Beam Modifiers

Beam modifiers are used to control the size, shape, and quality of the x-rays as it exits the x-ray tube, through the tube housing, and into the patient.  Beam modiers include:

  1. Collimators - controls the size and shape of the x-ray beam.
  2. Filtration - controls the quality of the beam.

Collimators

In the x-ray tube housing, there are shudders made of dense material, usually lead, that are used to define the x-ray beam. On the front of the x-ray tube housing, you will usually see 2 knobs.  Sometimes they are buttons.  These are used to control the size of the x-ray beam that will come out of the x-ray housing and expose the patient.  There is a light that defines the collimated area that the x-rays will expose.

Collimator 

In the above image, the collimator light has defined the x-ray field on this image receptor.  The importance of collimation is 2-fold.  Most importantly, it reduces patient dose.  The smaller you make the beam, the less the area is that is exposed.  Secondly, it increase image quality by reducing scattered radiation, which we will learn about later, and in the case of digital imaging, it affects the appearance of the radiograph because of the way the software works, which we also go over later.


Filtration

 filtration

When x-rays are produced, not all of the x-rays are useful.  When you select 100 kVp on the machine, that is the maximum energy of the photons.  kVp stands for kiloVolts PEAK.  X-rays are actually produced from 1 kV all the way to 100 kV.  The higher the kV, the more penetrating ability the x-ray beam has.  The lower energy photons do not contribute to the image.  They are not able to penetrate through the patient's body and get to the image receptor.  The patient ends up absorbing them, which does nothing more than add to the patient's dose.  In order to eliminate the low energy photons, aluminum (sometimes copper) is added between where the x-rays exit the x-ray tube and where the x-rays exit the collimator housing.  The aluminum will absorb the low energy photons while allowing the higher energies to reach the patient and image receptor. 

Bottom line - FILTRATION REDUCES PATIENT DOSE!