An ultrasonic assembly suited for attachment to a catheter, e.g. for medical treatment. The ultrasonic assembly includes an adjustable ultrasonic focus mechanism arranged in connection with the ultrasonic transducer to adjust focus of ultrasonic waves generated by the transducer. The ultrasonic focus mechanism includes a fluid focus lens with at least two fluids separated by an interface such that ultrasonic waves are substantially reflected at the interface. At least two electrodes are arranged in connection with the fluid focus lens so as to allow adjustment of the interface shape, e.g. a curvature of the interface, when a voltage is applied to the electrodes. In preferred embodiments the electrodes are arranged so as to allow adjustment of the fluid focus lens in an elevation direction as well as in a radial direction. In simple embodiments with rotational symmetric geometry with the transducer positioned in the center of the fluid focus lens, ultrasonic waves can be focused in an annular ring. This is e.g. suitable for ablative or coagulative necrotic treatment of human tissue, e.g. for atrial fibrillation treatment. In some embodiments, the assembly has a number of separate individually adjustable fluid focus lens compartments allowing a more detailed control of the ultrasonic focus pattern.