When x-rays are produced in an x-ray tube, not all of the x-rays are created
at the maximum, or peak, energy. A continuous x-ray spectrum is produced which
contains x-rays of energies from zero to the kVp. For most X-Ray spectra
the average energy of the X-rays produced are:
EAverage = 1/3 EMax
Since lower energy x-ray photons are unable to penetrate the patient, they
cannot add information to the x-ray image. These x-rays contribute radiation
dose to the patient, but do not enhance the diagnosis. Since these low energy
x-rays are less penetrating, they can be selectively removed from the x-ray beam
by adding filtration. The walls of the x-ray tube itself provide some
filtration, and additional filtration is typically added in the form of a few
millimeters of aluminum. This reduces unnecessary exposure to the patient.
Filtration will alter the x-ray spectrum, as shown below.