1. A method for registration of ultrasound device in three dimensions to a C-arm scan comprising: acquiring a baseline volume from a C-arm computed-tomography apparatus; inserting and navigating an ultrasound device to an organ in a patient's body; performing an initial registration of an ultrasound image acquired by said ultrasound device to said baseline volume; acquiring a plurality of images in which the ultrasound device is captured; performing a multimodal registration of the ultrasound volume to the baseline volume, by an iterative search that includes resampling data of the baseline volume in a neighborhood of the ultrasound device and optimizing an alignment of the baseline volume to an ultrasound volume using an image similarity metric, wherein said image similarity metric is arg ⁢ max ϕ ⁢ 1 N ⁢ ∑ i = 1 N ⁢ CC ⁡ ( U i , R · T i - 1 ) , wherein CC is a Pearson's correlation coefficient, N is the number of ultrasounds volumes U, T is a transformation matrix mapping the ultrasound volume U on the baseline volume R, and φ is a 6-vector that parameterizes T, and said optimization includes a search on a rotation about an axis of the ultrasound device followed by a Simplex direct search; and performing fusion of said registered ultrasound volume and baseline volume to display a view of the ultrasound device in the baseline volume, wherein the steps of performing an initial registration, performing a multimodal registration, and performing fusion are executed by a computer processor.