Anode heel effect

Anode heel effect refers to the lower field intensity oftowards theanode in comparison to the cathode due to lower x-ray beam, producedemissions from the X-ray tube, which is not uniform in all portions oftarget material at angles perpendicular to the electron beam.

Basic concept

In general,The conversion of the electron beam consistsinto x-rays doesn’t simply occur at the surface of a central raythe target material but deep within it. Because x-rays are produced deep in the target material they must traverse back out of it before they can proceed to the target field. More target material needs to be traversed at emission angles that are perpendicular to the electron beam (closer to the anode) than at those more parallel to it (closer to the cathode). This increase in material leads to more resorption of the x-rays by the target material resulting in fewer x-rays reaching the field at angles perpendicular to the electron beam. It also means that the x-rays emitted to angles closer to the incident beam travel through less target material and a diverging beamfewer are resorbed. The rays

The end result is that the field intensity towards the cathode end ofis more than that towards the tube have more intensity. This is because, in a diverging beamanode. 

Factors

• anode angle: by increasing the angle, the rays which are parallel or near parallelamount of target material perpendicular to the inclined/angulated anode get absorbed by the anode itself.

  Applications

  1. intensityis decreased resulting in less resorption of beam on the anode side is less than cathode side: therefore place the thicker part of the body on the cathode side, e.g. upper thoracic on the anode side and lower thoracic or upper abdomen should be placed on the cathode sidex-rays produced.
  2. target-to-film distance: increase in distance reduces heel effect.
  3. size of film (keeping target-to-film distance as constant ): the smaller film has lesser heel effect as the divergent beam does not reach the film and intensity by allowing more divergence of the beam iswhich produces a more uniform image.
  4. field size: the field will be more uniform at the centre than atdue to the peripherycollimator absorbing the peripheral variations.
• positioning: by aligning higher attenuating material towards the cathode and lower attenuating material towards the anode the resulting field is more uniform