RAD 120 - CH 15 Beam Restriction - Review Questions

Why does beam restriction reduce scatter radiation production?

Proper beam restriction will keep the total amount of tissue irradiated to a minimum. As the beam is restricted, fewer primary photons are emitted from the tube and collimator, and therefore fewer scattered photons will be created.

What are the two principal factors that affect the amount of scatter produced?

The principle factors that affect the amount of scatter produced are kilovoltage and the irradiated material.

How does the atomic number of a material affect the amount of scatter produced?

High Atomic Number materials have a greater number of electrons within each atom. Photons have a greater chance of striking an electron and interacting with these materials, thereby creating more scatter. More scatter is created in bone than in soft tissue for this reason.

How does a collimator restrict the primary beam field size?

A collimator has sets of lead shutters at right angles to one another that move in opposing pairs. Each set moves symmetrically from the center of the field and functions to restrict the beam. These lead shutters can be adjusted to correspond to an infinite number of square or rectangular field sizes.

What is PBL?

Positive Beam Limitation (PBL) devices are collimators with automatic controls. When an IR is placed in the bucky tray and secured in position, sensing devices determine the size and placement of the IR. The sensing devices activate an electric motor that drives the collimator lead shutters into proper position.

How does beam restriction affect patient dose?

As the beam is restricted, fewer primary photons are emitted from the tube and collimator, and therefore fewer scattered photons will be created. The decrease in the number of primary and scattered photons will result in a decrease in the dose to the patient.