NONINVASIVE THREE-DIMENSIONAL MYOCARDIAL ACTIVATION TIME IMAGING BY MEANS OF A HEART-EXCITATION-MODEL

B. He, G. Li
Department of Bioengineering, University of Illinois at Chicago, USA

Abstract: We have proposed a new method for imaging activation sequence within three-dimensional (3D) myocardium by means of a heart-excitation-model. The myocardial activation sequence is estimated from body surface ECG by minimizing multiple objective functions of the measured and the heart-model-generated BSPMs. Computer simulation studies have been conducted to evaluate the proposed 3D myocardial activation time imaging approach, using a pacing protocol. The present simulation results suggest the feasibility of noninvasive estimation of activation time throughout the ventricles from BSPMs, and that the proposed method may become an important alternative in imaging cardiac activation noninvasively.

INTRODUCTION

It is of importance for both basic cardiovascular research and clinical diagnosis and management of cardiac diseases to image noninvasively cardiac activation throughout the 3D myocardium. A number of efforts has been made to estimate equivalent cardiac electrical sources [1], which are associated with myocardial activation. Activation time has also been estimated over the heart surface [2]-[6] from the body surface potential maps (BSPMs).

In the present study, we propose a new approach, in which the activation time throughout the 3D myocardium can be estimated from body surface ECGs by means of a heart-excitation-model. Computer simulations have been conducted to test the performance of this new approach using a pacing protocol.  

METHODS

Principles of 3D Activation Time Imaging

In the proposed approach, information regarding activation time throughout the myocardium is obtained from parameters of a computer heart-excitation-model by means of a parameter optimization system. A 3D heart-excitation-model is constructed based on the knowledge of cardiac electrophysiology and geometric measurements via CT/MRI. The anisotropic nature of myocardium can be incorporated into this computer heart model and is implemented in the present computer simulation study. A preliminary classification system which approximately determines cardiac status based on a priori knowledge and the measured BSPMs is used to estimate the initial pattern of myocardial activation by means of an artificial neural network [7]. According to the output of the preliminary classification system, the parameters of the heart computer model are initialized, and the corresponding BSPM is calculated using the boundary element method [8]. Then a multi-objective nonlinear optimization procedure is employed to estimate the cardiac activation sequence. The activation sequence is estimated when the objective functions, that assess the dissimilarity between the measured and simulated BSPMs, are minimized. The activation sequence (i.e. activation time image) is then reconstructed from the heart model parameters.

Figure 1: Illustration of computer heart-torso modeling. (a) Heart model. (b) Anterior view of torso-heart model.

Simulation Protocols

A previously developed realistic geometry heart-torso model [9] (Fig. 1) has been recently modified by incorporating anisotropic propagation into the heart model for more accurate simulation of the body surface ECG and myocardial activation sequence. A pacing protocol was used to simulate initial cardiac activation. By setting pacing site in different myocardial regions of the heart model, sequential pace maps were obtained by solving the forward problem. For each pacing sequence, 200-lead (uniformly distributed over the torso surface) BSPMs were calculated within the whole cardiac excitation cycle using the boundary element method. Gaussian white noise of 10 mV was added to the BSPMs at each time instant after the onset of pacing, to simulate the noise-contaminated body surface potential measurements.

RESULTS

The performance of activation time imaging was initially tested by single-site pacing in selected sites in the ventricles. The 200-lead BSPMs at 10 time instants (from T₁ = 21 ms to T₂ = 48 ms after the onset of pacing, with time step of 3 ms) were used to inversely estimate the ventricular activation sequence following pacing. Fig. 2 shows a typical simulation example. The panel (A) shows the simulated “true” activation sequence, and the panel (B) shows the inversely estimated activation sequence. Each panel shows the activation sequence in 5 longitudinal sections ((b)~(f)) and 1 transverse section of ventricles ((a)). Five horizontal lines in the transverse section of ventricles from top to bottom respectively indicate the positions of the 5 longitudinal sections from (b) to (f). Fig. 2 indicates that the activation sequence can be well reconstructed by means of the proposed approach.

Figure 2: A typical example of activation time imaging results during single-site ventricular pacing. (A). “True” activation sequence. (B) Estimated activation sequence.

DISCUSSION

It has been a long term goal of estimating myocardial activation sequence from noninvasive body surface ECG measurements. Due to the technical difficulty associated with the inverse problem, attempts have been made in terms of equivalent physical source modeling, e.g. dipoles and epicardial potentials, to ensure a unique inverse solution, or image the activation time over the heart surface under the assumption of electrical isotropy within the myocardium.

In the present study, we have proposed a new approach for noninvasive 3D cardiac activation time imaging by means of an anisotropic-heart-excitation-model. Our approach is based on our observation that a priori information regarding cardiac electrophysiology should be incorporated into the cardiac inverse solutions in order to obtain useful information on the 3D cardiac activation from the two-dimensional electrical measurements over the body surface. In the present approach, the a priori information on cardiac electrophysiology is incorporated into the heart-excitation-model, which is not an equivalent physical source model but rather an equivalent physiological source model. By linking this physiological source model with body surface ECG measurements, we are able to directly estimate physiological parameters of interest from ECG recordings.

The present simulation results on single-site pacing suggest the feasibility of the present approach in imaging activation time for ventricular activation sequence.

In summary, we have proposed a new approach for noninvasive activation time imaging within the 3D myocardium by means of an anisotropic-heart-excitation-model. Our computer simulation studies have demonstrated the feasibility of the proposed approach to estimate activation time within 3D myocardium and enable the inclusion of myocardial anisotropy in the activation time imaging. The present promising results suggest this approach may become an important alternative for noninvasive imaging of cardiac activation for both basic cardiovascular research and clinical diagnosis and management of cardiac diseases.

ACKNOWLEDGEMENT

This work was supported in part by a grant from American Heart Association #0140132N and NSF CAREER Award BES-9875344.

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