The principles of Nuclear Magnetic Resonance (NMR) are used in a variety of applications for non-invasive analysis of the structure of materials.
NMR spectroscopy is used by chemists to study the structure of molecules. Laboratory medical applications of this can help in the development of drugs, analyzing their effect on proteins.
Some NMR applications use the signal from water to acquire spatial information about the spaces between molecules rather than the chemistry of the molecules themselves. One such application is analysis of the sizes of pore spaces in rocks, as described at SEED FAQ 1163.
Magnetic Resonance Imaging (MRI) also uses the NMR signal from water - in this case, the water inside a human body. Here, the word "nuclear" has nothing to do with ionizing nuclear radiation, which can damage cells. MRI measures the relaxation time of hydrogen nuclei when changing the orientation of a magnetic field. These hydrogen nuclei are already present in the body and MRI does not change them.
During MRI, the body being examined is submitted to a high magnetic field with strong gradients and a high radio frequency field. The impact of electromagnetic fields on the human body, such as from high voltage electrical lines, mobile phones and radio masts, has been widely researched. The frequency range used in MRI is lower than that of portable phones. The World Health Organization states that current evidence from research into the effects of medical applications of non-ionizing radiation does not confirm the existence of any health consequences, but recommends further research.
The strong magnetic fields will affect metals, so people with metal pins, shell fragments or electronic implants such as pacemakers are often unable to benefit from MRI.
Radio frequency (rf) fields will cause some heating, but MRI systems limit the amount of rf energy that can be deposited in tissue during a given period of time.
Changes in the magnetic field can induce currents in the body, which could stimulate nerves, but these are also regulated to safe levels in MRI systems.
MRI systems can create loud noises,particularly when the magnetic field is switched. Newer systems are designed to reduce these noises.
The main problems reported with MRI systems are discomfort, from having to lie still for a long time, and claustrophobia due to being inside a long narrow tube. These and many other issues are discussed at Wikipedia.