Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit

Abstract

Purpose: To examine gradient switching-induced heating of metallic parts. Materials and Methods:Copper and titanium frames and sheets (Ϸ50 ϫ 50 mm 2 , 1.5 mm thick, frame width ϭ 3 mm) surrounded by air were positioned in the scanner perpendicular to the static field horizontally 20 cm off-center. During the execution of a sequence (three-dimensional [3D] true fast imaging with steady precession [True-FISP], TR ϭ 6.4 msec) exploiting the gradient capabilities (maximum gradient ϭ 40 mT/m, maximum slew rate ϭ 200 T/m/ second), heating was measured with an infrared camera. Radio frequency (RF) amplitude was set to zero volts. Heating of a copper frame with a narrowing to 1 mm over 20 mm at one side was examined in air and in addition surrounded by several liters of gelled saline using fiber-optic thermography. Further heating studies were performed using an artificial hip made of titanium, and an aluminum replica of the hip prosthesis with the same geometry. Results:For the copper specimens, considerable heating (Ͼ10°C) in air and in gelled saline (Ͼ1.2°C) could be observed. Heating of the titanium specimens was markedly less (Ϸ1°C in air). For the titanium artificial hip no heating could be detected, while the rise in temperature for the aluminum replica was approximately 2.2°C. Conclusion:Heating of more than 10°C solely due to gradient switching without any RF irradiation was demonstrated in isolated copper wire frames. Under specific conditions (high gradient duty cycle, metallic loop of sufficient inductance and low resistance, power matching) gradient switching-induced heating of conductive specimens must be considered. ONE IMPORTANT ASPECT in MR safety testing of medical implants and instruments is the inspection of whether hazardous heating can occur in the electromagnetic environment of an MR scanner. In the standard ASTM F 2182-02a (1), heating resulting from the interaction with the radio frequency (RF) magnetic field is considered. Numerous examinations have been reported concerning this subject (e.g., Ref. 2-10). Regarding gradient switching, there are a few works that address the alteration of induced nerve stimulation near metallic implants (11,12). Two publications (13,14) ascribe heat sensations of patients with larger metallic implants to vibrations of these implants caused by gradient switching.In contrast to ASTM F 2182-02a and to those works, the present study is concerned with direct gradient switching-induced heating of medical implants or instruments made of electrically conducting material. Following Faraday's law, the change of the magnetic flux through such a device induces eddy currents in the device and the metal subsequently converts electric energy into thermal energy. This effect increases with distance from isocenter. To obtain insight into the order of magnitude of possible effects, quadratic wire frames and sheets made of copper and titanium were examined. Most experiments were performed in air, since the underlying cause for the heating is pure magnetic induction during gradien...

Safety of Simultaneous Scalp or Intracranial EEG during MRI: A Review

“…A large power is required to produce the RF B 1 field for refocusing pulses (that need to approach 180 • ) and many are applied in a short time period (order 1 per 10 ms) that leads to high RF power levels. This then causes increases in the heating effects because of the coupling between E-field and B-field in the RF band [79]. Gradient echo sequences involve a single RF pulse followed by a gradient reversal [80], and generally have a much lower SAR than SE.…”

“…The switching magnetic gradient fields can also affect the heating in the presence of metallic implants made of conducting materials [79]. The heating from the gradient switching fields can be determined by Faraday's law where the magnetic flux change produced by the scanner induces electrical eddy currents in conductive implants which is then converted to thermal energy.…”

“…The heating from the gradient switching fields can be determined by Faraday's law where the magnetic flux change produced by the scanner induces electrical eddy currents in conductive implants which is then converted to thermal energy. The magnitude of the flux change is strongly dependant on the position relative to the magnet's iso-center [20,79]; however this effect is minimal for EEG electrodes [20,55].…”

Mutual interferences and design principles for mechatronic devices in magnetic resonance imaging

“…Heating RF waves may resonate along conducting wires and build up strong electrical energy [24][25][26][27], and conductive components and loops can heat up due to induced currents [28].…”

MRI-Induced Tissue Heating at Metallic Sutures (Cerclages)

“…Metallic implants were considered acceptable for MRI based on calculated temperature elevations below 1.5˚C [1][2][3] while others concluded they were counterindicated because of calculated temperature elevations up to some 13˚C. Highest temperature increases were reported at tips of straight metallic wires [4][5][6][7][8]. Particular concern was also raised with regard to implanted metallic loops [9].…”