Filtration-MIDTERM

Filtration Overview

  • Definition: Filtration is the process of removing undesirable low-energy x-ray photons from the primary beam by using absorbing materials.

  • Purpose: Enhances image quality by selectively eliminating low-energy x-rays that are less likely to reach the image receptor.

  • Effect on Beam Quality:

    • Increases average energy and overall quality of the x-ray beam by removing low-energy photons.

    • Reduces patient dose by filtering out low-energy radiation.

Types of Filtration

  • Inherent Filtration:

    • Comes from the x-ray tube and housing materials.

    • Most is due to the glass envelope window, typically around 0.5 mm Al equivalent.

    • Increases with tube age due to deposition of vaporized tungsten from the target and filament.

  • Added Filtration:

    • Filtration applied externally before the primary beam reaches the image receptor.

    • Typically involves placing a sheet of aluminum between the tube housing and collimator.

Materials Used for Filtration

  • Common Filter Materials:

    • Aluminum (Z=13): Most common due to efficiency, cost, and ease of shaping.

    • Other materials include Copper (Z=29), Tin (Z=50), Gadolinium (Z=64), and Holmium (Z=67).

Half Value Layer (HVL)

  • Definition: The amount of attenuating material needed to reduce the beam intensity to half its original value.

  • Significance:

    • Used to measure x-ray beam quality; a higher HVL indicates better quality.

    • A typical diagnostic x-ray beam HVL ranges from 3 to 5 mm Al or 3 to 6 cm soft tissue.

    • For beams operated at 70 kVp and above, the minimum HVL is 2.5 mm - values below indicate insufficient filtration.

Total Filtration

  • Calculation:

    • Total filtration = Inherent Filtration + Added Filtration

      • Example: Inherent (0.5 mm Al) + Added filter (1.0 mm Al) + Mirror filter (1.0 mm Al) = 2.5 mm Al

Effects of Filtration

  • Output: Reduces patient exposure dose by removing low-energy x-rays from the beam, although some of the useful beam is also lost.

  • Density: As filtration increases, adjustments to technical factors (like exposure time) are necessary to maintain output density (OD).

Compensating Filters

  • Purpose: Designed to manage problems with unequal subject densities, ensuring more uniform exposure across the imaged area.

  • Materials: Typically made from aluminum or leaded plastics.

Types of Compensating Filters

  • Wedge Filters: Common for uneven body parts like the foot and used in imaging procedures such as AP-femur.

  • Bilateral Wedge Filter (Trough Filter): Similar to wedge filters but feature double wedges for compensating structures such as paired organs.

  • “Bow-Tie” Filters: Compensate for the varying shapes of the body in CT imaging.

  • Conic Filters: Used in digital fluoroscopy with circular image intensifiers.

  • Step-Wedge Filters: An adaptation of wedge filters, used in radiographic examinations requiring several image receptors across long anatomical sections.