MRI safety: RF induced heating on straight wires
Purdue Univ., West Lafayette, IN, USA IEEE Transactions on Magnetics
(Impact Factor: 1.39).
11/2005; 41(10):4197 - 4199. DOI: 10.1109/TMAG.2005.854803
One of the most crucial magnetic resonance (MR) safety concerns is related to excessive heating of metallic implants by the radio frequency (RF) magnetic field. In this study, heating by the MR imaging (MRI) RF magnetic field of bare and insulated wires was evaluated to model the heating of medical lead wires. Currents induced in the wire were calculated using the method of moments. The electric field in the tissue surrounding the wire was calculated to determine the power deposition. From the power, the temperature rise was calculated using the bioheat equation. For bare wires, the calculated and maximal temperature rise, which is about 28°C, occurred for a length of 20 cm. For lengths exceeding 20 cm, temperature rises for the insulated wires were greater, and the resonance length exceeded 40 cm.
Available from: Stefano Pisa
- "For typical RF sequences used in MRI the IEC limits are usually satisfied in terms of SAR and temperature    . However, if a subject with an implanted pacemaker is exposed to MRI radiofrequency (RF) fields, temperatures well above the IEC limits are reached at the catheter tip      . "
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ABSTRACT: In this paper a transverse electromagnetic (TEM) coil operating at 128 MHz in a 3-T magnetic resonance imaging system has been studied in terms of the interaction with patients with or without an implanted pacemaker. The pacemaker has been simulated as a copper box with a catheter constituted by an insulated copper wire with an uncapped tip and it has been placed inside either box or anatomical models of the thorax. Electromagnetic and thermal simulations have been performed by using finite difference time domain codes. The obtained results show that in the absence of the pacemaker, and for a radiated power producing in the box a whole body specific absorption rate (SAR) of 1 W/kg, that is a typical value for MRI examinations, the coil produces in the anatomical models peak temperature values lower than the limits issued by the International Electrotechnical Commission (IEC). In the presence of the pacemaker, temperature increments at the catheter tip in excess of those issued by the IEC standard are obtained when the MRI scanned area involves the pacemaker region. The 3-T coil produces lower SAR and temperature increments with respect to a 64-MHz (1.5-T system) birdcage antenna in patients with implanted pacemaker.
Available from: Jörg Müller
- "However, extended electrical wires for signal transmission during a catheter intervention inside an MRI could lead to serious tissue damage -. The maximum temperature rise allowed by a catheter inside a patient is 4°C . "
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ABSTRACT: Applying an active intravascular MR catheter device that allows signal transmission from the catheter tip requires special means to avoid radiofrequency-induced heating. This article presents a novel, miniaturized all-optical active MR probe to use with real-time MRI in minimally invasive interventions for catheter guidance and intravascular imaging. An optical link transmits the received MR signals from the catheter tip to the MR receiver with inherently radiofrequency-safe optical fibers. Furthermore, power is supplied optically to the transmitter as well. The complete integration into a small tube of 6-Fr (2-mm diameter) size with a 7-Fr (2.33-mm diameter) rigid tubing was realized using chip components for the optical modulator and a novel miniaturized optical bench fabricated from silicon substrates with 3D self-aligning structures for fiber integration. In MRI phantom measurements, projection-based tip tracking and high-resolution imaging were successfully performed with the optical link inside a 1.5-T MRI scanner. Images were obtained in a homogeneous phantom liquid, and first pictures were acquired from inside a kiwi that demonstrates the potential of the MR-safe optical link. The signal-to-noise ratio has significantly improved compared with former systems, and it is demonstrated that the novel optical link exhibits a signal-to-noise ratio comparable to a direct electrical link.
Available from: stanford.edu
- "The set of basis functions is the one described in . A more detailed description of the method can be found in  and . Compared to the previous work, where a uniform electric field was assumed, the calculations here make use of the nonuniform electric field in the phantom. "
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ABSTRACT: There are three principal magnetic fields in magnetic resonance imaging (MRI) that may interact with medical implants. The static field will induce force and torque on ferromagnetic objects. The pulsed gradients are of audio frequency and the implant may concentrate the induced currents, with a potential for nerve stimulation or electrical inference. The currents induced in the body by the radio frequency (RF) field may also be concentrated by an implant, resulting in potentially dangerous heating of surrounding tissues. This paper presents basic information about MRI interactions with implants with an emphasis on RF-induced heating of leads used for deep brain stimulation (DBS). The temperature rise at the electrodes was measured in vitro as a function of the overall length of a DBS lead at an RF frequency of 64 MHz. The maximal temperature rise occurred for an overall length of 41 cm. The method of moments was used to calculate the current induced in the lead. From the induced currents, the RF power deposition near the electrodes was calculated and the heat equation was used to model the temperature rise. The calculated temperature rises as a function of lead length were in good agreement with the measured values.
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