A simple and reliable process for activating a medical instrument, for example, one containing an anesthetic agent- or drug-dispensing unit as well as a respirator drive for respirating a patient is provided. The centering device has at least two struts extending between a proximal end and a distal end. Via a second clutch, power is transmitted from the transmission shaft to the input shaft, while power transmission vice versa is blocked. A system and method is also presented for providing a call log for identifying call activity associated with a plurality of communication lines. The compounds of the invention are of formula I wherein R1-R4 and Ar are as herein defined. The processor acquires second image data indicating an image in a second holding position after the first image data has been acquired.