Magnetocardiography (MCG) is a highly sensitive, non-invasive technique used for assessing the heart’s electrical function by measuring the magnetic fields it naturally produces. Every heartbeat is initiated by electrical currents flowing through the cardiac muscle, and these currents generate minute magnetic fields that extend outside the body. MCG uses advanced instrumentation to capture and map these fields across the chest, offering a detailed view of the heart’s electrophysiological processes. The technology allows for a precise, contactless recording of cardiac function, providing unique information about the heart’s electrical activity.
Measuring the Heart’s Magnetic Fields
The magnetic fields generated by the heart are extraordinarily weak, existing in the picotesla (\(10^{-12}\) Tesla) to nanotesla (\(10^{-9}\) Tesla) range. These fields are roughly one billionth the strength of the Earth’s natural magnetic field. Measuring such minute fluctuations requires specialized equipment and an environment that completely eliminates external magnetic interference.
To achieve the necessary signal clarity, MCG systems are housed within magnetically shielded rooms (MSRs). MSRs are designed to block environmental magnetic noise from urban sources like traffic, power lines, and hospital equipment. This shielding is necessary because even small external magnetic disturbances would completely overwhelm the heart’s faint signal. The core technology used for detection within this controlled environment is the Superconducting Quantum Interference Device (SQUID).
SQUIDs are ultrasensitive magnetometers that operate based on quantum mechanics, allowing them to detect the extremely small magnetic fields emanating from the body. Achieving this sensitivity requires the SQUID sensors be kept at cryogenic temperatures, typically by submerging them in liquid helium. This supercooling reduces electrical resistance to zero, enabling the precise measurement of magnetic flux changes corresponding to the heart’s electrical activity. The SQUIDs are often arranged in a multi-channel array, positioned near the patient’s chest without physical contact, to create a detailed map of the magnetic field over the cardiac region.
Diagnostic Uses of Magnetocardiography
MCG provides detailed, high-resolution spatial information valuable in the diagnosis and mapping of several cardiac conditions. One primary application is the non-invasive detection and localization of myocardial ischemia, a condition caused by a lack of blood flow to the heart muscle. The magnetic fields are particularly sensitive to changes in ion conduction and action potentials that occur in ischemic tissue, often allowing for the identification of compromised electrical activity at rest or at earlier stages of the disease.
The technology is also used for precisely locating the source of cardiac arrhythmias, or irregular heartbeats. By creating a detailed magnetic field map, MCG can localize the ectopic foci—the small areas of heart tissue that generate abnormal electrical impulses—with high spatial accuracy. This capability is beneficial for planning catheter ablation procedures, as the exact location of the problematic tissue must be identified before treatment.
MCG plays a role in the risk stratification for sudden cardiac death (SCD). The technique is sensitive to spatial dispersion patterns of cardiac currents, which are subtle indicators of an unstable heart rhythm that predisposes a person to lethal arrhythmias. Studies have shown that MCG may possess greater discriminating power than standard electrocardiography in identifying individuals at higher risk of experiencing these events.
How MCG Compares to Standard ECG
Magnetocardiography offers several unique advantages over the widely used standard electrocardiography (ECG), which records electrical potentials on the skin surface. A significant difference is that MCG is a non-contact method, meaning it does not require electrodes to be placed directly onto the patient’s skin. This makes it an ideal option for patients with fragile skin, burns, or other conditions that complicate the application of traditional electrodes.
The magnetic fields measured by MCG are much less distorted as they pass through body tissues like the lungs, bones, and chest wall compared to the electrical currents measured by ECG. This reduced distortion allows MCG to provide superior spatial resolution and a more accurate picture of the electrical activity originating directly from the heart muscle. Consequently, MCG can detect subtle changes in cardiac function that might be obscured or missed by ECG measurements.
MCG is uniquely sensitive to specific components of the heart’s electrical activity, such as tangential and vortex currents, which an ECG cannot easily measure. These additional measurements provide complementary information, particularly regarding myocardial ischemia, enhancing diagnostic capability. However, the technology has drawbacks, including the high cost of the equipment and the necessity of specialized, magnetically shielded facilities for operation. The complexity of the SQUID-based systems and the requirement for liquid helium cooling contribute to the difficulty and expense of widespread clinical implementation.