P波间期的心脏电流源重建及电活动磁成像

Current source reconstructing and magnetic imaging of cardiac electrical activity during P-wave

用超导量子干涉仪在人体胸腔表面测量的磁场数据重建心脏电流源及其成像,是一种无创获取心脏电活动信息的新技术.由于P波间期的心脏磁信号比R波峰值弱,信噪比较低,本文提出了一种可提高分布源空间谱估计强度对比度(IIC)的波束成形方法.该方法分两步:1)在空间滤波器的加权矩阵中引入导联场矩阵,使滤波器输出估计对磁场电流源及其分布比较敏感.通过求解逆问题,可以改进重建分布源强度的对比度;2)通过设置源强度阈值,提取每个时刻重建源中偶极矩强度极大的电流源,消除其他位置上相对弱的伪源,可提高P波间期电流源重建的精度.文中采用理论分析与仿真试验,比较了IIC与3种其他电流源重建方法的性能.结果表明,IIC的单源空间谱估计的强度对比度较高,电流源重建精度相对较好.文中还分析了2个健康人的61通道心磁测量数据,以及他们P波间期的心脏电活动成像.与其他3种方法相比,IIC的电流源成像结果最优.能够显示健康人P波峰时刻心房的电活动较心室强.P波间期右心房除极时,心脏电活动具有方向特征.

The current source reconstruction and magnetic imaging is a new technique to non-invasively obtain spatial information regarding cardiac electrical activity using magnetocardiogram(MCG)signals measured by the superconducting quantum interference device(SQUID)on the human thorax surface.Using MCG signals to reconstruct distributed current sources needs to solve the inverse problem of magnetic field.The beamforming is a type of spatial filter method that has been used for distributed source reconstruction and source imaging in electroencephalogram(EEG)and magnetoencephalogram(MEG).In this paper,the dipole moment of distributed current source is estimated with corresponding each spatial filter based on the cardiac source field model.The purpose is to enhance the intensity contrast of the dipole moment of distributed current sources in distributed source spatial spectrum estimation with beamforming,so that the reconstructed-pseudo sources beyond the heart can be removed for imaging cardiac electric activity well.A new beamforming method of improving intensity contrast(IIC)of distributed source spatial spectrum estimation is developed for imaging cardiac electric activity in P-wave,due to cardiac magnetic signals in Pwave lower than that of the peak value of R-wave,which has a relatively low signal-to-noise ratio(SNR).For enhancing the accuracy of current source reconstruction in P-wave,the IIC divided into two steps:firstly,to introduce the lead-field matrix,which represents the measurement sensor-array sensitivity to magnetic field current sources,into a weight matrix of the spatial filter for making the output estimation of the filter more sensitive to the current source distribution,so as to improve the intensity contrast of the reconstructed distributed sources.Secondly,by setting a threshold of source intensity from experience,to extract the reconstructed source with locally-maximal dipole strength at each time for eliminating the relatively weak pseudo sources in other locations,so as to enhance the accuracy of current source reconstruction during P-wave.In this paper,the IIC and three other methods,including minimum variance beamforming(MVB),suppressing spatial filter output noise-power gain(SONG)and trust region reflective(TRR),are compared by using the theoretical analysis and simulation experiments of MCG current source reconstruction during P-wave.The results show that the IIC has higher intensity contrast of the single source spatial spectrum estimation,and possesses better accuracy of the current source reconstruction.The 61-channel MCG signals of two healthy subjects and their imaging of cardiac electrical activity during P-wave also are analyzed.The result shows that the IIC is better than the other three methods.It is indicated that two healthy subjects have stronger electrical activity in the atrium than that in the ventricle at Ppeak time,also that the electrical activity has the direction feature when the right-atrium is depolarized during P-wave.In summary,the IIC is useful for imaging the cardiac electrical activity.However,it is needed to carry out a further research on patients with local myocardial ischemia and left or right coronary artery stenosis,and to establish the evaluation index for imaging of cardiac electrical activity in such patients.