1. A method for imaging iron in arterial plaque with an x-ray computed tomography (CT) system, the steps comprising: a) acquiring image data of the plaque using the CT system operating at a first x-ray energy level; b) acquiring image data of the plaque using the CT system operating at a second x-ray energy level; c) reconstructing two images from the data acquired in respective steps a) and b); d) normalizing the reconstructed images by adjusting the levels of pixel intensities such that pixels depicting calcium have substantially the same intensity; and e) producing an image depicting iron by subtracting the normalized images produced in step d).