1. A method for automatically selecting a region of interest covering a heated area in an MRI-guided high-intensity focused ultrasound system, said method comprising the steps of: operating a magnetic resonance data acquisition unit to implement a single acquisition of phase data of a scanned area of a subject while ultrasonically heating an area of the subject, to produce a heated area within the scanned area; providing the acquired phase data, acquired in said single acquisition, to a computerized processor and, in said processor, automatically identifying the heated area within the scanned area from the acquired phase data acquired in said single acquisition; and in said processor, automatically reconstructing an image from said phase data and from said processor, displaying said image on a display, as a displayed image, and in said displayed image, automatically demarcating a region of interest covering the heated area, identified from acquired phase data acquired in said single acquisition, according to a predetermined size of said region of interest that covers and is larger than said heated area.