A method is provided for calculating a region of interest in a second image using an initial region of interest in a first image. The first and second images are linked by a spatial transformation function. The method includes steps of (i) generating a first polygonal mesh enclosing the initial region of interest in the first image; (ii) computing a second polygonal mesh in the second image by applying the spatial transformation function to the first polygonal mesh, and (iii) identifying image elements within the second image, that belong to the area enclosed by the second polygonal mesh. Also provided are a computer program and a system.