CT: Computed tomography (CT) is a medical imaging method employing tomography created by computer processing. geometry processing is used to generate a three dimensional image of the inside of an object from a large series of two dimensional X ray images taken around a single axis of rotation. CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various bodily structures based on their ability to block the X ray beam. Although historically the images generated were in the axial or transverse plane,orthogonal to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures. CT can be used for detecting both acute and chronic changes in the lung parenchyma, that is, the internals of the lungs. It is particularly relevant here because normal two dimensional x rays do not show such defects. A variety of different techniques are used depending on the suspected abnormality. For evaluation of chronic interstitial processes (emphysema, fibrosis and so forth), thin sections with high spatial frequency reconstructions are used often scans are performed both in inspiration and expiration.