The electrical conductivity of living tissue: A parameter in the bioelectrical inverse problem

Electrically active cells within the human body generate currents in the tissues surrounding these cells. These currents are called volume currents. The volume currents in turn give rise to potential differences between electrodes attached to the body. When these electrodes are attached to the torso, electrical potential differences generated by the heart are recorded. The recording of these electrical potential differences as a function of time is called an electrocardiogram (ECG). ECG measurements can be used to compute the generators within the heart. This is called the solution of the ECG inverse problem. This solution may be of interest for diagnostic purposes. For instance, it can be used to localize an extra conducting pathway between atria and ventricles. This pathway can then subsequently be removed by radio-frequent ablation through a catheter. When the active cells are situated within the brain and the electrodes are attached to the scalp, the recording of the potential difference measured between two electrodes as a function of time is called an electroencephalogram (EEG). The EEG inverse problem can, for example, be used to localize an epileptic focus as part of the presurgical evaluation. The frequencies involved in electrocardiograms and electroencephalograms are in the range of 1-1000Hz. Therefore, the Maxwell equations can be used in a quasi-static approximation, implicating that capacitive and inductive effects and wave phenomena are ignored as argued by (1967).