Extraordinary progress has been made over the last two decades in the development and dissemination of new medical imaging technologies. The development of computed tomography, positron emission tomography, and magnetic resonance imaging, as well as major innovations to the conventional imaging modalities, have revolutionized medical diagnostic imaging. Despite their many differences, all of these modalities can be viewed from a common perspective: being described in terms of the underlying physical properties imaged, the type of radiation/detection system used to produce the images, and the imaging performance that the modality achieves, both in absolute terms and relative to that of a conceptual ideal observer. This is a US government work. There are no restrictions on its use.