Fabrication and Characterization of Optical Tissue Phantoms Containing Macrostructure

Summary

Optical tissue phantoms are essential tools for calibration and characterization of optical imaging systems and validation of theoretical models. This article details a method for phantom fabrication that includes replication of tissue optical properties and three-dimensional tissue structure.

Abstract

The rapid development of new optical imaging techniques is dependent on the availability of low-cost, customizable, and easily reproducible standards. By replicating the imaging environment, costly animal experiments to validate a technique may be circumvented. Predicting and optimizing the performance of in vivo and ex vivo imaging techniques requires testing on samples that are optically similar to tissues of interest. Tissue-mimicking optical phantoms provide a standard for evaluation, characterization, or calibration of an optical system. Homogenous polymer optical tissue phantoms are widely used to mimic the optical properties of a specific tissue type within a narrow spectral range. Layered tissues, such as the epidermis and dermis, can be mimicked by simply stacking these homogenous slab phantoms. However, many in vivo imaging techniques are applied to more spatially complex tissue where three dimensional structures, such as blood vessels, airways, or tissue defects, can affect the performance of the imaging system.

This protocol describes the fabrication of a tissue-mimicking phantom that incorporates three-dimensional structural complexity using material with optical properties of tissue. Look-up tables provide India ink and titanium dioxide recipes for optical absorption and scattering targets. Methods to characterize and tune the material optical properties are described. The phantom fabrication detailed in this article has an internal branching mock airway void; however, the technique can be broadly applied to other tissue or organ structures.

Introduction

Tissue phantoms are used widely for system characterization and calibration of optical imaging and spectroscopy instruments, including multimodality systems incorporating ultrasound or nuclear modalities^1,2,3,4. Phantoms provide a controlled optical environment for system characterization and quality control of multiple biological imaging techniques. Tissue-mimicking phantoms are useful tools in predicting system performance and optimizing system design for the physiological task at hand; for example, to predict the probing depth of spectros....

Protocol

1. Selection and Verification of Matrix Material Properties

1. Before starting the phantom fabrication process (Figure 1), find the absorption and reduced scattering coefficients for the biological tissue of interest at the imaging wavelength(s). Preliminary estimates may be found in the references^35,36. However, validation of the optical coefficients might be necessary.
2. Using the look-up tables for absorption coefficient, µ_a, and reduced scattering coefficient, µ_s', at 488, 535, 632, and 775 nm wavelengths (Tables 1&#....

Results

To demonstrate the phantom fabrication technique, mouse lung tissue phantoms were fabricated to simulate measured optical properties of excised healthy and inflamed murine lung tissue at 535 nm (Table 5). This wavelength of interest is the excitation wavelength for tdTomato fluorescent protein used in recombinant reporter strains of mycobacteria in previous studies³³. Optical measurements of mouse lung tissue were obtained with the same methods des.......

Discussion

We have demonstrated a method for creating optical phantoms to represent a murine lung with an internal branching structure to simulate the internal air-tissue interface. The optical properties of murine lung tissue are achieved by incorporating unique concentrations of optically scattering and absorbing particles distributed homogenously within the bulk matrix polymer. These optical properties can be tuned to mimic the physiological values within different spectral ranges of tissues in different states (i.e. he.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Dow Corning Sylgard 184 Silicone Encapsulant Clear 0.5 kg Kit	Ellsworth Adhesives	184 SIL ELAST KIT 0.5KG	Polydimethylsiloxane: polymer base for optical phantoms
White Rutile Titanium Dioxide powder	Atlantic Equipment Engineers	TI-602	Scattering particles for optical phantoms
Higgins Fountain Pen India Ink	Michaels Craft Stores	10015483	Absorbing particles for optical phantom
Heat Resistant tape	Uline	S-7595	Heat resistant tape for polymer molds
Fortus 360mc 3D printer	Stratasys	N/A	Able to switch build and support material with this model printer
ABS Ivory Model Material	Stratasys	SDS-000001	Material for printing mold parts and/or using as support for printing internal structure
SR-30 Soluble Support	Stratasys	400638-0001	Base soluble support material for printing internal structure
Flacktek Speedmixer	Flacktek Inc.	DAC 150.1 FV	For efficient mixing of polymer and particles
Integrating sphere	Edmund Optics	58-585	For measuring optical properties
Polycarbonate build plates (1 mm)	Stratasys	N/A	Used polycarbonate build plates from Stratasys printer can also be used