1. A method for detecting a three-dimensional position of a medical instrument introduced into a body region, using a radiological imaging system having at least one C-arm system with a C-arm having a radiation source and a radiation detector mounted thereon, comprising the steps of: obtaining a plurality of two-dimensional angiography projection images, using said radiological imaging system, of an examination region into which said instrument is to be introduced, and subsequently producing a three-dimensional volume reconstruction of said examination region from said two-dimensional angiography projection images; introducing said instrument into said examination region and, after introduction of said instrument, obtaining at least one pair of two-dimensional projection images of said examination region, using said radiological examination system, said two-dimensional projection images in said pair being disposed at an angle relative to each other; identifying spatial coordinates of a selected point of said instrument in said pair of two-dimensional projection images in a common coordinate system applicable to both said angiography projection images and said pair of two-dimensional projection images; representing said selected point of said instrument in a visual presentation of said three-dimensional volume reconstruction of said examination region in said common coordinate system; and repeatedly obtaining respective pairs of said two-dimensional projection images in sufficiently frequent succession to continuously represent said selected point of said examination instrument in said examination region in said visual representation.