1. A method for determination of 3-D structure of an object in biplane angiography, comprising: providing first and second imaging systems including first and second x-ray sources each having a focal spot and respective first and second x-ray sensitive receivers each defining an image plane, said first x-ray source arranged to transmit x-rays from its focal spot in a first direction through said object to the image plane of said first receiver and said second x-ray source arranged to transmit x-rays from its focal spot in a second direction arbitrarily selected with respect to said first direction through said object to the image plane of said second receiver; determining the distances (D, D') of perpendicular lines from the focal spot of each x-ray source to the image plane of the respective receiver; determining the points on respective image planes where respective perpendicular lines from the respective focal spots to the respective image planes intersect the respective image planes and defining said points on respective image planes as the origins of respective two dimensional image coordinate systems (uv), (u'v') at the respective image planes, wherein said focal spots and the respective image planes define respective first and second three-dimensional coordinate systems having respective z axes coincident with said perpendicular lines (D, D') in the directions of respective image planes, x axes parallel to respective of the image plane axes (u, u') and y axes parallel to respective of the image plane axes (v,v'), where the relative geometry of said first three dimensional coordinate system with respect to said second three dimensional coordinate system is defined by EQU x.sub.1 '=[R]{x.sub.1 =-t}, where x.sub.1 ', is the position vector of the object point (x.sub.i ', y.sub.i ', z.sub.i ') in said second three dimensional coordinate system, x.sub.1 is the position vector of the same object point (x.sub.i, y.sub.i,z.sub.i) in said first three dimensional coordinate system, [R] is a rotation matrix defining the rotation in three-dimensional space between the first and second three-dimensional coordinate systems and t expresses, in the first coordinate system xyz, a unit translation vector that moves the origin of the first coordinate system xyz to the origin of the second coordinate system; irradiating said object with x-rays from said x-ray sources and producing respective first and second images defined by digital image data based on the x-rays received by said first and second receivers; determining from each of said first and second images the image coordinates ((u, v), (u', v')) in the respective coordinate systems of N objects points, where N.gtoreq.8, which correspond to the same object points in the object; scaling the image coordinates (u.sub.i, v.sub.i), (u.sub.i ', v.sub.i ') of said eight points by respectively dividing said image coordinates by the respective distances (D, D') to obtain normalized image coordinates (.xi..sub.i, .eta..sub.i), (.xi..sub.i ', .eta..sub.i')); constructing N linear equations, one for each object point, containing only normalized image coordinates (.xi..sub.i, .eta..sub.i), (.xi..sub.i ', .eta..sub.i) and nine unknown elements (q.sub.kl), where q.sub.kl represent an unknown relative geometry between the two imaging systems in terms of nonlinear combinations of the elements of the translation vector (t) and the rotation matrix [R]; solving the N linear equations for eight of the q.sub.kl values relative to the ninth q.sub.kl value to produce a matrix [Q*]; and determining scaled three dimensional coordinate (x.sub.i, y.sub.i, z.sub.i) of said N object points expressed in units of a unit translation vector t from the product matrix [Q*].sup.T [A*].