d Original Contribution THE EFFECT OF THE SCANNING PATHWAY IN HIGH-INTENSITY FOCUSED ULTRASOUND THERAPY ON LESION PRODUCTION YUFENG ZHOU,* y STEVEN G. KARGL, z and JOO HA HWANG yz *School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore; y Division of Gastroenterology, Department of Medicine, University of Washington, Seattle, WA; and z Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA, USA (Received 9 February 2011; revised 23 May 2011; in final form 31 May 2011) Abstract—Because tumors are much larger in size compared with the beam width of high-intensity focused ultra- sound (HIFU), raster scanning throughout the entire target is conventionally performed for HIFU thermal ablation. Thermal diffusion affects the temperature elevation and the consequent lesion formation. As a result, the lesion will grow continuously over the course of HIFU therapy. The purpose of this study was to investigate the influence of scanning pathways on the overall thermal lesion. Two new scanning pathways, spiral scanning from the center to the outside and spiral scanning from the outside to the center, were proposed with the same HIFU parameters (power and exposure time) for each treatment spot. The lesions produced in the gel phantom and bovine liver were compared with those using raster scanning. Although more uniform lesions can be achieved using the new scanning pathways, the produced lesion areas (27.5 ± 12.3 mm 2 and 65.2 ± 9.6 mm 2 , respectively) in the gel phantom are significantly smaller (p , 0.05) than those using raster scanning (92.9 ± 11.8 mm 2 ). Further- more, the lesion patterns in the gel phantom and bovine liver were similar to the simulations using temperature and thermal dose-threshold models, respectively. Thermal diffusion, the scanning pathway and the biophysical aspects of the target all play important roles in HIFU lesion production. By selecting the appropriate scanning pathway and varying the parameters as ablation progresses, HIFU therapy can achieve uniform lesions while minimizing the total delivered energy and treatment time. (E-mail: yfzhou@ntu.edu.sg) Ó 2011 World Federation for Ultra- sound in Medicine & Biology. Key Words: High-intensity focused ultrasound, Thermal ablation, Lesion production, Scanning pathway, Thermal diffusion. INTRODUCTION High-intensity focused ultrasound (HIFU) is emerging as a new modality for ablating solid tumors, such as uterine fibroids and cancers of the prostate, kidney, liver, breast, and pancreas (Meaney et al. 2000; ter Haar 2001; Wu et al. 2001, 2004; Dubinsky et al. 2008). In China and Europe, more than 100,000 cases have already been treated using HIFU in clinics, with promising results. The principle of HIFU therapy is focusing a high- intensity ultrasound beam into a small region where a tumor is located under the guidance of either magnetic resonance imaging (MRI) or ultrasound imaging (Bailey et al. 2003). The acoustic intensity at the focus is several 1000 W/cm 2 so that the temperature can exceed 65  C within seconds to induce irreversible coagulation necrosis in the tissue. In comparison to traditional cancer treatment methods (i.e., open surgery, radiotherapy or chemo- therapy) and other physical methods for tissue ablation (i.e., laser, microwave or radio-frequency [RF]), HIFU ablation has the advantages of noninvasiveness, precise focusing and deeper penetration without exposing patients to ionizing radiation. Substantial evidence suggests that the risk of metastasis is not increased after HIFU treat- ment (Oosterhof et al. 1997; Wu et al. 2003; Kenney 2005), and fewer treatment related complications have been observed (Th€ uroff and Chaussy 2000; Aus 2006). Despite its uniqueness and encouraging preliminary clinical results, HIFU remains a therapeutic modality in development, with several technical problems limiting it from becoming a widely adopted procedure for both patients and physicians (Aus 2006; Rewcastle 2006; Zhou 2011). Engineers and scientists are devoting Address correspondence to: Yufeng Zhou, Ph.D., School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798. E-mail: yfzhou@ ntu.edu.sg 1457 Ultrasound in Med. & Biol., Vol. 37, No. 9, pp. 1457–1468, 2011 Copyright Ó 2011 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/$ - see front matter doi:10.1016/j.ultrasmedbio.2011.05.848