modality for visualization and delineation of the breast tumour margins [22–25]. Because MRI has an excellent anatomical resolution and high sensitivity for tumour detection, it offers accurate planning of the tissue to be targeted. An additional advantage of MRI over other imaging techniques is the possibility for MR-based tem- perature mapping by exploiting the temperature depen- dence of the water proton resonance frequency [26]. Proton resonance frequency shift thermometry allows monitoring temperature elevations during and after a focussed ultra- sound sonication. Damage to adjacent structures can be prevented by evaluation temperature elevations in the surrounding tissue. As a consequence, by combining focussed ultrasound as a noninvasive thermal therapy with MRI for planning, guiding, monitoring, and control- ling the focussed ultrasound beam, a real-time image- controlled non-invasive breast tumour ablation system is provided [13, 15]. The commercially available MRI- guided FUS system consists of a FUS ablation device, the ExAblate 2000™ (InSightec, Ltd., Haifa, Israel) integrated in a 1.5-T or 3-T MRI magnet bore [General Electric (GE) Medical Systems, Milwaukee, WI]. The MRI magnet and the focussed ultrasound system are integrated and controlled by a focussed ultrasound workstation and console (Fig. 2). The system provides programmable electronic control over the position and intensity of the focal point across the volume of interest with high flexibility, speed, and precision (Fig. 3). Diagnostic ultrasound can also be used to guide the FUS ablation procedure [21]. With US guidance, the field of the diagnostic transducer is overlying and arranged parallel to the therapeutic source and offers real-time visualization of the targeted volume [27]. The shortcoming of ultrasound for guiding ablation procedures in the breast is that in Fig. 2 The ExAblate 2000™ FUS ablation device (InSightec, Ltd., Haifa, Israel) with a built- in 3-T MRI system [General Electric (GE) Medical Systems[ in the Lucas MR Imaging Cen- tre, Stanford, CA 1433 general breast tumour size is underestimated with this technique [24]. Accurate delineation of tumour margins is a significant limiting factor of ultrasound FUS guidance as successful ablation of the tumour in its entire volume is mandatory for successful outcome [28, 29]. Another shortcoming of ultrasound guidance during breast tumour ablation is the formation of gas bubbles in ablated tissue. This results in reflection of the ultrasound waves and acoustic posterior shadowing that influences the accuracy for delineation of hypoechoic ablated tissue from hypo- echoic breast tumour tissue [28]. Finally, the use of ultrasound during breast tumour ablation is limited by its inability to monitor temperature. Despite these short- comings ultrasound has been used for noninvasive FUS ablation of breast cancer, although all reports in this area are from the same group [30–38]. Wu and colleagues also developed the only commercially available US-guided FUS ablation system (JC-HIFU system™, Haifu Technol- ogy Co. Ltd., Chongqing, China). MRI-guided FUS for breast tumour ablation This first experience of MRI-guided FUS for treatment of benign breast tumours was reported by Hynynen et al. in 2001 [39]. They described the use of MRI-guided FUS ablation for treatment of 11 histologically proven breast fibroadenomas. Treatment success was defined as complete or partial lack of contrast material uptake on post- procedural T1-weighted images, which was demonstrated in 8/11 (73%) lesions. In three lesions the treatment was not successful, which was attributed to low acoustic power and patient movement. This report was followed in the same year by the first case report of MRI-guided FUS for