Temperatures in Pigs During 3 T MRI Temperatures, Heart Rates, and Breathing Rates of Pigs During RF Power Deposition in a 3 T (128 MHz) Body Coil

Cho, C.‐H., Grosse‐Siestrup, C., Nadobny, J., Lojewski, C., Niehus, S.M., Taupitz, M., Hamm, B. and Schlattmann, P. (2020), Temperatures in Pigs During 3 T MRI Temperatures, Heart Rates, and Breathing Rates of Pigs During RF Power Deposition in a 3 T (128 MHz) Body Coil. Bioelectromagnetics. https://doi.org/10.1002/bem.22311

Abstract

Exposure to radiofrequency (RF) power deposition during magnetic resonance imaging (MRI) induces elevated body‐tissue temperatures and may cause changes in heart and breathing rates, disturbing thermoregulation. Eleven temperature sensors were placed in muscle tissue and one sensor in the rectum (measured in 10 cm depth) of 20 free‐breathing anesthetized pigs to verify temperature curves during RF exposure. Tissue temperatures and heart and breathing rates were measured before, during, and after RF exposure. Pigs were placed into a 60‐cm diameter whole‐body resonator of a 3 T MRI system. Nineteen anesthetized pigs were divided into four RF exposure groups: sham (0 W/kg), low‐exposure (2.7 W/kg, mean exposure time 56 min), moderate‐exposure (4.8 W/kg, mean exposure time 31 min), and high‐exposure (4.4 W/kg, mean exposure time 61 min). One pig was exposed to a whole‐body specific absorption rate (wbSAR) of 11.4 W/kg (extreme‐exposure). Hotspot temperatures, measured by sensor 2, increased by mean 5.0 ± 0.9°C, min 3.9; max 6.3 (low), 7.0 ± 2.3°C, min 4.6; max 9.9 (moderate), and 9.2 ± 4.4°C, min 6.1, max 17.9 (high) compared with 0.3 ± 0.3°C in the sham‐exposure group (min 0.1, max 0.6). Four time‐temperature curves were identified: sinusoidal, parabolic, plateau, and linear. These curve shapes did not correlate with RF intensity, rectal temperature, breathing rate, or heart rate. In all pigs, rectal temperatures increased (2.1 ± 0.9°C) during and even after RF exposure, while hotspot temperatures decreased after exposure. When rectal temperature increased by 1°C, hotspot temperature increased up to 42.8°C within 37 min (low‐exposure) or up to 43.8°C within 24 min (high‐exposure). Global wbSAR did not correlate with maximum hotspot.

Bioelectromagnetics. 2020; 1–14. © 2020 The Authors. Bioelectromagnetics published by Wiley Periodicals LLC on behalf of Bioelectromagnetics Society

https://onlinelibrary.wiley.com/doi/10.1002/bem.22311

CONCLUSION

RF exposure at 4 W/kg in 20 anesthetized pigs leads to hotspot temperatures exceeding 40°C, which is above accepted IEC limits. The hotspot temperature courses show four different curve shapes (sinusoidal, parabolic, plateau, and linear), which are influenced by breathing and heart rates. Rectal temperatures are lower and delayed compared to hotspot temperatures. Possibly, patient safety may be improved by online body core measurement to guide MRI, and individually adapt SAR exposure (higher or lower) for each patient.

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