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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.

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.

This study investigates the radio frequency (RF)-induced heating for a common medical implanted device, the trauma screw, in different human models under 1.5 T and 3 T magnetic resonance imaging (MRI) systems. Three different human models, Duke, Ella, and Fats from the virtual family project, are used in the simulations. The human models and the device are loaded in a generic 32 ports RF coil. The screws with suitable sizes are implanted in four regions (hip, knee, shoulder, and wrist) for each human model. From the simulation results, the RF-induced heating near the device shows significantly difference within the models. Consequently, the clinically relevant RF induced heating can be patient specific and needs to be investigated individually.

This paper presents a comparison study of searching strategies for the worst-case construct for an implantable internal fixation system under 1.5T MRI radio frequency (RF) birdcage coil. An example of the generic plate and screw system is used as the device of interest. Based on the combinations of the values of the six device parameters, 576 models are studied and the peak 1 g averaged specific absorption rate (SAR) near devices are extracted to build a dataset for searching. Three searching strategies, which are sequential one-dimension searching, searching based on various sequences and searching with multi-iterations, are investigated. It is found that the searching sequences and initial values of the device parameters have effects on the searched worst-case results. The possibility of getting the failed search results when searching with random sequence and initial parameter values can be high. Searching with multi-iterations can improve the accuracy of correctly identifying the worst-case heating construct.