The present invention relates to an apparatus for determining and/or monitoring foreign structures in a fluid or in a fluid flow, in particular blood or bloodstream, having at least one optical monitoring means, at least one ultrasound monitoring means and at least one signal evaluation means, wherein the fluid can be optically monitored at least by means of the optical monitoring means and at least by means of ultrasound by means of the ultrasound monitoring means, and wherein at least one foreign structure, in particular an air bubble and/or a solid body such as a blood clot, can be recognized in the fluid by the signal evaluation means with reference to the combination of the monitoring signals obtained herefrom, in particular with reference to the comparison of the monitoring signals acquired herefrom and/or can be distinguished from at least one second foreign body structure by means of the signal evaluation means.
