Controlling Scatter Radiation in Radiography: Techniques and Devices

What is scatter radiation?

X-rays that have interacted with an electron of an atom, altering their direction and energy.

Why is scatter radiation undesirable in radiography?

It decreases contrast on the radiograph and affects overall image quality.

What are the three possible interactions of an x-ray photon with a patient?

1) Absorption, 2) Passing through unaffected, 3) Scattering.

What are the two major controllers of scatter radiation?

1) Beam Restriction, 2) Grids.

What is the purpose of a grid in radiography?

To absorb scatter radiation.

Which two factors affect the amount of scatter produced?

1) kVp (kilovolt peak), 2) Irradiated material.

How does an increase in kVp affect scatter radiation?

Higher kVp increases beam energy, leading to greater scatter and less absorption by the patient.

What is off-focus radiation?

Photons not created at the focal spot but elsewhere in the x-ray tube, producing a ghost-like shadow on the radiograph.

What is the function of a collimator in radiography?

To regulate field size and reduce penumbra and off-focus radiation.

What is Positive Beam Limitation (PBL)?

A device that automatically collimates the x-ray beam to the size of the cassette.

What are lead blockers used for in radiography?

To absorb scatter from large body parts and aid in positioning.

How does the volume of irradiated material affect scatter?

Greater volume leads to more scatter due to increased tissue interaction.

What is the relationship between atomic number and scatter?

Higher atomic numbers lead to more absorption of x-rays, resulting in less scatter.

What is penumbra in radiography?

Geometric unsharpness around the periphery of the image caused by x-rays not having a point source.

What is the simplest beam restricting device?

Aperture diaphragm, which is a flat sheet of metal with a hole cut in the center.

What are the disadvantages of using an aperture diaphragm?

Different sizes are needed for different film sizes, and it can increase penumbra.

What happens when the field size is increased in radiography?

Increased volume of tissue is irradiated, leading to increased scatter and decreased contrast.

What occurs when the field size is decreased?

Decreased scatter and beam quantity, resulting in a shorter scale of contrast.

What is the effect of decreasing field size on image receptor exposure?

It decreases exposure as fewer photons reach the image receptor.

What must be adjusted when significantly reducing field size to maintain image quality?

An increase in mAs (milliampere-seconds) is required.

The amount of scatter created during an
interaction is affected by

The volume, and Atomic number (z) of the material
being irradiated.

The greater the volume =

more scatter.

Volume increases as

the field size increases or patient size increases

The higher the atomic number of the
irradiated material=

the more electrons around the nucleus = greater the chance of x-ray interaction due tophotoelectric absorption = less scatter$$

Bone absorbs more radiation than soft
tissue; Due to

the higher atomic number
more photons are absorbed and creates
less scatter on the radiograph

Types of beam restricting devices:

Aperture Diaphragm, Cones and Cylinders, Collimator, Lead Blocker/mask (Ancillary Device)

flat sheet of metal, usually lead, with a
hole cut in the center and attached to the x-
ray tube that Increased Penumbra

Aperture Diaphragm

photons not created at the focal spot but elsewhere in the tube. Produce a ghost like shadow on the
radiograph.

OFF-FOCUS RADIATION

Determining the diaphragm field size aids in determining

the projected image on the image receptor.

A diaphragm is placed 5” from the focal
spot. The diameter of the opening in
the aperture is a 2” circle. What would
be the projected image size if the SID
is 40”? N

(40 x 2)/5 = 80/5 = 16” diameter circle

Projected image size =

SID x diameter of diaphragm/
distance from focal spot to diaphragm

cylinder and cones do what to

cone - does not reduce pnuembra but cleans scatter

cylinder- Will reduce penumbra, Will reduce off-focus radiation

Formula for determining the cone field size
(projected image)=

SID x lower diameter of cone/
Distance from focal spot to bottom of cone

The diameter of the lower rim of a
cone is 3”. The bottom of the cone is
placed at a distance of 16” from the
focal spot. What will be the size of
the projected image at a 40” SID?

(40 x 3)/16 = 120/16 = 7.5” circle

Bottom set of shutters reduces-\

Upper shutters reduce the amount of-

U- off-focus radiation reaching the IR by
absorbing.

b-penumbra.

Decreasing Field Size =

increase beam restriction

increasing field size =

decrease beam restirction