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Method Article

Building Affordable, Durable, Medium-Fidelity Ballistic Gel Phantoms for Ultrasound-Guided Nerve Block Training

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DOI:

10.3791/66194

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February 9th, 2024

 ,  ,  ,  , 
1University of California, San Diego School of Medicine, 2Department of Anesthesia, University of California, San Diego Health, 3Department of Emergency Medicine, University of California, San Diego Health

* These authors contributed equally

In This Article

Summary

Here, we demonstrate the design and creation of four custom ballistic gelatin ultrasound phantoms for ultrasound-guided regional anesthesia training. We designed the phantoms using computer-aided design software, used 3D-printed models to create silicone molds, and then poured melted ballistic gel into the molds to create custom tissue layers.

Abstract

Ultrasound phantoms - alternatives to live human tissue - give learners the opportunity to practice ultrasound-guided regional anesthesia without introducing undue risk to patients. Gelatin-based phantoms provide educators with durable and reusable task trainers; however, commercially available gel-based phantoms are expensive. Here, we investigate the production of durable, low-cost, ballistic gel-based ultrasound phantoms for median, femoral, suprainguinal fascia iliaca plane, and serratus anterior plane nerve blocks, as well as a methodology for producing a phantom for any ultrasound-guided nerve block procedure.

Computer-aided design (CAD) software was utilized to design four phantoms replicating the anatomy of median, femoral, suprainguinal fascia iliaca plane, and serratus anterior plane nerve blocks, including relevant landmarks and tissue planes. Plastic models of the desired tissue planes were 3D printed and used to create silicone molds. Ballistic gel was melted and mixed with flour and dye to create a liquid, echogenic ballistic gel, which was poured into the silicone molds. Vessels were simulated by creating negative space in the ballistic gel using metal rods. Nerves were simulated using yarn submerged in ultrasound gel. Simulated bones were designed using CAD and 3D printed.

Ballistic gel is a versatile, durable medium that can be used to simulate a variety of tissues and can be melted and molded into any shape. Under ultrasound, these phantoms provide realistic tissue planes that represent the borders between different layers of skin, muscle, and fascia. The echogenicity of the muscle tissue layers, nerves, vessels, and bones is realistic, and bones have significant posterior shadowing as would be observed in a human subject. These phantoms cost $200 each for the first phantom and $60 for each subsequent phantom. These phantoms require some technical skill to design, but they can be built for just 4% of the cost of their commercial counterparts.

Introduction

Ultrasound phantoms - alternatives to live human tissue - give learners the opportunity to practice medical procedures, including ultrasound-guided regional anesthesia (UGRA), without introducing undue risk to patients1. While most commonly manufactured via injection molding of liquid silicone rubber, custom phantoms can be homemade using versatile materials at lower cost. Organic tissues such as tofu, pork, and beef are inexpensive but spoil quickly and are challenging to craft2. Human cadaveric tissue is ideal for anatomic accuracy but is difficult and costly to obtain and preserve1. More recently, virtual reality has been used to provide UGRA training; however, haptic feedback is a key component of procedural learning and is rarely implemented. Even when a hardware-software hybrid model provides high visual fidelity and tactile feedback, the hardware and software required to carry out such training are frequently cost-prohibitive3. Gelatin-based phantoms strike a balance between cost, longevity, and fidelity2.

Ballistic gelatin models are available commercially but are expensive for a perishable resource that is highly utilized in medical simulation centers. Small, simple, gel-based ultrasound phantoms with homogeneous parenchyma and two or three simulated vessels retail for hundreds of dollars. For example, the CAE Blue Phantom basic ultrasound training block costs upwards of $8004. Higher-fidelity phantoms specific to individual nerve block procedures cost thousands of dollars. The CAE Blue Phantom femoral regional anesthesia ultrasound training model costs $5,000 (Table 1)5. To bring down costs, educators have experimented with custom-made phantoms using gelatin or other low-cost or reusable materials6,7,8. Additives such as flour, corn starch, graphite powder, and Metamucil can be used to opacify the gelatin and customize the echogenicity of the phantom, thus increasing its fidelity8,9,10,11,12,13,14.

Previous attempts at homemade gelatin-based nerve block trainers were either unable to adequately recreate the appearance of nerves under ultrasound or utilized perishable items, thus limiting shelf life15,16. Even without these drawbacks, previous iterations did not include relevant anatomical landmarks and fascial planes that would allow trainees to practice specific nerve block procedures. Here, we investigate the production of durable, low-cost, ballistic gel ultrasound phantoms for median, femoral, suprainguinal fascia iliaca plane, and serratus anterior plane nerve blocks, as well as a methodology for producing a phantom for any ultrasound-guided nerve block procedure.

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Protocol

For this project, authors JR and PS volunteered as ultrasound subjects, and verbal consent was obtained from both. For those following this protocol, obtain approval from an ethics committee or institutional review board (IRB) prior to using patients or human volunteers as study subjects.

1. Phantom design and silicone mold creation

  1. Creation of a reference ultrasound image
    1. For each ultrasound phantom, have a physician with ultrasound sub-specialization and familiarity with the nerve block procedure being simulated by the desired phantom create a reference ultrasound image from a volunteer human subject (Figure 1). Ensure that this ultrasound image has a view transverse to the applicable nerve or tissue plane into which the anesthetic would be injected.
  2. Design and 3D printing of tissue layer models
    1. Draw cross-sectional designs of each phantom (Figure 2) and utilize computer-aided design (CAD) to design plastic models of desired tissue layers and an outer container for the overall shape of the phantom (Figure 3A, Supplemental File 1, Supplemental File 2, Supplemental File 3, and Supplemental File 4).
      NOTE: From a design standpoint, fascial plane blocks can be thought of as a series of tightly fitted prisms enclosed within a hollow rectangular prism with 5 mm walls. Walls less than 5 mm were too fragile for reliable production. This rectangular prism serves as the outermost layer of the phantom into which the other tissue layers are assembled and placed. The median nerve model (shown in Figure 3 and Figure 4) uses an arched container rather than a rectangular prism to simulate the shape of a human arm.
      1. In CAD software, create a flat canvas from an ultrasound, CT, or other target image with known dimensions. Click on Solid | Insert | Canvas, select File, click on XY plane, drag into the +X +Y space, and drag to scale such that the picture length is accurate to known units.
      2. Create a rectangular prism over the region of the image that outlines the model by clicking on Solid | Create Sketch | XY plane | 2-Point Rectangle tool. Drag the rectangle over the area and refine within the length/width boxes when close; press Enter; click on Finish Sketch; click on rectangle | Solid | Extrude; drag the rectangle to the desired height and refine with the height box when close; and press Enter.
        NOTE: Each of our models has a different length and width based on the anatomy they represent but we typically found ~100 mm to be an effective model height.
      3. Create another sketch on top of the rectangular prism by clicking on the top of the rectangular prism | Create Sketch and draw out the desired anatomy as well as the inner edge of the encasing rectangular prism by clicking on Sketch | Create and Sketch | Modify toolboxes. Use the canvas visible behind the prism to guide the design; if the canvas is not seen through the rectangular prism, change this via Display Settings. Once the sketch that represents the cross section of the model is created, click on Finish Sketch.
        NOTE: There is no specific ideal tool to draw the desired anatomy and the inner edge of the encasing prism. The above step is what was used in this protocol.
      4. Next, create each internal prism from the sketch by clicking on the shape in the sketch | Solid | Extrude; drag the shape back into the rectangular prism at the desired length, typically 5 mm less than the full rectangle length; and click on Operation = New Body | Enter. To view this new object, turn off the visibility of all other objects by clicking on Bodies | the eye symbol next to the new body's name.
        NOTE: Vessels and should be represented by circular or elliptical prism-shaped holes designed into the edges or centers of fascial plane bodies. At this point, if you view the virtual model en-face to the initial canvas, the individual model pieces and how they fit together can be visualized.
      5. Export each body individually for 3D printing by clicking on Bodies | the eye symbol next to every body except the one being exported; click on File | Export | Type = .stl file | Enter.
        NOTE: You should now have multiple .stl files, each representing a unique fascial plane or bone, as well as one additional .stl file representing the rectangular bounding box the fascial plane pieces will fit into.
    2. Open the STL file in a slicer software compatible with the 3D printer that will be used to print the models.
      1. Use the Place on face button to lay the model on the bed such that the bottom of the model is touching the print bed.
      2. Under Printer, select the printer. Under Print settings, select 0.20mm SPEED, and under Filament, select Generic PLA. Select 15-20% for Infill, select Everywhere on the Supports menu, and add a brim if necessary for print stability. Click Slice now.
      3. Export the G-code file to an SD card, plug it into the 3D printer, and print the file using polylactic acid (PLA) filament.
  3. Creation of silicone molds
    1. Glue each 3D-printed model to the bottom of a topless plexiglass container before submerging it in rapid-curing silicone rubber per manufacturer guidelines17.
    2. Once the silicone is set, remove the hard-plastic model and plexiglass case, leaving a flexible, durable, and reusable silicone mold of each desired tissue layer and container into which ballistic gel is poured (Figure 3B).
      NOTE: At this point, the protocol may be paused and restarted later.

Muscle ultrasound images; labeled anatomy, diagnostic technique, medical imaging analysis.
Figure 1: Representative ultrasound images obtained from a human subject. Representative images for the (A) median, (B) femoral, (C) suprainguinal fascia iliaca plane, and (D) serratus anterior plane nerve block models obtained from volunteer human subjects. Abbreviations: A = artery; V = vein; M = median nerve; F = femoral nerve; RAD = radius; U = ulna; AIIS = anterior inferior iliac spine; R =rib; SART = sartorius muscle; IL=Iliacus muscle; IO = internal oblique; SA = serratus anterior muscle; LD = latissimus dorsi muscle. Please click here to view a larger version of this figure.

Static equilibrium diagrams showing component forces; labeled geometric figures and measurements.
Figure 2: Cross-sectional schematics of nerve block ultrasound phantoms. (A) Median, (B) femoral, (C) suprainguinal fascia iliaca plane, and (D) serratus anterior plane nerve block ultrasound phantoms. Schematics were designed based on the representative human ultrasound images shown in Figure 1. Please click here to view a larger version of this figure.

Candle mold process: design diagram, silicone mold parts, wax pouring, final product detailing.
Figure 3: Creation of median nerve block phantom components. (A) Representative image of the computer-aided design file used to print plastic models of each tissue layer for the median nerve block phantom. (B) Silicone molds for each tissue layer of the median nerve block phantom, including metal rods inserted to create vessels within the ballistic gel. (C) Pouring hot, liquid, dyed ballistic gel into the silicone molds. (D) Sealing the open end of simulated vessels using liquid ballistic gel after vessels have been filled with simulated blood. Please click here to view a larger version of this figure.

2. Creation of other phantom landmarks

  1. Simulated bone design and creation
    1. If the region of the model designed in CAD represents bone instead of soft tissue, 3D print an imitation bone using the steps above, but use acrylonitrile butadiene styrene (ABS) filament instead.
      CAUTION: ABS filament fumes may contain volatile organic compounds (VOCs), which can cause physical discomfort, such as drowsiness, eye or respiratory tract irritation, nausea, and/or headaches18. ABS should be printed on an enclosed 3D printer or an unenclosed printer in a room with good ventilation and/or air filtration.
  2. Simulated nerve creation
    1. Submerge 80% acrylic, 20% wool yarn in a plastic cup filled with ultrasound gel. Place the cup is placed in a -1 atm pressure chamber.
    2. Use a single-stage vacuum pump to repeatedly build up and then release pressure in the chamber until all bubbles have been removed from the ultrasound gel, accomplished after approximately 4-6 cycles.
      ​NOTE: This step helps simulate nerves. At this point, the protocol may be paused and restarted later.

3. Ballistic gel melting and pouring

  1. Melting the ballistic gel
    1. Heat commercially available ballistic gel and dye at an approximate 20:1 volume ratio with intermittent stirring until the liquid reaches 132 °C in a commercial convection oven.
      CAUTION: Heated, liquid ballistic gel should be handled with caution due to the risk of burns associated with handling hot fluids. Use oven mitts when handling pans filled with liquid ballistic gel. Avoid direct contact between skin and liquid ballistic gel.
  2. Additives for echogenicity
    1. Stir approximately 4.5 g of finely granulated flour per kg of ballistic gel into the liquid ballistic gel. Leave the gel in the oven for at least 20 min with intermittent stirring to allow for even mixing and to allow any bubbles to escape.
    2. Add additional clear ballistic gel or dye as necessary to adjust the mixture's color to simulate human tissue.
  3. Ballistic gel pouring into silicone molds
    1. Insert solid steel rods of varying diameters into the designated locations on the reusable silicone molds, if appropriate for that specific nerve block model, to create channels in the final ultrasound phantoms, which will represent blood vessels (Figure 3C).
    2. Pour liquid ballistic gel, now colored by dye, with suspended flour particulates, and without retained air bubbles, into the silicone molds and allow it to cool.
    3. After cooling, remove the metal rods and the final ballistic gel tissue layers from the molds. When placed together with an ultrasound gel coating between them, adjacent tissue pieces align near-perfectly and together produce a simulated fascial plane on ultrasound.
      NOTE: Each ultrasound phantom requires approximately 0.7 kg of ballistic gel. Cooling time is dependent on the size of the tissue layer and varies from 20 min to 1.5 h.
  4. Addition of simulated blood and sealing of vessels
    1. For the tissue layers with simulated vessels, dip one side of the tissue layer into the liquid ballistic gel and allow it to cool, thus sealing off one side of the vessel channel.
    2. Hold these tissue layers upright and use a needle and syringe to introduce simulated blood into each vessel.
      NOTE: We used water with red or blue food coloring to represent arterial and venous blood, respectively.
    3. Use still-liquid ballistic gel to cover up the remaining vessel opening, thus sealing off each fluid-filled vessel completely (Figure 3D).
      ​NOTE: At this point, the protocol may be paused and restarted later; however, ballistic gel must be melted again to proceed to the following step.

4. Phantom assembly

  1. Assembly of tissue layers, nerves, and bones
    NOTE: Figure 4A depicts the individual components of the median nerve block phantom immediately prior to assembly, including the tissue layers, simulated nerves, and simulated bones.
    1. Assemble the phantoms by coating each component in ultrasound gel, assembling the components as shown by the cross-sections in Figure 2, and inserting them into their respective ballistic gel rectangular prisms (Figure 4B). Place any 3D-printed bones or yarn nerves properly at this step.
  2. Sealing the ends of the phantom
    1. Seal the models by dipping them on both sides into a pan filled with liquid ballistic gel (Figure 4C). Repeat the sealing process multiple times on each side.
    2. Finally, use a heat gun to smooth the edges of the phantom, removing bubbles and imperfections, as well as reinforcing the side seals.
  3. Addition of pseudo-skin (optional)
    NOTE: An augmentation to the fascial-plane models is the addition of pseudo-skin.
    1. Pour liquid ballistics gel over a sealed and cooled model, which has been covered loosely in ultrasound gel to prevent annealing between the newly poured skin layer and the existing model (Figure 4D and Supplemental Video S1).

Soap-making process; experiments for soap molding and shaping using molds and cutters in sequence.
Figure 4: Assembly of median nerve block ultrasound phantom. (A) Individual components of a disassembled median nerve block phantom, including ballistic gel tissue layers, 3D-printed radius and ulna, a yarn median nerve submerged in ultrasound gel, a bottle of ultrasound gel, and a pan filled with liquid ballistic gel. (B) Assembly of the median nerve block phantom, including insertion of tissue layers and simulated bones covered in ultrasound gel. (C) Sealing one end of the phantom by dipping into a pan of liquid ballistic gel. (D) Creating a layer of pseudo-skin by pouring liquid ballistic gel over a completed median nerve block phantom. Please click here to view a larger version of this figure.

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Results

Four ultrasound phantoms were successfully designed and built using the methods described above. An ultrasound cross-section of each model aligned with an ultrasound of equivalent human anatomy is shown in Figure 5. Under ultrasound, these phantoms provide realistic tissue planes, which represent the borders between different layers of skin, muscle, and fascia. The muscle tissue is appropriately and homogeneously echogenic. This echogenicity can be adjusted based on the amount of flour added...

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Discussion

These custom ballistic gel-based phantoms provide trainees with medium-fidelity median, femoral, suprainguinal fascia iliaca plane, and serratus anterior plane nerve block training for a fraction of the cost of commercially available nerve block phantoms (Table 1). Our first median and femoral nerve block trainers were made in-house for 12% and 9% of the price of the cheapest commercially available median and femoral nerve block trainers, respectively. None of the available femoral nerve block phantoms a...

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Disclosures

The authors of this paper do not have any conflicts of interest to disclose.

Acknowledgements

This project was funded by the Simulation Training Center (STC) at the University of California, San Diego School of Medicine in La Jolla, CA. We would like to thank Blake Freechtle for his contributions to Figure 5.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
ABS Filament - 1.75 m+B+A2:A14Hatchbox (Pomona, CA)https://www.hatchbox3d.com/collections/abs-1-75mm
Adobe PhotoshopAdobe (San Jose, CA)https://www.adobe.com/products/photoshop.html
Amber Tone DyeHumimic Medical (Greenville, SC)852844007925Ballistic gel dye; https://humimic.com/product/amber-tone-dye/
Fusion 360Autodesk (San Franciso, CA)Computer-assisted design (CAD) software; https://www.autodesk.com/products/fusion-360/overview?term=1-YEAR&tab=subscription&plc=F360
Gelatin #1 - Medical Gel by the PoundHumimic Medical (Greenville, SC)852844007406Ballistic gel; https://humimic.com/product/gelatin-1-medical-gelatin-by-the-pound/
Gluten-Free All-Purpose FlourArrowhead Mills (Hereford, TX)Flour for echogenicity; https://arrowheadmills.com/products/gluten-free/organic-gluten-free-all-purpose-flour/
Microsoft PowerPointMicrosoft (Redmond, WA)https://www.microsoft.com/en-us/microsoft-365/powerpoint
Mold Star 16 FAST Pourable Silicone RubberSmooth-On (Macungie, PA)Pourable silicone rubber; https://www.smooth-on.com/products/mold-star-16-fast/
Peach Tone DyeHumimic Medical (Greenville, SC)852844007895Ballistic gel dye; https://humimic.com/product/peach-tone-dye/
PLA Filament - 1.75 mmHatchbox (Pomona, CA)https://www.hatchbox3d.com/collections/pla-1-75mm
Prusa Original i3 MK3S+ printerPrusa Research (Prague, Czech Republic)3D printer; https://www.prusa3d.com/category/original-prusa-i3-mk3s/
Prusa Slicer 2.6.1Prusa Research (Prague, Czech Republic)3D printer slicer software; https://www.prusa3d.com/page/prusaslicer_424/
Wool-Ease Thick & Quick YarnLion Brand Yarn (Lyndhurst, NJ)640-610BYarn for simulated nerves; https://www.lionbrand.com/products/wool-ease-thick-and-quick-yarn?variant=32420963516509

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