood. The
profiles of detected signals obtained by compounding coherence tomography images of flowing blood-saline mixtures
with various blood concentrations are analyzed. The scattering properties of the studied mixtures influence the
corresponding profiles of the recorded signal. Monte Carlo simulations of light propagation through environments with
various scattering coefficients are used to support and to explain the experimental data.
Model consideration is given to explain observed multi-shell emission spectra from InAs quantum dots embedded in GaAs or InGaAs. The shell model is based on the quantization of kinetic energy of lateral motion of carrier in the dot. 2-D oscillator is calculated on the basis of effective mass approximation. Profiles of inter-level separation are classified into categories that are connected with the lateral confining potential. Comparison is carried with experimental data on InAs/InGaAs quantum dot structures of the DWELL type (dot-in-a-well).
A theoretical model for the dependence on temperature of the carrier behavior in a semiconductor structure containing InAs quantum dots grown inside a Ga0.85In0.15As quantum well is presented. The conditions, that have to be imposed in order to obtain analytical solutions with obvious physical interpretation are kept to minimum. Two temperature domains are approached in this model. In the low temperature case the equation system that describes the carrier behavior can be reduced to a cubic equation. One of the solutions of the equation represents the quantum dot photoluminescence yield. Also, a solution is obtained for the dot emission yield in the high temperature domain, where the carrier thermal escape from dots cannot be neglected. The solution depends, on the probabilities for electron and hole capture and reemission, and on the number of dot states occupied by electrons and holes. Temperature dependent measurements of the quantum dot photoluminescence are performed and the results are fit with the theoretical model.
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