The primary purpose of adding filtration to an x-ray beam is to increase the quality of an x-ray beam by selectively removing low-energy x-rays. Low-energy x-rays have no chance of reaching the image receptor and serve only to provide a radiation dose to the patient's skin. Filtration also makes the image a uniform density.
Filtration increases the average energy of the beam, sometimes called "hardening of the beam." The overall energy of the beam increases because the low-energy (soft) photons have been removed. Because the overall energy is higher, the half value layer (HVL) will subsequently increase. Filtration is expressed in Al (Aluminum) or AL/Eq (Aluminum Equivalency) and consists of inherent and added filtration. The combination of inherent and added filtration is called total filtration.
Inherent filtration (.5 mm Al/Eq) is filtration caused the x-ray tube itself. It consists of the tube glass (or envelope) and the cooling oil surrounding the tube. Together the glass and oil equal about .5 mm Al/Eq. Inherent filtration increases with age due to the deposition of tungsten from the filament and anode.
Added filtration measuring 2.0 mm Al/Eq is added to the x-ray beam. It consists of the collimator mirror and an added Al plate. The Al plate measures 1 mm of Al and the silver in the collimator mirror measures another 1 mm of Al/Eq. The National Council on Radiation Protection and Measurement (NCRP) Guidelines for Total Filtration are:
Many materials can be used as filters. As you have noticed in your clinical experience, certain materials such as wood and plastic from a backboard, sandbags, and saline will filter the beam. X-ray equipment usually uses 1 to 3 mm of aluminum (Al). One reason that aluminum is commonly used as a filter it that it has a low atomic number (Z = 13) which makes it a good absorber of low energy x-rays. Other reasons are that it is readily available; is inexpensive; and is easy to mold into different shapes.
Filters can also be used to absorb higher energy x-rays. These compensating filters are used to even out densities on a radiographic image. Compensating filters slide onto the collimator tube housing.
A wedge filter is thin on one edge and thick on the opposite edge. It is beneficial for t-spine, foot, shoulder, patella, and swimmers projections to name a few. With a trough filter, the center portion of the filter is thin and the outer edges are thicker for the lung fields. It is useful for chest exams to provide filtration over both lungs. If x-ray equipment does not have compensating filters, then a saline bag can be used in some circumstances. For example it can be used for a t-spine exam as an alternative to the wedge filter. Be careful when using a saline bag, the edges of the bag can cause an artifact on the radiograph.
The half-value layer is the amount (layer) of absorbing material that will reduce the intensity―and therefore the patient's exposure on their skin surface―of the primary beam to one-half its original value. It affects both the quantity and the quality of the primary beam. The HVL increases when filtration increases due to the hardening of the beam (increase in the average energy of the beam). The average HVL of diagnostic beam is from 3 to 5 mm Al.
Question
A general-purpose x-ray imaging system has a HVL of 2.2 mm Al. The exposure from this equipment is 2 mR/mAs at 100 cm SID. If 2.2 mm Al is added to the beam, what will be the x-ray exposure?
Answer
This is an addition of one HVL; therefore the x-ray exposure will be 1 mR/mAs
Density decreases as a result of a filtered useful beam. Therefore, technical factors must be increased to maintain density.
Contrast is slightly reduced due to hardening of the beam; however, the reduction is not really significant.
Filtering the useful beam also decreases skin dose because the overall energy is increased.