Safety of metallic implants in magnetic resonance imaging

Abstract

Magnetic resonance (MR) imaging has become a commonly accepted medical procedure. Manufacturers of medical implant devices are submitting claims that their devices are safe and effective in a MR environment. This paper concentrates on the issue of heating of patients due to the interaction of metallic implants with the strong radiofrequency (RF) magnetic field produced by the MR scanner. The commercially available program XFDTD was used to calculate the specific absorption rate (SAR) distribution in a realistic model of the human body. The body contained a metallic implant and was exposed to RF magnetic fields at 64 MHz from a model of a MR birdcage body coil. The results of the calculation showed that the magnitude of the increased heating of tissues due to the presence of the metallic implant depended on the dimensions, orientation, shape, and location of the metallic implant in the patient. This increased heating of surrounding tissues primarily concentrates in a small volume near the tip of the metallic wire. When the whole-body SAR was normalized to 1 W/kg, a calculated value of 41 W/kg was obtained at this location if the absorption was averaged over 1 g of tissue. However, a maximum value of 310 W/kg was calculated when the absorption was averaged over 1/8 g of tissue.

Evaluation of the Susceptibility Artifacts and Tissue Injury Caused by Implanted Microchips in Dogs on 1.5T Magnetic Resonance Imaging

“…Electrical properties such as the magnetic susceptibility, electrical resistivity, and thermal conductivity of metal materials of the implant have a significant influence on heat production. Other factors include the implant's size, shape, and orientation relative to the direction of the magnetic field, the magnetic field strength, and RF pulse duration and duty cycles [6,12]. In cases where the implant is elongated, the heat pri- marily concentrates at the tip of the implant, and such a heat concentration point is called a "hot spot" [6,12].…”

“…Other factors include the implant's size, shape, and orientation relative to the direction of the magnetic field, the magnetic field strength, and RF pulse duration and duty cycles [6,12]. In cases where the implant is elongated, the heat pri- marily concentrates at the tip of the implant, and such a heat concentration point is called a "hot spot" [6,12]. It has been shown that the tip of small metallic wire causes a major temperature rise in a static magnetic field of 1.5T [6].…”

Interaction between the RF Field of MRI Apparatus and Pacemakers

“…For bare catheters, the obtained temperature rise ranges from 1 to 28 °C depending on the catheter length and position. Moreover, a commercially available computer aided design (CAD) has been used for evaluating the SAR distribution produced in a realistic human model by a coarse catheter model (1 cm diameter, 8 cm length) implanted in the heart region (Ho, 2001). With this model, a peak SAR averaged over 1 g of 2 W/kg near the metallic implant (compared to the value of 1.8 W/kg at the same location without the metallic implant) has been computed and no appreciable heating has been obtained.…”

“…The most used is a simple bare cylindrical copper wire (see Fig. 11a) with various diameters and lengths (Ho, 2001;Golombeck & Dossel, 2004;Stuchly et al, 2006;Pisa et al, 2008), while a more realistic version is constituted by a copper wire covered with plastic (see Fig. 11b).…”

Safety of cardiovascular magnetic resonance in patients with cardiovascular implants and devices

“…The dimensions, orientation, shape, and location of the metallic implant all play a role. 4 Finally, exposure to the electromagnetic field used by CMR scanners may inhibit or modify the operational or functional settings of an electronic implant.…”