Magnetocardiography (MCG) is a technique to measure the magnetic fields produced by electrical activity in the heart using extremely sensitive devices such as the superconducting quantum interference device (SQUID). If the magnetic field is measured using a multichannel device, a map of the magnetic field is obtained over the chest; from such a map, using mathematical algorithms that take into account the conductivity structure of the torso, it is possible to locate the source of the activity. For example, sources of abnormal rhythms or arrhythmia may be located using MCG.
The first biomagnetic signal to be detected was the magnetocardiogram (MCG) by Baule and McFee (1963). The discovery raised a lot of optimism, as it was believed that MCG would provide as much new information about the heart's electric activity as had the ECG. Though this has been shown theoretically (Rush, 1975) and in practical clinical studies not to be true, there are still many potential clinical applications of the MCG. For instance, as will be discussed in Section 20.7, according to the present understanding, with the combined use of the ECG and the MCG, called electro magnetocardiogram, (EMCG), in some cardiac diseases the number of incorrectly diagnosed patients can be decreased by one half of that when using only the ECG is used.
Since the concept of the magnetic heart vector was introduced by Baule and McFee in 1970, studies have been conducted to detect the vector magnetocardiogram (i.e., in which the heart is considered as a magnetic dipole). Though the detection of the magnetic heart vector is an obvious selection as the first clinical tool, many of the MCG studies of today have been made by mapping the normal component of the magnetic field of the heart around the thorax.
There exist also many other kinds of trials for finding out clinical applications for the MCG - for example, testing the risk for sudden cardiac death and for rejection of an implanted heart. The localization of arrhythmogenic centers has also been a subject of intensive research. An overview of the methods for solving the biomagnetic inverse problem can be found in Swithenby (1987).
It is an axiom that electrical current always produces an associated magnetic field, and the electrical currents in the heart caused by depolarization-repolarization processes produce corresponding time-varying changes in magnetic field around the heart. Magnetocardiography is a noninvasive technique for recording local magnetic fields generated by the electrical activity of the heart. A magnetocardiogram (MCG) is nothing more than the recording of the heart’s electro-magnetic signals, hence its electrical activity. The EKG and the MCG seek to measure the same electrophysiological event. For over 100 years now, the first diagnostic test performed in the cardiac evaluation of a patient has been the electrocardiogram (EKG). It is inexpensive, non-invasive, and relatively portable so it can be used at the patient’s bedside, in the Emergency Room, or even carried by the patient ‘round-the-clock as in a Holter Monitor. But as ubiquitous and useful as the EKG is, it fails to provide complete information about the electrophysiological activity of the heart, and, at least for the detection of coronary artery disease, is grossly inadequate.
Electrical current in the heart is the driving force by which the heart operates, and the EKG is nothing more than a limited recording of the heart’s electrical activity. The waves that appear on an EKG primarily reflect the electrical activity of the myocardial cells, which comprise the vast bulk of the heart. Pacemaker cell activity and transmission by the conducting system are generally not seen on the EKG, because these events simply do not generate sufficient voltage to be recorded by surface electrodes. Nevertheless, perturbations in the normal electrical patterns as seen in an EKG allow one to diagnose many different cardiac disorders.
Measurement of the magnetic field of the heart, produced by the same ionic currents that generate the electrocardiogram, and showing characteristic P, QRS, T, and U waves.