Biomagnetism is the measurement and analysis of local magnetic fields emanating from living organisms and human organs. Magnetic fields may be spontaneously generated by physiological electric currents that flow in living tissues such as nerves and muscles or they may be induced by external excitation. Low-frequency signals from quasistatic to some kilohertz can be detected with superconducting quantum interference devices (SQUIDs).
SQUIDS are sensitive magnetometers and powerful signal amplifiers with applications in medicine, mining, cosmology and materials analysis. They have been used to measure brain activity, discover mineral deposits and detect faint light form early moments of the universe captured by telescopes. SQUIDs measure magnetic fields based on quantum properties which occur when a superconducting circuit loop where electricity flows without resistance is interrupted by sections of resistive material. The devices need to be cooled to cryogenic temperatures below -269 degrees Celsius with liquid helium.
The SQUID was invented by Ford Scientific Laboratories of Ford Motor Co. in the 1960s and further developed by NIST (then called the National Bureau of Standards). The RF-SQUID was developed by James Zimmerman and one of his SQUIDS is said to have launched the field of biomagnetism. A portable SQUID made by Zimmerman at NIST was used for the first magnetocardiogram, where he measured the magnetic signal of his own heart.
Clinical use of biomagnetic methods include magnetocephalography (MEG) on the human brain and liver susceptometry. Magnetocardiography is used for diagnostics on the human heart.
MEG non-invasively measures magnetic field produced by the human brain. Although MEG has been reported to have excellent temporal resolution, spatial resolution is more limited due to factors such as limited numbers of sensors, external noises and artifacts, inherently low signal-to-noise ratio and cancellation of magnetic fields produced by multiple sources. MEG systems have applications for sensing fine motor movement of hands which could be used for development of exoskeleton systems and brain-computer/machine interfaces (BCI/BMI).
Liver susceptometry can be used in certain diseases that result in iron overload. Liver iron concentration can be measured by placing a patient’s abdomen in a magnetic field and measuring the change produced by the magnetization of iron stored as ferritin or hemosiderin.
Biomagnetism therapy or magnetic therapy are terms used in alternative medical practices where magnets, integrated into bracelets, rings or shoe inserts are claimed to alleviate pain or other health conditions. Studies suggest that static magnetic devices are no more or less beneficial compares to sham devices devoid of magnet. A therapeutic magnet is claimed to increase blood flow, however iron in blood that is bound to hemoglobin is not ferromagnetic.