RESUMEN
In MRI examination, RF heating of implants will affect the safety of implant wearers. The conductivity of various tissues in the human body is significantly different, and the medium conductivity will affect the distribution of the RF electric field. Therefore, it is necessary to study the RF heating of different medium conductivity. Based on the analysis of the principle of MRI RF heating, this study build the model of the bird cage coil, ASTM standard phantom and lead, and the conductivity of several typical human tissues is selected as the conductivity in the experiment. Then calculate the power deposition of the lead at 64 MHz. The results show that the medium conductivity has no effect on the distribution of electric field and power deposition, and the hot spot distribution remains unchanged under different conductivity; The smaller the conductivity is, the larger the power deposition of the lead is, and the greater the temperature rise of the lead caused by RF heating is; The change of conductivity and power deposition is approximately linear. At the limit of 2 W/kg whole body specific absorption rate(SAR), the conductivity decreases, and the wire power deposition increases sharply.
RESUMEN
The complex electromagnetic field environments in magnetic resonance imaging system(MRI) can have a significant impact on patients carrying implants, the RF heating problems being particularly important. To ensure the safety of the patients, it is necessary to understand the distribution of tissue temperature in the MRI environment and its changes over time. Based on the analysis of tissue temperature rise in MRI, this paper constructs a bird cage coil for generating RF field in MRI system, and constructs ASTM standard/improved phantom and single-cavity pacemaker finite element models, use time-domain finite difference (FDTD) to simulate. Firstly, the correctness of the simulation software and simulation method was validated according to the method of ISO. Then the distribution of the electric field, SAR and temperature field and the temperature change with time were calculated in the environment of 64 MHz, 2 W/kg. The difference in temperature rise with blood heat exchange and no blood heat exchange (standard/improved phantom) was specifically compared. The simulation results show that there are electric field and SAR hotspots near the electrode tip, the wire tail and the case of pacemaker. There are high SAR values on both sides of the phantom, and the shorter the distance from the coil, the higher the SAR. The temperature field distribution is similar to the SAR distribution; the temperature is higher in the area around the end of the wire and the case of pacemaker because the heat accumulation is higher around this area. At the same time, blood heat exchange can reduce the temperature rise to a certain extent.