0730

Computational modeling of RF-induced heating due to a titanium-alloy rod: An Interlaboratory Comparison for the ASTM F2182 task group
Kyle Murdock1, David C. Gross1, Alan Leewood1, Peter Serano2, Marc Horner2, John Nyenhuis3, Payman Afshari4, Daniel Moreno4, Joshua White5, Rada Alnnasouri6, Pauline Ferry6, Yannick Ponvianne6, Mikhail Kozlov7,8, Claus Gerber9, Cesar Bibiano9, Sunder S Rajan10, and Leonardo M. Angelone10

1MED Institute Inc., West Lafayette, IN, United States, 2ANSYS, Inc., Canonsburg, PA, United States, 3Bemcalc, West Lafayette, IN, United States, 4DePuy Synthes, Raynham, MA, United States, 5Exponent, Philadelphia, PA, United States, 6Healtis, Nancy, France, 7Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 8MRCOMP, Gelsenkirchen, Germany, 9Stryker Trauma GmbH, Kiel, Germany, 10U.S. Food and Drug Administration, Center for Devices and Radiological Health, Silver Spring, MD, United States

Synopsis

Computational modeling of RF-induced heating due to a titanium-alloy rod was conducted by nine independent institutions with a primary goal to compare the impact of common, independent modeling choices on temperature rise results. Results showed that when the rod is located 2 cm from the enclosure, the temperature rise can be used to estimate the local background electric-field exposure. Temperature rise depends not only on the background exposure but also on the location of the rod within the phantom.

Introduction

Computational modeling of RF-induced heating due to a titanium-alloy rod was conducted by nine independent institutions with a primary goal to compare the impact of common, independent modeling choices on temperature rise results. The secondary goal was to provide a set of data of local and whole-phantom specific absorption rate (wpSAR) values in the ASTM F2182 phantom1. The findings in this study are currently being used in the revision of the ASTM F2182 Standard1 to provide additional context and information for users of this standard test method.

Methods

The geometrical dimensions and the material properties of the “phantom” (i.e., acrylic enclosure filled with a homogeneous conductive medium), and of the solid titanium-alloy rod1 used by each institution were constrained, as shown in Table 1. The initial background temperature of the conductive medium without thermal convection was set to 22°C. The exterior acrylic walls and the exposed top of the medium were thermally insulated. However, the implant fixtures and the temperature probes were excluded in the simulation. Additionally, the pilot holes of the titanium-alloy rod were filled with the medium. Simulations were performed at both 1.5T and 3T to calculate the temperature rise in the pilot holes of the rod (i.e., location where temperature is typically measured1). The RF exposure was continuous and the maximum temperature rise ΔT at six minutes inside the rod pilot-holes were compared across institutions.

The unconstrained parameters for each institution were: the specific RF exposure system and exact RF frequency used, the computational software platform, and the global (x,y,z) position of the phantom within the RF coil. As such, each institution implemented an RF source model representing either a clinical MR systems, generic birdcage coils, or simple plane-wave exposure. Each institution used commercially available or in-house software platform based on either the Finite Difference Time Domain (FDTD), the Finite Integration Technique (FIT), or the Finite Element Method (FEM).

For the institutions using an RF coil model loaded with the ASTM phantom, the titanium-alloy rod was centered in the phantom in the superoinferior direction and the longitudinal axis was centered in the mid-depth of the medium (i.e., 4.5 cm above the bottom of the acrylic enclosure). The pilot holes of the rod were parallel to the x-axis of the RF coil. The rod was placed at three different positions along the x-axis: 2 cm, 5 cm, and 8 cm from the phantom enclosure. The results were scaled to provide an average background incident ||E||rms=150 V/m at 2 cm from the phantom enclosure on the same plane where the titanium-alloy rod was subsequently placed.

For the institutions using a plane-wave model, the presence of the phantom was not required. The titanium-alloy rod was placed in a homogeneous conductive medium with an incident electric plane wave parallel to the rod axis with ||E||rms=150 V/m . Only a single position was evaluated, given the spatially homogeneous exposure.

Results and Discussion

Overall, the nine institutions provided a total of twelve datasets. The software methodology is shown in Table 2 along with the RF coil dimensions (where applicable). The background electric fields are shown in Table 3. For the models including an RF coil, the ΔT at 2 cm location was 11.3±0.36oC at 1.5T and 13.7±0.50oC at 3T (Table 4). at 1.5 T, the values of ΔT were reduced at 5 cm and 8 cm, as expected due to the smaller values of background ||E||rms. Conversely, at 3T the ΔT at 2 cm and at 5 cm locations was similar, despite different values of background ||E||rms. Whole-phantom SAR values were 3.82 ±0.29 W/kg at 1.5T and 4.39±0.64 W/kg at 3T (Table 3). Finally, for the plane-wave models ΔT was 13.4±1.0oC at 1.5T and 14.4±1.5oC at 3T (Table 5).

Conclusions

When the local exposure (V/m) is constrained for a given titanium-alloy rod size and position within the phantom, with an RF coil used as quadrature exposure, the maximum temperature rise inside the holes after six minutes was consistent across all institutions and datasets, independently on the coil dimensions. Therefore, the temperature rise due to the titanium-alloy rod placed at 2cm location can be used to estimate the local background ||E||rms, within the expected measurement uncertainty. The temperature rise with the rod placed at 5cm was similar to the one at 2cm, despite different background ||E||2rms. Thus, the temperature rise depends not only on ||E||2rms but also on the location of the rod. Additionally, the standard deviation of temperature rise with plane wave exposure was typically higher compared to the coil exposure.

Acknowledgements

No acknowledgement found.

References

1. ASTM F2182-11 - Standard Test Method for Measurement of Radio Frequency Induced Heating On or Near Passive Implants During Magnetic Resonance Imaging

Disclaimer: The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the Department of Health and Human Services

Figures

Table 1. A description of the dimensions and material properties of the phantom and the titanium-alloy rod.

Table 2. A list of software methodologies and dimensions of the RF coils (where applicable).

Table 3. Background ||E||rms for models using RF coil exposure at different locations within the phantom.

Table 4. RF coil exposure - The maximum temperature rise (°C) in the pilot holes of the titanium-alloy rod after six minutes of RF heating at 1.5 T and 3.0 T. The RF exposure was normalized to provide ||E||rms=150 V/m along a 1 cm line centered head-to-foot, mid-depth and 2 cm from the box enclosure. The whole-phantom SAR (wpSAR) at 1.5 T and 3.0 T (without the rod in place) is shown for each dataset.

Table 5. Plane-wave RF exposure - The maximum temperature rise (°C) in the pilot holes of the titanium-alloy rod after six minutes of RF heating at 1.5 T and 3.0 T. The RF exposure was calibrated to provide ||E||rms=150 V/m along a 1 cm line centered head-to-foot, mid-depth and 2 cm from the box enclosure.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
0730