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Computed tomography and pulmonary measurements.

Computed tomography (CT) was first developed to measure accurately radiographic density, rather than as a method of imaging. 19 Although subsequent development concentrated on the imaging potential of the method, some recent studies have used the ability of the method to quantify radiographic density, to provide precise measurements of lung regions and infer differences in regional properties. CT measures the radiographic density of an array of small discrete parts of the body. Usually, each of these parts in a single tomographic slice is represented as a pixel, and these pixels are displayed as the units of a two-dimensional image. The operator allocates a range of grey shades between black and white (tone range) to a particular range of radiographic densities, so that the densities of interest lie between black and white in the visual image. For example, when the details of lung tissue are of interest in the image, the density window (measured in Hounsfield units, HU) often chosen is from 500 (black) to 200 (white). These settings can be expressed in terms of the window centre and width. Lung tissue, containing a large proportion of air, has a range of radiographic densities much less than other body tissues, for example the soft tissues of the mediastinum. Consequently, details of lung structure can be shown more clearly by expressing this narrow range of low densities over the whole visual range from black to white. The grey scale that shows how the black and white visual image is related to radiographic density is displayed at the edge of most CT images, and can be altered by the operator to enhance the features of interest. Each CT image therefore consists of an array of small squares, each with a discrete grey value, that represents the mean radiographic density of a cylinder of tissue, or voxel. The end is represented in position and area by the pixel, and the length is the thickness of the CT slice. Although the visual image is often altered to improve the contrast of specific features, the original radiographic density values can be used to obtain more specific information about the properties of the materials studied. Radiographic density is the property of materials to absorb electromagnetic radiation. This radiation is exponentially reduced on passage through materials, so that the intensity (I) after passing a distance (x)