Impact of Patient Body Posture on RF-Induced Energy Absorption by Orthopedic Plates

Yang, Xiaolin; Zheng, Jianfeng; Kainz, Wolfgang; Chen, Xuemin; Chen, Ji

doi:https://doi.org/10.1155/2024/7418643

Concepts in Magnetic Resonance Part B, Magnetic Resonance Engineering

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Abstract Introduction Methods Results Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2024 | Article ID 7418643 | https://doi.org/10.1155/2024/7418643

Impact of Patient Body Posture on RF-Induced Energy Absorption by Orthopedic Plates

Xiaolin Yang,¹Jianfeng Zheng,¹Wolfgang Kainz,²Xuemin Chen,³and Ji Chen¹

Academic Editor: Giulio Giovannetti

Received20 Feb 2024

Revised01 Apr 2024

Accepted17 Apr 2024

Published30 Apr 2024

Abstract

This study investigates variations in radiofrequency- (RF-) induced energy absorption by orthopedic plates within the human body during 1.5T and 3T magnetic resonance imaging (MRI) scans, considering diverse postures. Using the poseable Duke model, we developed typical postures (O-posture, X-posture, Y-posture, and Z-posture) and placed anatomically correct representations of various orthopedic plates within these postures. Numerical simulations were conducted to evaluate electromagnetic fields and RF-induced energy absorption in these postures near orthopedic plates during MRI scans. Comparing RF-induced energy absorption (peak spatial averaged SAR over 1 g, pSAR_1g) in postured models to the original posture reveals substantial variations. The pSAR_1g differences for X-posture, Y-posture, and Z-posture reach 48%, 134%, and 32% at 1.5T, and 36%, 83%, and 101% at 3T, respectively. Changing posture can lead to higher or lower pSAR_1g. These findings underscore the impact of patient posture on RF-induced energy absorption in orthopedic plates on the ulna bone. The study recommends considering representative body postures in future evaluations for MR conditional labeling of passive implants. Until then, maintaining a neutral posture during MR scans is advised to mitigate unforeseen RF-induced heating risks.

1. Introduction

Magnetic resonance imaging (MRI) is a safe technique for obtaining high-resolution images of soft tissues inside the human body in a noninvasive manner, without the use of ionizing radiation [1, 2]. To ensure high-quality images of target regions, patients are often instructed to maintain specific postures during MRI scans [3, 4]. However, despite instructions to maintain neutral postures, patients’ body conditions and personal habits can lead to some variations in their postures during scans.

Improper postures during MRI scans can lead to severe tissue damage, as evidenced by MRI-related accident reports from the United States Food and Drug Administration (FDA) [5, 6]. For instance, a third-degree burn occurred on the skin of the right hand when it came into contact with the pelvis during an MRI scan, while a second-degree burn occurred on the right elbow when it touched the bore wall of the scanner [7]. Numerical simulations have also confirmed higher radiofrequency (RF)-induced heating due to posture changes [8]. When a closed electrical loop is formed by the patient’s body parts, significant local RF-induced energy absorption of up to 500 W/kg of peak spatial-averaged specific absorption rate over 1 gram (peak SAR1 g) can be induced at the contact points under normal operating conditions (with a whole-body averaged specific absorption rate (SAR) of 2 W/kg) [9]. The temperature rise at the contact points between the elbow and the bore wall can exceed 15°C after 1 minute of RF exposure [10]. Variations in patients’ postures can result in variations in RF-induced heating by affecting the incident fields inside the human body [11].

While the impacts of postures on RF-induced heating are well-documented, there is a gap in research concerning patients with implantable medical devices. This gap is particularly concerning given that some implants are susceptible to high RF-induced heating themselves [12–15]. Orthopedic implants are used as examples since they are often positioned on or near movable limbs to treat various bone fractures. Consequently, diverse limb positioning leads to varying postures. Prior studies have addressed safety concerns regarding patients with orthopedic implants during MRI scans [16–22]. The interaction between the RF field generated by the MRI RF coil and metallic implants can result in high local energy absorption, reaching peak SAR_1g levels of up to 1000 W/kg near the implants [23]. The extent of RF-induced heating in orthopedic implants is heavily influenced by implant dimensions, orientations, and their location within the patient [24]. According to the American Society for Testing and Materials (ASTM) F-2182 [25], passive implants are placed inside the ASTM phantom to evaluate the RF-induced heating. Based on the simulation and experimental results, conditional requirements (including the input power and scan time) of MRI are restricted to ensure the safety of patients. Due to the limitation of homogeneous phantom [26, 27], the anatomically correct human body model is suggested to assess the RF-induced energy absorption. The human body models with neutral body posture were used in the previous studies [18, 24]. While neutral body postures are often representative, variations in patient posture can alter the field distribution inside the human body, thus significantly impacting RF-induced heating in orthopedic implants. This fluctuation in RF-induced heating results in the varying conditional requirements, posing a substantial safety concern for patients with orthopedic implants, necessitating thorough investigation.

Currently, there are few published studies investigating the impact of patient body posture on RF-induced heating for medical implants [28]. One potential challenge is the development of a diverse set of body postures. On one hand, the human body’s flexibility allows for a multitude of postures. On the other hand, creating new body postures is not only complex and time-consuming but also demands significant computational resources and time to simulate the electromagnetic field within the human body with medical implants. As a result, it becomes impractical to assess RF-induced heating across a wide range of postures and implants. Therefore, there is a need to systematically develop a set of typical body posture models based on poseable human body models with an original neutral body posture.

The objective of this study is to develop a set of posture models and evaluate the variation in RF-induced energy absorption of orthopedic implants due to changes in body posture. The methodology involves several steps. First, orthopedic plates of various dimensions are designed to address diverse clinical scenarios. Subsequently, utilizing advanced poseable high-resolution anatomically correct human body models with a neutral posture, a set of postured models is developed to represent patients with orthopedic plates. Following this, numerical simulations are conducted for four imaging landmarks, calculating RF-induced energy absorption based on the simulated electromagnetic field and comparing it across the set of postured models. Finally, the study evaluates the variation in RF-induced energy absorption of orthopedic plates resulting from changes in patient posture.

2. Methods

2.1. Orthopedic Plate Models

Given the arm’s flexibility and its potential to result in various postures within an MRI coil, a bridge plate implanted on the ulna bone serves as the representative orthopedic implant in this study. This bridge plate functions to stabilize multifragment fractures of long bones by connecting two pieces of healthy bone, resembling a bridge [29]. In clinical practice, the plate’s dimensions are typically determined by the length of the fractured bone to ensure proper screw insertion into healthy bone segments. Generally, no more than half of the plate holes are filled with screws, and multiple plate holes are positioned over each main fracture fragment [30].

This study models a set of orthopedic plates with different dimensions based on clinical conditions. The front and side views of these plates are depicted in Figures 1(a) and 1(b), respectively. Their lengths vary: 140 mm, 120 mm, 100 mm, and 80 mm. Each plate features four screws positioned on its two sides. The plate’s width and thickness measure 10 mm and 2 mm, respectively, while the screw’s length and diameter are 14 mm and 4 mm, respectively. In simulations, the plate material is assumed to be a perfect electrical conductor (PEC). Although typical material used for implants is stainless steel or titanium, the relative difference in RF-induced energy absorption near device is less than 1% under both 1.5T and 3T [31]. The placement of the plate on the ulna within the human body is illustrated in Figure 1(c).

(a)

(b)

(c)

2.2. Human Posture Models with Orthopedic Plates

To simulate various postures, the poseable advanced high-resolution anatomically correct human body model (Duke) from the virtual population is utilized. The Duke model’s original posture, which is also the neutral posture, is depicted in Figures 2 A(1), B(1), and C(1), denoted as “O” (for “Original”). Positioned in the Cartesian coordinate system, the Duke model faces in the y-direction, with the x-direction from right to left and the z-direction from foot to head. Modifications to the body posture are achieved through a computational “poser” tool. Pivoting points are added to the joint locations, enabling adjustments to the arm using the poser tool. Typical postures of concern or interest to researchers and radiologists are selected for investigation.

The accidental shifting of a patient’s arm, particularly when it contains implants, towards the MRI bore wall is a cause for concern. In this study, a scenario is considered where an orthopedic plate is placed on the left arm and the arm is horizontally rotated away from the body, resulting in what we refer to as the X-posture. When the human model is in its original posture (O-posture), the horizontal distance between the left arm and the MRI coil is 80 mm. However, as depicted in Figures 2A(2)–2A(4), for human models adopting X-postures (labeled as “X1,” “X2,” and “X3”), these distances are reduced to 40 mm, 20 mm, and 10 mm, respectively.

Furthermore, a patient’s arm may be raised by some objects such as wearing a cast during the MRI scan, which also brings the implants to be closer to the coil. To replicate this scenario, the orthopedic plate is once again positioned on the ulna, and the entire arm is vertically rotated. In the O-posture model, the vertical distance between the left arm and the coil measures 310 mm. As depicted in Figures 2B(2)–2B(4), the arm-to-coil distances for the Y-posture models, designated as “Y1,” “Y2,” and “Y3,” are 170 mm, 100 mm, and 60 mm, respectively.

Moreover, a previous study has determined that RF-induced heating for implants positioned perpendicular to the axis of the coil, known as the z-axis, is reduced within the ASTM phantom. This behavior of RF-induced heating warrants validation in a human body model. Accordingly, the left arm is rotated 90° above the abdomen to align the plate vertically with the RF coil axis, as depicted in Figure 2C(4). In addition, postures involving a 30° and a 45° rotation are developed, as shown in Figures 2C(2)–2C(3). These rotations adjust the arm’s position along the z-direction, denoted as “Z1,” “Z2,” and “Z3.”

After creating the array of postured body models, the orthopedic plates are implanted into these models. To ensure consistent implantation of the orthopedic plate at a fixed position across all posture models, one bounding box is employed for the ulna bone and another bounding box is used to include the orthopedic plate. Across the O-posture, the X-posture, the Y-posture, and the Z-posture models, the relative positions between two bounding boxes remain same in terms of the orientation and the center coordinate.

2.3. Numerical Simulations

A body coil, denoted as G32 [31], with a diameter measuring 630 mm and a length of 650 mm, as depicted in Figure 3(a), serves as the RF source. This coil can produce an RF field identical to that found within a physical MR RF coil, namely, a circularly polarized and uniformly distributed B1 field inside the coil. To power the coil, sixteen voltage sources are positioned at both end rings. On each end ring, adjacent sources have a phase delay of 22.5°. In addition, the voltage sources located at the top and bottom rings exhibit opposite phases. All sources operate with the same amplitude. The operational frequency is 64 MHz at 1.5T and 128 MHz at 3T.

(a)

(b)

Imaging landmark significantly affects RF-induced heating in patients with passive devices [34]. Typically, higher RF-induced heating occurs when the implant resides within the RF coil. Hence, four distinct imaging landmarks are selected for simulation with the ulna plate inside the generic RF coil. The landmark is placed at the isocenter of the coil. As illustrated in Figure 3(b), landmark LM4 for the human body model is positioned at the junction between the humerus and the ulna. Progressing downwards along the z-direction, landmarks LM3, LM2, and LM1 are marked at intervals of 100 mm, spanning 300 mm.

The simulations are conducted using the commercial software SEMCAD X, employing the finite-difference time-domain (FDTD) method. To balance computational time with resolution, the mesh size of the human body model is set at 2 mm. For proper voxelization of the plate, the ulna plate employs a smaller mesh size of 0.7 mm. With approximately 90 million cells, the final computational space achieves a compromise between model resolutions and computational time. The platform for simulations was a Dell XPS desktop with processor of Intel® Core™ i7-6700 and RAM of 16.0 GB. The NVIDIA TESLA C2070 GPU was employed to run the FDTD simulation, and each simulation can be finished in 8 hours. To truncate the simulation domain, the perfectly-matched-layer boundary condition was used to absorb the outgoing electromagnetic signals.

In all results below, we followed the IEC 60601-2-33 to limit the input power using the whole-body (WB) SAR at 2 W/kg or head SAR at 3.2 W/kg for normal operating mode. While the averaged whole-body SAR and head SAR values limit the total input power, the RF-induced heating is inhomogeneous inside the human body and is also closely related to the peak local SAR of 1 g or 10 g [35, 36]. These local SAR are evaluated around a volume of 1 cm cube (1 g peak SAR) or 2.1 cm cube (10 g peak SAR). The local average SAR over 1 gram (SAR1 g) is employed in the study since the device’s cross-section is less than 1 cm². SAR1 g can be calculated by determining the average SAR within a 1 gram tissue volume.where σ is the tissue conductivity, ρ is the mass density of the tissue, V is the volume of a mass of 1 gram, and E is the root mean square of E-field strength. The maximum (peak) SAR_1g is extracted and denoted as pSAR_1g. The pSAR_1g is normalized to the limits of the normal operating mode with a whole-body averaged SAR of 2 W/kg based on the SAR limits specified in the International Electrotechnical Commission (IEC) standard 60601-2-33.

3. Results

Comparisons of pSAR_1g of orthopedic plates for human body models with original posture and adjusted postures under 1.5T and 3T MRI systems are presented as follows.

3.1. RF-Induced Energy Absorption at 1.5T

At 1.5T, the pSAR_1g near the orthopedic plates with different dimensions inside the human postured body models is shown in Figure 4.

Figure 4

Comparison of pSAR_1g for various orthopedic plates inside the human models with different postures for four imaging landmarks at 1.5T. (A1)–(A3): Comparison of the pSAR_1g for the orthopedic plates with a length of 140 mm. (B1)–(B3): Comparison of the pSAR_1g for the orthopedic plates with a length of 120 mm. (C1)–(C3): Comparison of the pSAR_1g for the orthopedic plates with the length of 100 mm. (D1)–(D3): Comparison of the pSAR_1g for the orthopedic plates with the length of 80 mm.

With changes in imaging landmarks, the peak SAR averaged over 1 gram (pSAR_1g) decreases for both the O-posture and the X-posture across all orthopedic plates in Figures 4(A1)–4(D1). However, the reduction of pSAR_1g is more pronounced for the X-posture compared to the O-posture. Notably, in comparison to the O-posture, the pSAR_1g of the X-posture is higher at LM1 but lower at the other three landmarks. This observation suggests that as the patient’s left arm approaches the coil, the imaging landmarks can have a larger impact on the RF-induced heating.

In Figures 4(A2)–4(D2), the trends observed for the Y-posture differ from those of the O-posture as the imaging landmark is altered. Typically, the pSAR_1g shows a pattern of initial decrease followed by an increase, with its minimum occurring at LM2 or LM3 and its maximum at LM4. Substantial disparities in pSAR_1g between the O-posture and the Y-posture are evident. Notably, for a patient with an orthopedic plate measuring 140 mm in length, the pSAR_1g decreases from 106 W/kg to 55 W/kg when transitioning from the O-posture to the Y2-posture at LM2, as illustrated in Figure 4(A2). Conversely, for a patient with an orthopedic plate measuring 100 mm in length, the pSAR_1g increases from 57 W/kg to 134 W/kg at LM4 when adopting the Y2-posture instead of the O-posture, as depicted in Figure 4(C2). When the patient’s left arm is raised, the RF-induced energy absorption for the orthopedic plate can increase by 77 W/kg or decrease by 51 W/kg, with its magnitude highly dependent on the height of the raised arm and the dimensions of the orthopedic plate.

As shown in Figure 4(D3), the pSAR_1g values exhibit a decrease in the Z1-posture, and the Z3-posture as the imaging landmark is altered. Particularly, the pSAR_1g for the Z1-posture demonstrates a more significant reduction compared to that of the O-posture. Moreover, for all orthopedic plates except the one measuring 80 mm in length, the pSAR_1g of the Z3-posture is lower than that of the O-posture. In contrast, the variations in pSAR_1g for the Z2-posture remain within 10 W/kg across changes in the imaging landmark. When the orthopedic plate is perpendicular to the coil axis, altering the patient’s posture can lead to a substantial variation in RF-induced energy absorption, especially for orthopedic plates with dimensions ranging from 100 mm to 140 mm.

3.2. RF-Induced Energy Absorption at 3 T

At 3 T, the pSAR_1g of the orthopedic plates with different dimensions inside human posture models is shown in Figure 5.

Figure 5

Comparison of the pSAR_1g for various orthopedic plates inside the human models with different postures for four imaging landmarks at 3T. (A1)–(A3): Comparison of the pSAR_1g for the orthopedic plates with a length of 140 mm. (B1)–(B3): Comparison of the pSAR_1g for the orthopedic plates with a length of 120 mm. (C1)–(C3): Comparison of the pSAR_1g for the orthopedic plates with a length of 100 mm. (D1)–(D3): Comparison of the pSAR_1g for the orthopedic plates with a length of 80 mm.

The X-posture generally results in a reduction in the pSAR_1g for all orthopedic plates across all four imaging landmarks. However, there are three exceptions to this trend: (1) the X2-posture for the 140 mm plate at LM1, as depicted in Figure 5(A1); (2) the X2-posture; and (3) the X3-posture for the 120 mm plate at LM1, as shown in Figure 5(B1). The most significant increase in pSAR_1g can reach up to 20 W/kg, occurring for the 140 mm plate in the X2-posture at landmark LM1. When the patient’s left arm approaches the coil, RF-induced energy absorption for the orthopedic plate can generally be reduced during MRI scans from landmark LM2 to LM4. However, exceptions to this reduction may occur in certain scenarios, as indicated by the examples mentioned above.

With the change of imaging landmark, the pSAR_1g of the O-posture increases first and then decreases as shown in Figures 5(A2)–5(D2). Conversely, for the Y-posture, the pSAR_1g decreases, with the highest and lowest values observed at landmarks LM1 and LM4, respectively. When transitioning from the O-posture to the Y-posture, the pSAR_1g can either increase from 57 W/kg to 99 W/kg at LM1 or decrease from 77 W/kg to 16 W/kg at LM4. These findings suggest that as the patient’s left arm is raised, RF-induced energy absorption for the orthopedic plate increases at LM1, remains relatively constant at LM2, and decreases at LM3 and LM4.

Figures 5(A3)–5(D3) illustrate that the Z-posture tends to increase the pSAR_1g of orthopedic plates. However, as the imaging landmark changes, the pSAR_1g decreases for the Z-posture, resulting in reduced differences in pSAR_1g between the Z-posture and the O-posture. Particularly at landmark LM1, the pSAR_1g for the Z-posture can be up to 2 times higher than that for the O-posture for plates measuring 140 mm and 100 mm in length. When the patient’s left arm is perpendicular to the coil axis, the RF-induced energy absorption increases.

3.3. Relative Difference

From the results above, patients’ postures can significantly affect the RF-induced energy deposited near orthopedic plates. The variation in RF-induced energy near orthopedic plates depends on the type of posture, the dimensions of the orthopedic plate, and the operating frequency of the MRI system. To quantitatively illustrate the variation of pSAR_1g for the orthopedic plate caused by different postures, the relative difference of pSAR_1g between O-posture and each posture are calculated (all the relative differences are in Supplementary Materials 1). The maximal relative difference of pSAR_1g between the O-posture and X, Y, and Z postures for 1.5T and 3T MRI systems are shown in Tables 1 and 2, respectively. Notably, the highest relative difference of pSAR_1g for various orthopedic plates is observed for the Y-posture at 1.5T and the Z-posture at 3T. At 3T, the Z-posture increases the pSAR_1g substantially for the orthopedic plate for all imaging landmarks, which can become a significant safety concern. At 1.5T, the effect of the Y-posture on the pSAR_1g depends much on the length of the orthopedic plates. For a long orthopedic plate, such as the 140 mm plate, the Y-posture often reduces the pSAR_1g. However, the pSAR_1g for a short orthopedic plate will increase significantly for the Y-posture. In addition, the X-posture also has a non-negligible effect on the pSAR_1g for orthopedic plates with a relative difference up to 48% and 36% for 1.5T and 3T MR systems, respectively.

4. Discussion

The variations in pSAR_1g are caused by patient posture because the incident RF field in the vicinity of the orthopedic plate changes due to the arm movement. The electromagnetic field distributions are simulated for patients with different postures after removing orthopedic plates. At landmark LM4, the magnitude of E-field distributions for the four postures is compared with the O-posture at both 1.5T and 3T, as shown in Figure 6. For the X3, Y2, and Z3 postures, the lower magnitude of E-field inside the forearm is observed compared with that for O-posture, resulting in the decrease of pSAR_1g for the plate.

Among the human posture models examined in this study, the variation of pSAR_1g for the orthopedic plate reaches up to 100% compared to the original posture. Such significant variation will result in varying conditional requirements (input power and scan time) of the MRI, indicating that conditional requirements under neutral posture do not guarantee the safety of patients. It is strongly recommended that patients maintain a consistent neutral posture during MRI examinations to mitigate potential risks of unexpected heating, which may not have been accounted for during the determination of MRI conditional labeling. In addition, it highlights that the original posture may not represent the worst-case scenario when assessing RF-induced heating for orthopedic plates within the human body model. Conducting comprehensive studies involving various postures is essential, with further research needed to identify the posture that presents the highest risk. Furthermore, the RF-induced energy absorption could potentially be reduced by adopting specific postures, such as the Z3-posture in 1.5T and X1-posture in 3T. However, this observation is specific to the plate and human body model utilized in this study. Additional research is warranted to elucidate the underlying mechanisms, potentially guiding the way for clinical applications aimed at preventing tissue heating damage caused by orthopedic plates.

5. Conclusion

This study investigated the RF-induced energy absorption of orthopedic plates with various dimensions within human models, considering both the original posture and a range of typical body postures for both 1.5T and 3T MR systems. Significant variations in RF-induced energy absorption were observed across different postures. Specifically, at 1.5T, the relative differences in pSAR_1g for the X-posture, Y-posture, and Z-posture were 48%, 134%, and 32%, respectively. At 3T, these differences were 36%, 83%, and 101%, respectively. It is noticed that three postures studied in this paper had a notable impact on the pSAR_1g of the orthopedic plates, indicating the need to prioritize these postures when assessing RF-induced heating. The distribution of the electric field (E-field) significantly changes when the patient’s arms are repositioned, resulting in variations in pSAR_1g for the orthopedic plates. Thus, considering various posture models is crucial when evaluating RF-induced heating for passive implants to mitigate unexpected heating risks. At present, posture variations are not typically factored into the standard assessment of RF-induced heating for MR conditional labeling of passive implants. Further research is necessary to explore special postures that may result in maximal or minimal RF-induced heating. Given the observed disparities in pSAR_1g in our study, we recommend that patients maintain a consistent posture during MRI scans.

Data Availability

The data used to support the findings of this study are available in the supplementary materials of this article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Science Foundation [grant 1922389].

Supplementary Materials

All the relative differences for the pSAR_1g between the O-posture and other postures for various orthopedic plates at 1.5T and 3T are calculated to support the maximum relative difference. (Supplementary Materials)

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Copyright © 2024 Xiaolin Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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