Filtration in Radiography

Key Terms

  • Added Filtration: Measures to increase x-ray beam quality by removing low-energy photons.
  • Compensating Filter: Filters designed specifically to balance exposure for varying tissues/densities during imaging.
  • Compound Filter: A combination of materials providing enhanced absorption of x-ray energies.
  • Filter: Material that selectively absorbs photons from the x-ray beam.
  • Half-Value Layer (HVL): The thickness of material that reduces x-ray beam intensity to half its original value.
  • Inherent Filtration: Filtration due to the structure and material of the x-ray tube and its components.
  • K-edge Filter: A specialized filter that uses materials with distinct atomic numbers to enhance photon absorption.
  • Thoraeus Filter: A specialized compound filter composed of tin, copper, and aluminum.
  • Total Filtration: The sum of inherent and added filtration in an x-ray system.
  • Trough Filter: A compensating filter that balances the density difference between tissues such as mediastinum and lungs.
  • Wedge Filter: A compensating filter used to adjust exposure for different tissue thicknesses in imaging.

Objectives

Upon completion of this chapter, students will be able to:

  • Define all major types of filtration related to radiography.
  • Explain half-value layer (HVL) concepts and equivalencies in measurements of filtration.
  • Evaluate the best filtration methods for different clinical scenarios.
  • Describe how filtration affects the overall x-ray beam.

Types of Filtration

Filtration in radiography occurs at various points between the x-ray tube and the image receptor. It is classified into two main types: inherent filtration (part of the x-ray tube components) and added filtration (introduced after the x-ray tube before reaching the patient).

Inherent Filtration

Inherent filtration is due to the materials used in constructing the x-ray tube and its housing. This filtration occurs naturally and is an integral part of the system's design. Common contributors to inherent filtration include:

  • Thickness of the glass envelope of the x-ray tube.
  • Dielectric oil surrounding the tube.
  • Glass window of the housing.

Typical inherent filtration for x-ray tubes ranges from 0.5 to 1 mm Al/Eq. For mammography units, special materials like beryllium may be used to reduce inherent filtration further (to as low as 0.1 mm Al/Eq).

Added Filtration

Added filtration refers to any filtration materials placed outside of the x-ray tube before the image receptor. In diagnostic radiology, aluminum is the most frequently used filter. The standard measurement for all added filtration is given in terms of aluminum equivalency (e.g., a collimator's mirror may contribute 0.5 mm Al/Eq). This added filtration aims to maximize the transmission of useful high-energy photons while attenuating low-energy, non-contributory rays.

Measurement and Half-Value Layer (HVL)

The half-value layer (HVL) measures the penetration ability of the x-ray beam and is defined as the thickness of material that reduces the x-ray beam intensity to half its original value. The HVL is an important parameter for determining total filtration effectiveness and compliance with regulatory standards. The federal government specifies minimum HVL requirements for diagnostic x-ray tubes, ensuring adequate patient protection from unnecessary radiation exposure.

Example Table of HVL Requirements:

  • HVL below 51 kVp should be 0.3-0.5 mm Al,
  • Between 51 to 70 kVp, it should be 1.2-1.5 mm Al,
  • For voltages above 70 kVp, 2.1-2.7 mm Al is required.