1. A method for reducing noise in a medical diagnostic image, said method comprising: acquiring an initial three-dimensional (3D) volume of projection data; de-noising the initial 3D volume of projection data to generate a 3D set of de-noised projection data; generating an initial 3D volume of image data using the 3D set of de-noised projection data; generating a de-noised image volume using the initial 3D volume of image data; generating an estimated noise volume in image space using the initial 3D volume of image data and the de-noised image volume; forward projecting the estimated noise volume to provide a noise projection estimate; generating a set of revised projection data using the noise projection estimate and the 3D set of de-noised projection data; and reconstructing the image using the set of revised projection data.