Magnetic Measurement of Cardiac Volume Changes

Magnetic susceptibility plethysmography (MSPG) is a non-invasive technique for detecting changes in the total cardiac volume. The MSPG signal is observed with a sensitive magnetometer when an external magnetic field is applied to the thorax. The signal is due to the differences in magnetic susceptibilities of intracardiac blood, heart muscle, and surrounding tissues. In this work the spatial distribution of the MSPG signal across the chest of normal subjects is presented. The ventricular ejection of blood was clearly exhibited in signals measured above the heart. Relatively large MSPG signals were also observed on the upper and lower thorax during the ventricular ejection, related to the blood flow in the aortic arch and in the abdominal aorta, respectively tively. The temporal behavior of the MSPG is compared with the results obtained from existing clinical methods. The origin of the MSPG signal is analyzed theoretically. When a fixed cardiac magnetic dipole model was used, overall change in cardiac volume as a function of time could be evaluated with reasonable accuracy when the proximity effect of the torso was taken into account.