The important difference between the contactless collimator and the contact collimator is that it greatly reduces the contribution of the scattered electrons in the collimator wall. Based on this, the plate collimator or the diaphragm collimator is developed.
The use of composite scattering filters or scattering compensation foil measures can also ensure uniform dose distribution without the use of collimator walls to scatter electrons, the required radiation field area is controlled by the diaphragm, the lower part of the collimator is kept at a certain distance from the skin, and the patient is squeezed. The danger of pressure is greatly reduced. In order to reduce the weight of the collimator, the upper and lower pairs of secondary collimators in the radiation head are usually used for preliminary collimation. Since the lower part of the collimator is a certain distance from the skin, the penumbra is slightly larger than the full release of the skin, but at the depth of the target area, the effect of side scatter is much greater than the effect of the lower part of the collimator.
In general, the collimation of the beam current cannot be handled simply as a ray concentrating problem due to multiple scattering of electrons in the air and inevitable scattering of collimator walls. The transverse beam of the electron beam from the collimator is generally significantly larger than the cross section of the electron beam, the purpose of which is to achieve dynamic equilibrium of electrons scattered and scattered at the edge of the beam.
In order to adjust the shape and size of the electron radiation, a collimator of different sizes is arranged.