Abstract: An image display system is provided with an image display apparatus (1) for displaying an image, the image being dependent on the position and orientation of the image display apparatus and independent of the position and posture of a user of the image display apparatus. An image guided surgery system is provided with such an image display system in order to show the user, during a medical treatment utilizing an instrument, where within the patient the instrument is situated, without the user having a direct view of said instrument.

Abstract: An image-guided surgery system is used to show a user, for example a surgeon, a position of a surgical instrument in an operating area in the body of a patient during a surgical operation. The image-guided surgery system includes an alignment system for deriving an alignment line through a target position, for selecting a starting plane through an end of the surgical instrument, for deriving a starting point on the alignment line and in the starting plane, and for reproducing a position of said end and of said starting point relative to one another in the starting plane. For example, the positions of the end and the starting point in the starting plane are reproduced in a guide image.