2870
A New Method to Improve RF Safety of Implantable Medical Devices using Inductive Coupling at 3.0T MRI
Bu S Park1, Joshua Guag2, Sunder Rajan2, and Brent McCright3
1FDA, Silver Spring, MD, United States, 2Division of Biomedical Physics (DBP), FDA, Silver Spring, MD, United States, 3Division of Cellular and Gene Therapies (DCGT), FDA, Silver Spring, MD, United States
1FDA, Silver Spring, MD, United States, 2Division of Biomedical Physics (DBP), FDA, Silver Spring, MD, United States, 3Division of Cellular and Gene Therapies (DCGT), FDA, Silver Spring, MD, United States
Synopsis
Keywords: Safety, Electromagnetic Tissue Properties
This study describes a new method to improve RF safety of implantable medical devices located outside of the imaging region by using a secondary resonator (SR) to reduce electric fields and corresponding specific absorption rate (SAR) during MRI. The SR is designed to produce opposing electro-magnetic fields (EM-fields) compared to the EM-fields made by a body coil at 3.0T. This study was performed using numerical simulations with ASTM phantom and human models, and corresponding experimental verifications with the ASTM phantom.
Keywords: RF safety, secondary resonator
INTRODUCTION: A significant
temperature rise or high SAR values during high-field MRI can cause RF safety
issues in human studies and should be addressed based on several guidelines1.
Following the guidelines, many methods 2-3 have been
developed to improve RF safety in MRI without or with active implantable
medical devices (AIMD). Building on the previous research2-3, we propose a
new method using a simple RF resonator of loop element (secondary resonator, or
SR) to reduce RF exposure within the VoI including an implant
region during an MRI. It is
assumed that the VoI is located outside of
the imaging region to minimize any unwanted interaction with MR images.
Numerical simulations of SR were studied using both the ASTM phantom and human
models of Ella and Duke. The corresponding experimental verifications using the
ASTM phantom and designed SRs were performed. The EM effects on a copper tube
construct representing a medical stent4 were evaluated using
numerical simulations.
METHODS: This study was performed using numerical simulations with ASTM phantom and two adult human models of Ella and Duke, and corresponding experimental results using the ASTM phantom (Fig. 1). The first body coil (“610 mm body coil”) has an inner diameter (ID) of 610 mm, inner length (L) of 570 mm, outer length of 620 mm, width (W) of the copper strip for end ring and each rod of 25 mm, and an RF shield (ID = 660 mm, L = 1220 mm). Tuning capacitors (CT_Body) were placed in the end rings and had a value of 18.5 pF (Fig. 1 (a)). The circular SR was designed using parameters of ID = 150 mm, and W = 6 mm. The ASTM phantom was designed based on the ASTM standard test method and has parameters of L = 650 mm, W = 420 mm, and Height (H) = 90 mm with conductivity (s) = 0.47 S/m and relative permittivity (er) = 80. The experimental measurements were conducted using 3T MITS body coil (Zurich Med Tech, ID = 746 mm), EM field mapping probes (ER3DV6 and H3DV7), and ASTM phantom (Fig. 1 (b)). . RESULTS: Fig. 2 shows numerical simulation (a) and corresponding experimental results (b) of D||E|| without and with the SR of opposing (red lines) and enhancing (black lines) using ASTM phantom and body coil. Figures 3-4 show numerical simulation results of |B1+|, D|B1+| (Fig. 3), SAR1g, and DSAR1g (Fig. 4) within the Ella model at different landmark positions of Sternum (first column), Neck (second column) and Knee (third column) at 128 MHz. The effect of SR was shown more at Neck and Knee landmarks than that of Sternum landmarks, e.g., Mean D|B1+| were -8.09 % (Neck landmark), and -22.3% (Knee landmark), whereas -3.34% (Sternum landmark) with the SR making opposing magnetic fields.
DISCUSSION: The primary novelty of this study is that a new method using an SR designed to make opposing magnetic fields and lower SAR distributions has been proposed. The effect of SR was more obvious in the region having uniform and the same directional magnetic field components as the magnetic fields made by the SR, e.g., the central region of the body coil, because of fewer interactions with unwanted electromagnetic field components. Whereas the designed RF magnetic fields made by the SR were mainly BY in this study. Therefore, interactions with RF magnetic fields made by the SR and (BX, and BZ) made by the body coil would result in unwanted EM-field distributions. That would be the main reason that the effect of SR was not so obvious in some regions.
CONCLUSION: A new method using the designed SR making opposing magnetic fields to partially shield a sample has been proposed to improve RF safety at the VoI through numerical simulations with different simulation conditions at 3.0T.
METHODS: This study was performed using numerical simulations with ASTM phantom and two adult human models of Ella and Duke, and corresponding experimental results using the ASTM phantom (Fig. 1). The first body coil (“610 mm body coil”) has an inner diameter (ID) of 610 mm, inner length (L) of 570 mm, outer length of 620 mm, width (W) of the copper strip for end ring and each rod of 25 mm, and an RF shield (ID = 660 mm, L = 1220 mm). Tuning capacitors (CT_Body) were placed in the end rings and had a value of 18.5 pF (Fig. 1 (a)). The circular SR was designed using parameters of ID = 150 mm, and W = 6 mm. The ASTM phantom was designed based on the ASTM standard test method and has parameters of L = 650 mm, W = 420 mm, and Height (H) = 90 mm with conductivity (s) = 0.47 S/m and relative permittivity (er) = 80. The experimental measurements were conducted using 3T MITS body coil (Zurich Med Tech, ID = 746 mm), EM field mapping probes (ER3DV6 and H3DV7), and ASTM phantom (Fig. 1 (b)). . RESULTS: Fig. 2 shows numerical simulation (a) and corresponding experimental results (b) of D||E|| without and with the SR of opposing (red lines) and enhancing (black lines) using ASTM phantom and body coil. Figures 3-4 show numerical simulation results of |B1+|, D|B1+| (Fig. 3), SAR1g, and DSAR1g (Fig. 4) within the Ella model at different landmark positions of Sternum (first column), Neck (second column) and Knee (third column) at 128 MHz. The effect of SR was shown more at Neck and Knee landmarks than that of Sternum landmarks, e.g., Mean D|B1+| were -8.09 % (Neck landmark), and -22.3% (Knee landmark), whereas -3.34% (Sternum landmark) with the SR making opposing magnetic fields.
DISCUSSION: The primary novelty of this study is that a new method using an SR designed to make opposing magnetic fields and lower SAR distributions has been proposed. The effect of SR was more obvious in the region having uniform and the same directional magnetic field components as the magnetic fields made by the SR, e.g., the central region of the body coil, because of fewer interactions with unwanted electromagnetic field components. Whereas the designed RF magnetic fields made by the SR were mainly BY in this study. Therefore, interactions with RF magnetic fields made by the SR and (BX, and BZ) made by the body coil would result in unwanted EM-field distributions. That would be the main reason that the effect of SR was not so obvious in some regions.
CONCLUSION: A new method using the designed SR making opposing magnetic fields to partially shield a sample has been proposed to improve RF safety at the VoI through numerical simulations with different simulation conditions at 3.0T.
Acknowledgements
No acknowledgement found.References
1. Medical electrical equipment - Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis, IEC60601-2-33, 2020
2. W. Mao, M. B. Smith, and C. M. Collins, "Exploring the Limits of RF Shimming for High-Field MRI of the Human Head," Magn Reson Med, vol. 56, pp. 918–922, 2006
3. H. Merkle et al., "Transmit B1-field correction at 7 T using actively tuned coupled inner elements.," Magn Reson Med, vol. 66, no. 3, pp. 901-10, 2011
4. L. Winter et al., "On the RF heating of coronary stents at 7.0 Tesla MRI," Magn Reson Med, vol. 74, 4, pp. 999-1010, 2015
Figures
Figure 1 Geometrical models used for this study. (a): Model of
the 16-rod HP birdcage body coil, ASTM phantom (yellow), and circular secondary
resonator (SR) at 3.0T (128 MHz). (b): Experimental setup with a designed SR (c): Ella model with RF shield,
16-rod birdcage RF coil, and SR. Different landmark
positions of Neck (d), Sternum (e), and Knee (f). The Ella model with a copper
tube (Length = 15 mm, Diameter = 3 mm (inner)/6 mm (outer)) represents a broad
spectrum of medical stent structures ((g) and (h)).
Figure 2 Numerical simulations
(first row) and corresponding experimental results (second row) of D||E|| without and with designed SRs of opposing (red line) and
enhancing (black line) at three different measurement lines (Z = 0, 2.54, and
5.08 [cm].
Figure 3 Numerical simulation
results of |B1+| and D|B1+| within the Ella model with conditions
of without SR (first row), with SR of enhancing (second and fourth rows) and
opposing (third and fifth rows) at different landmark positions of Sternum
(first column), Neck (second column) and Knee (third column) at 128 MHz. The
white rectangular bars in the first row indicate the location of the designed SR.
Figure 4 Numerical simulation
results of SAR1g and DSAR1g within the Ella model with different landmark positions. Other
parameters are the same as in Fig. 3.
DOI: https://doi.org/10.58530/2023/2870