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

Abstract

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. K E Y W O R D S antenna effect, capacitive mutual coupling effect, magnetic resonance imaging (MRI), RF-induced heating, wire-based sternal closure 1056 | ZHENG Et al.

The effect of frequency (64–498 MHz) on specific absorption rate adjacent to metallic orthopedic screws in MRI: A numerical simulation study

“…A motivating factor for this study was the scarce and contradictory data in the published literature describing the effect of increasing frequency on the RF‐induced SAR in tissue close to electrically conductive implants. For example, in comparisons between 64 and 128 MHz, some studies reported higher SAR levels at higher frequencies while others reported the opposite 3,14–18 . Similarly, in a recent study investigating SAR for various DBS lead trajectories across field strengths (1.5 T to 10.5 T), higher SAR was reported at higher fields when the input power to the RF transmitters was adjusted to produce a B 1 + field of 2 μT, while lower SAR was reported up to 7 T, increasing at 10.5 T, when the input power was adjusted to produce a global average SAR of 3 W/kg 20 .…”

“…For example, in comparisons between 64 and 128 MHz, some studies reported higher SAR levels at higher frequencies while others reported the opposite. 3,[14][15][16][17][18] Similarly, in a recent study investigating SAR for various DBS lead trajectories across field strengths (1.5 T to 10.5 T), higher SAR was reported at higher fields when the input power to the RF transmitters was adjusted to produce a B 1 + field of 2 μT, while lower SAR was reported up to 7 T, increasing at 10.5 T, when the input power was adjusted to produce a global average SAR of 3 W/kg. 20 A contributing factor to the different apparent…”

“…For example, a 78 mm screw alternatively produces either a higher or lower SAR at 297 MHz compared to 128 MHz depending on the material in which it is embedded. Previous studies only investigated one or a small range of implant lengths, [14][15][16][17][18][19][20]31,32 which could lead one to erroneously conclude that one frequency or the other produced the worst SAR.…”

“…Many prior peer-reviewed studies 3,[8][9][10][11][12][13] have investigated RF-induced tissue specific absorption rate (SAR) adjacent to conducting implants in a variety of both physical and simulation environments, with only a hand-ful reporting SAR at several frequencies/magnetic field strengths, mainly 1.5T/3T. 3,[14][15][16][17][18][19][20] A limitation of these studies was a lack of knowledge of or control over the incident electric field along the full length of the implant. The highly heterogeneous spatial distribution of the electric fields produced by the birdcage-type RF coils 21 used in those studies inevitably resulted in a high (and unknown) degree of variability of the resulting SAR, invalidating a direct comparison of SAR across field strengths.…”

“…Many prior peer‐reviewed studies 3,8–13 have investigated RF‐induced tissue specific absorption rate (SAR) adjacent to conducting implants in a variety of both physical and simulation environments, with only a handful reporting SAR at several frequencies/magnetic field strengths, mainly 1.5T/3T 3,14–20 . A limitation of these studies was a lack of knowledge of or control over the incident electric field along the full length of the implant.…”