1. A method for intraoperatively generating and updating a volume dataset, comprising the steps of: (a) acquiring a series of n 2D x-ray projections of biological tissue, in an initial position, of a patient respectively at n different projection angles with an x-ray system, each of said projections having an associated projection geometry selected from the group consisting of known projection geometries and determinable projection geometries, said n 2D x-ray projections forming a volume dataset for said biological tissue; (b) reconstructing image information of said volume dataset representing said biological tissue from said n 2D x-ray projections using said associated projection geometries; (c) after a positional change of said biological tissue, intraoperatively acquiring a series of m 2D x-ray projections of said biological tissue respectively at m different projection angles with said x-ray system, wherein m <n, said m 2D x-ray projections forming a further volume dataset; (d) for each of said intraoperatively acquired m 2D x-ray projections, obtaining an associated projection geometry; (e) segmenting image information of said further volume dataset so that a 2D contour of the biological tissue is present in each of said m 2D x-ray projections; and (f) using the respective projection geometries obtained in step (d) back-projecting said 2D contours that are segmented in the respective m 2D x-ray projections into the image information of said volume dataset reconstructed in step (d), to produce an updated volume dataset containing a 3D contour of said biological tissue after said position change.