A medical image can be roughly described in terms of the three basic features given in Chapter 1: contrast, spatial resolution, and noise. Spatial resolution or clarity refers to the spatial detail of small objects within the image. Noise refers to the precision within the image; a noisy image will have large uctuations in the signal across a uniform object, while a precise signal will have very small uctuations. e subject of this chapter, contrast, relates to the dierence in signals between a structure and its immediate surroundings. For example, if circles are displayed against a black background, a white circle will have larger contrast relative to the background when compared to gray circles (Figure 4.1). One  uses the dierences in gray shades to “visually” distinguish dierent tissue types, determine anatomical relationships, and sometimes assess their physiological functions. e larger the contrast between dierent tissue types, the easier it is to make distinctions clinically. It is oen the objective of an imaging system to maximize the contrast in the image for a particular object or tissue of interest, although this is not always true since there may be design compromises where noise and spatial resolution are also very important. e contrast in an x-ray image depends on both physical characteristics of the object and properties of the device(s) used to image the object. e focus of this chapter is x-ray image contrast and we discuss the physical determinants of contrast, including material properties, x-ray spectra, detector response, and the role of perturbations such as scatter radiation and image intensier veiling glare. We include physical determinants of contrast for several other medical imaging modalities, including those used in nuclear medicine, magnetic resonance imaging, and computed tomography to round o the discussion.