RF‐induced temperature elevation along metallic wires in clinical magnetic resonance imaging: Influence of diameter and length

Abstract

With the development of interventional MRI, heating of biological tissues along the metallic wires in the MRI scanner has become an important issue. To assess thermal response to RF exposure during MRI, we studied the temperature elevation near nonmagnetic metallic wires. All tests were performed on a 1.5 T clinical scanner. Four experiments were conducted to investigate the effects of the wire diameter, the excitation flip angle, the temperature distribution along the wire, and the wire length. Electromagnetic simulations of the experimental setup were made with the use of commercial method of moments In the last few years, interest in interventional MRI has grown considerably (1,2). Since the use of metallic guidewires or catheters is necessary in interventional MRI, the security aspects of such devices are of great importance. The use of endoluminal coils also raises specific safety issues. Such coils are generally made of metallic parts both at the top (loop as sensor) and along the catheter (coaxial wires for transmitting RF signal) (3). Since metallic wires are located in the MRI tunnel, heating hazards can occur inside the patient's body. Thus, along with the development of endoluminal sensors, patient safety must be ensured with respect to international standards. Previous attempts to characterize patient safety showed that heating hazards are difficult to predict. For this reason, it is important to focus first on the simple model of wires inside an MRI tunnel before studying the endoluminal sensor.The major issue is to ensure patient safety against potential heating of tissues located in the vicinity of the metallic wire (4 -6). During RF excitation, in addition to the magnetic field B 1 , an electric field E induces currents (at the same frequency) in the metallic wire placed in the MR scanner. Furthermore, the metallic wire concentrates the RF electric field, and may lead to a significant temperature increase.Beyond the well-known case of metallic implants (even nonferromagnetic ones) that can cause heating hazards (7,8), several authors have investigated the specific case of metallic wires during interventional MRI (9 -12). Most studies have shown that the specific absorption rate (SAR) measured in the presence of a metallic wire can overcome the SAR limitation of 2 W/kg (1,13-15). This SAR elevation induces an increase of temperature in the near vicinity of the wire. The heating close to the wire follows the tissue bio-heat law established by Pennes in 1948 (16). According to this law, local heating can be expressed as the algebraic sum of the following energies: 1) the metabolic activity of the cells forming the surrounding tissues, 2) the energy evacuated by the blood flow irrigating the tissues, and 3) the energy concentrated locally by the SAR. Without RF stimulation, the first two terms equilibrate the temperature of the region at a steady level equal to the blood flow temperature. During a clinical MRI examination, the third term adds local heating by SAR. Indeed, since the metallic wire conc...

Evaluation of RF Heating on Humerus Implant in Phantoms during 1.5T MR Imaging and Comparisons with Electromagnetic Simulation

“…Research concerning implants in the human body has focused primarily on durability and biocompatibility, and research into their safe use in MR imaging is insu‹cient. However, the in‰uence of RF heating of the implant on the human body during MR imaging has been examined, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and in most reports, the heating is small. [1][2][3][4][5][6][7][8][9] RF heating is attributed mainly to electric currents induced by RF irradiation and switching of a gradient magneticˆeld.…”

“…14,16 In addition, there are some reports of burn accidents caused by the loop formation during MR procedures. [17][18][19] Extra caution is still needed when an implant is in the loop.…”

Specific Absorption Rate Analysis of Heterogeneous Head Models with EEG Electrodes/Leads at 7T MRI

“…The interactions of the EEG leads and RF coils will induce changes in the EM field inside the head (i.e., "shielding effect" of the EEG leads) [Hamblin 2007] and local SAR enhancement at the interface between electrodes and skin [Armenean 2004, Yeung 2002 Table 1) determines an electric field enhancement at the interface between leads and head surface (i.e., epidermis). This observation is in line with theoretical models [Guy 1975] and physical evidence of reports of burns due to "antenna-effect" of leads [Dempsey 2001].…”

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Role of the lead structure in MRI‐induced heating: In vitro measurements on 30 commercial pacemaker/defibrillator leads

“…In particular, the RF-induced heating in tissues with metal implants is a serious concern as it can pose severe health risks (12)(13)(14)(15)(16).…”