NON-INVASIVE zyxwvu IN-VIVO TEMPERATURE MAPPINGOF ULTRASOUND HEATING USING FLUORINE-BASED MAGNETIC RESONANCE IMAGING AGENTS zyx Andrew G. Webb [l] Nadine Barrie Smith [2], D. Scott Ellis [2] and William D. O'Brien, Jr.[2] zyxw [ zyxwvu 11 Magnetic Resonance Engineering Laboratory, Department of Electrical and Computer Engineering [2] Bioacoustics Research Laboratory, University of Illinois at Urbana-Champaign. IL 61801. USA ABSTRACT zyxwvut The aim zyxwvuts of this study was to investigate the feasibility of using fluorine based magnetic resonance to measureaccuratelyhepatic heating from a focused ultrasound transducer in rats. This newmethod uses the fluorine chemical shifts within a single temperature dependence of the difference in for complicating physiological effects. molecule to provide internal compensation Female Sprague-Dawley rats were exposed to 5 minutes of CW irradiation at 1.37 MHz from a focused transducer. Theoretical calculations based on the monopole source solution to the bio-heat transfer equation gave a volume-averaged temperature rise of 2.1 "C. Preliminary experiments using protein encapsulated perfluoro- octylbromide targeted to the liver gave an empirical rise of 2.0 f0.4 'C. I. INTRODUCTION. A numberofprotonmagneticresonance imaging (MRI) techniques have been developed to map temperature: the relevant parameters being the chemical shift of the time (2), and the diffusion coefficient of the water peak (l), the proton T I relaxation encapsulatedcobaltcomplexeshavealso water protons in tissue (3). Liposome been shown to be effective in-vivo (4). In- vitro results using phase transition compounds (5) and liquid crystals (6) have also been reported. The main advantages of the MRI techniques are their non-invasive character, and the high spatial and temporal 0-7803-2940-6/95/$4.00 zyxwvut 0 1995 IEEE resolution attainable. Using such MRI methods, the field of mapping temperature in-vivo has shown a recent rapid expansion. The proton based methods of mapping temperature are particularly suited to studies ofmuscleandotherrelativelystationary organs. In contrast, we are interested in the reticuloendothelial system (RES) which presents particular problems due to motion artifacts and local changes in magnetic susceptibility due to respiration. Relaxation time and diffusion mapping require a number of images to be acquired. Motion artifacts can be reduced by using high-speed imaging techniques such as echo-planar, but these require specialized hardware. The proton reference frequency method can also be affected by motion. Our new approach here is to use an internal reference such that motion or changes in magnetic susceptibility are internally compensated. In order to achieve this we use an agent which contains two chemical shifts, where the difference in chemical shifts is temperature dependent. 11. MATERIALS & METHODS A. PERFLUOROCARBON PROPERTIES Perfluorooctylbromide (PFOB) has been used as a blood-substitute, and as an For MRI there isno fluorine background imaging agent in MRI, ultrasound and CT. contrast for ultrasound imaging, and the signal, an increased echogenicity gives electron-rich bromine group results in attenuation of x-rays. Here, we use the 1995 IEEE ULTRASONICS SYMPOSIUM - 1609