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Gradient dB/dt Heating Measurements of Copper, Steel, and Titanium Annuli: Assessment of Measurement Repeatability, and Identification of a Potential Reference Standard1Physics and Astronomy, University of Western Ontario, London, ON, Canada
Synopsis
The purpose of this work is to develop a simple reference standard to be used for gradient-induced heating tests to verify test system operation. A second objective is to provide data on measurement repeatability of heating in these reference objects. Two different dB/dt exposures (270 Hz and 1000 Hz) were used to measure the gradient-induced temperature rise of three MRI compatible metals using a laboratory dB/dt exposure system. Based on the temperature rise, an MRI compatible metal of distinct dimensions and geometry was determined to be most suitable as a dB/dt heating standard across a frequency range of 270-1000 Hz.
Introduction
Changes in magnetic field through time (dB/dt, measured in T/s) induce eddy currents on conductive materials, and this is of particular significance for both passive and active implantable devices (AIMDs). Device heating is an important consequence of the induced eddy currents [1]. The purpose of this work is to develop a simple reference standard to be used for gradient-induced heating tests to verify test system operation. A second objective is to provide data on measurement repeatability of heating in these reference objects.Methods
The gelled hydroxyethyl cellulose (HEC) phantom material was prepared in accordance with [2], consisting of 31 g/L HEC powder and 1.52 g/L NaCl in distilled water. The gelled HEC serves to simulate human tissue, sharing similar conductive and thermal properties, alongside limiting internal conduction and minimalizing thermal convection. Four copper annuli, two stainless-steel annuli, and two titanium annuli were measured for temperature change when subjected to gradient field exposure. All annuli had a 5-cm outer diameter with varying thickness and varying inner diameters. A water-cooled solenoid electromagnet, an amplifier (Copley Controls Corp., Model 266), and a waveform generator were integrated into a dB/dt exposure system and used to produce gradient dB/dt field exposures as specified in Clause 9 of ISO 10974 [3]. Two different dB/dt exposures were used: a continuous 270 Hz sinusoidal wave (dB/dt value of 42 T/s rms), and a continuous 1000 Hz sinusoidal wave (dB/dt value of 42 T/s).
Temperature was measured by two fiber optic temperature probes (Neoptix Inc., T1C-15548A) placed in 1-mm holes on the outer edges of each annuli. The metal annuli were centered in a phantom with a 13-cm diameter and a 9-cm gel height. The metal annuli underwent three repeated exposures, each involving a 20-second gradient exposure followed by 160-seconds of cool-down time. Two fiber optic probes collected temperature readings of the annulus during each trial, totaling six temperature readings per three trials for each condition. A third probe was placed in the gelled HEC ensuring the media did not significantly heat up and lose its thermal properties. A fourth probe was placed on the boundary surrounding the gradient coil measuring the ambient temperature, ensuring tests were being conducted in a stable environment.
Results and Discussion
Heating is greatest when the metal annulus is oriented orthogonally to the vector of the gradient field and positioned at the radial centre of the phantom [3]. Of the 3 metals tested, the copper annuli heated up the greatest due its conductive properties (Table 1). This was expected, as copper is the best conductor of heat and has a lower specific heat capacity as compared to the other materials. A larger inner diameter, i.e., Copper Annulus #2 and #4, heated up greater than copper annuli of lesser inner diameter. For all annuli tested at the two frequencies, temperatures were higher at 270 Hz compared to 1000 Hz. The ratio of heating at 270 Hz to that at 1 kHz is geometry dependent, with the greatest difference for the annuli with the most cross-sectional area. We believe that the Copper Annulus #4 represents the best single candidate for a reference standard object for these tests, since the heating is significant across the frequency range of interest gradient dB/dt in MRI (300 to 1200 Hz [2]), allowing for temperature measurements to be made with an SNR of at least 20 across the range. The copper annulus is also practical, readily available, easy to manufacture, and low-cost.Conclusion
The gradient-induced temperature rise of three MRI compatible metals was measured using a laboratory dB/dt exposure system. Based on the temperature rise, Copper Annulus #4 would likely be most suitable as a dB/dt heating standard across the frequency range of interest as defined in [3].Acknowledgements
This work was funded by NSERC Industrial Research Chairs Program, Ontario Research Fund Research Excellence Program, and Canadian Foundation for Innovation.References
1] B.J. Peng, et al., Phys. Med. Biol., vol. 55, 2010. [2] ASTM F2182-11a. [3] ISO/TS 10974:2018. [4] K. E. Bannan, et al., IEEE/ASME Transactions on Mechatronics, vol. 18, 2013.Figures
Boxplots summarizing temperature change for all copper annuli at 270 Hz over 5 sessions (i.e., 15 trials, i.e., 30 temperature readings)
Boxplots summarizing temperature change for all copper annuli at 1000 Hz (1 kHz) over 2 sessions (i.e., 6 trials, i.e., 12 temperature readings)