Estimation of 3D left ventricular deformation from echocardiography

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Abstract

The quantitative estimation of regional cardiac deformation from 3D image sequences has important clinical implications for the assessment of viability in the heart wall. Such estimates have so far been obtained almost exclusively from Magnetic Resonance (MR) images, specifically MR tagging. In this paper we describe a methodology for estimating cardiac deformations from 3D echocardiography (3DE). The images are segmented interactively and then initial correspondence is established using a shape-tracking approach. A dense motion field is then estimated using a transversely isotropic linear elastic model, which accounts for the fiber directions in the left ventricle. The dense motion field is in turn used to calculate the deformation of the heart wall in terms of strain in cardiac specific directions. The strains obtained using this approach in open-chest dogs before and after coronary occlusion, show good agreement with previously published results in the literature. They also exhibit a high correlation with strains produced in the same animals using implanted sonomicrometers. This proposed method provides quantitative regional 3D estimates of heart deformation from ultrasound images.

Introduction

A fundamental goal of many efforts in the cardiac imaging and image analysis communities is to assess the regional function of the left ventricle (LV) of the heart. The general consensus is that the analysis of heart wall deformation provides quantitative estimates of the location and extent of ischemic myocardial injury. Regional left ventricular deformation can be determined using all of the principal imaging modalities, including contrast angiography, echocardiography, radionuclide imaging, cine computed tomography (CT), and magnetic resonance (MR) imaging. There have been considerable efforts within the medical image analysis community aimed at estimating this deformation from each of these imaging modalities. Much of the effort has been confined to analysis of two-dimensional images or projections of the heart. Although, recently significant effort has been directed at a more comprehensive analysis of left ventricular deformation in all three dimensions.

Left ventricular deformation can be assessed in three-dimensional space using ECG-gated single photon emission computed tomography (SPECT) Shen et al., 1999, Cwajg et al., 1999, Faber et al., 1999, Calnon et al., 1997, Berman and Germano, 1997, Germano et al., 1995, Cooke et al., 1994 or positron emission tomography (PET) Miller et al., 1994, Yamashita et al., 1989, Buvat et al., 1997. However, both of these radionuclide methods have a restricted ability to assess left ventricular deformation, secondary to the limited spatial and temporal resolution of these approaches. These radionuclide methods have involved both count-based Shen et al., 1999, Calnon et al., 1997, Cooke et al., 1994 and geometry-based approaches Cwajg et al., 1999, Faber et al., 1999, Germano et al., 1995.

Cine MR imaging has emerged as a more comprehensive approach to assess myocardial deformation in three-dimensional space (Moore et al., 2000). MR imaging offers improved spatial resolution. Unique to cine MR imaging is the ability to track deformation of myocardial tissue within the wall as well as on the endocardial and epicardial surfaces. However, the analysis of mid-wall myocardial deformation requires special cine MR imaging sequences, including MR tissue tagging Amini et al., 1998, Haber et al., 1998, Kerwin and Prince, 1998, Park et al., 1996, Prince and McVeigh, 1992, Young et al., 1995 and others, or MR phase contrast velocity imaging Pelc, 1991, Duncan et al., 1998, Meyer et al., 1996, Zhu et al., 1997. While these newer MR approaches offer a comprehensive analysis of regional left ventricular deformation, wide application of MR imaging remains limited by cost and the difficulty in routinely applying these MR approaches to critically ill cardiac patients.

Echocardiography offers significant advantages over both radionuclide imaging and MR imaging. Echocardiographic images can be acquired on critically ill patients in an emergency room or at the patient’s bedside in the intensive care unit, and this can be accomplished at a reduced cost. Comprehensive analysis of left ventricular deformation is now feasible using echocardiography, with the advent of newer three-dimensional acquisition systems (von Ramm and Smith, 1990). Recently, commercial software has become available to automatically assess global and regional left ventricular function (Lang et al., 1996). However, these newer automated echocardiographic approaches have not been fully validated. Hence, development of automated analysis of echocardiographic images is attracting an increasing amount of attention in the literature Chuang et al., 1999a, Chuang et al., 1999b, Chuang et al., 2000, Sheehan et al., 1998, Angelini, 1999, Brandt et al., 1999, Montagnat et al., 1999, Jacob et al., 1999. However, none of these methods is capable of estimating dense maps of three-dimensional deformation from echocardiographic images comparable to those obtained from the analysis of MR tagging images.

In this paper we describe, test and present prelinary validation for an approach to estimate the regional three-dimensional deformation of the left ventricle using echocardiography. We use a biomechanical model to describe the myocardium and shape-based tracking displacement estimates on the endocardial and epicardial walls to generate the initial displacement estimates. These are integrated in a Bayesian estimation framework and the overall problem is solved using the finite element method. This method produces quantitative regional 3D cardiac deformation estimates from ultrasound images which up to now was thought to be only possible using magnetic resonance and especially MR tagging. We validate these estimates by comparing them to invasive measurements performed simultaneously using implanted sonomicrometers. The fast improving quality of ultrasound images with the introduction of harmonic imaging (Caidahl et al., 1999) and contrast agents (Porter et al., 1994) should make it possible to obtain even more accurate estimates of 3D left ventricular deformation in the future.

Section snippets

Our approach

We estimate a dense displacement field within a Bayesian estimation framework which consists of a data term and a model term. These are described in Sections 2.1 and 2.2, respectively. The data term captures the image-derived information about the problem. We segment the images interactively and then proceed to extract initial displacement estimates using a shape-tracking approach. We then model the noise in these estimates using a Gaussian noise model. The model term captures our prior beliefs

Animal experiments

To evaluate the efficacy of using image-derived in vivo deformation estimates to measure regional LV function we conducted experiments on fasting, anesthetized, open chest, adult mongrel dogs with approval of the Yale University Animal Care and Use Committee. In this preliminary work, we report results from four animals. The 3DE images were obtained either before (D1 and D2) or after occlusion of the left anterior descending coronary artery (D3 and D4), using the procedure described in Section

Results

The potential of our methodology is illustrated in Fig. 9, which shows a cut through our tracked 3D mesh overlaid on a slice through the original 3DE image data over time. This could be seen as a form of software-derived, 3DE-based ‘tissue tagging’ somewhat in the sense of MR tagging. Note the spreading grid lines near the endocardium on the right as the LV thickens from end diastole to end systole.

The quantitative results are summarized in Table 1. Function in the risk area, which was

Validation using implanted sonomicrometers

In an effort to obtain an independent source of in vivo strain values for validation of image-derived strains, we have developed an independent approach for strain measurement using cubic arrays of sonomicrometers implanted in the canine LV myocardium (see details in (Dione et al., 1997)). The efficacy of this technique was illustrated by additional work (Meoli et al., 1998) that showed that the distances obtained with sonomicrometers compared favorably (r=0.992) with those obtained using the

Conclusions

In this work we have demonstrated that estimates of 3D cardiac deformation can be obtained from ultrasound images. These estimates are generally consistent with values reported in the literature. Further, we validate such estimates of regional deformation directly by comparing them to strains measured concurrently from implanted sonomicrometers. While many problems remain to be solved, such as improving and speeding up the segmentation process, we are confident that this approach has the

Movies

There are three movies included with this paper (available as electronic annexes via www.elsevier.com/locate/media). The first movie, papad1.mov, corresponds to Fig. 9. The second and third movies, papad2.mov and papad3.mov, correspond to the top and bottom parts of Fig. 10, respectively.

Supplementary data

Acknowledgements

The first author would also like to thank professors Turan Onat and Gary Povirk from the Department of Mechanical Engineering at Yale University for many useful discussions. Additional thanks to Farah Janzad, David Meoli, Jason Soares and Jennifer Hu for their help with segmenting the images, processing the sonomicrometer data, tissue processing and surgical preparation respectively. We would like to acknowledge support from the National Institutes of Health under grant NIH-NHLBI RO1-HL44803

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