The wires enter the MRI RF transmitter coil’s effective exposure volume and operate as an antenna, the performance of which depends on the relative positioning of the wires and the human body, patient landmark position, and the quality of the electrical contact between the electrode and skin. For example, a tDCS setup includes two external wires and electrodes. This requires a dedicated setup of wires and electrodes that are in contact with human skin. Īn increasing number of MRI investigations that study the human brain employ multimodal setups, where additional devices are used to record complementary information or manipulate brain states, examples include electroencephalography (EEG), transcranial magnetic stimulation (TMS), and transcranial direct current stimulation (tDCS). Safety limits based on the SAR in MRI are typically derived from three-dimensional (3D) numerical EM simulations of MRI RF transmit coils loaded with human body models. The specific absorption rate (SAR) used as the safety limit in MRI is also subject specific, especially at RF above 100 MHz. The interaction of radio frequency (RF) electromagnetic (EM) fields with the human body during magnetic resonance imaging (MRI) is complex and subject specific. Here, we report (i) our workflow used to generate the surface meshes of a head and torso model from the segmented AustinMan dataset, (ii) head and torso model mesh optimization for three-dimensional EM simulation in ANSYS HFSS, and (iii) several case studies of MRI RF coil performance and safety assessment. These requirements can be fulfilled with anatomically correct surface-based human models and EM solvers based on unstructured meshes.
The electrical contact within and between tissues, as well as between an electrode and the skin, must also be preserved. Examples of such devices include those used during electroencephalography, transcranial magnetic stimulation, and transcranial direct current stimulation, which record complementary information or manipulate brain states during MRI measurement. The dimensions of mesh elements used for discretization of the EM simulation domain must be adequate for correct representation of the MRI coil elements, different types of human tissue, and wires and electrodes of additional devices. MRI radiofrequency (RF) coil performance and safety assessment typically includes numerical EM simulations with a set of human body models. The interaction of electromagnetic (EM) fields with the human body during magnetic resonance imaging (MRI) is complex and subject specific.
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