Experimental investigation and histopathological identification of acute thermal damage in skeletal porcine muscle in relation to whole-body SAR, maximum temperature, and CEM43 °C due to RF irradiation in an MR body coil of birdcage type at 123 MHz

Abstract

Purpose: This study is an investigation of the relationship between several characteristic parameters and acute thermal damage in porcine skeletal muscle. Material and methods: Fourteen pigs under injection anaesthesia were placed into a magnetic resonance body coil and exposed for different time durations to different specific energy absorption rate (SAR) levels at 123 MHz. Local temperatures were measured using four temperature sensors. Sensors 1–3 were placed in skeletal muscle and one sensor was placed in the rectum. Sensors 1 and 2 were placed in hot-spot areas and sensor 3 was placed at the periphery of the animals. The pigs were exposed to whole-body SAR (SAR-wb) between 2.5 W/kg and 5.2 W/kg for 30 or 60 min. Three animals received no SAR. After each experiment, muscle samples adjacent to the positions of sensors 1–3 were taken for frozen section analysis. Three characteristic parameters were chosen for investigation: SAR-wb, maximum sensor temperature (T-max), and cumulative equivalent minutes at 43 °C (CEM43 °C). Results: Histopathological criteria were established to detect acute thermal tissue damage in frozen sections such as widening of intercellular space between the muscle fibres and loss of glycogen. Clear tissue damage thresholds were found for T-max and CEM43 °C, though not for SAR-wb. For all animals with high thermal exposure, damage was also found for muscle samples adjacent to the peripheral sensor 3. Conclusions: Both T-max and CEM43, are able to predict thermal damage in porcine muscle. However, CEM43 is the less ambiguous parameter. The reasons for the occurrence of the aforementioned damage at low local temperatures at the animals’ periphery remain unclear and further investigations are needed.

• Acute thermal damage
• CEM43°C
• frozen muscle sections
• MR body coil
• thermal damage thresholds

Acknowledgements

The authors are deeply indebted to Eckart Stetter (Siemens) and Peter Wust (Charité) for initiating the study and for organising the financial, technical and logistic support from Siemens and Charité. Furthermore, we would like to thank Christian Grosse-Siestrup (Charité) who was responsible for the official authorisation to carry out the animal experiments and for supervising this project in the field of veterinary medicine, as well as for recruiting the veterinary staff listed below. We thank Chie Hee Cho (Charité) and Peter Wust for valuable contributions to the study with their experience in hyperthermia. The practical realisation of experiments could not have been achieved without Chie Hee Cho and the competent and skilful assistance of Sarah Ribéreau, Friederike Kobelt and Markus Piaskowski (all three students of the Veterinary Faculty of the Free University Berlin).

Declaration of interest

This research was supported by Siemens Healthcare Sector, Erlangen, and Charité Berlin. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.