title: Temperature-density hysteresis in X-ray CT during HIFU thermal ablation: heating and cooling phantom study.
abstract: OBJECTIVE
  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.
  
  
  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 °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.
authors: Department of Biomedical Engineering, Technion IIT , Haifa , Israel .
authors: Noam
authors: N
authors: Weiss
authors: S Nahum
authors: SN
authors: Goldberg
authors: Jacob
authors: J
authors: Sosna
authors: Haim
authors: H
authors: Azhari
mesh_terms: D000818:Animals; D002417:Cattle; D002645:Chickens; D005229:Fatty Acids, Monounsaturated; D057086:High-Intensity Focused Ultrasound Ablation; D008099:Liver; D019047:Phantoms, Imaging; D000074262:Rapeseed Oil; D012709:Serum Albumin; D013696:Temperature; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*; D014867:Water
chemical_list: D005229:Fatty Acids, Monounsaturated; D000074262:Rapeseed Oil; D012709:Serum Albumin; D014867:Water