A signal processing unit configured to: cause a first ultrasound emitter, when attached to a wall of a vessel, to emit a first ultrasound test signal; receive, from at least one ultrasound receiver, when attached to the wall of the vessel, the first ultrasound test signal; detect a time of flight of the received first ultrasound test signal; and determine that an acoustic coupling of the first ultrasound emitter and a first ultrasound receiver of the at least one ultrasound receiver to the wall of the vessel is intact if the detected time of flight corresponds to a length of a path in the wall of the vessel from the first ultrasound emitter to the at least one ultrasound receiver.