Characteristics and control of contrast in CT.

Understanding how contrast is produced and controlled in computed tomography (CT) is essential to proper application of this modality. In the typical CT scanner, a narrow x-ray beam in the section thickness direction and an air gap in the section plane are used to reduce scatter and improve contrast. High- and low-contrast detectability of a CT scanner are important performance parameters contributing to optimal image quality. The limits of detectability of high-contrast objects (ie, spatial resolution) are affected by detector aperture size, pixel size of the image, algorithm used to reconstruct the image, and section thickness. Visibility of low-contrast objects is limited by image noise and the algorithm. Contrast in CT images can be controlled by the window level and window width settings used to display the image. These settings dictate how the actual measurements of tissue attenuation are translated into a gray-scale image. Wide window widths can be used to provide an accurate representation of bone, and narrow widths are more useful for visualizing soft tissues.