Abstract: The invention relates to a fixation system for attaching a medical image registration matrix (40) to a patient holder (2) which is to be covered by a sterile drape (10), wherein the fixation system comprises at least one fixation set (L, R) comprising a lower part (6) which is adapted to be connected to the patient holder (2) and an upper part (26) which is adapted to be connected to the registration matrix (40) or one of its supporting elements, wherein: the lower and upper parts (6, 26) can be connected to each other by a connector (25) which does not penetrate the drape (10); the lower and upper parts (6, 26) comprise mutually engageable profiled interfaces (8, 28) which are formed so as to accommodate and clamp the drape (10) between them; and wherein the profiled interfaces (8, 28) fit together over a continuous zone of engagement.