Intravascular Ultrasound Image-Based Finite Element Modeling Approach for Quantifying In Vivo Mechanical Properties of Human Coronary Artery

Quantifying the mechanical properties of coronary arteries is of ultimate importance in cardiovascular biomechanical research. However, coronary bodies typically work with ex vivo coronary tissues. This work introduce the finite element model-based updated approach to quantifying the coronary material properties in vivo based on specific intervascular ultrasound images.

Current mechanical approaches to quantify the mechanical properties of coronary vessel walls, such as planar vessel testing and intention testing, can only work with isolated coronary artery tissues ex vivo. This approaches is not suitable to monitor the coronary mechanical properties in patient-specific safety. The introduced finite element model-based updated approach could quantifying the material properties of coronary artery tissue in vivo based on patient-specific intervascular ultrasound images and blood pressure measurements.

To begin, access the folder containing saved virtual histology intravascular ultrasound, or VH IVUS, and Cine IVUS images obtained from coronary artery disease patients in DICOM format for offline analysis. Open the DICOM files using the viewer. Double click the corresponding sequence name to open the image.

Select export, then choose export images to save each Cine IVUS frame and VH IVUS frame as individual BMP images, ensuring each image is in a 500 to 500 pixel resolution. Next, examine each Cine IVUS image frame by frame to locate consecutive frames captured at the preselected plaque site during one cardiac cycle. Examine all the generated VH IVUS images to identify the VH IVUS image obtained at the given plaque site.

In the ImageJ software, for the Cine IVUS image, select straight, followed by the segmented line tab to delineate the contours of vessel boundaries and plaque component boundaries. Next, go to image, select overlay, and click add selection to overlay the traced contours onto the original image. Open the to ROI manager menu to manage contours.

In the properties tab, assign green, blue, and red colors to the lumen, outer boundary, and lipid contours respectively with a line width of three. Select each contour and smooth it by clicking edit, selection, and fit spline from the command bar. Next, sequentially select file, save as, XY coordinates tab to save the point coordinates of each contour, including the lumen, outer boundary, and plaque component, in a separate text file.

To reconstruct finite element mesh generation on VH IVUS and Cine IVUS image, create two auxiliary contours by linearly interpolating the lumen and outer boundary contours with weights of one third and two thirds respectively for each layer. Divide the vessel area into eight circumferential parts and three radial parts by connecting the lumen and outer boundary to the nearest point on the lipid contour or two auxiliary contours with radial lines. Next, connect all points between layers with straight lines to form a three-dimensional structure with three by eight volumes.

Divide each volume using hexahedral elements to generate the finite element mesh and different material groups. Next, using a modified anisotropic Mooney-Rivlin material model, define the material properties of the coronary vessel wall. Prescribe patient-specific blood pressure waveforms on the lumen surface to simulate physiological conditions.

To obtain patient-specific blood pressure waveforms, scale a typical aortic pressure waveform with systolic and diastolic pressure values measured by arm cuff. Write all the commands required to create the thin layer structure-only model into a batch file using MATLAB. Load the batch file using the advanced user interface to generate the model.

Generate the model by selecting data file solution, then solve the thin layer structure-only model and save it as a dat file. Export results of node coordinates to a TXT file by navigating list, value list, and zone, and selecting X position, Y position, and Z position in variables to list under coordinate. Click apply and export to export the coordinate results.

The in vivo material parameters for the coronary vessel based on the Mooney-Rivlin material model were determined within two hours, demonstrating the method's efficiency and suitability for clinical use. The material property results showed a stiffness factor of 1.125 and a stability factor of 98%with both circumferential and axial material curves displaying a J shape. Simulation results demonstrated that stress distribution under systolic pressure was higher than under diastolic pressure, with localized stress concentration seen in regions of thin vessel walls or fibrous caps.