Nuclear Magnetic Resonance (NMR) In Vivo Studies

Abstract

Sources (3) of harmful health effects from NMR in vivo techniques were examined. Static magnetic fields showed that harmful effects on humans and reproducible cellular, biochemical or genetic effects had not been observed at fields < Tesla [T] (20,000 gauss). Changing magnetic fields showed that the threshold for effects of induced currents was above that produced from < 1-100 Hz sinusoidal field changes with a maximum field of 5 mT (50 gauss). Waveform, repetition rate, maximum B field and duration of exposure were parameters requiring further study. Radiofrequency (RF) heating showed that a practical upper level for absorbed power was 4 W/kg in medically important studies of short duration (< 10 min). For long-term studies, 1.5 W/kg was a reasonable level in low humidity environments. The power absorbed by the subject was estimated by measuring the RF coil Q before and after the subject was placed in the NMR instrument. Large metal objects absorbed power in proportion to the conductivity of the device or prosthesis.