Finite-element-method (FEM) model generation of time-resolved 3D echocardiographic geometry data for mitral-valve volumetry

Abstract

Introduction: Mitral Valve (MV) 3D structural data can be easily obtained using standard transesophageal echocardiography (TEE) devices but quantitative pre- and intraoperative volume analysis of the MV is presently not feasible in the cardiac operation room (OR). Finite element method (FEM) modelling is necessary to carry out precise and individual volume analysis and in the future will form the basis for simulation of cardiac interventions.

Method: With the present retrospective pilot study we describe a method to transfer MV geometric data to 3D Slicer 2 software, an open-source medical visualization and analysis software package. A newly developed software program (ROIExtract) allowed selection of a region-of-interest (ROI) from the TEE data and data transformation for use in 3D Slicer. FEM models for quantitative volumetric studies were generated.

Results: ROI selection permitted the visualization and calculations required to create a sequence of volume rendered models of the MV allowing time-based visualization of regional deformation. Quantitation of tissue volume, especially important in myxomatous degeneration can be carried out. Rendered volumes are shown in 3D as well as in time-resolved 4D animations.

Conclusion: The visualization of the segmented MV may significantly enhance clinical interpretation. This method provides an infrastructure for the study of image guided assessment of clinical findings and surgical planning. For complete pre- and intraoperative 3D MV FEM analysis, three input elements are necessary: 1. time-gated, reality-based structural information, 2. continuous MV pressure and 3. instantaneous tissue elastance. The present process makes the first of these elements available. Volume defect analysis is essential to fully understand functional and geometrical dysfunction of but not limited to the valve. 3D Slicer was used for semi-automatic valve border detection and volume-rendering of clinical 3D echocardiographic data. FEM based models were also calculated.

Method: A Philips/HP Sonos 5500 ultrasound device stores volume data as time-resolved 4D volume data sets. Data sets for three subjects were used. Since 3D Slicer does not process time-resolved data sets, we employed a standard movie maker to animate the individual time-based models and visualizations. Calculation time and model size were minimized. Pressures were also easily available. We speculate that calculation of instantaneous elastance may be possible using instantaneous pressure values and tissue deformation data derived from the animated FEM.

Figures

Figure 1

Strategy to process the data sets. The processing diagram shows the workflow used in the study beginning with the echocardiographic data acquisition and ending with the geometry model generation with 3D Slicer 2 software package.

Figure 2

Section through left atrium and ventricle shown schematically. US indicates the position of the TEE transducer and the beam direction. AV is aortic valve, MV is mitral valve and Ap is apex.

Figure 3

Ultrasound data of after preprocessing. The ultrasound data are shown from patient 1. Shown is a standard visualization of one time step during the heart cycle with 3D Slicer 2 software. The three planes for each spatial direction are shown in the lower three frames. In the frame above one plane is shown together with the set of four fiducials. These four points are defined for all time steps.

Figure 4

Ultrasound data after applying the elliptical region-of-interest (ROI). Ultrasound data of the same patient as in Figure 2 after applying the elliptical region-of-interest (ROI) defined by the four fiducials (above frame). The three planes for each spatial direction are shown in the lower three frames.

Figure 5

3D FEM models at systole for the three data sets. (a) patient 1, (b) patient 2, (c) and (d) patient 3 at two different time steps in the cardiac cycle. Here a smoothing factor [13] of 20 and a decimation factor [13] of 20 were applied in order to obtain FEM models with a reasonable number of FEM triangles for further processing. 3D FEM model of the same data set as in Figures 3 and 4. The three planes (some with level sets) for each spatial direction show the cutting planes through the model. No smoothing or decimation was applied.

Figure 6

Models from a patient using different parameters and techniques. In this figure FEM models from patient 1 are shown in different manners. (a) is the smoothed and decimated model shown in Figure 5 (a). In addition the yellow line indicates the leaflets border. The red arrows indicate the prolapse of the mitral valve leaflet. (b) is a model where no ROI was applied so the full (and unnecessary) ultrasound cone is visible. (c) shows a model based on a very time consuming manual segmentation.

Figure 7

Rotating 3D model to get the best point of view. This figure illustrates the choice of the best point of view from a single model. (a) The 3D model in the viewer window can be moved and rotated in order to get the best view to the morphological findings. (b) shows the back view.