Filtration in Radiography

Filtration Overview

  • Process Description
    • Eliminates undesirable low-energy x-ray photons by inserting absorbing materials into the primary beam.
    • Allows radiographers to shape the emission spectrum.

Effect of Filtration

  • "Hardening" the Beam

    • Removes low-energy "soft" photons, which increases the average beam energy.
    • Soft tissue penetration requires approximately 30-40 keV photons.

  • Low Energy Photons

    • Cannot penetrate the part, contributing only to patient dose, rather than diagnostic value.

Measurement of Filtration

  • Standard Filtering Material: Aluminum.
  • Filtration Measurement: Expressed as Al/Eq (aluminum equivalent).
  • Half-Value Layer (HVL): The thickness of material needed to reduce the x-ray beam to half its original intensity.
HVL Values for Different kVp Levels:
  • Below 50 kVp: 0.3 - 0.5 mm Al
  • 50 – 70 kVp: 1.2 - 1.5 mm Al
  • 70 – 150 kVp: 2.1 – 4.1 mm Al
  • Each generator type has its own HVL assigned.

Types of Filtration

  • Inherent Filtration:

    • Comprises materials within the x-ray tube design, such as the glass envelope and dielectric oil bath.
    • Aging of the tube increases inherent filtration due to vaporized tungsten coating the tube window.

  • Added Filtration:

    • Any filtration outside the tube and housing, typically around 2 mm Al/Eq (includes additional aluminum or copper).

  • Total Filtration:

    • Total filtration = inherent + added filtration.
    • Does not consider compound or compensating filtration.

  • Compound Filtration:

    • K-edge filters made from two or more materials, absorbing characteristic photons from previous layers (e.g., copper, aluminum).

  • Compensating Filtration:

    • Evens out radiographic density with uneven tissue thickness or densities using filters such as wedges or troughs.

Equipment Considerations

  • Effect on Tube Output:
    • Ideally, filtration removes low-energy photons without removing significant high-energy photons.
    • Removes some high-energy photons, leading to a decrease in radiographic density which requires compensation with increased exposure techniques.

Special Filters

  • Flattening Filters:

    • Used in linear accelerators to reduce central exposure relative to beam edges, designed to maintain penetration power and reduce dose distribution (PDD).

  • Scattering Foil:

    • Device used to expand a thin pencil beam of electrons.
    • Made of tin, lead, or aluminum, it can cause beam attenuation and bremsstrahlung radiation generation, although it is less prone to mechanical errors and less costly than scanning beam systems.
Conclusion
  • Understanding these filtration processes and materials is essential for optimizing x-ray image quality and minimizing patient dose.