Dependence of RF Heating on SAR and Implant Position in a 1.5T MR System

Abstract

Purpose: We evaluated radiofrequency (RF) heating of a humerus implant embedded in a gel phantom during magnetic resonance (MR) imaging for the speciˆc absorption rate (SAR), angle between the implant and static magneticˆeld (B 0 ), and position of the implant in the irradiation coil.Methods: We embedded a stainless steel humerus implant 2 cm deep in tissue-equivalent loop and mass phantoms, placed it parallel to the static magneticˆeld of a 1.5T MR scanner, and recorded the temperatures of the implant surface with RF-transparentˆberoptic sensors. We measured rises in temperature at the tips of the implant by varying the SAR from 0.2 to 4.0 W/kg and evaluated RF heating of the implant for its angle to B 0 and its displacement along B 0 from the center of the RF irradiation coil.Results: RF heating was similar for the loop and mass phantoms because the eddy current ‰ows through the periphery of both. As the SAR increased, the temperature at the implant tip increased, and there was a linear relationship between the SAR and temperature rise. The values were 6.49 C at 2.0 W/kg and 12.79 C at 4.0 W/kg. Rise in temperature decreased steeply as the angle between the implant and B 0 surpassed 459 . In addition, as the implant was displaced from the center of the RF coil to both ends, the rise in temperature decreased.Conclusion: The rise in temperature in deep tissue was estimated to be higher than 1.09 C for SAR above 0.4 W/kg. RF heating was greatest when the implant was set parallel to B 0 . In MR imaging of patients with implants, there is a risk of RF heating when the loop of the eddy current is formed inside the body.

Use of Brain MRI after Deep Brain Stimulation Hardware Implantation

“…More recently, Tagliati et al (2009) reported that 3,304 DBS patients had brain MRI at 40 centers responding to a questionnaire. Despite many centers reporting use of postoperative brain MRI, including use of the SP, V, or VP platforms (Duffner, Schiffbauer, Breit, Friese, & Freudenstein, 2002;Elias, Fu, & Frysinger, 2007;Hamid et al, 2005;Larson et al, 2008;Mehdorn et al, 2005;Pillon et al, 2000;Pollo et al, 2007;Schrader, Hamel, Weinert, & Mehdorn, 2002), as well as series involving 100 or more DBS leads (Benabid et al, 2002;Elias et al, 2007;Funkiewiez et al, 2004;Goodman et al, 2006;Hemm et al, 2005;Larson et al, 2008;Mehdorn et al, 2005;Oh, Abosch, Kim, Lang, & Lozano, 2002;Umemura et al, 2003) including the body weight entered, sequence parameters, and also by the specific object within the field as well as the precise location and positioning of the object in the field Mattei et al, 2008;Muranaka et al, 2007). For example, displayed SAR H is increased in our experience for a given patient in the setting of a stereotactic head ring.…”

“…Specific absorption rate (SAR) is defined as the RF power absorbed per unit mass of an object (International Electrochemical Commission (IEC) Standard 60601-2-33, 2004). A linear relationship between SAR and DBS lead-contact heating has been suggested in laboratory models (Baker, Tkach, Phillips, & Rezai, 2006;Muranaka et al, 2007). Routine brain imaging employs scans with SAR H values up to 30 times greater than the SAR H recommended in manufacturer guidelines in the setting of implanted DBS hardware (IEC Standard 60601-2-33, 2004;U.S.…”

Evaluation of RF Heating due to Various Implants during MR Procedures

“…These results were previously reported. 22 Comparison of RF heating between 1.5-and 3.0-T MR systems…”

MRI-Induced Tissue Heating at Metallic Sutures (Cerclages)

“…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]. The diverging results demonstrate the need for case-by-case risk assessment.…”