Filtration
Filtration
Filtration is the process of eliminating undersirable low-energy x-ray photons by the insertion of absorbing materials into the primary beam.
The primary reason for filtration is the elimination of photons that would cause increased radiation dose to the patient (damage to the skin) but would not enhance the radiographic image.
Measurement
Aluminum is the most common filter material used, although other materials, such as glass, oil, copper, and tin, are used as or become filters in various instances.
Aluminum is considered the standard filtering material and all filtration can be expressed in terms of the thickness of aluminum equivalency (Al/Eq).
The half-value layer (HVL) is that amount of absorbing material that will reduce the intensity of the primary beam to one-half its original value.
Half-blue layers are usually expressed in terms of aluminum filtration equivalency.
Inherent Filtration
Filtration that is a result of the composition of the tube and housing is often called inherent filtration because it is a part of these structures. (The x-ray tube)
Most of the inherent filtration comes from the window of the glass envelope
As tubes age they become gassy, the anode begins to pit, and the glass envelope may gain a mild coating of vaporized metal.
All of these factors will cause an increased radiation dose in the inherent filtration, thus reducing the tube efficiency.
Added filtration (colimater)
The collimation device also adds filtration to the beam and is considered to be added filtration
Compound filtration
A good example of a compound filter is the Thoraeus filter used in radiation therapy.
This filter combines tin, copper, and aluminum, in that.order.
Compensation
The thicker portions of the filter are matched to the less-dense patient body parts.
Total filtration
The total filtration is equal to the sum of inherent and added filtration and does not include any compound or compensation filters that may be added later.
Above 70 kVp is 2.5-mm aluminum
Although the exposure needs to be increased to maintain exposure, there is a greater decrease in overall exposure to the patient.
Filtration benefits the patient; radiation patient does is kept low.
Chapter 15
Controlling scatter
Scatter radiation is produced during a Compton interaction
Scattered photons are not a part of the useful beam, and will impair image quality by placing exposures on the image receptor that are unrelated to patient anatomy
This can be best accomplished by restricting the x-ray beam and by using a grid
Grids are devices that are placed between the patient and the image receptor to absorb scatter radiation.
The principle factors that affect the amount of scatter produced are
Kilovoltage (kVp)
The irradiated material
Because Compton interactions create scatter, as kilovolt age increases, the percentage of primary photons that will undergo scattering also increases.
Scattered portions from Compton interactions are of no use in demonstrating th structures of interest.
Merely add unwanted exposure.
As the volume of irradiated tissue increases, the amount of scatter increases.
Volume increases as the field size increases or as the patient thickness increases. Or both.
Therefore, the higher the atomic number of a material is, the greater will be the number of photoelectric absorption interactions, and the less scatter.
The foremost method of restricting the primary beam field size is the use of a device known as a collimator.
Aperture diaphragms and cones/cylinders are also devices that historically were used to restrict the beam, but are not in common use today.
These lead shutter can be adjusted to correspond ti an infinite number of square or rectangle field sizes.
The bottom shutters reduce penumbra along the periphery of the beam because of their greater distance from the focal spot.
The upper shutter of the collimator help in reducing the amount of off-focus (stem) radiation reaching the image receptor by absorbing this radiation before it exits the tube.