Noninvasive mapping of transmural potentials during activation in Swine hearts from body surface electrocardiograms

The three-dimensional cardiac electrical imaging (3DCEI) technique was previously developed to estimate the initiation site(s) of cardiac activation and activation sequence from the noninvasively measured body surface potential maps (BSPMs). The aim of this study was to develop and evaluate the capability of 3DCEI in mapping the transmural distribution of extracellular potentials and localizing initiation sites of ventricular activation in an in vivo animal model. A control swine model (n = 10) was employed in this study. The heart-torso volume conductor model and the excitable heart model were constructed based on each animal's preoperative MR images and a priori known physiological knowledge. Body surface potential mapping and intracavitary noncontact mapping (NCM) were simultaneously conducted during acute ventricular pacing. The 3DCEI analysis was then applied on the recorded BSPMs. The estimated initiation sites were compared to the precise pacing sites; as a subset of the mapped transmural potentials by 3DCEI, the electrograms on the left ventricular endocardium were compared to the corresponding output of the NCM system. Over the 16 LV and 48 RV pacing studies, the averaged localization error was 6.1±2.3 mm, and the averaged correlation coefficient between the estimated endocardial electrograms by 3DCEI and from the NCM system was 0.62±0.09. The results demonstrate that the 3DCEI approach can well localize the sites of initiation of ectopic beats and can obtain physiologically reasonable transmural potentials in an in vivo setting during focal ectopic beats. This study suggests the feasibility of tomographic mapping of 3D ventricular electrograms from the body surface recordings.     

Three-dimensional imaging of ventricular activation and electrograms from intracavitary recordings

Three-dimensional (3-D) mapping of the ventricular activation is of importance to better understand the mechanisms and facilitate management of ventricular arrhythmias. The goal of this study was to develop and evaluate a 3-D cardiac electrical imaging (3DCEI) approach for imaging myocardial electrical activation from the intracavitary electrograms (EGs) and heart-torso geometry information over the 3-D volume of the heart. The 3DCEI was evaluated in a swine model undergoing intracavitary noncontact mapping (NCM). Each animal's preoperative MRI data were acquired to construct the heart-torso model. NCM was performed with the Ensite 3000 system during acute ventricular pacing. Subsequent 3DCEI analyses were performed on the measured intracavitary EGs. The estimated initial sites (ISs) were compared to the precise pacing locations, and the estimated activation sequences (ASs) and EGs were compared to those recorded by the NCM system over the endocardial surface. In total, six ventricular sites from two pigs were paced. The averaged localization error of IS was 6.7 ± 2.6 mm. The endocardial ASs and EGs as a subset of the estimated 3-D solutions were consistent with those reconstructed from the NCM system. The present results demonstrate that the intracavitary-recording-based 3DCEI approach can well localize the sites of initiation and can obtain physiologically reasonable ASs as well as EGs in an in vivo setting under control/paced conditions. This study suggests the feasibility of tomographic imaging of 3-D ventricular activation and 3-D EGs from intracavitary recordings.     

Three-dimensional cardiac electrical imaging from intracavity recordings

A novel approach is proposed to image 3-D cardiac electrical activity from intracavity electrical recordings with the aid of a catheter. The feasibility and performance were evaluated by computer simulation studies, where a 3-D cellular-automaton heart model and a finite-element thorax volume conductor model were utilized. The finite-element method (FEM) was used to simulate the intracavity recordings induced by a single-site and dual-site pacing protocol. The 3-D ventricular activation sequences as well as the locations of the initial activation sites were inversely estimated by minimizing the dissimilarity between the intracavity potential "measurements" and the model-generated intracavity potentials. Under single-site pacing, the relative error (RE) between the true and estimated activation sequences was 0.03 +/- 0.01 and the localization error (LE) (of the initiation site) was 1.88 +/- 0.92 mm, as averaged over 12 pacing trials when considering 25 microV additive measurement noise using 64 catheter electrodes. Under dual-site pacing, the RE was 0.04 +/- 0.01 over 12 pacing trials and the LE over 24 initial pacing sites was 2.28 +/- 1.15 mm, when considering 25 microV additive measurement noise using 64 catheter electrodes. The proposed 3-D cardiac electrical imaging approach using intracavity electrical recordings was also tested under various simulated conditions and robust inverse solutions obtained. The present promising simulation results suggest the feasibility of obtaining 3-D information of cardiac electrical activity from intracavity recordings. The application of this inverse method has the potential of enhancing electrocardiographic mapping by catheters in electrophysiology laboratories, aiding cardiac resynchronization therapy, and other clinical applications.     

Constrained registration for motion compensation in atrial fibrillation ablation procedures

Fluoroscopic overlay images rendered from preoperative volumetric data can provide additional anatomical details to guide physicians during catheter ablation procedures for treatment of atrial fibrillation (AFib). As these overlay images are often compromised by cardiac and respiratory motion, motion compensation methods are needed to keep the overlay images in sync with the fluoroscopic images. So far, these approaches have either required simultaneous biplane imaging for 3-D motion compensation, or in case of monoplane X-ray imaging, provided only a limited 2-D functionality. To overcome the downsides of the previously suggested methods, we propose an approach that facilitates a full 3-D motion compensation even if only monoplane X-ray images are available. To this end, we use a training phase that employs a biplane sequence to establish a patient specific motion model. Afterwards, a constrained model-based 2-D/3-D registration method is used to track a circumferential mapping catheter. This device is commonly used for AFib catheter ablation procedures. Based on the experiments on real patient data, we found that our constrained monoplane 2-D/3-D registration outperformed the unconstrained counterpart and yielded an average 2-D tracking error of 0.6 mm and an average 3-D tracking error of 1.6 mm. The unconstrained 2-D/3-D registration technique yielded a similar 2-D performance, but the 3-D tracking error increased to 3.2 mm mostly due to wrongly estimated 3-D motion components in X-ray view direction. Compared to the conventional 2-D monoplane method, the proposed method provides a more seamless workflow by removing the need for catheter model re-initialization otherwise required when the C-arm view orientation changes. In addition, the proposed method can be straightforwardly combined with the previously introduced biplane motion compensation technique to obtain a good trade-off between accuracy and radiation dose reduction.     

A new method for incorporating weighted temporal and spatial smoothing in the inverse problem of electrocardiography

In this paper, we present a method for incorporating temporal smoothing (TS) into the estimate of epicardial potentials from body surface potential data. Our algorithm employs a different spatial smoothing parameter, chosen by the composite residual error and smoothing operator criteria, at each time step in the sequence. The total spatial smoothing term is then simply partitioned between temporal and spatial smoothing. The algorithm appears to be quite robust with regard to this partitioning. The new method was evaluated in the setting of additive Gaussian noise, but otherwise realistic conditions of body geometry and reference epicardial potentials. In examining the match between estimated and measured electrograms, or the match between estimated isopotential maps and measured isopotential maps, the estimates constructed using the new TS algorithm produced consistently smaller relative errors than those constructed using a quasi-static (QS) algorithm or those constructed by postprocessing the QS estimate with a moving average filter.     

Demonstration of “discontinuities” in the timederivatives of body surface potentials, and their prospective role innoninvasive imaging of the ventricular surface activation map

The ill-posed nature of the inverse electrocardiography problem has necessitated use of regularization techniques as a means of rendering epicardial potential maps. Significant inaccuracies are introduced by such methods (on the order of 50% error relative to the actual map), due to imposition of the regularizing functionals. It has been previously shown that the generator of topological changes in epicardial potential maps during ventricular activation is of much lower order than the epicardial maps themselves, yet sufficiently constrains the full ventricular surface activation mapping problem so as to obviate the need for further regularization. The mathematical formalism for reconstructing this generator has previously been presented under the assumption of negligible depolarization wavefront thickness, and requires identification and processing of “step discontinuities” predicted to occur in body surface potential derivatives. Using principal component analysis and spatial averaging methods, we have examined this fundamental prediction of the formalism in the context of a nonzero depolarization wavefront thickness (leading to prediction of dominant body surface potential derivative deflections with rise times comparable to those of ventricular electrogram intrinsic deflections). Studies of the body surface potential data obtained in three normal subjects supports the predictions of the theory, thereby suggesting the possible usefulness of the approach as an alternative to traditional regularization methods     

Noninvasive localization of the site of origin of paced cardiac activation in human by means of a 3-D heart model

A recently developed heart-model-based localization approach is experimentally evaluated in noninvasively localizing the site of origin of cardiac activation in a patient with a pacemaker. The heart-torso model of the patient was constructed from the contrast ultrafast computed tomography images. The site of initial paced activation in the patient was quantitatively localized and compared with the tip position of the pacemaker lead. The localization error of the inverse estimation was found to be 5.2 mm with respect to the true lead tip position. The promising result of this pilot experimental study suggests the feasibility of localizing the site of origin of cardiac activation in an experimental setting. The heart-model-based localization approach may become an alternative tool in localizing the site of origin of cardiac activation.     

Noninvasive reconstruction of three-dimensional ventricular activation sequence from the inverse solution of distributed equivalent current density

We propose a new electrocardiographic (ECG) inverse approach for imaging the three-dimensional (3-D) ventricular activation sequence based on the modeling and estimation of the equivalent current density throughout the entire volume of the ventricular myocardium. The spatio-temporal coherence of the ventricular excitation process has been utilized to derive the activation time from the estimated time course of the equivalent current density. In the present study, we explored four different linear inverse algorithms (the minimum norm and weighted minimum norm estimates in combination with two regularization schemes: the instant-by-instant regularization and the isotropy method) to estimate the current density at each time instant during the ventricular depolarization. The activation time at any given location within the ventricular myocardium was determined as the time point with the occurrence of the maximum local current density estimate. Computer simulations were performed to evaluate this approach using single- and dual-site pacing protocols in a physiologically realistic cellular automaton heart model. The performance and stability of the proposed approach was evaluated with respect to the various levels of measurement noise (0, 5, 10, 20, 40, and 60 microV), the various numbers of ECG electrodes and the modeling errors on the torso geometry and heart position. The simulation results demonstrate that: 1) the single-site paced 3-D activation sequence can be well reconstructed from 200-channel body surface potential maps with additive Gaussian white noise of 20 microV (correlation coefficient = 0.90, relative error = 0.19, and localization error = 5.49 mm); 2) a higher imaging accuracy can be obtained when the activation is initiated from the left/right ventricle (LV/RV) compared to from the septum; 3) the isotropy method gives rise to a better performance than the conventional instant-by-instant regularization; 4) a decreased imaging accuracy results from a larger noise level, a fewer number of electrodes, or the volume conductor modeling errors; however, a reasonable imaging accuracy can still be obtained with a 60 microV noise level, 64 electrodes, or mild errors on both the torso geometry and heart position, respectively; 5) the dual-site paced 3-D activation sequence can be imaged when the two sites are paced either simultaneously or with a time delay of 20 ms; 6) two pacing sites can be resolved and localized in the imaged 3-D activation sequence when they are located at the contralateral sides of ventricles or at the ventricular lateral wall and the apex, respectively.     

Wavefront-based models for inverse electrocardiography

We introduce two wavefront-based methods for the inverse problem of electrocardiography, which we term wavefront-based curve reconstruction (WBCR) and wavefront-based potential reconstruction (WBPR). In the WBCR approach, the epicardial activation wavefront is modeled as a curve evolving on the heart surface, with the evolution governed by factors derived phenomenologically from prior measured data. The body surface potential/wavefront relationship is modeled via an intermediate mapping of wavefront to epicardial potentials, again derived phenomenologically. In the WBPR approach, we iteratively construct an estimate of epicardial potentials from an estimated wavefront curve according to a simplified model and use it as an initial solution in a Tikhonov regularization scheme. Initial simulation results using measured canine epicardial data show considerable improvement in reconstructing activation wavefronts and epicardial potentials with respect to standard Tikhonov solutions. In particular the WBCR method accurately finds the anisotropic propagation early after epicardial pacing, and the WBPR method finds the wavefront (regions of sharp gradient of the potential) both accurately and with minimal smoothing.     

A method for determining high-resolution activation time delays inunipolar cardiac mapping

Presents a method for determining activation time delays in unipolar cardiac mapping data to resolutions considerably smaller than the sample interval. The method involves taking two filtered, differentiated electrograms and computing the Hilbert transform of their cross correlation, which exhibits a negative-to-positive zero crossing at the delay time between the signals. Simultaneous endocardial/epicardial recordings of sinus rhythm were made in the swine right atrium using identical, precisely superpositioned electrode arrays. Data were amplified, lowpass filtered, and digitized at 1000 Hz. A window of data was chosen around each electrogram in an endocardial/epicardial electrogram pair. The windowed electrograms were differentiated and highpass filtered, and the Hilbert transform of the cross correlation between the electrograms was computed. The activation time delay was taken to be the first negative-to-positive zero crossing. Average activation time delays (±SD) were computed for 4-s sinus rhythm recordings from each endocardial/epicardial electrode pair. For a representative site, the average transmural activation time delay was 0.71±0.06 ms (n=10 electrograms). Time delays estimated using the Hilbert transform method were compared with time delays estimated using the maximum negative slope criterion. The Hilbert transform results exhibited much smaller standard deviations, indicating that the Hilbert transform method may produce more accurate time delay estimates than the maximum negative slope method     

Application of sonomicrometry and multidimensional scaling tocardiac catheter tracking

This paper describes a technique for tracking the three-dimensional (3-D) position of a cardiac catheter using sonomicrometry and the mathematical method of multidimensional scaling (MDS). Sonomicrometry is used to measure the distances between ultrasonic transceivers. MDS is then used to calculate the 3-D coordinates of the ultrasonic transceiver locations, including the catheter tip, from the measured distances. Feasibility of catheter tracking was initially studied using simulated data from a geometric model in which the actual coordinates of all transceivers were known. The method was then shown to be feasible in vivo by tracking a catheter-mounted piezoelectric transducer using seven reference crystals sewn to the epicardial surface of a sheep heart. Simulation results indicate that a catheter can be tracked with a root-mean-square (rms) error of 1.51±0.05 mm and an average-distance error of e=1.06±0.27 mm using 12 reference points. In vivo results showed acceptable stress values (G<0.05) for 95% of the data samples with an average-distance error of e=0.52±0.66 mm. These simulation and experimental results show that sonomicrometry and MDS can be used to accurately localize the 3-D position and track the motion of a catheter tip within the heart     

Three-dimensional surface reconstruction and panoramic optical mapping of large hearts

Optical mapping of electrical activity from the surface of the heart is a powerful tool for studying complex arrhythmias. However, a limitation of traditional optical mapping is that the mapped region is restricted to the field of view of the sensor, which makes it difficult to track electrical waves as they drift in and out of view. To address this, we developed an optical system that panoramically maps epicardial electrical activity in three dimensions. The system was engineered to accomodate hearts comparable in size to human hearts. It is comprised of a surface scanner that measures epicardial geometry and a panoramic fluorescence imaging system that records electrical activity. Custom software texture maps the electrical data onto a reconstructed epicardial surface. The result is a high resolution, spatially contiguous, mapping dataset. In addition, the three-dimensional positions of the recording sites are known, making it possible to accurately measure parameters that require geometric information, such as propagation velocity. In this paper, we describe the system and demonstrate it by mapping a swine heart.     

An augmented reality system for patient-specific guidance of cardiac catheter ablation procedures

We present a system to assist in the treatment of cardiac arrhythmias by catheter ablation. A patient-specific three-dimensional (3-D) anatomical model, constructed from magnetic resonance images, is merged with fluoroscopic images in an augmented reality environment that enables the transfer of electrocardiography (ECG) measurements and cardiac activation times onto the model. Accurate mapping is realized through the combination of: a new calibration technique, adapted to catheter guided treatments; a visual matching registration technique, allowing the electrophysiologist to align the model with contrast-enhanced images; and the use of virtual catheters, which enable the annotation of multiple ECG measurements on the model. These annotations can be visualized by color coding on the patient model. We provide an accuracy analysis of each of these components independently. Based on simulation and experiments, we determined a segmentation error of 0.6 mm, a calibration error in the order of 1 mm and a target registration error of 1.04 +/- 0.45 mm. The system provides a 3-D visualization of the cardiac activation pattern which may facilitate and improve diagnosis and treatment of the arrhytmia. Because of its low cost and similar advantages we believe our approach can compete with existing commercial solutions, which rely on dedicated hardware and costly catheters. We provide qualitative results of the first clinical use of the system in 11 ablation procedures.     

Statistical accuracy of a moving equivalent dipole method to identify sites of origin of cardiac electrical activation

While radio frequency (RF) catheter ablation (RCA) procedures for treating ventricular arrhythmias have evolved significantly over the past several years, the use of RCA has been limited to treating slow ventricular tachycardias (VTs). In this paper, we present preliminary results from computer and animal studies to evaluate the accuracy of an algorithm that uses the single equivalent moving dipole (SEMD) model in an infinite homogeneous volume conductor to guide the RF catheter to the site of origin of the arrhythmia. Our method involves measuring body surface electrocardiographic (ECG) signals generated by arrhythmic activity and by bipolar current pulses emanating from a catheter tip, and representing each of them by a SEMD model source at each instant of the cardiac cycle, thus enabling rapid repositioning of the catheter tip requiring only a few cycles of the arrhythmia. We found that the SEMD model accurately reproduced body surface ECG signals with a correlation coefficients > 0.95. We used a variety of methods to estimate the uncertainty of the SEMD parameters due to measurement noise and found that at the time when the arrhythmia is mostly localized during the cardiac cycle, the estimates of the uncertainty of the spatial SEMD parameters (from ECG signals) are between 1 and 3 mm. We used pacing data from spatially separated epicardial sites in a swine model as surrogates for focal ventricular arrhythmic sources and found that the spatial SEMD estimates of the two pacing sites agreed with both their physical separation and orientation with respect to each other. In conclusion, our algorithm to estimate the SEMD parameters from body surface ECG can potentially be a useful method for rapidly positioning the catheter tip to the arrhythmic focus during an RCA procedure.     

双激光雷达的水平风场估计方法

针对民航机场区域内单部激光雷达探测水平风场存在较大误差的问题,提出一种基于支持向量回归的双激光雷达水平风场估计模型。该模型以两部激光雷达重叠扫描区域风速为基础,对非重叠区雷达径向的其他数据点水平风速进行估计。首先,提取重叠区的径向风速、水平风速和距离三个特征,以重叠区域数据点为训练集,在同一维度规范化后设定惩罚因子和核函数参数,用支持向量回归得到初始估计值。然后,以单部激光雷达的径向风速为先验条件,估计出非重叠区相邻径向点水平风速。将估计的结果扩展为新的训练集,依次逐步扩大训练集进而估计出非重叠区的水平风速。最后,通过实测数据分析了该方法逐步估计的误差,分析了风速大小和回波信噪比对该方法估计性能的影响,结果表明该方法估计的风场的均方根误差较单部雷达的均方根误差更小,减小了水平风速误差,扩大了双激光雷达探测水平风场范围,提高了雷达的利用率。     

The feasibility of endocardial propagation mapping using magnetic resonance guidance in a Swine model, and comparison with standard electroanatomic mapping

The introduction of electroanatomic mapping (EAM) has improved the understanding of the substrate of ventricular tachycardia. EAM systems are used to delineate scar regions responsible for the arrhythmia by creating voltage or activation time maps. Previous studies have identified the benefits of creating MR-guided voltage maps; however, in some cases voltage maps may not identify regions of slow propagation that can cause the reentrant tachycardia. In this study, we obtained local activation time maps and analyzed propagation properties by performing MR-guided mapping of the porcine left ventricle while pacing from the right ventricle. Anatomical and myocardial late gadolinium enhancement images were used for catheter navigation and identification of scar regions. Our MR-guided mapping procedure showed qualitative correspondence to conventional clinical EAM systems in healthy pigs and demonstrated altered propagation in endocardial infarct models.     

Model-based imaging of cardiac electrical excitation in humans

Activation time (AT) imaging from electrocardiographic (ECG) mapping data has been developing for several years. By coupling ECG mapping and three-dimensional (3-D) + time anatomical data, the electrical excitation sequence can be imaged completely noninvasively in the human heart. In this paper, a bidomain theory-based surface heart model AT imaging approach was applied to single-beat data of atrial and ventricular depolarization in two patients with structurally normal hearts. In both patients, the AT map was reconstructed from sinus and paced rhythm data. Pacing sites were the apex of the right ventricle and the coronary sinus (CS) ostium. For CS pacing, the reconstructed AT pattern on the endocardium of the right atrium was compared with the CARTO map in both patients. The localization errors of the origins of the initial endocardial breakthroughs were determined to be 6 and 12 mm. The sites of early activation and the areas with late activation were estimated with sufficient accuracy. The reconstructed sinus rhythm sequence was in good qualitative agreement with the pattern previously published for the isolated Langendorff-perfused human heart.     

Mapping of Cardiac Electrophysiology Onto a Dynamic Patient-Specific Heart Model

Electrophysiological cardiac data mapping is an essential tool for the study of cardiac rhythm disorders, such as atrial fibrillation. Over the past decade, various advanced cardiac mapping systems have been developed to create detailed cardiac maps and assist physicians in diagnosis and therapy guidance. While these systems have increased the ability to study and treat cardiac arrhythmias, inherent limitations exist. The objective of this paper is to describe and evaluate a system that extends current approaches to cardiac mapping, to create a dynamic cardiac map, using patient-specific cardiac models. This paper details novel approaches to collecting a stream of electrophysiological cardiac data, registering the data with patient-specific dynamic cardiac models, and displaying the data directly on the dynamic model surface, giving a more accurate and comprehensive visualization environment when compared to current systems. To validate the system, a series of laboratory and in vivo experiments were conducted. In the laboratory studies, the system was used to test the user's ability to accurately locate a landmark in physical space, as well as their ability to accurately navigate to a virtual location. In the in vivo studies the overall system performance was compared to an existing electrophysiological recording system, where right atrial cardiac maps were created during sinus and paced cardiac rhythms. The results showed that the new dynamic cardiac mapping system was able to maintain high accuracy in locating physical and virtual landmarks, while being able to create a dynamic cardiac map displayed on a dynamic cardiac surface model.      

An improved method for the estimation and visualization of velocity fields from gastric high-resolution electrical mapping

High-resolution (HR) electrical mapping is an important clinical research tool for understanding normal and abnormal gastric electrophysiology. Analyzing velocities of gastric electrical activity in a reliable and accurate manner can provide additional valuable information for quantitatively and qualitatively comparing features across and within subjects, particularly during gastric dysrhythmias. In this study, we compared three methods of estimating velocities from HR recordings to determine which method was the most reliable for use with gastric HR electrical mapping. The three methods were 1) simple finite difference (FD) 2) smoothed finite difference (FDSM), and 3) a polynomial-based method. With synthetic data, the accuracy of the simple FD method resulted in velocity errors almost twice that of the FDSM and the polynomial-based method, in the presence of activation time error up to 0.5 s. With three synthetic cases under various noise types and levels, the FDSM resulted in average speed error of 3.2% and an average angle error of 2.0° and the polynomial-based method had an average speed error of 3.3% and an average angle error of 1.7°. With experimental gastric slow wave recordings performed in pigs, the three methods estimated similar velocities (6.3-7.3 mm/s), but the FDSM method had a lower standard deviation in its velocity estimate than the simple FD and the polynomial-based method, leading it to be the method of choice for velocity estimation in gastric slow wave propagation. An improved method for visualizing velocity fields is also presented.     

Reduced-rank channel estimation for time-slotted mobile communication systems

In time-slotted mobile communication systems with antenna array at the receiver, the space-time channel matrix is conventionally estimated by transmitting pilot symbols within each data packet (or block). This work is focused on reduced rank (RR) estimation methods that exploit the low-rank property of the space-time channel matrix to estimate single or multiple user channels from the observation of single or multiple training blocks. The proposed RR methods allow to improve the estimate accuracy by reducing the set of unknown parameters (rank reduction) and extending the training set (multiblock processing). The maximum likelihood RR estimate is obtained as the projection of the prewhitened full-rank (FR) estimate onto the spatial or temporal signal subspace. The paper shows that, even for time varying channels, these subspaces can be considered to be slowly varying, and therefore, they can be estimated with increased accuracy by properly exploiting training signals from several blocks. The analytical and numerical performance in terms of mean square error for the RR estimate shows that the main advantage of the proposed method with respect to the conventional FR one can be ascribed to the reduced complexity of the channel parameterization.