Microscopy Focus
Multilayer Tissue-Like Optical Phantom; a Model for Skin in Optical Coherence Tomography Imaging
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Optical coherence tomography (OCT) is an advanced high-resolution non-invasive imaging tool, which delivers three-dimensional (3D) images from the microstructure compartments within the skin tissue. Using OCT one can extract optical properties (scattering coefficient and anisotropy factor) of normal or diseased skin to generate an optical model for skin, which can be used for diagnosis. To verify and validate the optical properties extraction algorithm a tissue-like optical phantom is needed. If the phantom can be multilayer, it can model the skin more accurately.
WHAT IS PHANTOM AND WHY IS IT NEEDED?
Phantom is a virtual tissue with well-controlled optical properties (refractive index, scattering coefficient, anisotropy factor and absorption coefficient) and can be constructed in solid or liquid states depending on its application. Phantoms are being used for testing the design of the system, system optimisation, and performance evaluation of the system. In optical imaging modalities in particular, phantoms are used for measurement/evaluation of the system parameters such as longitudinal and transversal resolutions, image contrast, point spread function (PSF), system sensitivity, system differentiability between two types of tissues, and system detectability of a certain concentration. In this study, we used phantoms to evaluate the accuracy and precision of the optical properties extraction algorithm in calculating the values of scattering coefficient and anisotropy factor.
HOW CAN WE EXTRACT OPTICAL PROPERTIES FROM OCT IMAGES?
Multilayer phantoms can be a good way of evaluation of optical coherence tomography and its related algorithms.
The optical properties (scattering coefficient and anisotropy factor) can be extracted from the OCT images by fitting the OCT signal which is obtained based on extended Huygens- Fresnel (EHF) principle, onto the averaged A-line vector in a specific region. The fitting procedure and corresponding curves are shown in Figure 1 and Figure 2, respectively. One can generate an optical model for skin by specifying the range of variation of the optical properties for the healthy skin. Then the values out of this range can be used for the diagnosis of diseased skins.
layer of skin, we use scatterer, absorber and hardener with determined concentrations. Different combinations of scaterrer and absorber can be used to construct the phantom according to the application. We have
experimented phantoms composed of one of TiO2, Iron, super white polystyrene microsphere, white and black polystyrene microspheres and gold microsphere embedded in epoxy-resin or Agarose. Among them, we used polystyrene microsphere with combination of Agarose to construct a multilayer solid phantom.
HOW IS THE STRUCTURE OF SKIN?
Figure 1: Stages of the optical properties extraction algorithm
Skin is composed of four layers; Stratum Corneum (SC), Epidermis (ED), Dermis (D), and hypodermis (HD) (Figure 3). The layers are with different structures and compartment sizes. Looking at the literature, we extracted the information of the structure of the layers; viscosity and concentration. The amount of hardener, Agarose, and the scatterer were then computed accordingly. The conclusion was as follows. Multilayer phantoms can be a good way of evaluation of optical coherence tomography and its related algorithms. This means higher than normal (1 portion Agarose, 100 portion of water). Also as there is no melanin in the SC, there is no absorber needed in this layer. The largest size of our microspheres was used in this layer. Higher concentration of the hardener also resembles the diffusivity of this layer, which is the major feature of this layer. Epidermis is about 0.1mm thick; it however varies in different sites throughout the body. Epidermis consists of mainly connective tissue and cells so called melanosomes that produce pigment melanin. The major property of this layer is the fact that this layer is highly absorbing due to melanin. The melanin was modelled in the phantom by using Proj90N that is molecular nano-size absorber. To model the epidermis, we used smaller size of microspheres with less concentration and higher absorption than those have been used in SC. Dermis is composed of two sub layers; papillary dermis and reticular dermis. However in this study we considered dermis as only one layer. Papillary dermis composed of dense collagen tissue, which reflects most of the incident light reflected from this layer. Reticular dermis consists of dense irregular connective tissue, which is made up of mainly loose connective tissue. Blood vessels and the nerve endings are in this layer. To model the dermis, the concentration of Microspheres considered to be higher and the size of the scatterers smaller than those of the two top layers. We modelled the blood vessels in dermis using very narrow fibres. The summary of the material selection is given in Table 1.
Author Details:
Mohammad R. N. Avanaki1 Email:
mn96@kent.ac.uk, Ali Hojjat1
, and Adrian G. Podeleanu2
1. Neurosciences and Medical image computing (NMIC) group, school of Biosciences, University of Kent, Canterbury, Kent, United Kingdom
2. Applied optics group (AOG), school of Physical sciences, University of Kent, Canterbury, Kent, United Kingdom
Figure 2. Left: OCT image of polystyrene microsphere embedded in epoxy-resin.
HOW IS A PHANTOM DESIGNED?
To design a phantom one would need to know the interaction of particles within the phantom with the incident light, which can be simply described by Mie theory. Mie theory is a common approach to model the scattering in tissue, which treats scatterers as isolated spheres with given size and refractive index relative to the surrounding medium. For this reason we need to understand the structure of the skin and size of the compartments in different depths within the skin. In each
Figure 3. Left: Skin layers including sub-layers in epidermis and dermis, right: OCT B-scan of dorsal skin of hand from a 25-years old girl
Table 1. Relative concentration of scaterrer, absorber and hardener used to model the skin layers
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