Compensating filters

Compensating filters are external, uniquely shaped; typically aluminum mounts often affixed to the collimator of the x-ray tube. The filters are used to limit the primary beam at an anatomical region during a specific examination that requires fine radiographic detail at both the 'denser' and 'less dense' regions, e.g. a horizontal beam lateral hip requiring detail at both the proximal and distal portion of the image.

The application determines the shape of the filter. The three most common compensating filters are: 

• boomerang filter
  • shaped like the indigenous Australian boomerang
  • traditionally used during shoulder examination, it is placed behind the patient, with the thickest portion of the superior-lateral aspect of the shoulder
  • boomerang filtration balances out the density of the soft tissue often during the shoulder examination
• trough filter 
  • channel shaped filter used during chest radiographs 
  • affixed to the collimator of the x-ray tube
  • the thicker outer regions of the trough corresponding to the less dense aspects of the peripheral lungs 
• wedge filter 
  • a wedge-shaped filter affixed to the collimator of the x-ray tube 
  • the thicker portion of the wedge is placed in the primary beam which passes through the less dense parts of the anatomy being imaged
  • common uses
    • horizontal beam hips, the thick portion of the wedge over the distal aspect of the femur
    • AP foot, the thick portion of the wedge over the distal aspect of the foot