Guidelines on Exposure to Electromagnetic Fields from Magnetic Resonance Clinical Systems - Safety Code 26

2. Exposure Levels For Typical Devices

Presently available commercial human imaging systems produce a magnetic field inside the magnet bore with flux densities ranging from 0.02 T (Tesla) to 2 T, depending on the system. The magnetic fields are produced by a permanent magnet, by a resistive magnet (only below 0.3 T), or by a superconducting magnet. Outside the magnet bore the magnetic flux densities decrease with distance away from the magnet.

The magnetic flux density outside the system depends on the field strength of the magnet as well as the system design (the bore size, shielding, etc.). Measurements around the FONAR QED-80 imaging system (0.04 T) indicate that the magnetic flux density changes from 0.04 T in the imaging volume to 0.6 mT at the end of the patient table⁽²⁾. The level in the control room was about 0.4 mT⁽²⁾. Measurements performed by the Bureau have shown that for a 0.15 T MRI system (Teslacon Technicare, TM) the magnetic flux density at the entry to the magnet is 0.1 T, decreasing to 15 mT at a distance of about 1 m from the surface of the magnet housing, and to less than 1 mT at 3 m. For a 0.5 T MRI system (Philips Gyroscan, 515) the magnetic flux density at the magnet entry is about 0.12 T, 30 mT at a distance of 1 m, and 3 mT at 3 m from the surface of the magnet housing. For a 1.9 T MRS system (Oxford Research TMR 32/20) with a small bore (0.26 m) the magnet flux density at the entry is about 0.8 T, decreasing to 12 mT at a distance of 1 m, and less than 1 mT at 3 m.

Time-varying magnetic fields are superimposed on the static magnetic field to obtain spatial information in MR imaging and spectroscopy devices. These fields are of low magnitude compared with the static field.

RF fields are produced inside the magnet bore by transmitting coils. The RF fields are pulsed, and various pulse sequences are used by different systems. Several sequence options are available in each system. The frequency of RF fields depends on the strength of the static magnetic field. In MRI systems imaging protons the frequency ranges from about 6.4 MHz for a 0.15 T system to about 85 MHz for a 2 T system. Various frequencies are used in MRS. The average RF power or MR systems varies from a few to a few tens of watts (W). The peak RF power of the pulses may reach a few kW. Outside the magnet housing, intensities of RF fields are very low. Measurements by the Bureau performed for 0.15, 0.5, and 1.9 T systems have indicated that the RF magnetic field strength is below 0.05 A/m (the sensitivity of the survey instrument used) anywhere outside the magnet housing.

On the basis of the available data it can be evaluated that operators of MR clinical devices are likely to be exposed to magnetic fields below 2 mT for long time periods. This is because the operator spends a lot of time at a console containing Cathode Ray Tubes (CRTs), which produce distorted pictures in magnetic fields above about 0.5 mT. The actual exposure level depends on the magnetic field and the siting of the system. For short periods of time, while placing the patient in the imaging device, the operators and other personnel may be exposed to much stronger fields. Hands and arms may be exposed to the nominal magnetic flux density of the system when placed inside the magnet bore.