Agarose-based Tissue Mimicking Optical Phantoms for Diffuse Reflectance Spectroscopy

Summary

Here, we demonstrate how agarose-based tissue-mimicking optical phantoms are made and how their optical properties are determined using a conventional optical system with an integrating sphere.

Abstract

This protocol describes how to make agarose-based tissue-mimicking phantoms and demonstrates how to determine their optical properties using a conventional optical system with an integrating sphere. Measuring systems for the acquisition of the diffuse reflectance and total transmittance spectra are constructed with a broadband white light source, a light guide, an achromatic lens, an integrating sphere, a sample holder, an optical fiber probe, and a multi-channel spectrometer. An acrylic mold consisting of two rectangular acrylic pieces and a U-shaped acrylic piece is constructed to create an epidermal phantom and a dermal phantom with whole blood. The application of a sodium dithionite (Na₂S₂O₄) solution to the dermal phantom enables the researcher to deoxygenate hemoglobin in red blood cells distributed in the dermal phantom. The inverse Monte Carlo simulation with the diffuse reflectance and total transmittance spectra measured by a spectrometer with an integrating sphere is performed to determine the absorption coefficient spectrum µ_a(λ) and the reduced scattering coefficient spectrum µ_s'(λ) of each layer phantom. A two-layered phantom mimicking the diffuse reflectance of human skin tissue is also demonstrated by piling up the epidermal phantom on the dermal phantom.

Introduction

Optical phantoms are objects mimicking the optical properties of biological tissues and have been widely used in the biomedical optics field. They are designed so that the optical properties, such as light scattering and absorption coefficients, match with those of living human and animal tissues. Optical phantoms are generally used for the following purposes: simulating the light transport in biological tissues, calibrating a newly developed optical system design, evaluating the quality and performance of existing systems, comparing the performance between systems, and validating the ability of the optical methods to quantify the optical properties^1....

Protocol

1. Construction of a Conventional Diffuse Reflectance and Total Transmittance Spectroscopic System

Note: Construct the measuring systems for the diffuse reflectance and total transmittance spectra using a broadband white light source, a light guide, an achromatic lens, an integrating sphere, a sample holder, an optical fiber, and a multi-channel spectrometer. The role of the light trap is to remove the specular reflection component from the reflectance spectrum. The sample holder of the integrating sphere consists of a mounting plate and a dovetail and spring-loaded clamp assembly that holds the sample against the port. The dovetail and sprin....

Results

Figure 3 shows the representative estimated spectra of the reduced scattering coefficient and the absorption coefficient for the epidermal phantom and dermal phantom. The results shown in Figure 3 are the averages of ten measurements of both reflectance and transmittance spectra. The reduced scattering coefficient µ_s' has a broad scattering spectrum, exhibiting a higher magnitude at shorter wavelength.......

Discussion

The most critical step in this protocol is the temperature control of the base material. The temperature to maintain the base material ranged from 58 to 60 °C. If the temperature is more than 70 °C, a denaturation of both the lipid emulsion and the whole blood will occur. As a consequence, the optical properties of the phantom will deteriorate. If the temperature is less than 40 °C, the base material will be ununiformly gelled and, thus, the light scattering and absorption agents will be heterogeneously di.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
150-W halogen-lamp light source	Hayashi Watch Works Co., Ltd, Tokyo, Japan	LA-150SAE
Light guide	Hayashi Watch Works Co., Ltd, Tokyo, Japan	LGC1-5L1000
Integrating Sphere	Labsphere Incorporated, North Sutton, NH, USA	RT-060-SF
Port adapter	Labsphere Incorporated, North Sutton, NH, USA	PA-050-SMA-SF
Light trap	Labsphere Incorporated, North Sutton, NH, USA	LTRP-100-C
Spectralon white standard with 99% diffuse reflectance	Labsphere Incorporated, North Sutton, NH, USA	SRS-99-020
Optical fiber	Ocean Optics Inc., Dunedin, Florida, USA	P400-2-VIS-NIR
Miniature Fiber Optic Spectrometer	Ocean Optics Inc., Dunedin, Florida, USA	USB2000
Achromatic lens	Chuo Precision Industrial Co.,Ltd, Tokyo, Japan	ACL-50-75M
Intralipid	Fresenius Kabi AB, Uppsala, Sweden	Intralipid 10%
Coffee (Blendy Mocha Blend Regular Coffee)	Ajinomoto AGF, Inc. Tokyo, Japan	Unavailable
Whole blood	Nippon Bio-Test Laboratories Inc. Saitama, Japan	0103-2
Agarose	Nippon Genetics Co., Ltd, Tokyo, Japan	NE-AG02
Cooking heater	TOSHIBA CORPORATION Tokyo, Japan	HP-103K