Noninvasive magnetic detection of cardiac mechanical activity: experiments

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 thus 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. A differential magnetometer, a liquid helium dewar, and a superconducting persistent-current magnet have been developed to detect the x component of the MSPG signal. The theoretical analysis of this instrument is presented to understand the spatial sensitivity of an MSPG susceptometer with a nonuniform magnetizing field. Preliminary measurements are shown to be in fair agreement with the model calculations. Several possible improvements of the technique are discussed.