material addresses items on which BMETs or clinical engineers actually work. Because
of this, an attempt is made to use clearer language.
Collimation of the X-ray beam starts with the tube design, specifically the focal spot
track, which focuses the majority of the X-rays generated in one direction. The tube housing
further reduces stray radiation and continues the collimation process. The beam then
exits the tube housing through the “top hat,” a lead cone with a rectangular hole in it that
sets the maximum film size that can be exposed at a given distance, also called “sourceimage
distance” or “focal-film distance.” This also shapes the beam and helps to reduce
stray radiation. The collimator mounting block (which is made from aluminum) and the
rotation ring secure the collimator to the tube housing. These items should be checked
regularly to ensure that the hardware is secure. One old trick is to use a dab of nail polish
on the screw head to housing surface. Cracked nail polish will indicate that the hardware
is starting to loosen.
On some collimators, there is a slot in the housing, near the mounting block, that allows
for the placement of filters in the X-ray beam. These are thin sheets of copper or aluminum
(but other material could be used) that are inserted to remove spikes in the radiation
intensity that are generated by characteristic radiation, and to smooth the remaining
radiation so that it is consistent. This is also referred to as “hardening the beam” or removing
soft radiation. These filters should be added or removed only as directed by the physicist
doing the radiation certification on the unit. The certification might need to be redone
after major repairs to the generator, but otherwise it is done as required by local and federal
regulations. The maximum length of certification is three years. Commonly, technologists
readjust their techniques after filtration is added or removed. In addition, it is
common to experience an increase in “retake” films immediately after changes are made
in the system, based on the requirements of the physicist. CEs and BMETs need to work
closely with the department supervisors and technologists to ensure that the retake rate
returns to, or decreases below, the previous level.