The invention concerns an imaging system to monitor at least one surgical instrument in an operative site inside a volume of the body of an animal, comprising: at least one endoscopic camera to obtain endoscopic data on the operative site, at least one ultrasound imaging device to obtain ultrasound data on the operative site, and a processing device to process the endoscopic and ultrasound data. The imaging system further comprising at least three markers intended to be positioned in the operative site, said markers being mobile relative to the instrument, each marker being adapted to be detected both by the endoscopic camera and by the ultrasound imaging device, so as to permit cross-mapping of the endoscopic and ultrasound data by the processing means.