1. A method for analyzing a multi-dimensional structure which includes a tubular structure having local sizes, from two-dimensional x-ray image datasets for pre-determined projection directions, the method comprising using a computer to perform the steps: identifying at least one pair of corresponding initial projected centre points of the tubular structure in two respective initial two-dimensional x-ray image datasets, identifying at least one further projected centre point corresponding to said initial projected centre points in at least one further two-dimensional x-ray image dataset, deriving a three-dimensional spatial position of the centre point of the tubular structure corresponding to said initial and said at least one further projected centre points, identifying projected edges of the tubular structure locally in said initial two-dimensional x-ray image datasets and in said further two-dimensional x-ray image dataset near the respective projected centre points, wherein identifying includes identifying on the basis of local differences between (i) brightness or grey values or (ii) local gradients of brightness or grey values in the corresponding two-dimensional x-ray image datasets and deriving a local size of the tubular structure at the three-dimensional spatial position of the centre point of the tubular structure from (i) said projected edges and (ii) the predetermined projection directions, wherein deriving the local size yields an inscribed polygon of local cross-sectional area of the tubular structure locally at the centre point at issue, edges of said inscribed polygon corresponding to said projection edges, a number of vertices of said inscribed polygon corresponding to a number of the two-dimensional x-ray image datasets.