Having read the excellent review article by Manohin and Manohin  elucidating important physical principles in anaesthesiology, we were disappointed that magnetic resonance imaging (MRI) was not mentioned. Anaesthesiologists are facing increasing demands for anaesthesia to assist MRI examination and MRI-aided interventional procedures often in critically ill patients. This is a complex physical environment for an anaesthesiologist to be confronted with and it may be appropriate to contribute some additional remarks to this important issue.
The MRI environment is characterized by a powerful magnetic field (usually about 1.5 T in current MRI machines) and alternating radio frequency fields (usually at 64 MHz). In MRI, any conductive wire placed inside the bore can be excited by the surrounding electromagnetic field and become a dipole antenna [2,3]. An induced voltage and current occur in this wire and are manifested as waves travelling along the antenna in response to the alternating field. These waves are reflected from the wire ends and its length determines whether so-called resonating waves occur. Thus, the antenna length is the critical factor for possible excessive heating due to resonance phenomena. This can lead, in the worst-case scenario to a conductive material in the MRI scanner forming circuits or antennas that are susceptible to resonant excitement by the field resulting in thermal injuries . Temperature increases of 63.5°C in copper wire antennas of 2200 mm length have been reported under resonant conditions . Minor heating effects due to Ohmic heating can occur even in conductors of inappropriate length for resonant excitement [2,4,5].
When patients undergoing MRI are exposed to magnetic fields and radio frequency fields, monitoring devices containing wires of conductive metallic material may be either dislocated due to translational attraction and torque caused by the magnetic field or may lead to thermal lesions due to heating effects . The latter is particularly relevant under general anaesthesia when sensations of heat cannot be reported by the patient.
Assuming perfect alignment of an antenna and radio frequency field-lines, the length l of a wire that will resonate can be calculated according to the antenna theory  as the multiple of half the radio frequency wavelength λ as follows: Equation (1)
where c is the velocity of light, f is the radio frequency and ε is the relative dielectric constant of the surrounding medium .
As the wavelength determines the possible resonant length of a conductor exposed to the radio frequency field, it is important to note that equation 1 can be transformed to:
As c is constant, the wavelength λ for resonance is determined by the dielectric constant ε of the surrounding medium and the radio frequency f.
In MRI, the dielectric constant ε as a feature of the material surrounding the wire is the variable that primarily determines the resonant length of a conductor (equation 1). Therefore, the situation of an exposed wire in air (e.g. ECG or EEG cables) has to be distinguished from a wire embedded in conductive material such as tissue or blood (e.g. endovascular devices or pacemaker leads).
With respect to a wire in air, under current MRI conditions, (1.5 T and 64 MHz operating frequency), the length needed for resonant excitement can be calculated from equation 1 to be just under 2350 mm (using c = 3 × 108 ms−1, ε = 1 and f = 64 MHz). This has been validated in an experimental setting . As long as the dielectric constant of the surrounding medium is low (e.g. ε = 1 for air), displacement currents are unlikely to occur and any temperature increase will be due to Ohmic heating of the excited conductor.
With respect to a wire in a conductive material, if there is contact between the wire and the surrounding medium, with regard to equation 1 the length needed for resonance is only 235 mm (using c = 3 × 108 ms−1, f = 64 MHz and assuming a tissue dielectric constant of ε = 100) . According to the law of continuity , induced currents will be zero at the ends of the wire. Since the electric field strength is maximal at the wire ends, electric displacement currents and radio frequency power deposition occurs at these points of discontinuity and may cause significant heating of tissue that directly surrounds the wire ends. This explains the documented observation of heat induction at wire ends [2,5].
Hardly any endovascular catheter device would have the length necessary for resonant excitement in the 1.5 T MRI environment, unless attached to monitoring or connecting cables. These are exposed partially to the radio frequency field, making it difficult to determine a potentially resonant length. MRI associated burns reported so far have been almost exclusively in association with the presence of monitoring or connecting cables that incidentally formed conductive circuits . Thus, disconnection and removal of monitoring cables connected to endovascular devices is obligatory, unless they are approved for MRI use according to FDA or CE regulations. Removal of the device itself may not be necessary, but this decision should be subject to thorough enquiry according to current MRI safety guidelines .
P. H. Tonner
Department of Anaesthesiology and Intensive Care Medicine; University Hospital of Schleswig-Holstein; Christian-Albrechts University; Kiel, Germany
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Mechanism of burn injury during magnetic resonance imaging (MRI) - simple loops can induce heat injury. Front Med Biol Eng
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