RADT 1010- Filters

Filtration in Radiologic Technology (RADT 1010)

Introduction to Filtration

  • Definition: Filtration is the process of increasing the mean energy of a heterogeneous beam of radiation by passing it through an absorber.

  • Purpose: It is used to selectively remove low-energy x-rays from the beam.

Effects of Filtration

  • Impact on Beam Characteristics:

    • Changes both the quantity and quality of the radiation beam.

    • Reduces the patient dose by filtering out less effective radiation.

    • “Hardens” the beam, which refers to the increase in the mean energy of the x-ray photons in the beam.

Common Beam Filtering Materials

  • Materials Used in Filtration:

    • Brass

    • Copper

    • Aluminum

    • Lead

    • Tin

    • Molybdenum

    • Tungsten

    • Titanium

    • Zirconium

    • Additional other materials may be included.

Types of Filtration

  • Total Filtration:

    • Calculated as the sum of inherent filtration and added filtration.

Inherent Filtration

  • Definition: Refers to the filtration that results from the absorption of x-rays as they pass through the tube and housing; this is a constant that the technologist cannot modify.

  • Components Contributing to Inherent Filtration:

    • Envelope of the tube

    • Oil surrounding the tube

    • Port or exit window of the tube

  • Aging Impact on Filtration:

    • Inherent filtration increases as the tube ages because vaporized tungsten from the anode and cathode deposits on the inside of the envelope, which further attenuates the beam.

  • Measurement of Inherent Filtration:

    • Generally falls between 0.5 – 1 mm Al equivalent.

Aluminum Equivalent

  • Definition: This describes the thickness of aluminum that has the same radiation attenuation characteristics under specified conditions as that of the material being considered.

Added Filtration

  • Definition: Refers to the additional layers of metal placed in the beam, located outside the true tube housing, primarily made of aluminum, and sometimes copper.

Types of Filtration Devices

Compound Filters

  • Composition:

    • Contains three layers: tin, copper, aluminum.

  • Functionality:

    • Material with a higher atomic number (Z) is positioned closer to the tube.

    • Each layer absorbs the characteristic photons created in the previous layer.

  • Atomic Numbers:

    • Tin (Z = 50)

    • Copper (Z = 29)

    • Aluminum (Z = 13)

Compensating Filters

  • Purpose: These filters are utilized to compensate for parts of non-uniform thickness or density in radiographic imaging.

  • Functionality:

    • They rectify variations in anatomy and promote a more consistent image brightness across the radiographic image.

  • Materials Used:

    • Opaque plastics

    • Aluminum

    • Various other metals

  • Shapes Available:

    • Wedge

    • Step wedge

    • Bilateral wedge

    • Trough

    • Boomerang shape

Conclusion

  • Future Topics: Information on other devices related to filtration techniques will be discussed in upcoming sessions (TBL next week).

Data Representation

  • Photon Numbers and Energy Spectrum: Illustrates characteristic L-shell x-rays, Bremsstrahlung x-rays, and K-shell x-rays across energy levels. The x-ray spectrum reveals significant differences before and after inherent and additional filtration.

    • Energy Levels (keV): Illustrates the distribution and maximum energy like 20, 40, 60, 80, and 100 keV depending on filtration steps.