MRI is absolutely contraindicated in patients who have any strongly magnetized metal object in their heads. This includes aneurysm clips, reconstructive metal plates, or traumatically embedded metal fragments. MRI is also contraindicated in patients who have implanted electronic devices, such as cardiac pacemakers. It is advisable to screen all subjects undergoing MRI by questionnaire for possible contraindications. A skull radiograph is a useful preliminary examination if there is any doubt about the presence of intracranial metal. All subjects need to provide informed consent in writing.
Static magnetic fields as strong as 2 T cause no harmful effects to biological tissue. Rapid switching of field gradients can induce electrical currents in tissue, but at the switching speeds used in MRI these induced currents are several times less than needed for muscle contraction. Since radiofrequency energy can cause heating, limits to the amounts of energy absorbed are set by national standards.
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