1. A method for reducing noise in a medical diagnostic image, said method comprising: acquiring an initial three-dimensional (3D) volume of projection data; generating a projection space noise estimate using the 3D volume of projection data; generating an initial 3D volume of image data using the 3D volume of projection data; generating an image space noise estimate using the 3D volume of image data; generating a noise projection estimate using the projection space noise estimate and the image space noise estimate; and reconstructing an image using the generated noise estimate.