Magnetic field imaging | |
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Purpose | non-invasive cardiac diagnostic method |
Magnetic Field Imaging (MFI) is a non-invasive and side-effect-free cardiac diagnostic method. In more recent technology, magnetocardiography (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 array. The electric signals are known from the ECG. 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.
The general principal of MFI is based on two facts:
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), [1] [2] can be acquired with an MFI system, but cannot be recorded electrically on the body surface (First experimental hint by Brockmeier et al. 1994 [3] and comprehensive demonstration Brockmeier et al. 1997). [4]
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, SQUIDs (superconducting quantum interference devices), which are cooled down to 4 K (-269 °C) with liquid helium, are used to acquire the signals.
The main fields of use are the risk stratification of ventricular tachycardia (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.
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.
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Impedance cardiography (ICG) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), cardiac output (CO), ventricular ejection time (VET), pre-ejection period and used to detect the impedance changes caused by a high-frequency, low magnitude current flowing through the thorax between additional two pairs of electrodes located outside of the measured segment. The sensing electrodes also detect the ECG signal, which is used as a timing clock of the system.
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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 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.
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Coronary ischemia, myocardial ischemia, or cardiac ischemia, is a medical term for abnormally reduced blood flow in the coronary circulation through the coronary arteries. Coronary ischemia is linked to heart disease, and heart attacks. Coronary arteries deliver oxygen-rich blood to the heart muscle. Reduced blood flow to the heart associated with coronary ischemia can result in inadequate oxygen supply to the heart muscle. When oxygen supply to the heart is unable to keep up with oxygen demand from the muscle, the result is the characteristic symptoms of coronary ischemia, the most common of which is chest pain. Chest pain due to coronary ischemia commonly radiates to the arm or neck. Certain individuals such as women, diabetics, and the elderly may present with more varied symptoms. If blood flow through the coronary arteries is stopped completely, cardiac muscle cells may die, known as a myocardial infarction, or heart attack.
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John Peter Wikswo, Jr. is a biological physicist at Vanderbilt University. He was born in Lynchburg, Virginia, United States.
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Jaakko A. Malmivuo is a Finnish engineer, academic, author, and opera singer. He was a professor of Bioelectromagnetism at Tampere University of Technology (TUT) from 1976 to 2010, an adjunct professor in the Faculty of Medicine at the University of Tampere as well as a visiting professor in the Faculty of Electrical Engineering and Computer Science, Electronics, and Medical Signal Processing at Technische Universität Berlin. Moreover, he was a director of the Ragnar Granit Institute at TUT from 1992.