Received: 24 June 2010, Revised: 17 August 2010, Accepted: 9 September 2010, Published online in Wiley Online Library: 12 December 2010 In vivo characterization of tissue thermal properties of the kidney during local hyperthermia induced by MR-guided high-intensity focused ultrasound Franc¸ois Cornelis a , Nicolas Grenier a , Chrit T Moonen a and Bruno Quesson a * The purpose of this study was to evaluate quantitatively in vivo the tissue thermal properties during high-intensity focused ultrasound (HIFU) heating. For this purpose, a total of 52 localized sonications were performed in the kidneys of six pigs with HIFU monitored in real time by volumetric MR thermometry. The kidney perfusion was modified by modulation of the flow in the aorta by insertion of an inflatable angioplasty balloon. The resulting temperature data were analyzed using the bio-heat transfer model in order to validate the model under in vivo conditions and to estimate quantitatively the absorption (a), thermal diffusivity (D) and perfusion (w b ) of renal tissue. An excellent correspondence was observed between the bio-heat transfer model and the experimental data. The absorption and thermal diffusivity were independent of the flow, with mean values (W standard deviation) of 20.7 W 5.1 mm 3 KJ S1 and 0.23 W 0.11 mm 2 s S1 , respectively, whereas the perfusion decreased significantly by 84% ( p < 0.01) with arterial flow (mean values of w b of 0.06 W 0.02 and 0.008 W 0.007 mL S1 mL s S1 ), as predicted by the model. The quantitative analysis of the volumetric temperature distribution during nondestructive HIFU sonication allows the determination of the thermal parameters, and may therefore improve the quality of the planning of noninvasive therapy with MR-guided HIFU. Copyright ß 2010 John Wiley & Sons, Ltd. Keywords: MRI; high-intensity focused ultrasound (HIFU); thermometry; perfusion; absorption; thermal diffusivity; kidney INTRODUCTION Noninvasive tumor ablation with high-intensity focused ultra- sound (HIFU) is being used increasingly (1–7). MR thermometry, employing the proton resonance frequency (PRF) shift technique (8,9), provides online monitoring of the temperature distribution in the targeted tissue (8,10), increasing patient safety and opening up perspectives for the better control of local heat deposition during treatment (11). The treatment outcome can be assessed from the calculation of the accumulated thermal dose (12) derived from the temporal integration of temperature curves in each pixel of the MR temperature maps. The use of fast imaging MR acquisition sequences and associated real-time data processing (13) recently allowed for real-time volumetric monitoring of the temperature in the liver and kidney (14). In the latter study, it was reported that a large amount of HIFU energy was required to achieve ablation because of rapid heat evacuation, presumably arising from the influence of blood perfusion, in agreement with other studies (15,16). This substantial energy deposition increases the risks of inducing adjacent adverse effects in surrounding tissues (such as skin burns or bowel lesions) (17–19). Therefore, treatment planning may be facilitated by a precise knowledge of the thermal response of tissue to increases in local temperature. For this purpose, different physical models have been proposed to account for the evolution of temperature in biological tissues (20–23). Recently, Dragonu et al. (24) demonstrated, on an ex vivo perfused pig kidney, that the bio-heat transfer (BHT) model (25) can accurately describe the spatiotemporal evolution of temperature at the macroscopic scale. This method is based on the processing of the three-dimensional temperature distribution in the kidney, and allows for a quantitative estimation of the thermal tissue parameters, namely the absorption, thermal diffusivity and perfusion. The authors reported that the thermal diffusivity and absorption were independent of the arterial flow, whereas perfusion was linearly dependent on the flow in the renal artery. The objectives of the current study were to evaluate in vivo the validity of the BHT model in pig kidney subjected to HIFU heating, and to estimate quantitatively the tissue thermal parameters from the real-time volumetric MR thermometry images. (wileyonlinelibrary.com) DOI:10.1002/nbm.1624 Research Article * Correspondence to: B. Quesson, Laboratory for Molecular and Functional Imaging, UMR5231, CNRS/Universite´ Victor Segalen Bordeaux 2, 146 rue Le´o Saignat, 33076 Bordeaux cedex, France. a F. Cornelis, N. Grenier, C. Moonen, B. Quesson Laboratory for Molecular and Functional Imaging, CNRS/Universite´ Bordeaux 2, Bordeaux, France Abbreviations used: BHT, bio-heat transfer; HIFU, high-intensity focused ultrasound; PRF, proton resonance frequency; Wac, watts of acoustic power. NMR Biomed. 2011; 24: 799–806 Copyright ß 2010 John Wiley & Sons, Ltd. 799