MRI safety: RF-induced heating near straight wires
ABSTRACT 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.
- SourceAvailable from: Jörg Müller[Show abstract] [Hide abstract]
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.Magnetic Resonance in Medicine 01/2012; 67(1):148-55. DOI:10.1002/mrm.23002 · 3.40 Impact Factor
- [Show abstract] [Hide abstract]
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.IEEE Transactions on Device and Materials Reliability 10/2005; DOI:10.1109/TDMR.2005.859033 · 1.54 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: In this paper Birdcage and TEM coils operating at 128 MHz have been studied and compared in terms of their interaction with pacemaker holders. The pacemaker (PM) 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 a box model of the thorax. Electromagnetic and thermal simulations have been performed by using CST microwave studio. The obtained results show that in the absence of the PM the two coils produce SAR values lower than the ICNIRP limits while, in the presence of the PM, the birdcage antenna overcomes the ICNIRP limit on the SAR as averaged over 10 g while both coils give rise to temperature increments at the catheter tip in excess of those issued by the ICNIRP standard. In general the TEM coil produces lower SAR and temperature increments with respect to the birdcage antenna.