1.5 T RF Birdcage Simulations, 64 MHz

Model

The simulations in this study are based on a CAD model of an experimental 1.5 T birdcage developed by Zurich Med Tech for experimental assessment of SAR, safety issues for implants and leads, etc. The .IGES CAD model was imported directly. A leg feed excitation is achieved by introducing 16 edge sources at the center of the legs. Matching components are introduced in the rings between the legs.

A high resolution adult male model consisting of more than 100 tissues is also used to study absorption in the body and compare local peak SAR hotspots with whole body averages.

Simulation

The operational frequency of the MRI system is 64 MHz so the structure is not electrically large. However, the curvature and complexity of the model still requires that it is necessary to use a fine grid to resolve the structure. Furthermore, small matching network components applied between the legs must be resolved by the grid so that when voxelled the correct connectivity is achieved. Furthermore, when including the human model, a max. grid step of 3 mm is enforced to ensure an accurate representation of localized absorption effects in the body. 

The grid size for the empty birdcage model is 8.1 Mcells while when including the human model the grid increase to  88 Mcells due to the high resolution used in the patient. Both simulations are run on the Acceleware cluster-in-a-box CiB 1000 with 8 GB of RAM. The resulting simulation times are 38min and 5hr 50 respectively.

Results

The H- and B-field homogeneity can be studied from the empty birdcage simulations. Furthermore, by animating the phase change of the H-field vector the rotation of the H-field can be nicely illustrated.

Localized SAR hotspots are studied from the birdcage coil including the patient. Comparing whole-body, 10 g and 1 g spatial peak ave. SAR to illustrate the need to address localized effects rather than only whole-body. Coupled EM-thermal simulation can also be used to study temprature distributions in the body.