In conventional x-ray radiography (e.g., a chest radiograph), a patient is illuminated by the penetrating x-rays. e patient attenuates the x-rays, and so casts a shadow on the detector. Stronger shadows correspond to strongly attenuating (usually dense) materials such as bone, while many so-tissue features cause little to no attenuation and so do not cast meaningful shadows. Conventional x-ray radiography is thus an example of “attenuation contrast” imaging: our ability to resolve features in the radiograph is entirely dependent on the (harmful) absorption of x-rays by the patient.