The science of biomagnetism refers to the measurement of magnetic fields produced by living organisms. These tiny magnetic fields are produced by naturally occurring electric currents resulting from muscle contraction, or signal transmission in the nervous system, or by the magnetization of biological tissue. The first observation of biomagnetic activity in humans was the recording of the magnetic field produced by the electrical activity of the heart, or magnetocardiogram, by Baule and McFee in 1963 (1). In 1968, David Cohen (2) at the Massachusetts Institute of Technology reported the first measurement of the alpha rhythm of the human brain, demonstrating that it was possible to measure magnetic fields of biological origin that are only several hundred femtotesla in magnitude (1 femtotesla=10-15 T) -- more than 1 million times smaller than the earth's magnetic field (~ 5×10-5 T). These early measurements were achieved using crude instruments consisting of inductance coils of 1-2 million windings in magnetically shielded enclosures and using extensive signal averaging. Instruments with increased sensitivity and performance based on the superconducting quantum interference device, or SQUID became available shortly after these pioneering measurements. The SQUID is a highly sensitive magnetic flux detector based on the properties of electrical currents flowing in superconducting circuits, as predicted by Nobel laureate Brian Josephson in 1962 (3). The SQUID was soon adapted for use in biomagnetic measurements (4) and by the early 1970s, measurements of the spontaneous activity of the human heart (5) and brain (6) had been achieved without the need for signal averaging using superconducting sensing coils coupled to SQUIDs immersed in cryogenic vessels containing liquid helium. Thereafter, the field of biomagnetism continued to expand with the further development of SQUID based instrumentation during the 1970s and 1980s. The introduction in 1992 of multichannel biomagnetometers capable of simultaneous measurement of neuromagnetic activity from the entire the human brain (7,8) has resulted in widespread interest in the field of magnetoencephalography or MEG as a new method of studying human brain function.
Biomagnetic measurements are considered to have a number of advantages over more traditional electrophysiological measurements of heart and brain activity, such as the electrocardiogram or electroencephalogram. One significant advantage is that propagation of magnetic fields through the body is less distorted by the varying conductivities of the overlying tissues in comparison to electrical potentials measured from the surface of the scalp or torso, and can therefore provide a more precise localization of the underlying generators of these signals. In applications such as MEG and magnetocardiography (MCG), these measurements are completely passive and can be made repeatedly without posing any risk or harm to the patient. Also, biomagnetic signals are a more direct measure of the underlying currents in comparison to surface electrical recordings that measure volume conducted activity that must be subtracted from a reference potential at another location complicating the interpretation of the signal. In addition, magnetic measurements from multiple sites can be less time consuming since there is no need to affix electrodes to the surface of the body. As a result, biomagnetic measurements provide an accurate and noninvasive method for locating sources of electrical activity in the human body. The development of multichannel MEG systems has dramatically increased the usefulness of this technology in clinical assessment and treatment of various brain disorders. This has resulted in the recognition of routine clinical procedures by health agencies in the United States for the use of MEG to map sensory areas of the brain or localize the origins of seizure activity prior to surgery. Clinical applications of MCG have also been developed although to a lesser extent than MEG. This includes the assessment of coronary artery disease and other disorders affecting the propagation of electrical signals in the human heart. Another biomagnetic technique, known as biosusceptometry, involves measuring magnetized materials in the human body by measuring their moment as they are moved within a strong magnetic field.
These measures can provide useful information regarding the concentration of ferromagnetic or strongly paramagnetic materials in various organs of the body, such as iron particles in the lung or iron-containing proteins in the liver. In addition, novel biomagnetometer systems are now available for the assessment of fetal brain and heart function in utero, and may provide a new clinical tool for the assessment of fetal health. Currently, there are >100 multichannel MEG systems worldwide and advanced magnetometer systems specialized for the measurement of magnetic signals from the heart, liver, lung, peripheral nervous system, as well as the fetal heart and fetal brain are currently being commercially developed. Although biomagnetism is still regarded as a relatively new field of science, new applications of biomagnetic measurements in basic research and clinical medicine are rapidly being developed, and may provide novel methods for the assessment and treatment of a variety of biological disorders. The following section reviews the current state of biomagnetic instrumentation and signal processing and its application to the measurement of human biological function.