title: Nuclear magnetic resonance (NMR) in vivo studies: known thresholds for health effects.
abstract: Three sources of harmful health effects from nuclear magnetic resonance (NMR) in vivo techniques have been examined with the following conclusions: (a) Static magnetic fields. Harmful effects on humans and reproducible cellular, biochemical, or genetic effects have not yet been observed at fields less than 2 Tesla (20,000 gauss). (b) Changing magnetic fields. The threshold for effects of induced currents is above that produced from less than 1 to 100 Hz sinusoidal field changes with a maximum field of 5 mT (50 gauss). Waveform, repetition rate maximum B field, and duration of exposure are parameter requiring further study. (c) Radiofrequency (RF) heating. A practical upper level for absorbed power is 4 W/kg in medically important studies of short duration (less than 10 min). For long-term studies, 1.5 W/kg is a reasonable level in low humidity environments. The power absorbed by the subject can be estimated by measuring the RF coil Q before and after the subject is placed in the NMR instrument. Large metal objects will absorb power in proportion to the conductivity of the device or prosthesis.
mesh_terms: D000818:Animals; D001812:Blood-Brain Barrier; D002468:Cell Physiological Phenomena; D004562:Electrocardiography; D004798:Enzymes / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics / Q000009:adverse effects*; D009431:Neural Conduction; D012306:Risk
chemical_list: D004798:Enzymes