To diagnose abnormalities in certain organ systems, one optimally would want to know the electrical state of each region in the organ system at each instant in time. Example organ systems about which one would wish to know the electrical state include the heart, the brain, and skeletal muscles.
Previous investigators have attempted to localize electrical activity in the brain. See, F. H. Duffy, et al., "Brain Electrical Activity Mapping (BEAM): A Method for Extending the Clinical Utility of EEG and Evoked Potential Data," Annals of Neurology, 5, p. 309, 1979 and G. Raviv et al., U.S. Pat. No. 4,649,482. Their approach involves displaying on a video screen surface potentials recorded from multiple electrodes placed on the surface of the head. Galambos et al., "Cortical Localization of Pure Tone Responses Using a Laplacian Electrode," Federation Proceedings, 12, 48, 1953, reported using a set of five electrodes to compute the Laplacian of the surface potential at a single location on the scalp to localize brain responses to auditory tones. They did not generate images. Similarly, Hjorth in "An On-Line Transformation of EEG Scalp Potentials into Orthogonal Source Derivations," Electroencephalography and Clinical Neurophysiology, 35, 526-530, 1975, attempted to measure the Laplacian of the scalp surface potentials in order to localize brain electrical activity using nineteen widely spaced unipolar electrodes. He did not generate images. R. Srebro in "Localization of Visually Evoked Cortical Activity in Humans," J. Physiology, 360, 233-246, 1985 also localized cortical activity in the brain using a set of five electrodes (comprising the "Laplacian electrode"), to compute the Laplacian of the surface potential at a single site. He moved this set of electrodes to different sites and measured brain activity in response to visual stimulii. Because of poor signal to noise characteristics he measured only an average response to stimuli repeated 128 times. He did not localize spontaneous brain electrical activity. Srebro constructed a fixed image based on recordings from multiple placements of the Laplacian electrode. The physical separation of the electrodes was 2.5 cm yielding an effective "Laplacian electrode" diameter of 5 cm thereby providing only crude localization.
F. Perrin et al., "Scalp Current Density Mapping: Value and Estimation from Potential Data," IEEE Transactions on Biomedical Engineering, 34, 283-288, 1987, conducted a theoretical analysis of "scalp current density" which is proportional to the two-dimensional Laplacian of potentials measured on the scalp.
Gevins, in "Analysis of the Electromagnetic Signals of the Human Brain: Milestones, Obstacles and Goals," IEEE Transactions on Biomedical Engineering, 31, 833-850, 1984, discusses the improved spatial resolution that the Laplacian of surface potentials recorded from the head provides for identifying sources of brain electrical activity. Gevins indicates that if one uses an array of unipolar electrodes that the Laplacian cannot be computed at points corresponding to the periphery of the array. W. F. Poole et al. in "Method for Modelling the Potential Sensed by a Concentric Multi-Ring Electode Set from a Moving Depolarization Wave," IEEE EMBS 11th Annual International Conference, 1297-1298, 1989, simulate the signal sensed by concentric ring electrodes but no method or apparatus is proposed for using such electrodes to record and display images of bioelectrical sources in the body.
Previous investigators have also devoted effort to measuring cardiac electrical activity. As is well known, the cyclic process of electrical activation (depolarization) and deactivation (repolarization) of the heart muscle triggers the mechanical contraction and relaxation of the heart muscle. To diagnose abnormalities in cardiac electrical activity one would wish to know the electrical state of each region of myocardial tissue at each point in time. The standard clinical means of assessing abnormalities of cardiac electrical activity involves body surface electrocardiography. In this technique, one records the instantaneous electrical potential difference between two points on the body surface. This potential difference fluctuates in time due to the electrical activity of the heart. The interface between each region of depolarized and repolarized tissue generates an equivalent electric dipole. These electric dipoles cause currents to flow in the body which acts as a volume conductor. This process results in an electric potential distribution within the body and on the body surface. The electric potential distribution changes as the state of cardiac depolarization and repolarization evolves during the cardiac cycle.
Conventional electrocardiography involves displaying versus time the electrical potential difference for one or more pairs of electrodes on the body surface. For each pair of electrodes, one obtains a signal which for each point in time represents a summation of the contributions of all the spatially separated electric dipoles in the myocardium. By use of multiple pairs of electrodes, one can estimate the instantaneous magnitude of each of the three vector components of a net "cardiac dipole" located theoretically in the center of the heart. However, one may not localize spatially distributed dipole sources in the heart by means of conventional body surface electrocardiography.
Attempts have been made to obtain spatial information on cardiac electrical activity. One technique that has been used is body surface mapping in which a large number of electrodes are placed on the body surface (primarily on the torso) and contour maps of the potential distributions on the body surface are made at different time points during the cardiac cycle. Unfortunately, the ability to interpret these potential maps has been very poor in terms of identifying electric dipole source distributions within the heart. Thus, this technique has not been adapted into routine clinical practice.
An underlying problem in terms of determining electrical source distributions within the heart from body surface potentials is the non-uniqueness of the electrocardiograph inverse problem. One can show mathematically that even if one could measure the potential distribution arbitrarily accurately everywhere on the body surface, and one could assume uniform conductivity of the body contents, one could not uniquely determine the three-dimensional distribution of sources within the heart. See, R. M. Gulrajani et al., "The Inverse Problem in Electrocardiography: Solution in Terms of Equivalent Sources," Critical Reviews in Biomedical Engineering, 16, 171-214, 1988. Attempts at solving the ECG inverse problem by constraining the number and location of cardiac dipoles has also generally proved unsatisfactory.
Mapping of cardiac electrical activity can be performed in experimental animals or in patients by applying electrodes directly to the endocardial surface, epicardial surface, or in the myocardium itself by means of needle electrodes. This approach is highly invasive and suffers from the fact that the electrograms, especially those recorded from bipolar electrodes, represent electrical activity localized to the region of the electrodes. Thus, it is difficult to map conduction paths because one is recording only from a finite number of points and conduction may progress between recording sites. Further, conventional bipolar electrodes may impose a directional bias on the recordings. F. J. L. van Capelle et al., "Conduction in Ischemic Myocardium," Normal and Abnormal Conduction in the Heart, A. P. de Carvalho, B. F. Hoffman, and M. Lieberman, Eds., Futura Publishing, Mount Kisco, New York, 1982, recorded electrical potentials directly from the surface of the heart and computed the Laplacian of the array of electrodes. They reported a single "source current" map based on these measurements.
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