Temperature-density hysteresis in X-ray CT during HIFU thermal ablation: Heating and cooling phantom study

Purpose: This paper investigated the effects of thermal ablation treatment on imaged X-ray computed tomography (CT) Hounsfield units (HU), for the purpose of monitoring tissue denaturation and coagulation. Materials and methods: Eight phantoms of water, oil, and chicken serum albumin as well as 15 ex vivo tissue samples were heated by applying high intensity focused ultrasound (HIFU) for 10 to 29 min to obtain denaturation temperatures, (i.e. >50 degrees C). X-ray CT scanning was performed simultaneously during heating and post-ablation cooling stages, and the HU at the focal zone were registered. The temperature profile versus time was also monitored under similar conditions using a thermocouple probe. The results were plotted and correlated as curves of HU versus temperature. Results: In all specimens studied, HU values depicted an exponential curve as a function of temperature during the heating stage. However, linear behaviour was observed during the cool-down stage for both chicken serum albumin and ex vivo bovine liver. Thus, a hysteresis phenomenon occurred only when the thermal conditions induced irreversible changes in the sample with quantification demonstrating high correlation with the maximal temperature reached during treatment (R-2>0.9) for the chicken serum albumin. Conclusions: Our results demonstrate a HU-temperature hysteresis phenomenon for HIFU ablation, which is detectible by X-ray CT. This hysteresis is related to the amount of heat induced into the tissue and could potentially indicate irreversible tissue damage. Accordingly, this measurable phenomenon can be utilised as a quantitative method for non-invasive monitoring of thermal ablation.