X-ray imaging modalities such as Computed Tomography (CT) provide images at length scales of meters with penetration deep into tissues and organisms. Contrast in CT images is measured in terms of X-ray attenuation; the unit for describing X-ray attenuation is the Hounsfield Unit (HU), which provides a quantitative scale for describing radiodensity. The HU scale is defined by the radiodensity of distilled water (0 HU), air (-1000 HU). For biological tissues, typical HU values range from -700 (lungs) to +1900 (bone), with most soft tissues (fat, muscle, organs) having HU values between -120 and 60. Inorganic materials attenuate X-rays much more effectively than biological tissues, with pure copper attenuating at 14,000 HU and gold representing 30,000 HU (an upper measurable limit). For inorganic nanoparticles, X-ray attenuation depends on particle concentration.