1. A method for non-rigid multi-modal registration of digitized images comprising the steps of: providing a reference image and an alignment images acquired from different imaging modalities to a graphics processing unit (GPU), wherein each said image comprises a set of intensities defined on a 3-dimensional (3D) grid of points; initializing a deformation field for registering said reference image and said alignment image; computing marginal and joint intensity histograms of the reference image and the alignment image as registered by said deformation field; computing gradients of the reference and registered alignment images and of their respective marginal and joint intensity histograms; smoothing said histograms and gradients using Gaussian filters; calculating a new deformation field using said smoothed gradients; and registering said alignment image to said reference image using said deformation field.