Injuries from burns caused by certified magnetic resonance (MR) compatible equipment for patient monitoring is now rare. Unfortunately it is still possible. This case report is a reminder of the necessity for meticulous attention to detail such as line placement and sudden onset of artifacts in vital signs while monitoring patients during a MR examination. Furthermore, we need to be certain whether or not conductive lines for electrocardiography (ECG) monitoring or pulse oximetry are still the technology of choice in the setting of the increasing power used by modern MR scanners.
Magnetic resonance imaging allows excellent visualization of soft tissue areas, which may contain malignancy or infection. Within a strong magnetic field (1.0-2.0 T) hydrogen atoms are stimulated using radio frequency pulses. The energy emitted by the stimulated atoms is detected and allows computer reconstruction of anatomical images . Certain metal implants are thought to be incompatible with MR imaging since induced electric currents may lead to high local temperature levels with subsequent burn injuries .
Patient monitoring and management during MR examinations have been challenging because the strong magnetic field and high-energy radio frequency impulses interfere with electronic equipment. Patients cannot be visually observed in the scanner tube. Most surveillance devices have to be placed outside the scanner room to avoid hazards for the patient by a mobile metallic device on the one hand and artifacts on the MR images on the other hand. Meticulous attention must be paid to monitoring cables, which may form an inductive loop causing energy induction and heat production. In recent years, MR compatible devices have been introduced for use with MR scanners .
A 4-yr-old girl was admitted for a day case MR examination of the abdomen and the pelvis at the end of a combined course of surgical and chemotherapy treatment for rhabdomyosarcoma of the bladder. Various therapeutic and diagnostic procedures, including MR, had been carried out under general anaesthesia without problems. Clinically the girl presented with no signs of current infection or immunosuppression. Induction of anaesthesia and endotracheal intubation were uneventful. Monitoring included non-invasive blood pressure, ECG and pulse oximetry with special MR compatible equipment.
A new monitoring device (DATEX-Ohmeda MR Compatible Monitor S/5) in a special cage had been installed in the MR suite (CV/i 1.5T; General Electric, Milwaukee, USA) allowing ECG monitoring over a carbon fibre cable. Approximately 30 min after the start of the examination the ECG record worsened and the examination was paused. The skin under the MR compatible carbon electrodes was found to have second to third degree burns. Additionally, there were two small stripes of burned skin at the left side of the neck where the ECG cable had come into direct contact with the skin of the patient. The examination and the anaesthetic were aborted immediately and the burned skin was cooled. The child was transferred to the recovery room, seen by a paediatric surgeon and conservative treatment was initiated. A scar at the lateral side of the neck and above the right clavicle persisted, while the other skin defects healed over a time of more than 6 months.
There are two possible explanations for this unfortunate accident. The twisted high-resistance cable, which was specially designed for the use in the MR environment, may have inadvertently formed an inductive loop as the child was moved into the tube. Alternatively, one of the carbon electrodes may not have been fastened properly, thus allowing heat development. No defects were detected at the cable itself, which later was checked and replaced by the manufacturer.
Dempsey and colleagues [4,5] revealed that forming a conducting resonant loop rather than forming a conductive loop seems likely to cause heat development and thus burn injuries in conductive lines. Electric current and heat are induced and augmented if the formed inductive loop of the cable is in resonance with the applied radio frequency pulses and/or imaging gradients. Burn injuries during MR examination have repeatedly been reported, such as injuries by temperature monitoring devices or fingertips for respiratory monitoring  and even with tattoos made of metal-containing dark blue colours .
We now use a different monitoring system for ECG recording utilizing fibre-optic signal transmission (Multigas Monitor with Fox-module; Medrad). This ECG device was specifically designed for the use in cardiac MR imaging. The ECG current is converted into light pulses immediately at the patient's chest and transmitted via fibre-optic, non-conductive cables to a monitor outside the magnet. Pulse oximetry is recorded using the same principle. Systems utilizing fibre-optic transmission provide an ECG signal which is quite resistant to artifacts induced by the imaging gradients of the MR scanner. Furthermore, they are unable to induce local electric currents that may be harmful to the patients even in the setting of a high-performance gradient system with gradient power of 30-50 mT.
Choosing the correct equipment for monitoring patients during sedation or anaesthesia is crucial for the safety of patients undergoing MR examination, since anaesthesia as well as sedation impair the sensation of heat development and pain. Using conductive lines for patient monitoring in the setting of a high-field MR scanner requires meticulous attention to the details of line placement and to the limits of the energy transfer during the imaging process allowed by the manufacturer of the monitoring equipment. Non-conductive equipment utilizing fibre-optic signal transfer out of the magnet does not require this high level of attention to guarantee the safety of patients.
We are indebted to our patient and her family for encouraging us to report on this adverse event.
*Department of Anaesthesiology, Hannover Medical School, Hannover, Germany
†Department of Diagnostic Radiology, Hannover Medical School, Hannover, Germany
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