Thermal effects of RF for foreign metal bodies in MRI Scan

Metal devices also absorb RF energy and make become hotter than the surrounding tissue. Notably, Shellock(2000) reports that there have been no reports of burns associated with internal medical devices. However, this potential exists at high field strengths.

Certain looped configurations, particularly from wire leads, can act as a RF antenna, and thus focus RF energy to a small locus. Thus, the most significant safety risk for the RF electromagnetic fields used in MRI is a local burn (note that induced currents in conductors and loops due to time-varying magnetic fields associated with gradient coils can also result in heating).

These incidents reported to the FDA illustrate potential problems.

• MDR-711781 An electrically conductive lead was looped and placed against bare skin causing a burn on the patient's upper arm. (5/19/95)
• MDR-591457: A child received a burn to the right hand from an ECG cable while the patient was anesthetized. A skin graft was required to treat the affected area. (1/26/95)
• MDR-246106: A patient received a 1.5" x 4" blistered burn to the left side of the back near the pelvis from an ECG gating cable. (9/23/91)
• MDR-701219: A patient received blistered burns on the finger where a pulse oximeter was attached during MR scanning. A skin graft was required to treat the affected area. (2/27/95)
• MDR-391667: A patient received small blistered burns to the left thumb and left thigh. Reportedly, the operator input an inaccurate patient weight resulting in an incorrect SAR value. (2/10/93)

The potential for burns is greatest when the patient is uncommunicative, sedated, or otherwise compromised. The best methods for avoiding burns are (1) screening subjects to exclude those that have metal devices or wires implanted within their bodies, (2) ensuring that subjects remove all metal prior to entering the scanner � including non-ferromagnetic jewelry such as necklaces, piercings, and earrings, (3) make sure that any wire leads (such as ECG, EMG, or EEG leads) are not looped and that wires are not run over bare skin.
Source: http://www.biac.duke.edu/

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