Magnetic studies on mechanical activity of the heart

Magnetic measurement offers a new noninvasive technique for monitoring cardiac mechanical activity and for evaluation of important hemodynamic parameters, e.g., heart volume changes and cardiac output. The magnetic signal is observed with a sensitive magnetometer when an external magnetic field is applied to the thorax. The magnitude of the signal is usually in the order of a few picoteslas when an applied magnetic field of about 250 A/m is used. This signal is due to the differences in magnetic susceptibilities of intracardiac blood, heart muscle, and surrounding lung tissues. In the present paper, the origin of the magnetic susceptibility signal is theoretically analyzed and various measurement techniques are reviewed. The measurement results reported by different authors are compared and discussed. The spatial variation of the magnetic signal is analyzed in terms of mathematical models and its temporal behavior is compared with the results obtained from other measurement techniques. Three independent experimental methods are proposed in order to separate the magnetic plethysmogram from the other components in the total magnetic susceptibility signal recorded above the heart. The results suggest that usually about 65 to 80% of the signal originates directly from cardiac volume changes, and that the cardiac plethysmogram can be separated easily by surrounding the thorax with the medium whose magnetic susceptibility is about -5.10(-6), or by preventing motion of the chest wall of the subject.