Noninvasive magnetic detection of cardiac mechanical activity: theory.

Because the magnetic susceptibility of the heart and the intracardiac blood is substantially greater than that of the surrounding lung tissue, the periodic redistribution of blood and tissue during the cardiac cycle will alter an externally applied magnetic field. These magnetic field changes can be detected by a magnetometer outside the body and have been termed magnetic susceptibility plethysmography (MSPG) signals. This paper presents a theoretical analysis of the MSPG technique. A single moving sphere model is used to explain the susceptibility induced field changes and the detection of the MSPG signals. The susceptibility distribution within the thorax and a more detailed model of the heart are then used to predict the MSPG waveform. Several other contributions to the MSPG signal are analyzed. This model has demonstrated that the time dependence of certain MSPG components closely resembles that of the total heart volume while other components have waveforms similar to the displacement of the cardiac center of mass. The magnitude and direction of the MSPG field has been shown to depend on the location of the field point, the volume change of the heart, and the motion of the heart's center of mass. It has also been shown that 90% of more of the MSPG signal recorded near the heart but outside the body will be produced by cardiac volume changes. For these reasons, the MSPG technique may provide a noninvasive method of measuring changes both in total cardiac volume and in the position of the cardiac center of mass throughout the cardiac cycle.