Impact of RF Shimming on RF-Induced Heating Near Implantable Medical Electrodes in a 3T MRI Coil

Radio frequency (RF) shimming is a widely used method to improve the MRI quality by adjusting the magnitude and phase of the transmit elements in the RF coil and eliminating the field inhomogeneity in 3T MRI system. This process alters the field distribution inside the MRI borehole and changes both the level and location of the absorbed RF energy in patients. It is necessary to assess the impact of RF shimming on the risk of RF-induced heating for patients especially those who have implantable longitude medical electrodes in place during the MRI procedure. This paper investigates the impacts of RF shimming on the RF-induced heating of a generic pacemaker electrode using a transfer function method. Two different shimming schemes were studied. The first excitation scheme was designed to obtain a homogeneous B
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field in the image region-chest region. The second condition maximized the temperature rise around the generic pacemaker electrode with different implant trajectories. All the results were normalized to 2 W/kg average whole-body specific absorption rate (SAR) and a Q-matrices method was used to improve the SAR calculation efficiency. The average temperature rise at the electrode tip under quadrature excitation was 3.7 °C while under optimized excitation was 2.3 °C. The worst-case temperature rise for a selected trajectory was found to be 33.17 °C, which was almost 6 times of the temperature rise under quadrature excitation. Therefore, the RF-shimming condition can significantly change the RF-induced heating of electrodes. To ensure patient safety under MRI, RF-shimming condition should be considered and thoroughly integrated into the RF heating assessment.