1. A method of registering 3-dimensional digitized images to 2-dimensional digitized images during a medical procedure comprising the steps of: providing a pair of correctly-registered training images L={l r , l f }, wherein l r and l f are reference and floating images, respectively, and their joint intensity distribution p l (i r , i f ), wherein i r and i f are reference and floating images, respectively; providing a pair of observed images O={o r , o f }, wherein o r and o f are reference and floating images, respectively, and their joint intensity distribution p o (i r , i f ); mapping a marginal intensity distribution of the observed pair O={o r , o f } to a marginal intensity distribution of the training pair L={l r , l f }; and estimating a set of parameters T that registers image o f to image o r by maximizing a weighted sum of a Jensen-Shannon divergence (JSD) of a joint intensity distribution of the observed pair and a joint intensity distribution of the training pair and a similarity measure between the observed images.