We have developed a robotic system to assist doctors when they are moving ultrasonic probes on a patient's skin while exerting a given effort. The probes are used to monitor arteries for cardiovascular disease prevention, namely to reconstruct the three-dimensional profile of arteries. A preliminary feasibility study making use of an industrial robot has been made to validate the force control scheme. It has proven the interest of the robotized approach for such medical applications where force control is needed. In order to comply with safety constraints, a dedicated robotic system 'Hippocrate' has been designed. This paper describes the arm and the controller architectures, with emphasis on design strategies selected to meet safety requirements. Preliminary in vivo results are presented as well as a possible new application of Hippocrate as a tool for reconstructive surgery.