Abstract : From the original concept of ultrasound probe holder robot dedicated to mobile tele-echography medical application, a conception study of kinematic structures with optimised spherical modules is proposed. The medical gestures, performed during a tele-echography act, are analysed; the results are used in the definition of the robot kinematic structure specifications. A synthesis study based on Lie groups of rigid body displacements is carried out to establish a set of proposed mechanisms for the two modules forming this medical robot: planar-translations positioning module and spherical displacements orientating module. Multi-criteria optimization is proposed for spherical structures and is applied to three structures: serial, hybrid and parallel. Kinematic performance and workspace volume, in addition to indexes expressing compactness and lightness of the manipulator, play a part in the optimization. The ability of the optimised kinematic structures to travel through their workspace is studied and compared. The kinematic structures of the probe holder robots used in “Otelo” system are presented.