MRI Safety Course

Module 5- Magnetic Fields and Radiofrequency

Performance Objective

At the end of Module 5, the student will be able to describe various magnet types, radiofrequency concerns, and SAR limitations

Radiofrequency(RF) concerns

RF energy has the potential to harm a patient by raising the patient’s core body temperature or indirectly by coupling enery into coils and cables and causing a burn to the patient’s skin.

MRI Saftey Reiew Progam

Specific Absorption Rate(SAR)

SAR is a unit of measurement used by the FDA to ensure that too much RF energy is not trasmitted into the patient’s body.

SAR is measured in unites of watts per kilogram

SAR regulates the amount of RF energy allowed per kilogram of patient body weight

SAR con’t

All MR systems have SAR limitations SAR is determined by the system

manufacturer FDA regulates that the SAR limitation be less

than .4watts per kilogram At this level the patient’s temperature will not

elevate more than 1 degree C.

Magnets

Magnetic field is measured in Testa and Gauss

Field strength is about 25,000 x earth’s magnetic field

5T and 1.5 T are the most common strengths

Stronger magnets produce better quality images.

Magnet Types

Permanent for example a refrigerator magnet Resistive – science class; nail/wire/battery Superconductive – supercooled with

cryogens, e.g. liquid helium

Magnetic Field Concerns

FDA concerns– Regards the maximum static magnetic field– Regards the time varying magnetic fields

The magnetic field must not exceed a change greater than 6 tesla per second or is should be ensured that the rate of change of the magnetic field will not cause peripheral nerve stimulation in the patient