Safety And Exposure

MRI is widely regarded as a safe, innocuous imaging procedure that can be repeated at virtually any desirable interval. It is generally deemed as being safe for use with normal subjects within the context of research studies. MRI is accomplished without the use of ionizing radiation (X-rays). In addition to exposing the subject to a static magnetic field of considerable strength, the imaging process also requires that the subject be exposed to magnetic fields that oscillate at a frequency used in radio and television broadcasting and also to switched magnetic field gradients. No studies have demonstrated that MRI exposure has any adverse affects on health in the short or long term. Of course, failure to detect an effect does not necessarily

Figure 15 Photograph of a typical commercial MRI scanner.

rarely, if ever, turned off. Accordingly, the MRI scanner must be located in a controlled environment from which iron and steel objects are excluded. The magnetic field can also affect the operation of heart pacemakers and certain other electromagnetic pros-theses. Individuals who rely on these devices should be excluded from the MRI environment. The magnetic field may also apply torque to certain types of aneurysm clips; therefore, subjects who have undergone vascular surgery must be carefully evaluated before being imaged with MRI. The switched magnetic field gradients used in MRI can induce electric current flow in the body and affect neuromuscular function. Accordingly, the limits of field gradient strength and gradient switching rates have been established by the appropriate regulatory authorities. Alternating RF electric currents can be induced in the body by the RF pulses used in MRI. Such currents dissipate energy in the form of heat and this can be a source of local tissue burns. This hazard becomes more probable when metallic objects (e.g., electrocardiographic electrodes) are located in or on the body in the vicinity of the RF coil. Therefore, the RF energy that can be used is regulated. With appropriate precautions, it is exceedingly rare for subjects to experience injury during MRI scanning.

Undergoing MRI is not necessarily a pleasant experience despite its innocuous health effects. MRI scanning is a relatively slow process during which the subject must remain motionless within the tight confines of the scanner hardware. Some subjects experience claustrophobia during MRI scanning. In addition, the magnetic field gradient pulses are accompanied by a high level of audio noise, which some subjects intensely dislike.

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