Table 3.1 Technological Improvements in the First 20 Years of Computed Tomography
three-dimensional representation of anatomical structures.
The attenuation of monochromatic x-rays in homogeneous objects is governed by
where I is the x-ray intensity behind the object, I 0 is the x-ray intensity without the object, x is the length of the x-ray path through the object, and a is the linear attenuation coefficient of the material for the x-ray energy employed.
For inhomogeneous objects like the human body, the attenuation of x-rays consequently can be described by
This model is explored further in section 14.1.1.
In CT scanners the x-ray 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: