An Optically Coupled System for Quantitative Monitoring of MRI-Induced RF Currents Into Long Conductors

• Published in: IEEE transactions on medical imaging Vol. 29; no. 1; pp. 169 - 178
• Main Authors: Zanchi, M.G., Venook, R., Pauly, J.M., Scott, G.C.
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.01.2010; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Conductors; Current sensing; Equipment Design; heating; High speed optical techniques; Hot Temperature; Humans; interventional devices; Magnetic resonance imaging; magnetic resonance imaging (MRI); Magnetic Resonance Imaging - adverse effects; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; Monitoring; Optical attenuators; Optical coupling; Optical receivers; Optical sensors; Optical transmitters; Phantoms, Imaging; Radio frequency; Radio Waves - adverse effects; Reproducibility of Results; safety; Transducers
• ISSN: 0278-0062; 1558-254X
• DOI: 10.1109/TMI.2009.2031558

The currents induced in long conductors such as guidewires by the radio-frequency (RF) field in magnetic resonance imaging (MRI) are responsible for potentially dangerous heating of surrounding media, such as tissue. This paper presents an optically coupled system with the potential to quantitatively measure the RF currents induced on these conductors. The system uses a self shielded toroid transducer and active circuitry to modulate a high speed light-emitting-diode transmitter. Plastic fiber guides the light to a photodiode receiver and transimpedance amplifier. System validation included a series of experiments with bare wires that compared wire tip heating by fluoroptic thermometers with the RF current sensor response. Validations were performed on a custom whole body 64 MHz birdcage test platform and on a 1.5 T MRI scanner. With this system, a variety of phenomena were demonstrated including cable trap current attenuation, lossy dielectric Q -spoiling and even transverse electromagnetic wave node patterns. This system should find applications in studies of MRI RF safety for interventional devices such as pacemaker leads, and guidewires. In particular, variations of this device could potentially act as a realtime safety monitor during MRI guided interventions.