Filtration,
removal of parts of the X ray spectrum using absorbing materials in the X-ray beam. The X-ray spectrum reaching the patient is filtered by attenuating material in its path. Filtering of the beam is used in order to modify the spectral or spatial distribution of X-rays, or both. Filtration is in principal divided in two parts: inherent filtration and added filtration. Among those filters added are compensation and equalization filters. Examples of compensation filters are variations of the wedge filter, which is used to compensate for the otherwise uneven X-ray fluence generated by objects with a wide thickness variation, such as hands and feet. Equalization filters follow a similar principle and are sometimes used to compensate for more irregular absorption variations in the object, such as the mediastinum in a chest frontal image.

Inherent filtration,
the filtration of an X-ray beam by any parts of the X ray tube or tube shield through which the beam must pass. The parts include the glass envelope of the X-ray tube, the oil cooling the tube and the exit window in the tube housing. The inherent filtration corresponds to approximately 0.5–1 mm of aluminium. The total filtration of the X-ray beam before it reaches the patient consists of the inherent filtration plus the added filtration.

Added filtration,
commonly metallic filters inserted into the X-ray beam. The inherent filtration normally consists of the filtration of the X-ray beam from the glass envelope of the X-ray tube, the oil cooling the tube and the exit window in the tube housing. In excess of this, added filtration is almost always considered needed. This filtration is for normal X-ray purposes commonly made of aluminium or copper. The purpose of inserting such extra filtration into the X-ray beam serves the following purposes:
· To remove the low-energy photons that never would have been able to reach the film and produce an image. These photons would, if present, only increase the radiation dose given to the patient.

· To remove those low-energy photons that otherwise would have reached the film but would have given rise to too high contrast in the image. The classical example of this is in chest imaging, where the contrast from ribs and shoulder blades must be reduced.

In other cases, extra filtration can fulfil other purposes:

· In mammography, where a molybdenum anode is used, the added filtration normally is made of the same material (molybdenum Mo ). It is a fact that a material is particularly translucent to its own characteristic radiation, therefore giving an X-ray spectrum with as much (monoenergetic) characteristic X-rays from Mo as possible and filtering more selectively on both the high- and low-energy side of the Mo characteristic X-rays.

· For very special purposes, special filters can be used that will create a shape of the X-ray spectrum that — to some extent — will “match” the absorption characteristics of the X ray contrast medium, thereby selectively increasing their contrast properties.
 

Coppper & Aluminium Filtration

All the radiation absorbed inside the body, without having a chance of penetration and forming an image, is harmfull radiation only!
In order to make the radiation "less harmfull", filters are used. The soft radiation is absorbed inside the filter while the hard radiation passes only slightly effected.

As seen in the left graph, Aluminum attenuates the very soft radiation drastically. The radiation spectrum shown is the result of 100kV tube voltage in combination with a filtration equivalent to 2.5mm Aluminum.
According to international regulations, this is the minimum amount of filtration and must be guaranteed by the tube assembly.

Additional filtration with copper can be employed to make the radiation "safer". Notice the shift of the peak intensity to higher keV by absorbing the lower energies. So, the radiation quality is hardened-up by increasing the amount of filtration
 

Additional filtration
The use of additional Copper filters presents a simple way of reducing patient Entrance Surface Dose by removing low energy photons from the x-ray spectrum. Typical ESD reductions from this are:

 Copper also has a beam hardening effect (seen as the shift of the peak in the spectrum to the right), resulting in a loss of contrast. X-ray output is also reduced (seen as the reduced area under the curves with Copper) so tube loading will be increased. Specially designed x-ray tubes and specific kV-mA curves are needed to overcome these issues