Magnetic resonance imaging (MRI) is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error. During the last 150 years, thousands of papers focusing on the effects or side effects of magnetic or radiofrequency fields have been published. They can be categorized as incidental and physiological. Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel and metallic foreign bodies in the eyes. The safety of MRI during the first trimester of pregnancy is uncertain, but it may be preferable to other options. Since MRI does not use any ionizing radiation, its use generally is favored in preference to CT when either modality could yield the same information. (In certain cases, MRI is not preferred as it may be