The concept of optical coherence tomography (OCT) for high-resolution imaging of tissues in vivo is introduced. Analytical and numerical models for describing and understanding the light propagation in samples imaged by OCT systems are presented. An analytical model for calculating the OCT signal based on the extended Huygens-Fresnel principle and valid both for single and multiple scattering regimes is outlined. From this model, an algorithm for extracting tissue optical properties for multi-layered tissues is derived. The algorithm is first verified for various optical properties and geometries using solid phantoms and numerical simulations. The applicability of the algorithm for extraction of tissue optical properties is then demonstrated for vascular tissue samples ex vivo. With the use of data from numerical phantoms, the validity of the OCT extraction algorithm for a two-layer geometry is further supported. It is concluded that by using optical properties extracted from OCT images of human tissues, the clinical utility of OCT imaging can be substantially increased.
KEYWORDS: Optical coherence tomography, Scattering, Tissue optics, Signal attenuation, Monte Carlo methods, Computer simulations, Tissues, Extremely high frequency, In vivo imaging, Heterodyning
A recently developed analytical optical coherence tomography (OCT) model (L. Thrane et al., J. Opt. Soc. Am. A 17, 484 (2000)) allows the extraction of optical scattering parameters therby enabling attenuation compensation in OCT images. By expanding this theoretical model, we have delevoped a new method for extracting optical scattering parameters from multi-layered tissue structures in vivo. To verify this, we have used a Monte Carlo (MC) OCT model as a numerical phantom to simulate the OCT signal for heterogeneous multi-layered tissue. Excellent agreement is obtained between the extracted values for the optical scattering properties of the different layers and the corresponding input reference values of the MC simulation, which demonstrates its feasibility for in vivo applications. This is the first time such a verification has been obtained, which holds promise of expanding the funtional imaging capabilities of OCT.
A Monte Carlo model of light scattering in tissue is described and used to estimate the signal-to-noise-ratio that can be obtained with an optical coherence tomography (OCT) system. By utilizing the correspondence between the Wigner phase-space distribution and the specific intensity in the small-angle approximation, the novel Monte Carlo model is valid for light reflected both from and outside the focal plane of the system. The model is compared with experiments and an analytical model and good agreement is found. It is expected that the model can be used to examine how multiple scattering affects the axial resolution for a given source bandwidth.
KEYWORDS: Optical coherence tomography, Scattering, Tissues, Monte Carlo methods, Heterodyning, Light scattering, Systems modeling, Extremely high frequency, Multiple scattering, Backscatter
We review a new theoretical description of the optical coherence tomography (OCT) geometry for imaging in highly scattering tissue. The new model is based on the extended Huygens-Fresnel principle, and it is valid in the single and multiple scattering regimes. Furthermore, we simulate the operation of the OCT system using a specially adapted Monte Carlo simulation code. To enable Monte Carlo simulation of the coherent mixing of the sample and reference beams the code uses a method of calculating the OCT signal derived using the extended Huygens-Fresnel principle. Results obtained with the Monte Carlo simulation and the new theoretical description compare favorably. Finally, the application of the extended Huygens-Fresnel principle for extracting optical scattering properties is used to obtain a so-called true reflection algorithm.
Optical coherence tomography (OCT) images of basal cell carcinomas (BCCs) have been acquired using a compact handheld proble with an integrated video camera allowing the OCT images to be correlated to a skin surface image. In general the healthy tissue of the skin has an obvious stratified structure, whereas the cancerous tissue shows a more homogeneous structure. Thus it was demonstrated that it is possible to distinguish BCCs from healthy tissue by means of OCT. Furthermore different histological types of BCC were identified. Comparison of OCT images taken prior to and immediately after photodynamic theory clearly shows the tissue response to the treatment, and indicates local oedema in the treated area.
We have developed a new theoretical description of the optical coherence tomography (OCT) geometry for imaging in highly scattering tissue. The new model is based on the extended Huygens-Fresnel principle, and it is valid in the single and multiple scattering regimes. The so-called shower curtain effect, which manifests itself in standard OCT systems, is an inherent property of the extended Huygens-Fresnel model. We compare the theoretical analysis with experiments carried out on samples consisting of aqueous suspensions of microspheres and solid phantoms. We calculate the signal-to-noise ratio, and provide an estimation of the maximum attainable probing depth for shot-noise limited detection. Furthermore, we investigate the focusing of the Gaussian probe beam in the tissue using Monte Carlo simulations, and compare it to the extended Huygens-Fresnel model. Finally, we simulate the operation of the OCT system using a specially adapted Monte Carlo simulation code.
In the later years, a great effort has ben put into simulation of the geometry of an optical coherence tomography (OCT) system. Recently, a new analytical model of the OCT geometry has been developed based on the extended Huygens-Fresnel (EHF) principle. Although advanced, the result of the model are surprisingly simple and easy to handle for e.g. system optimization. To validate this model, new features have been added to the Monte Carlo (MC) simulation program MCML, which is widely used and recognized for its credibility. We have incorporated the true shape of a focused Gaussian beam including the finite size of the beam waist, which previously has been approximated by a point. This enables us to do high-resolution comparison of the intensity distribution in the focus plane and excellent agreement is found between the EHF model and the MC simulations. Results are also compared with previously published modeling result and it is shown that there are substantial differences. We emphasize the importance of the so-called shower curtain effect (SCE), which is an inherent - but often overlooked - effect in light propagation through random media. Finally, we calculate the OCT signal using MC simulation. This is done by keeping track of the path length traveled by each photon packet and restricting its access back into the OCT system from the sample using the antenna theorem. The degradation of the detected signal due to scattering is determined, and compared with the EHF model and experiments. The comparison of MC simulations with EHF allows us to show that the SCE is an inherent effect in MC simulation, and that for common tissue parameters, the EHF model yields the same results as the MC simulation but with faster computation time and with field and phase information available.
We have developed a new theoretical description of the optical coherence tomography (OCT) geometry for imaging in highly scattering tissue. The new model is based on the extended Huygens-Fresnel principle, and it is valid in the single and multiple scattering regimes. The so-called shower curtain effect, which manifests itself in standard OCT systems, is an inherent property of the extended Huygens- Fresnel model. We compare the theoretical analysis with experiments carried out on samples consisting of aqueous suspensions of microspheres and solid phantoms. We calculate the signal-to-noise ratio, and provide an estimation of the maximum attainable probing depth for shot-noise limited detection. Furthermore, we investigate the focusing of the Gaussian probe beam in the tissue using Monte Carlo simulations, and compare it to the extended Huygens-Fresnel model. Finally, we simulate the operation of the OCT system using a specially adapted Monte Carlo simulation code.
In recent years, models based on the "Extended Huygens-FresnelÓ principle has been applied in the description of light propagation in tissue. This principle was originally developed for light propagation through aerosols and clear air turbulence. An adaptation of this principle to tissue for the analysis of Optical Coherence Tomography (OCT) systems is advantageous, because of the convenient description of complex optical systems through implication of the ABCD-matrix formalism, and because -contrary to transport theory and diffusion theory- the phase of the light can be modeled. OCT is often combined with con focal microscopy by focusing the probing beam. It is therefore interesting to gauge the performance of the Extended-Huygens Fresnel theory for a geometry with a focused gaussian beam against a (for tissue) more well established model such as diffusion theory. A new model of a focused beam in a semi-infinite slab of scatterers using the diffusion theory is presented here. The model is developed using a GreenÕs function and a Hankel transform. The performance of both models is gauged against Monte Carlo simulations for a medium with low loss and highly forward scattering particles, which is typical of tissue. The model based on diffusion theory utilizes an exponential phase function, and the implication of using this phase function instead of the traditionally used Henyey-Greenstein phase function is investigated. Finally, reasonable qualitative agreement between the three models is shown and discussed.
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