1. A computed tomography (CT) system comprising: a radiation source configured to emit radiation; a detector positioned to receive the radiation; and a computer coupled to the source and the detector, the computer configured to: access CT image data over a plurality of slices of an object; reconstruct an image using the CT image data; determine a total attenuation of the reconstructed image; estimate noise in the reconstructed image using the determined total attenuation and based on one of a simulation, a theoretical analysis, and data from at least two scanned phantoms; and apply an anisotropic diffusion algorithm to the reconstructed image based on the estimate of the noise level in the reconstructed image.