2.7 Safety Considerations in MRI

Since the technique of MRI is used to image humans, it is important to keep the safety of the subjects as a high priority. Since MRI does not use any form of ionising radiation, it is considerably safer than x-ray or radio-isotope techniques. However it is important, especially in a research setting, that the potential hazards of any new developments are carefully considered. In this section the major safety aspects are outlined.

2.7.1 Static Magnetic Fields

Most scanners used for MRI use magnets with field strengths of anything from 0.1 T to 4 T. There are many opinions on the effect of magnetic fields on biological tissues, and many studies carried out on the subject, ranging from epidemiological human studies, to the investigation of the development of animal embryos in high fields [32]. It is however currently concluded that there is no adverse biological effect from the static magnetic field used in MRI. Further experimentation will no doubt be carried out, and this view can be altered in the light of any new discoveries.

By far the more serious effect of the static magnetic field is the response of ferromagnetic objects to such fields. It is essential that no free ferromagnetic object is allowed near the magnet since the field will turn it into a projectile. In a laboratory setting this means that most tools, connectors, and other equipment to be used in the vicinity of the field must not be ferromagnetic. Subjects must be screened for objects like keys, pens, belts and other metal on clothing, as well as the possibility of surgical implants. Before scanning a subject it is also necessary to check that there would be no ill effects from exposure to the magnetic field. It is common to exclude people who are in the early stages of pregnancy, people who may have any kind of metal fragments in them, and those suffering from certain conditions such as epilepsy.

2.7.2 Time Varying Magnetic Fields

As well as the high static magnetic field used in MRI, it is possible that the two time varying fields, namely the gradients and the r.f. radiation, could affect the subject in the scanner.

The rapid switching of the field gradients, particularly in EPI, produce two safety concerns. Firstly there is the possibility of inducing voltages in tissue by Faraday's law. The current induced in a loop of tissue is dependent on the rate of change of the field (dB/dT), the conductivity of the tissue, and the cross section of the loop. Calculations by Mansfield and Morris [33] show that for dB/dT = 1.0 Ts-1, the currents induced are of the order 1 mAcm-2. Cohen [34] reports of subjects experiencing mild neural stimulation at gradient field variations of 61 Ts-1, which is higher than the rates in normal use. It is wise however, if high switching rates are used, that subjects are warned of the possible effects, and monitored during the imaging.

A second safety concern with the gradients is that of acoustic noise levels. Since large currents are flowing through wires in a large magnetic field, a force is exerted on the wires. When the currents oscillate at audio frequencies, the resulting noise can be in excess of 100 dB. Subjects therefore must wear suitable ear protection during scanning, to reduce the noise to an acceptable level.

The heat that can be dissipated by r.f. fields is a further source of concern. The currents induced in tissues by such fields are dissipated as heat. Although most tissue has adequate blood flow to carry the heat away, some anatomical regions such as the eye do not. It is sensible to keep the heating due to r.f. radiation to a minimum, and the NRPB [35] guidelines state that the body temperature, or any mass of tissue, should not rise by more than 1 degrees centigrade. This is achieved by limiting the mean absorption rate in the whole body to 0.4 Wkg-1 and in any mass of tissue to 4 Wkg-1.

It is also essential that there are no conductive items touching the subject's skin, since the heating of such objects by the r.f. radiation can cause serious burns.

2.7.3 Other Safety Considerations

Claustrophobia, and other psychological problems, can prevent a subject from being able to enter the scanner, and should be screened for before attempting to scan. It is necessary to check the subject is fully informed as to the nature of the experiment and happy to proceed. Depending on the medical condition of the subject, it may be necessary to monitor them closely during the scanning, and communication is important so that the subject does not feel isolated.

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