1432 of small carcinomas at time of diagnosis [5]. This further facilitated the implementation of breast-conserving surgery in clinical practice. However, the cosmetic outcome of breast-conserving surgery is often suboptimal, this being due to the fact that the resection of the tumour necessitate a margin of 1 cm normal breast tissue and use of post- operative radiation. Although breast-conserving surgery carries a relatively low morbidity rate, a variety of complications such as bleeding (2%-10%) and infections (1%-20%) can occur [4]. Technological advances over the last decade have fuelled interest in even less invasive treatment of patients with localized breast cancer. Currently available minimally invasive image-guided tumour ablation techniques include radiofrequency ablation, cryoablation, laser ablation, mi- crowave ablation, and focussed ultrasound (FUS) ablation [6]. Different imaging modalities, including fluoroscopy, ultrasound (US), and magnetic resonance imaging (MRI), are used to guide instruments, to monitor the therapeutic procedure and assess treatment response [7, 8]. Compared with traditional surgical methods, minimally invasive image-guided ablation therapies potentially offer several advantages including reduced recovery time and hospital stay, decreased complication risk (infections, bleeding), and the ability to be performed under conscious sedation in an outpatient setting, all factors leading to significant cost reduction [7]. One of the most attractive image-guided ablation therapies is FUS ablation. FUS ablation is a non- invasive procedure, i.e., requires no probe insertion and utilizes focussed ultrasound energy to coagulate tissue [9]. FUS ablation offers a promising method for noninvasive treatment of benign or malignant breast tumours. The breast is an organ with an excellent soft-tissue window that is required for the ultrasound beam to reach the target volume; furthermore, the breast can be easily immobilized. This review outlines the current status and future directions of image-guided FUS ablation for treatment of breast cancer. The basic concept of FUS ablation Ultrasound beams are generated by a piezoelectric ultra- sound transducer and propagate through tissue as a high- frequency pressure wave. By focussing the ultrasound beams to a focal spot at a certain distance from its source, acoustic energy is converted to heat, and a sharp circum- scribed lesion caused by thermal coagulation will be produced [10]. The skin and tissue surrounding the tissue lesion will be unaffected or show negligible temperature rise [11]. The induced tissue lesion has a typical elliptical shape and a volume of 50–300 mm (Fig. 1). For FUS ablation, ultrasound beams with frequencies in the range of 0.5 to 4 MHz are used depending on the application type and penetration depth that is to be attained, i.e., typical penetration depth is 20 cm at 1.5 MHz. The temperature Fig. 1 The typical elliptical shape of coagulated tissue after focussed ultrasound ablation reached within the focal point during a single sonication should be between 60 and 95°C. The mechanism for cell damage is primarily thermal [12]. Rapid increase in tissue temperature above 56°C for 1 s results in immediate protein denaturation and coagulative necrosis. The extent of cellular damage is determined both by the end temperature achieved and length of time for which it is maintained [13]. At high ultrasound intensity levels, in addition to thermal effect, mechanical stresses also occur, resulting in acoustic cavitation and more extensive cell necrosis. In clinical practice acoustic cavitation should be avoided because it may result in unpredictable thermal lesions. Since a single sonication creates a rather small tissue lesion and the need for cooling between sonications necessary to protect tissue heat accumulation and overheating, the time for treatment of a breast tumour of several cm is rather long and ranges from 45 min to 2.5 h. Several recent articles have reviewed the basic physics of FUS for tissue ablation [13–15]. Choice of image guidance for FUS ablation The potential application of FUS ablation for thermal tumour destruction was demonstrated as early as 1942. Lynn et al. reported focal lesions after FUS ablation in mammalian brain tissue [16]. However, initial clinical success of FUS ablation for treatment of neurological conditions and solid tumours was hampered for decades by the lack of reliable treatment monitoring and guidance technology [17–20]. Developments in imaging technology advanced the ability to use FUS effectively, and non- invasive FUS ablation regained widespread scientific momentum in the 1990s after integration of the ultrasound treatment device with modern imaging system. This was achieved in one of two ways, either by using real-time US or MRI [9, 21]. Today, MRI is the most sensitive technique for diagnosing breast cancer with a sensitivity approaching 100%. Furthermore, it is the most accurate imaging