Method Article

Fabrication and Characterization of Optical Tissue Phantoms Containing Macrostructure

DOI:

10.3791/57031

⸱

February 12th, 2018

 ,  ,  ,  ,  , 
1Department of Biomedical Engineering, Texas A&M University, 2Deparment of Molecular Pathogenesis and Immunology, Texas A&M College of Medicine

In This Article

Summary

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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

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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

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Tissue phantoms are used widely for system characterization and calibration of optical imaging and spectroscopy instruments, including multimodality systems incorporating ultrasound or nuclear modalities1,2,3,4. 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 spectroscopic probes for assessing tumor margins5. Optical properties and structural design of the phantoms can be tuned to mimic the specific physiological environment in which the instrument will be used, therefore allowing for both feasibility studies and verification of system performance3,6,7. Verification of imaging system performance with realistic optical phantoms prior to entering pre-clinical or clinical trials reduces the risk of malfunction or acquisition of unusable data during in vivo studies. The reproducibility and stability of optical phantoms make them customizable calibration standards for optical techniques to monitor intra- and inter-instrument variability, particularly in multicenter clinical trials with different instruments, operators, and environmental conditions8,9.

Tissue-mimicking phantoms also serve as tunable and reproducible physical models for validation of theoretical optical models. Simulations aid in the design and optimization of in vivo optical instruments, while reducing the need for animal experiments10,11. The development and validation of optical simulations to accurately represent the in vivo environment can be encumbered by the complexity of the tissue structure, the biochemical content, and the location of the target or tissue within the body. Variability between subjects makes validation of theoretical models challenging using animal or human measurements. Polymer optical tissue phantoms allow for validation of theoretical models by supplying a known and reproducible optical environment in which to study photon migration12,13,14,15.

For the purpose of system calibration, solid optical phantoms may consist of a single homogeneous slab of cured polymer with the optical scattering, absorption, or fluorescence tuned for the wavelengths of interest. Layered polymer phantoms are frequently used to mimic the depth variance of the tissue optical properties in epithelial tissue models16,17. These phantom structures are sufficient for epithelial imaging and modeling, because the tissue structure is fairly homogeneous through each layer. However, larger scale and more complex structures affect radiative transport in other organs. Methods to create more complex phantoms have been developed to simulate the optical environment of subcutaneous blood vessels18,19 and even whole organs, such as the bladder20. Modeling light transport in the lungs provides a unique problem due to the branching structure of the air-tissue interface; a solid phantom would not likely replicate radiative transport in the organ accurately21. To describe a method for incorporating complex structure into an optical phantom, we describe a method to create an internal, reproducible fractal tree void that represents the three-dimensional (3D) macroscopic structure of the airway (Figure 1).

In the past few decades, 3D printing has become a predominant method for rapid prototyping of medical devices and models22, and optical tissue phantoms are no exception. 3D printing has been used as an additive manufacturing tool for fabricating optical phantoms with channels23, blood vessel networks24, and whole-body small animal models25. These methods use one or two printing materials with unique optical properties. Methods have also been developed to tune the optical properties of the printing material to mimic general, turbid biological tissue25,26. However, the range of achievable optical properties are limited by the printing material, usually a polymer such as acrylonitrile butadiene styrene (ABS)26, so this method is not suitable for all biological tissues. Polydimethylsiloxane (PDMS) is an optically clear polymer that can be readily mixed with scattering and absorbing particles with a higher level of tunability27,28. PDMS has also been used to mold phantoms with aneurysm models for deployment of embolic devices29,30. These phantoms also utilize a dissolvable 3D printed part, but remain optically clear for visualizing device deployment. Here, we combine this method with tunability of the optical properties of PDMS with scattering and absorbing particles to fabricate a preliminary model of the tissue and airways of the murine lung.

While the phantom presented here is specific to the lungs, the process can be applied to a variety of other organs. 3D printing of the internal structure of the phantom allows the design to be customizable for any purpose and printable scale, whether it be a blood or lymph vessel network, bone marrow, or even the four chambered structure of the heart31. Because we are interested in optical imaging and modeling of the lung32,33,34, we have opted to use a four-generation fractal tree as the internal structure to replicate within the polymer phantom. This structure was designed to approximate the branching structure of the airway and have break-away support material for the 3D printing process. A more anatomically correct airway could be printed if break-away support material is not necessary. Although this particular model represents an airway, the internal structure of the phantom does not have to remain a material void. Once the surrounding polymer is cured and the 3D printed part is dissolved, the internal structure can be used as a flow pathway or as a secondary mold for a material with its own unique absorption and scattering characteristics. For example, if the internal structure from this protocol was designed as a digital bone rather than an airway, the bone structure could be 3D printed, molded with PDMS with optical properties of the finger, and then dissolved out of the phantom. The void could then be filled with a PDMS mixture with different optical properties. Additionally, each mold is not limited to a single dissolvable part. A phantom of the finger could be created to include bone, veins, arteries, and a general soft tissue layer, each with its own unique optical properties.

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Protocol

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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 references35,36. However, validation of the optical coefficients might be necessary.
  2. Using the look-up tables for absorption coefficient, µa, and reduced scattering coefficient, µs', at 488, 535, 632, and 775 nm wavelengths (Tables 1–4 and Figures 2–3), select the concentrations of India ink and titanium dioxide (TiO2) that approximate the desired optical properties. These recipes are specific to phantoms fabricated with PDMS. As these tables provide experimental data at discrete wavelengths, optimization of the recipe may be required for the specific application.
  3. Fabricate a polydimethylsiloxane (PDMS) slab of the selected recipe for confirmation of optical properties.
    1. Using a 10:1 ratio in weight of PDMS resin to curing agent, pour ingredients into the mixing cup in the following order: PDMS resin, TiO2, India ink, PDMS curing agent.
      NOTE: Here, we test two recipes: 1) 2 mg TiO2 + 3.5 µL India ink per g PDMS and 2) 1 mg TiO2 + 10 µL India ink per g PDMS. For each recipe, 4.5 g PDMS resin and 0.45 g PDMS curing agent are used with the corresponding amounts of optical particles.
    2. Mix in a speed mixer (see Table of Materials) for 60 s. If TiO2 particles stick to the mixing cup (probable with high concentrations of TiO2), mix by hand to remove the particles from the base of the cup, and mix in the mixer for another 30 s.
    3. Pour the mixture into wells or Petri dishes to make thin (0.1-1 mm) slabs of the mixture.
    4. Degas the slabs for 10 min by placing them in an air-tight negative pressure chamber, then place in a pre-heated oven at 80 °C for 30-60 min. Remove from oven and let cool.
    5. Remove the cooled polymer slab from its container. Trim off the edges to leave a flat, uniform slab. Measure the thickness of the slab using calipers.
  4. Measure transmittance (T) and reflectance (R) of slab(s) using an integrating sphere. Additional details and instructions can be found in the Inverse-Adding Doubling (IAD) manual37.
    1. Turn on the light source and spectrometer of the integrating sphere setup. Check the alignment of the system to ensure a small, collimated beam is centered on the entry and exit ports of the integrating sphere.
    2. Calibrate the integrating sphere system.
      1. Turn off the source, cap the exit port of the integrating sphere, and record three dark spectra.
      2. Turn the source back on to obtain the transmission reference with the exit port capped and the entrance port empty. Record three spectra.
      3. Obtain reflectance reference measurements using reflectance standard(s). Place each standard at the exit port of the sphere. Record three spectra for each reflectance standard.
    3. Measure the transmittance of the slab. With the cap on the exit port, place the slab on the entry port of the integrating sphere for the transmission measurement. Record three spectra.
    4. Measure the reflectance of the slab. Remove the exit port cap and place the slab on the exit port for the reflectance measurement. Record three spectra.
  5. Determine optical properties using IAD software. A full tutorial on the software can be found in the IAD manual with the software download37,38.
    1. Average the three spectra acquired for each measurement.
    2. Using the equations in the IAD manual37, convert these measurements to R and T values. If necessary, condense the files by reducing the sampling rate along the spectrum.
    3. Prepare the input .rxt file (Supplemental Material 1) for IAD with the wavelengths, reflectance, transmittance, and sample thickness as described in the IAD manual37. Using the command prompt (Windows OS) or terminal (Mac OS), navigate to the correct path. Type "iad 'input file name'" to run IAD. The software will produce an output text file with the estimated optical properties.
  6. If the optical properties are not within an acceptable range (~15%) of the desired values, modify the recipe accordingly and repeat steps 1.3–1.5.

2. Preparation of Dissolvable 3D Printed Internal Structure

  1. Design internal structure using computer aided design (CAD) software. Convert structure solid model to a stereolithography file for fabrication on a 3D printer. If available, a segmented CT scan can also be converted into a stereolithography file rather than drawing a solid model of the internal structure.
    NOTE: The CAD file for the fractal tree structure used here is provided in Supplemental Material 2. The printer used in this paper is an extruding printer, so the part was designed to have break-away support material.
  2. Select a dissolvable material for printing, such as poly-vinyl alcohol (PVA) or high-impact polystyrene (HIPS) (see Table of Materials). Print the solid model in this dissolvable material.
  3. When printed parts are sufficiently cooled, break, dissolve, or machine the support material off of the printed part. File or sand off any large imperfections.
  4. Vapor polish the printed part to reduce surface roughness.
    1. With the printed part secured in a vice, drill a through hole with clearance for a thin steel or nitinol wire in the base of the printed part.
    2. Thread a stainless steel or nitinol wire through the hole. Bend ends of wire and hook together. This will allow for the part to be fully immersed in acetone vapor within the beaker. Set wire and part aside.
    3. Fill a large beaker roughly 10% full of acetone. Place beaker on a hot plate while heating to 100 °C. CAUTION: Perform this step in a fume hood to prevent inhalation of acetone vapor.
    4. When acetone vapor condensation reaches about halfway up the wall of the beaker, hang the looped wire with the mock airway on a second wire and suspend in acetone vapor for 15–30 s. Ensure printed parts do not touch the beaker walls or each other (if vapor polishing multiple parts at once).
    5. Remove printed part and suspend over empty beaker or container. Let part dry for at least 4 h.
  5. Verify the dimensions of the internal structure are within tolerance to the CAD design, as needed. Depending on accuracy requirements, calipers or a 3D laser scanner can be used to measure the structure.

3. Construction of Heat Resistant Mold

NOTE: Prepare a leak-proof, heat-resistant mold to form the PDMS phantom. Select a mold geometry to best fit the final phantom design. Here, a reusable rectangular mold is described.

  1. Design a solid model base of the mold to 3D print. This mold is designed for a phantom with a base of 1.17 cm x 1.79 cm. The base of the mold has a 1 mm thick and 5 mm deep recess with inner dimensions matching the base of the phantom. This allows the mold to sidings to be removed and the mold to be disassembled and re-used.
  2. Print a base for the mold with an inset of sufficient width to secure the sidings of the mold.
  3. Place sidings in the recesses of the mold base. Here, 1 mm thick polycarbonate sheets are used as mold sidings.
  4. Using heat resistant tape, seal the edges of the mold. It is imperative that all corners and edges are sufficiently sealed with no bubbles in the tape to prevent any leakage during the molding process.
  5. Place a polycarbonate base plate inside the mold prepared in step 3.4. This base plate is the same 1 mm thick polycarbonate sheet as the mold siding and gives the phantom base a smooth surface without the roughness of the 3D printed surface of the mold base. Glue the fully-dried vapor polished part to the base plate. Allow sufficient time for glue to dry.

4. Fabrication of Polymer Phantom

NOTE: Use the verified recipe for the bulk matrix material determined in step 1 for the specific application. The protocol here provides the steps for a healthy murine lung tissue phantom at 535 nm with µs' of 40 cm-1 and µa of 2 cm-1. It may be useful to fabricate a second phantom with no optical particles to use as a reference in the fabrication process.

  1. Pour 9.1 g of PDMS resin into a plastic mixing cup. Add 20 mg of rutile TiO2, followed by 35 µl of India ink. Finally add 0.91 g of curing agent to the top of the mixture. Follow the mixing protocol in step 1.3.2.
  2. Pour final polymer mixture into the heat-resistant mold.
  3. Pour a small amount of the mixture into a separate container to create a polymer slab for confirmation of material optical properties. Ensure enough polymer is poured to have a slab of at least 100 µm thickness.
  4. Place both the mock airway mold and the separate slab into a bell jar for degassing. Begin vacuum process. If the polymer in the mock airway mold starts to rise, let the air back into the bell jar to burst the surface bubbles, then begin to pull air again. Repeat this process until the polymer does not rise significantly. This will take between 5-10 min depending on how much air was trapped during step 4.2. Once the PDMS no longer rises, continue to degas for another 15 min.
  5. After degassing, slowly let the air back into the chamber. Remove both the mock airway phantom and the polymer slab and place in level oven at 80 °C for 2 h.
  6. Remove the phantom and slab from the oven and let cool for 20 min. Disassemble the polymer mold with a scalpel without cutting the cured polymer. Snap the base plate off of the mock-airway base.
  7. Place phantom in a heated (60 °C) ~0.5 M sodium hydroxide (NaOH) base bath until the internal part is fully dissolved. An optically clear reference phantom may help to determine the dissolving time for the internal component. Once internal structure is dissolved, take phantom out of the bath and let fully dry (~24 h) before taking any optical measurements.

5. Verification of Phantom Fabrication

  1. Verify phantom geometry using high resolution magnetic resonance imaging (MRI) or micro-computed tomography (CT) imaging, if desired. These methods provide a 3D verification of internal structures within turbid material with axial resolutions of <400 µm39,40. Alternatively, an optically clear reference phantom can be optically imaged for verification that the printed part is fully dissolved and the remaining void is the correct geometry.
    NOTE: We have verified the internal geometry of an optically opaque phantom (2 mg TiO2 + 3.5 µl India ink) with micro-CT on a North Star Imaging (NSI) X50. The phantom was imaged with 20 µm resolution in all dimensions (Supplemental Materials 3, 4).
  2. Verify optical properties of the phantom using the polymer slab and the integrating sphere (described in steps 1.5–1.6).

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Results

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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...

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Discussion

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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...

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Disclosures

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The authors have nothing to disclose.

Acknowledgements

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This work was supported by the National Science Foundation CAREER award no. CBET-1254767 and National Institute of Allergy and Infectious Diseases grant no. R01 AI104960. We gratefully acknowledge Patrick Griffin and Dan Tran for their assistance with characterization measurements and the Texas A&M Cardiovascular Pathology Laboratory for micro-CT imaging.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Dow Corning Sylgard 184 Silicone Encapsulant Clear 0.5 kg KitEllsworth Adhesives184 SIL ELAST KIT 0.5KGPolydimethylsiloxane: polymer base for optical phantoms
White Rutile Titanium Dioxide powderAtlantic Equipment EngineersTI-602Scattering particles for optical phantoms
Higgins Fountain Pen India InkMichaels Craft Stores 10015483Absorbing particles for optical phantom
Heat Resistant tapeUlineS-7595Heat resistant tape for polymer molds
Fortus 360mc 3D printerStratasysN/AAble to switch build and support material with this model printer
ABS Ivory Model MaterialStratasysSDS-000001Material for printing mold parts and/or using as support for printing internal structure 
SR-30 Soluble SupportStratasys400638-0001Base soluble support material for printing internal structure
Flacktek SpeedmixerFlacktek Inc.DAC 150.1 FVFor efficient mixing of polymer and particles 
Integrating sphereEdmund Optics58-585For measuring optical properties
Polycarbonate build plates (1 mm)StratasysN/AUsed polycarbonate build plates from Stratasys printer can also be used

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PDMS Fabrication3D Printed StructureIntegrating SphereOptical PropertiesTissue MimickingVapor PolishingDissolvable MaterialLight TransportAirway Model

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