Experimental Investigation of Secondary Flow Structures Downstream of a Model Type IV Stent Failure in a 180° Curved Artery Test Section

This video shows how to use Particle Image Velocimetry, and Coherent Structure Detection, to detect secondary flow or wortical structures in a curved artery model, to better understand the clinical relevance of Type IV stent failures. This method can help answer key questions in cardiovascular hemodynamics. Such as, how stents, which can deteriorate over time and ultimately fracture, can influence blood flow patterns and affect disease progression in vascular remodeling.

The main advantage of this technique is that it facilitates investigation of in-vivo and in-vitro measurements in complex physiological arterial flows involving stent and stent fractures. Demonstrating procedure will be Mohammad Najjari, a graduate student, and Jessica Hinke, an undergraduate student. Prepare a blood-analog solution comprised of 79%saturated sodium iodide solution, 20%glycerol, and 1%deionized water.

Start with saturated sodium iodide solution, then, add the glycerol in small increments using a syringe until the entire volume of glycerol has been added. Between additions, wait for the solution to become visibly homogenized, and take note of the volume of each glycerol addition. Next, add the required volume of water, continue stirring until the blood-analog solution is visibly homogenized.

Finally, add very small quantities of sodium thiosulfate, using a spatula until the solution is optically clear. To begin, create high resolution STL files from the straight and curved stent Cad models. In the software, select Export and Model from the file menu.

Choose the STL option, set the Chord Height to zero, and set the Angle Control to one. The angle control value regulates the amount of tessellation along the surface with small radii, and the setting can be adjusted between zero and one. Then, click OK to create the STL file.

Next, fabricate the stent models on a Rapid Prototyping machine. To do this, start the 3D printing software and click on Insert to locate the STL file on the 3D printer computer. Then, select the desired file and drive the mouse to place the STL file on a virtual platform on the screen.

Next, using the 3D-printed parts, install the stent into the curved artery test section to recreate an Idealized Type IV fracture scenario. Entailing a complete transverse fracture of stents and linear displacement of fragmented parts. Now, assemble the setup on an optical table.

Connect the straight acrylic pipes to the inlet and outlet of a 180 degree curved artery test section. To begin, grossly adjust the laser's position by illuminating a small piece of paper. Make the laser sheet approximately 2mm thick by adjusting the focus.

Next, target the laser sheet along the 90 degree portion of the measurement region with the sheet perpendicular to the table. Then, place a camera near the zero degree or 180 degree location to view the cross sectional region illuminated by the laser sheet. Incrementally adjust the alignment of the laser and camera so the camera views the circular cross section of the curved artery with minimal particle distortion, which is assessed using the Grab function in the software.

Once the camera and laser are in position, acquire images of the secondary flow fields. First, turn on the programmable pump. In the pump instrument control program, set the Amplitude to one volt.

Set the DC Offset to zero volts. Set the number of timed steps to 1000, and set the time period to four seconds. Additional settings are discussed in the Text Protocol.

Then, load the text file that has the values for the voltage time wave form, and run the program to supply the blood-analog fluid to the experiment. In the PIV Data Acquisition program, after clicking the New Recordings tab, select the device under the Settings section, and confirm that the laser is set to On with the appropriate power settings. Navigate to laser control to confirm the values.

Navigate to Timing and set it to External Cyclic Trigger. Click on Acquisit, select Table Scan, Edit Table Scan, Append Scan, and populate scan settings by entering start time representative of systolic deceleration. Time increment and end time of PIV data acquisition.

Reference Time, DT1, or the time between laser pulses should be adjusted. Then, click on Close. Under Acquisit, click on image acquisition and enter the number of images as 200.

Now, the PIV system is ready to acquire data. Select Start Recording to acquire phase-wise measurements, using the trigger signal from the pump instrument control. The set number of plane or velocity fields at each time instance, and at the prearranged 90 degree test section location will then be acquired.

When the recording is done, power down the laser. Turn off the pump, turn off the camera, and replace the lens cover. Using the supplemental code file, compute the Phase Averaged and RMS Secondary Flow Velocities, Vorticity, and Swirling Strength fields.

In the software, initialize setting pertaining to defined scale and send to Mask Definition. Review the Supplemental Code file for details. In the software, right click on any PIV data, and initiate Batch Processing.

Select the operation Vector Statistics, Vector Field Results from the group Statistics, and click on Parameter in the Dialog box. Toggle the average V and RMSV options under the Vector Field section. Then, select the operation Vorticity from the group Extract Scalar Fields Rotation and Shear.

This finds the two dimensional vorticity in the planar cross section. Now, start the processing by making a right click on any PIV data under the Project window, and selecting Hyperloop All Sets. To compute the Swirling Strength fields for the detection of Secondary Flow Structures, right click on any PIV data, select Velocity Vector Field in the tree, and initiate batch processing.

Then, select the operation Swirling Strength from the group Extract Scalar Fields Rotation and Shear, and click on Close. Now, right click on the same Velocity Vector Field, select Hyperloop All Sets, and adjust the parameters for Batch Processing as before. Finally, execute the computation of the Swirling Strength fields.

Secondary Flow Velocity data at the 90 degree cross-sectional location, was acquired from the 2C 2D PIV system. The inflow condition supplied to the curved artery test section with an Idealized Type IV stent fracture was the carotid artery wave form. The Secondary Flow Velocity field data was generated using the PIV technique, via synchronization of Trigger produced by the pump instrument control computer.

Post-processing of the data was applied to the pixelated data, to determine the Q-criterion and Swirling Strength or Landis ci-criterion during the systolic deceleration. The Swirling Strength and Q-criterion-based flow fields were compared. The wortical patterns marked with DLW characters, representative of deformed Dean-Lynn-and Wall-type DLW wortical patterns, and strain-dominated regions, were examined at four distinct cross-sectional locations during systolic deceleration.

The continuous Wavelet Transformed-based method was applied to the secondary flow vorticity field, that enabled vortex detection with greater detail in shape, size, and strength. Finally, the Q-criterion, Swirling Strength, and Wavelet Transformed-based flow fields were compared. The Wavelet Transformed-based method revealed the presence of complex wortical patterns with great detail in size, scale, and strengths.

Once mastered, this technique can be done in approximately eight hours, including PIV data acquisition and post-processing, after the model stents are printed on the 3D printer. While attempting this procedure, it's important to remember to make sure the blood-analog fluid refractive index is adequately matched using minute quantities of sodium thiosulfate. In future, to improve paratheory result accuracy, and minimize the optical distortion, we will be using a PIV calibration map like this.

Following this procedure, flows in patient-specific blood vessels can be evaluated in order to answer additional questions pertaining to pathological conditions, allowing unprecedented access to hemodynamics of prosthetics, stent implants, and fractured stents.