Basics of X-ray and Mammographic Systems

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The Role of Filters in Diagnostic X-Ray Beam Production

Unfiltered Diagnostic X-Ray Beam

Combination of

Bremmstrahlung photons

Continuous spectrum of Energies

0 - kVp set on generator

Characteristic x-rays

Discrete energies

Correspond to differences of binding energies for various electron shells in target atoms

Most x-ray photons are very low energy

Filter Function

Sprawls Educational Foundation (www.sprawls.org)

Modifies x-ray energy spectrum
Attenuates low energy x-ray photons that
- Lack energy to penetrate patient
- Increase patient dose
Increases average beam energy

Inherent Filtration

Attenuation occuring as x-ray photons emerge from tube

glass insert
oil in housing
housing window
- Place in tube housing through which diagnostic beam emerges
- Usually made of glasss
  - Allows viewing of x-ray target & filament by service

Filter Locations

Anything through which x-rays pass will filter the beam

Filters sometimes installed between tube and collimator


Mirror in collimator (used to direct light beam toward patient) filters beam Some collimators contain a slot into which additional filters can be placed Some collimators contain selectors which add or remove filtration

Filter Materials

Radiography

Usually aluminum
Copper

Mammography
- Molybdenum
- Rhodium
- Aluminum

Filter Effects

Decreases beam intensity
Increases mean beam energy
- Slightly decreases contrast
Advantages
- Gross reduction in patient dose over unfiltered beam
Disadvantages
- Reduction in beam intensity
- X-ray technique must be increased to compensate
  - Increased tube loading
    - Longer exposure times
      - Patient motion