Rad Physics Chapter 10: Filtration

The process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials (e.g. aluminum) into the primary beam.

When filtration is added the overall quantity of photons go down (filtering out low energy ones) therefore the overall quality of the x-ray spectrum goes up.

Because it filtration removes low energy ("soft") photons and increases the average beam energy

30-40 keV

Low energy photons cannot exit the patient so they only contribute to patient dose without doing anything to the x-ray image

Measured in aluminum/equivalency because aluminum is the standard filtering material

The Half-Value Layer is the amount of filtration required to reduce the original intensity of the beam in half

1. Inherent filtration
2. Added filtration
3. Compound filtration
4. Compensating filtration
5. Total filtration

Filtration that results from the composition of the tube and the tube housingi.e. the glass envelope, dielectric oil bath, and the glass window of the housing

As the tube ages it becomes gassy, the anode becomes pitted and vaporized tungsten can deposit on the glass envelope/window which can lead to an increase in inherent filtration and therefore reduce the tube effiency. HVL testing can compare the filtration from when the tube was new til now.

Any filtration that occurs outside the tube and tube housing and before the IR

1. Sheets of aluminum between x-ray tube housing and the collimator
2. Collimators (1mm al/eq) because of the silver on the back of the mirrors that direct the beam

Also called K-edge filtersFilters that use two or more materials; where each layer absorbs characteristic photons created from the previous layerThis is why it is layered from highest atomic number to lower atomic number (closest to the patient)

• lead, copper (#29), aluminum (#13)
• the characteristic photons created from aluminum has such lower energy (1.5keV) that they are absorbed in the air before it reaches the patient

Designed to solve the problem of parts with uneven tissue thickness or densities (i.e. foot, thoracic spine, shoot through hip)