Mri imaging : danger level
Tuesday, October 01, 1991
The National Radiological Protection Board has sounded a warning about magnetic resonance imaging, the sensitive x-ray technique that produces detailed cross sectional images of the body's soft tissues, including nervous tissue and other areas too re
MRI works by having the patient placed inside a magnetic field some 50,000 times stronger than the earth's magnetic field. The effect of this to to excite the nuclei of atoms within body cells. These hyped up nuclei produce radiofrequency signals, which get translated into images on the computer. Some 50,000-75,000 people in the UK and 3 to 4 million in the US undergo MRI every year.
Concern has been raised about the heating effect of the magnetic field and its ability to influence magnetic matter inside the body.
With high field intensities in MRI equipment being investigated, the NRPB has now recommended a two tier standard for measuring which levels of exposure are safe.
The report concludes that a magnetic field of 2.5 tesla is safe for all patients. Between 2.5 and 4 T, evidence of harm is doubtful, but about 4 T, likely to occur.
The NRPB also recommends that pregnant women, patients with pacemakers and those with metal prostheses like artificial hips or retained shrapnel avoid MRI. Some research on chick embryos has demonstrated that babies are at risk with the increased temperatures. In the States, several patients with pacemakers died when the magnetic forces altered them. Metal in the body exposed to MRI heats up and can cause tissue damage or move under the influence of the magnetic field.