Medicine
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Development and Evaluation of 3D-Printed Cardiovascular Phantoms for Interventional Planning and Training
Chapters
Summary January 18th, 2021
Here we present development of a mock circulation setup for multimodal therapy evaluation, pre-interventional planning, and physician-training on cardiovascular anatomies. With the application of patient-specific tomographic scans, this setup is ideal for therapeutic approaches, training, and education in individualized medicine.
Transcript
The presented method allows for the creation of patient-specific anatomical cardiovascular models for in vitro testing, teaching, and planning of procedures. This method offers a standardized approach to creating 3D printable individualized anatomical models based on radiological data sets that can be easily included in flow loops or training setups. While this modeling approach is focused on the cardiovascular system, it can be transferred to other anatomical structures.
The quality of the radiological data set has a big influence on the difficulties encountered during modeling. For the first models, use a dataset with minimal movement artifacts and a high spatial resolution. To begin, define a range of Hounsfield unit values by opening the thresholding tool resulting in a combined mask of the contrast enhanced blood volume and bone structures.
Remove all bone parts that are undesirable in the final 3D model by using the Split Mask tool, which enables the marking and separation of multiple areas in overall slices based on Hounsfield values and location. Following this separation, ensure that a mask containing the contrast enhanced blood volume remains. This can be done by scrolling through the coronal and axial planes and matching the created mask with the underlying dataset.
From this mask, calculate a rendered 3D polygon surface model. Click the Local Smoothing tool to adjust the surface of the segmented model manually and locally. Focus on removing rough polygon shapes, single peaks, and rough edges created by previous trimming operations.
To allow the later connection of the model to a flow loop, include tubular parts with defined diameters adjusted to the available hose connectors and tube diameters. To place a datum plane parallel to the opening cross-section of the vessels, select the tool, Create Datum Plane, and use the preset 3 Point Plane. Next, click on three equally spaced points on the vessel's cross-section to create the plane.
Input an offset of 10 millimeters in the command window and confirm the operation. Select the draw sketch tool from the menu and choose the previously created datum plane as the location of the sketch. In the sketch, place a circle roughly on the center line of the vessel and set the radius constraint to match the outer diameter of the hose connector.
From the created sketch, use the Extrude tool to create a cylinder with a length of 10 millimeters. Orient the extrusion to move away from the vessel opening to create a distance between the cylinder and the vessel cross section of 10 millimeters. Then use the Loft tool to create a connection between the vessel ending and the geometrically defined cylinder.
Ensure a smooth transition between the two cross sections, thereby avoiding turbulence and low flow areas in the final 3D flow model. Finally, use the Hollow tool to make a hollow blood space in the command window, and put the required wall thickness and set the direction of the hollowing process to move outward. Confirm the selection to execute the hollowing process.
After uploading the printing file from the slicing software to the 3D printer, ensure that the amount of printing material and support material in the printer's cartridges is sufficient for the 3D model and start the print. Following the printing process, remove the support material from the finished model. First, remove the support material manually by gently squeezing the model.
Place the model on the sink, and then immerse it in water or a respective solvent after removing the cover. Dry the model in an incubator set to 40 degrees Celsius overnight. On the next day, embed the model in 1%agar.
Use a plastic box with at least two centimeter side margins around the model and drill holes into the walls to allow the tubes to be connected from the vessels to the pump and the reservoir. Add agar to water and bring it to a boil. After stirring the mixture, let it cool for five minutes and pour it into the box to create a bed of at least two centimeter height.
While the agar bed sets, connect the model to a non-compliant PVC tube using commercial hose connectors at every opening. Use zip ties to fix the connection between the hose connectors and the 3D model and ensure that there is no fluid leakage. Guide the PVC tubes through the drilled holes into the box, then place the model on top of the set agar bed.
To prevent agar leaking from these holes, use heatproof modeling clay to seal it. Next, fill the box with agar and cover the model by adding a two centimeter layer on top. Allow the agar to fully cool and set for an hour at room temperature.
Agitate the ventricle using a piston pump with a stroke volume of 120 to 150 milliliters. For CT imaging, place the entire flow loop within the CT scanner with the drive unit standing close by. Connect the contrast agent pump directly to the reservoir of the flow loop so the flooding of the model with contrast agent can be simulated during scanning.
This is especially useful for visualizing vascular pathologies. Perform CT as a dynamic scan over the whole model to visualize contrast agent inflow. Inject 100 milliliters of one to 10 diluted iodinated contrast agent into the model's reservoir at a speed of four milliliters per second.
Start the scan using bolus triggering in the leading tube with a 100 Hounsfield unit threshold in a four second delay. To perform sonography, put a small amount of ultrasonic gel on top of the agar block to reduce artifacts. Start the pump and use the ultrasonic head to locate the anatomical structure of interest.
Use 2D echo mode to evaluate leaflet movement as well as the opening and closing behavior of the valve. Use color Doppler to evaluate blood flow across the valve and spectral Doppler to quantify the flow velocity following the heart valve. Insert an access port into the PVC tube directly below the 3D model to allow for an easier access of the anatomy with a cardiac catheter or guide wire.
After starting the flow loop, check for leakage at the port entrance point. If necessary, use a two component adhesive to seal the opening. Place the 3D model on the patient table underneath the C arms of the x-ray machine.
Use x-ray imaging to guide the catheter and guide wires through the anatomic structure. For 4D MRI, use a 1.5 Tesla scanner and ensure that the acquisition protocol consists of a non-contrast enhanced MRA in the 4D flow sequence. Acquire an isotropic dataset with 25 phases in a slice thickness of 1.2 millimeters.
Set the velocity encoding at 100 centimeters per second. Perform the 4D flow image analysis with a commercially available software. First, import the 4D MRI dataset by selecting it from the flash drive, then perform semi-automated offset correction and correction of aliasing to improve image quality.
The center line of the vessel will be automatically traced, and the software extracts the 3D volume. Finally, perform quantitative analysis of flow parameters by clicking on the individual tabs in the analysis window. Flow visualization, path line visualization, and flow vector can be visualized without further input.
For quantification of pressure and wall shear stress in the representative tab, place two planes by clicking on the Add Plane button. Move the planes to the ROI by dragging them along the center line so that one plane is placed at the beginning of the ROI and one at the end. The pressure drop across the ROI and wall shear stress will be visualized and quantified in the diagram next to the 3D model.
The presented 3D printed models offer a wide range of possibilities in CT imaging. The printed material can be easily distinguished from the surrounding agar and possible metallic implants. Therefore, the use of a contrast agent is normally not required, except for generating dynamic imaging sequences.
When using ultrasonic imaging, it is possible to distinguish between the model's wall, the surrounding agar, and thin dynamic objects like heart valve leaflets. The agar layer on top of the model provides realistic haptic feedback during the scanning process. Flow analysis within the flow loop offers a wide range of possible applications and pre-interventional imaging.
4D MRI sequence enables visualization of fluid flow, turbulences, and wall shear stress within the 3D printed model, making it possible to analyze flow patterns following artificial heart valves. This workflow can be transferred to different interventional medical procedures for training or planning purposes. The technique allows for a closer in vitro examination of the flow behavior in large cardiovascular vessels and offers great potential for individualized therapy planning.
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