Filtration

What is a filter? Well, if you're a coffee drinker, you are familiar with filters. A nice brew is created by placing a filter in the coffee maker and filling the filter with ground coffee beans, allowing the desired coffee (created when hot water soaks the grounds) to flow through to the pot, while preventing the unwanted portion (the grounds) from trickling down. This is not so different from radiographic filters at all.

An x-ray filter is composed of Aluminum equivalent material (Al - not to be confused with Pb for shielding) and is between the target and the patient for the purpose of preventing unwanted photons (the grounds) from passing through while allowing the desired photons (the coffee) to pass through toward the patient.

So which photons do we want to keep and which ones do we want to get rid of? Our duty as Radiologic Technologists is to keep radiation exposure levels of the patient at a minimum (ALARA standards). So, in any x-ray exposure, there is a portion of the beam that will be at a very low energy for the part being x-rayed. It is so low, that it does not even contribute to the useful beam, and it ends up getting absorbed in the patient contributing to radiation dose - BAD photon!

A filter allows us to remove a majority of the bad photons while allowing the good photons (higher energy) to get through to the film. The line of thinking goes something like this: "If the low energy photons are not going to contribute to our image anyways, why not remove them?" Take a look at the picture of an unfiltered beam:



Here I have randomly selected five photons with varying energy levels (we all know that just because we select 70 kV, it doesn't mean all photons produced are at their peak). To calculate the average energy of the beam using these photons, you add them all together and divide by 5, which gives you an average energy of 50 keV. The 30 and 40 keV photons are probably going to be absorbed by the body to contribute to radiation dose to the patient. Now, lets look what happens when we add filtration:



Notice, the two lower energy photons are removed, and the remaining photons have a higher average energy of 60 keV. This is also referred to as "hardening" the beam. Note that any time you add filtration without changing any other factors, you are reducing the intensity of your beam, so an increase in technique is always required when adding any absorbent material. The resulting energies are shown in the following graphic representation of exposures made at 120 kVp:



HVL - half value layer is any amount of material (or in this instance, filtration) that reduces the intensity of your beam to half its original value. Consequently the TVL (tenth value layer) is the amount of material that reduces the intensity to one tenth the original value, and so on.

Types of filtration:

Inherent filtration is any filter that is present as part of the radiographic equipment, and usually includes the glass envelope surrounding the tube, as well as any oil around it. This usually makes up about .5 - 1.0 mm Al equivalency.

Added filtration is just as it is described - anything added to what filtration already exists within your equipment. It usually resides between the tube housing and the collimator box. See the following picture from "Principles of Radiographic Imaging" Carlton/Adler 4th edition:



Total filtration = inherent filtration + added filtration. According to the National Council on Radiation Protection (NCRP), total filtration must be a minimum amount depending on the kVp range you are using:

Below 50 kV - 0.5mm Al
50 to 70 kV - 1.5mm Al
Above 70 kV - 2.5mm Al

Compensating filters are for another post... to be continued...