MRI is a fast advancing diagnostic imaging modality and is generally considered as being safe. Nevertheless hazards in the MR environment have to be taken into account, especially hazards related to implants. Some guidance has been devised to determine if an implant can be considered safe in the MR environment (ASTM, ISO/TS standards).
ASTM tests standards mention that implants have to be tested regarding displacement force, torque, radiofrequency-induced heating and artifacts to attain an MR Safe, the MR Conditional or an MR Unsafe label.
Some other interactions are necessary to be tested such as device malfunction for active implants or gradient-induced voltages and gradient-induced heating for both passive and active implants. In the past years, an ISO/IEC joint working group has coded these new testing obligations into standardized test procedures which have been published as an ISO Technical Specification (ISO/TS 10974).
Furthermore ASTM standard test methods for radiofrequency-induced heating are designated for field strengths of either 1.5 or 3 T. However, the development of field strengths of 7 T and beyond should be taken into account due to the tremendously shorter wavelength within body tissue at these field strengths and the higher inhomogeneity of the electromagnetic field distribution.
The role of MR:comp in the IIIOS project focuses on the development of a new method to assess RF heating of implants in the MR environment. This work is performed with the active collaboration of the Erwin L.Hahn Institute in Essen, Germany. For this purpose a 7 T scanner from Siemens was used and the first implant chosen was an aneurysm clip.
A detailed study of the effect of field polarization has been conducted. The clip used was the model 07-934-02 (Mizuho, Medical Inc., Tokyo, Japan) made of titanium alloy. To perform this study, both RF field measurements and simulations were done. SAR elevation was calculated for both measurement and simulation showing a maximum elevation for a parallel polarization of the E-field regarding the major axis of the aneurysm clip (Noureddine Y, Kraff O, Ladd ME, Schaefers G, Wrede K, Bitz AK. RF Safety of Aneurysm Clips at 7 Tesla: Effect of Field Polarization, Proc. Intl. Soc. Mag. Reson. Med. (ISMRM) 20 (May 2012): 2767.).
Figure 1: SAR elevation for parallel and orthogonal polarization
Results acquired in this study will be evaluated by temperature simulation under consideration of bio-heat transfer mechanisms. Furthermore, realistic field distributions produced by typical RF transmit coils will be used (Orzada S, Kraff O, Schäfer LC, Brote I, Bahr A, Bolz T, Maderwald S, Ladd ME, Bitz AK. 8-Channel Transmit/Receive Head Coil for 7 T Human Imaging Using Intrinsically Decoupled Strip Line Elements with Meanders. Proc. Intl. Soc. Mag. Reson. Med. (ISMRM) 17 (Apr 2009): 3010.).
Figure2: RF-transmit coil with head phantom (left) and RF-transmit coil with a human model (right) Software used: SEMCAD-X, SPEAG, Switzerland
The IIIOS project has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no 238802