Magnetic Field Imaging (MFI) is a non-invasive and side-effect-free cardiac diagnostic method. In more recent technology,

magnetocardiography Magnetocardiography (MCG) is a technique to measure the magnetic fields produced by electrical currents in the heart using extremely sensitive devices such as the superconducting quantum interference device (SQUID). If the magnetic field is meas ...

(MCG) has become the clinically predominant application for recording the heart's magnetic signals. that detects and records the electromagnetic signals that are associated with the heartbeat using a multi-channel

magnetic sensor A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...

array. The electric signals are known from the

ECG Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...

. In the 1990s and beyond, more recent technology has supplanted the MFI, particularly MCG (xref. Cardiomag Imaging, Inc.). Through clinical research in Europe, Asia, and the U.S. (see publications in footnotes), MCG has been proven to have practical application for diagnosis of cardiac disease, and has become the clinically predominant application for recording the heart's magnetic signals. In comparison to MCG, MFI, among others, records the whole relevant area above the chest of the person.

Background

The general principal of MFI is based on two facts: *Cell activity in the human body is connected to electric activity (based on

Galvani Luigi Galvani (, also ; ; la, Aloysius Galvanus; 9 September 1737 – 4 December 1798) was an Italian physician, physicist, biologist and philosopher, who studied animal electricity. In 1780, he discovered that the muscles of dead frogs' legs t ...

, Italy 1786). *Electric current is associated with a corresponding magnetic field (based on Ørsted, Denmark 1819). The difference between the electric and the magnetic signals: In comparison to the electric signals, which are influenced by the differently conductive tissue of the body and varying resistance of the skin before they can be recorded, the magnetic signals travel through the body almost without disturbance. The differences in the electric potentials, that are recorded by the ECG, are directly depending on the inhomogeneity and geometry of the thorax, the magnetic signals outside of the thorax depend primarily on the intracellular currents of the cardiac tissue and only secondarily on the secondary currents generating the electric signal. Furthermore, the magnetic signals of so-called vortex currents, which occur regularly in every heartbeat and include important information for an advanced and more accurate cardiac diagnosis (first theoretically described by

John Wikswo John Peter Wikswo, Jr. (born October 6, 1949) is a biological physicist at Vanderbilt University. He was born in Lynchburg, Virginia, United States. Wikswo is noted for his work on biomagnetism and cardiac electrophysiology. Graduate school ...

), can be acquired with an MFI system, but cannot be recorded electrically on the body surface (First experimental hint by Brockmeier et al. 1994 and comprehensive demonstration Brockmeier et al. 1997).Brockmeier, K. et al "Magnetocardiography and 32 lead potential mapping: Repolarisation in normal subjects during pharmacologically induced stress" in ''J. Cardiovasc. Electrophysiol.'' Vol. 8 pp. 615 - 625, 1997 Recording technology The magnetic field changes detected by the MFI are about one million times weaker than the magnetic field of the earth. High end acquisition electronics and noise reduction concepts are necessary. As sensors the most sensitive magnetic sensors presently available,

SQUID True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...

s (superconducting quantum interference devices), which are cooled down to 4 K (-269 °C) with liquid helium, are used to acquire the signals.

Applications

The main fields of use are the risk stratification of

ventricular tachycardia Ventricular tachycardia (V-tach or VT) is a fast heart rate arising from the lower chambers of the heart. Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple episodes over a short perio ...

(VT) and the detection of stress induced ischemia. The MFI system can detect the onset of arrhythmic and ischemic diseases in a very early stage with high accuracy for both acute and asymptomatic patients. *Early detection of

arrhythmia Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults ...

ischemia Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...

angina pectoris Angina, also known as angina pectoris, is chest pain or pressure, usually caused by insufficient blood flow to the heart muscle (myocardium). It is most commonly a symptom of coronary artery disease. Angina is typically the result of obstr ...

, cardiac microvascular diseases *Direct diagnosis of heart function after

myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which ...

(MI) and surgery of heart transplantation *Ongoing monitoring of patients with heart surgical intervention: patients with a stent or who underwent a balloon dilatation, post-bypass patients, post heart transplantation patients As MFI is absolutely risk free and harmless for the patient, the procedure can be repeated without any negative effects for the patient, which gives the cardiologist the opportunity to observe a patient's progressive changes. The non-invasiveness of MFI makes it an ideal tool for the diagnosis of pregnant women as well as it can in addition detect the cardiac signal of a fetus starting from the 4th month of pregnancy.

References

{{Electrodiagnosis Diagnostic cardiology Medical imaging