Non-invasive temperature monitoring and hyperthermic injury onset detection using X-ray CT during HIFU thermal treatment in ex vivo fatty tissue

Purpose: This paper examines X-ray CT, to serve as an image-guiding thermal monitoring modality for high intensity focused ultrasound (HIFU) treatment of fatty tissues. Materials and methods: Six ex vivo porcine fat tissue specimens were scanned by X-ray CT simultaneously with the application of HIFU. Images were acquired during both heating and post-ablation stages. The temperature at the focal zone was measured simultaneously using a thermocouple. The mean values of the Hounsfield units (HU) at the focal zone were registered and plotted as a function of temperature. Results: In all specimens studied, the HU versus temperature curves measured during the heating stage depicted a characteristic non-linear parabolic trajectory (R² > 0.87). The HU-temperature trajectory initially decreased to a minimum value at about 44.5 °C and then increased substantially as the heating progressed. The occurrence of this nadir point during the heating stage was clearly detectable. During post-ablation cooling, on the other hand, the HU increased monotonically with the decreasing temperature and depicted a clearly linear trajectory (R² 0.9). Conclusions: Our results demonstrate that the HU-temperature curve during HIFU treatment has a characteristic parabolic trajectory for fat tissue that might potentially be utilised for thermal monitoring during HIFU ablation treatments. The clear detection of 44.5 °C, presumably marking the onset of hyperthermic injury, can be detected non-invasively as an occurrence of a minimum on the HU-time curve without any need to relate the HU directly to temperature. Such features may be helpful in monitoring and optimising HIFU thermal treatment for clinically applicable indications such as in the breast by providing a non-invasive monitoring of tissue damage.