Evaluation of the RF-induced lead-tip heating of AIMDs using a Volume-Weighed Tissue-Cluster Model for 1.5T MRI.

The RF-induced lead-tip heating of AIMDs is related to the tangential electric field distribution along the AIMD lead paths in patients and the electromagnetic behavior (represented by the transfer function model) of the AIMDs. To evaluate the in-vivo RF-induced lead-tip heating of AIMDs using in-vitro methods, the electric field distribution is critical. In this paper, we proposed a Volume-Weighed Tissue-Cluster Model, a feasible bench method, to simplify the evaluation of the in-vivo RF-induced lead-tip heating of AIMDs. The incident electric field distribution inside this simplified model is highly correlated to that of the original inhomogeneous human body model. Compared to the RF-induced lead-tip heating results in the original model, the maximum error of the lead-tip heating in this Volume-Weighed Tissue-Cluster Model is less than 1 °C. The correlation coefficients of the temperature rise between the two models are higher than 0.997.Clinical Relevance- Simplified and accurate anatomical models can be used to emulate the in-vivo heating assessment for MRI safety.

Effects of patient orientations, landmark positions, and device positions on the MRI RF-induced heating for modular external fixation devices.

PURPOSE: This paper studies the RF-induced heating for modular external fixation devices applied on the leg regions of the human bodies. Through numerical investigations of RF-induced heating related to different patient orientations, landmark positions, and device positions under 1.5T and 3T MRI systems, simple and practical methods to reduce RF-induced heating are recommended. METHODS: Numerical simulations using a full-wave electromagnetic solver based on the finite-difference time-domain method were performed to characterize the effects of patient orientations (head-first/feet-first), landmark positions (the scanning area of the patient), and device positions (device on left or right leg) on the RF-induced heating of the external fixation devices. The G32 coil design and three anatomical human models (Duke model, Ella model, and Fats model) were adopted to model the MRI RF coil and the patients. RESULTS: The relative positions of the patient, device, and coil can significantly affect the RF-induced heating. With other conditions remaining the same, changing the device position or patient orientation can lead to a peak 1-g averaged spatial absorption ratio variation of a factor around four. By changing the landmark position and the patient orientation, the RF-induced heating can be reduced from 1323.6 W/kg to 217.5 W/kg for the specific scanning situations studied. CONCLUSION: Patient orientations, landmark positions, and device positions influence the RF-induced heating of modular external fixation devices at 1.5 T and 3 T. These features can be used to reduce the RF-induced heating during MRI simply and practically.

Wire-based sternal closure: MRI-related heating at 1.5 T/64 MHz and 3 T/128 MHz based on simulation and experimental phantom study.

PURPOSE: The paper investigates factors that affect the RF-induced heating for commonly used wire-based sternal closure under 1.5 T and 3 T MRI systems and clarifies the heating mechanisms. METHODS: Numerical simulations based on the finite-difference time-domain method and experimental measurements in ASTM (American Society for Testing and Materials) phantom were used in the study. Various configurations of the wire-based sternal closure in the phantom were studied based on parameter sweeps to understand key factors related to the RF-induced heating. In vivo simulations were further performed to explore the RF-induced heating in computational human phantoms for clinically relevant scenarios. RESULTS: The wire-based sternal closure can lead to peak 1-g averaged spatial absorption ratio of 106.3 W/kg and 75.2 W/kg in phantom and peak 1-g averaged specific absorption rate of 32.1 W/kg and 62.1 W/kg in computational human models near the device at 1.5 T and 3 T, respectively. In phantom, the simulated maximum temperature rises for 15-minute RF exposure are 9.4°C at 1.5 T and 5.8°C at 3 T. Generally, the RF-induced heating will be higher when the electrical length of the device is close to the resonant length or when multiple components are spaced closely along the longitudinal direction. CONCLUSION: The RF-induced heating related to wire-based sternal closure can be significant due to the antenna effect and capacitive mutual coupling effect related to the specific geometries of devices.