1. A method of automatically initializing pose for registration of 2D fluoroscopic abdominal aortic images with a 3D model of an abdominal aorta, comprising: detecting a 2D iliac bifurcation and a 2D renal artery bifurcation from a sequence of 2D fluoroscopic abdominal aortic images obtained from a patient; detecting a spinal centerline in a 2D fluoroscopic spine image; providing a 3D iliac bifurcation and a 3D renal artery bifurcation from a 3D image volume of the patient's abdomen, and a 3D spinal centerline from the 3D image volume of the patient's abdomen; and determining pose parameters {x, y, z, θ}, wherein (x, y) denotes the translation on a table plane, z denotes a depth of the table, and θ is a rotation about the z axis, by minimizing a cost function (C A +C S ), wherein C A is defined as C A = ∑ i = 1 2 ( B i 2 D - P 1 ( H · B i 3 D ) ) 2 , wherein P 1 is a projection of the 3-D bifurcations to an aortic image plane, B 1 2D and B 1 3D are the respective 2-D and 3-D bifurcations of the iliac arteries, B 2 2D and B 2 3D are the respective 2-D and 3-D bifurcations of the renal arteries, and H ( x , y , z , θ ) = [ cos θ - sin θ 0 0 sin θ cos θ 0 0 0 0 1 0 x y z 1 ] is the homogenous transformation matrix of the registration parameters, and C S is defined as C S =Dist(L spine , P 2 (H·B 3 3D )), wherein P 2 is a projection of the 3D spine centerline to a spinal image plane, B 3 3D is a 3-D spinal landmark that is a point a predetermined distance behind the renal artery bifurcations, L spine is the 2-D spinal centerline, and Dist( , ) is a point-to-line distance in 2-D.