Filters are types of metal sheets that are usually used in an x-ray beam and are placed between the window and the patients. These filters are used to assuage the soft x-ray photons from the spectrum. Filtering is done to remove the soft (low-energy) x-rays from the spectrum, which don’t contribute to the image quality and add to the patient’s dose and disperse. If filtering is not done, the low-energy x-ray photons absorb the shallow structures of the patient’s body and have a say to Entrance Surface Dose (ESD). As absorbed by the shallow structures, they contribute to image formation minimally.
X-rays are produced internally of an x-ray tube when there are high energy electrons from filaments that tend to interact with the anode. The necessary conditions for the production of x-rays are the anode, source of electrons, high potential difference, and deceleration of projectile electrons.
There are basically two different types of filtration:
- Inherent filtration from various components of x-ray tube-like window, cooling oil, and housing.
- Added filtration from metal sheets those are interchangeable like Al, Cu, and more.
You can get total filtration by a combination of inherent and added filtration. The added filtration’s components are personalized, like the type of metal and filter thickness for the procedures and examinations such as fluoroscopy. It tends to benefit from characteristics of specific metals filtration to improve the contrast and image quality. You can get to know more about filters from https://www.evaporatedcoatings.com/beamsplitters/
How is the filtration of the beam accomplished?
- Inherent filtration
In inherent filtration, the primary beam of radiation is passed through a glass wall of the aperture window, tube, and a layer of oil. As it passes through all these, some amount of filtration takes place, which is known as inherent filtration.
- Additional filtration
In additional filtration, the additional materials are put in the path of the primary beam to get done the further filtration.
- Total filtration
Total filtration is the actual addiction to inherent filtration and additional filtration.
The requirement of filter material:
The material of filtration must be able to distinguish against the photons of lower energy. The absorption edge must not be close to photons’ energies as they need to be used in materials. The material needs to be thick as it has been studied that if the thickness of the material is small, the beam produced is non-uniform.
Materials used for filtration
The materials that need to be used highly depends on whether the beam is of low or high energy, thus depending on the generating voltage used in filtration. The most commonly used and accepted x-range range for filtration materials is 100-250kV Copper, which has appropriate backing, 30-120 kV Aluminium, most preferred, 200-600 kV Tin, and last one 600-2 mV Lead.
Effects of Filtration
The major effect of filtration is that it eliminates unwanted radiations and tends to harden the beam by increasing its penetrating power. A profile wedge filter acts as an absorbing device that involves a vertical bar which is wedge-shaped and reduces the redundant less penetrating radiation which occurs to the patient and has no or less diagnostic value. The position of the thin edge of the filter is above the bone region of the patient as it absorbs fewer x-rays, whereas the thick edge is placed in the soft tissue region. The wedge is made up of aluminium and can be placed in the cassette in the x-ray beam at the time of x-ray and need to be placed between the x-ray source and patient.
Factors that affect the quality and quantity of X-ray beam
The total number of x-ray photons in the beam refers to the quantity of the X-ray beam. As there is an increment in the number of photos, the beam intensity also tends to increase, and other factors that affect the photons in the x-ray beam influence the quantity of beam. Quantity of beam is affected by changes in filtration, changes in tube current (mA), changes in the kind of waveform used, change in the distance from the tube (FFD), and changes in the material of the target (Z number).
Quality of X-ray beam
The overall energy of the beam refers to the quality of the X-ray beam. The X-ray beam is polyenergetic; thus, any factor that tends to increase or decrease the energy of photons affects the beam quality of x-ray. There are some factors that directly affect the quality of beam-like changes in the material of target material (atomic number Z), changes in filtration, changes in kVp, and the type of waveform used highly affect the quality of the beam.
These are some of the things that make you clear your doubts about the beam filters. You can check out the factors and the materials and enhance your knowledge about it.