High intensity focused ultrasound (HIFU) is a noninvasive treatment modality that focuses acoustic energy to the targeted area of tissue within the body without damage to intervening tissue.[1] This technique has been used clinically for ablating solid tumors of the prostate,liver,breast,kidney,and brain.[2-5] In most of the existing clinical HIFU devices,ultrasonic energy may be delivered in two modes,i.e.sequential discrete mode and continuous scanning mode.[6] In the former,a single lesion is created by each ultrasound exposure (typically 2-10 s exposure).The transducer is required to move in discrete steps and fire at each position,leading to either overlapping or separated lesions.The latter strategy is to move the transducer in pre-determined trajectories (e.g.linear tracks or spirals) to conform to the required treatment volume.