Electromagnetic and modeling analyses of an implanted device at 3 and 7 Tesla

To study the specific absorption rates (SAR) associated with implantable devices at 3T and 7 T.

Studies were carried out utilizing a finite difference time domain (FDTD) model that treats the radio frequency (RF) coil and an anatomically detailed human head mesh as a single system. Analyses were performed at 3 T and 7 T for different orientations and positions of an implanted (in the brain) aneurysm clip. Studies were also performed for two different types of FDTD mesh of the same aneurysm clip.

The results showed that: 1) the electromagnetic effects of implanting the aneurysm clip (in the brain) is mostly local on SARs; and 2) orientations of the implanted aneurysm clip have considerable effect on the local SARs near the implanted clip; the level of such an effect can also vary significantly between 3 T and 7 T.

In general, the presented study shows that the local SARs (in 1 g and in 10 g of tissue) near the implanted aneurysm clip are lower than the peak SARs (due to the standard RF coil operation) in other regions of the human head mesh/brain. For specific orientations, however, if the aneurysm clip is implanted in a region in which the brain peak-SAR occurs due the standard RF coil operation, the brain peak SAR increases further. This is more prevalent at 7T compared to 3T. Additionally, it was also found that basic structured and Cartesian FDTD modeling produces relatively higher local SARs than that obtained with simple non-Cartesian FDTD modeling.