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  • Title: Offline impedance measurements for detection and mitigation of dangerous implant interactions: an RF safety prescreen.
    Author: Ellenor CW, Stang PP, Etezadi-Amoli M, Pauly JM, Scott GC.
    Journal: Magn Reson Med; 2015 Mar; 73(3):1328-39. PubMed ID: 24623586.
    Abstract:
    PURPOSE: The concept of a "radiofrequency safety prescreen" is investigated, wherein dangerous interactions between radiofrequency fields used in MRI, and conductive implants in patients are detected through impedance changes in the radiofrequency coil. THEORY: The behavior of coupled oscillators is reviewed, and the resulting, observable impedance changes are discussed. METHODS: A birdcage coil is loaded with a static head phantom and a wire phantom with a wire close to its resonant length, the shape, position, and orientation of which can be changed. Interactions are probed with a current sensor and network analyzer. RESULTS: Impedance spectra show dramatic, unmistakable splitting in cases of strong coupling, and strong correlation is observed between induced current and scattering parameters. CONCLUSIONS: The feasibility of a new, low-power prescreening technique has been demonstrated in a simple phantom experiment, which can unambiguously detect resonant interactions between an implanted wire and an imaging coil. A new technique has also been presented which can detect parallel transmit null modes for the wire.
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