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Table 3.1 Technological Improvements in the First 20 Years of Computed Tomography

threedimensional representation of anatomical structures.
The attenuation of monochromatic xrays in homogeneous objects is governed by
where I is the xray intensity behind the object, I _{0} is the xray intensity without the object, x is the length of the xray path through the object, and a is the linear attenuation coefficient of the material for the xray energy employed.
For inhomogeneous objects like the human body, the attenuation of xrays consequently can be described by
This model is explored further in section 14.1.1.
In CT scanners the xray attenuation according to equation 3.2 is measured along a variety of lines within a plane perpendicular to the long axis of the patient with the goal of reconstructing a map of the attenuation coefficients a for this plane. The resulting attenuation coefficients are usually expressed with reference to water: