Control of Scatter Radiation
Beam Restricting Devices
By Prof. Stelmark
PRODUCTION OF SCATTER RADIATION
Two types of x-rays are responsible for the optical density and contrast on a
radiograph: those that pass through the patient without interacting, and those
that are scattered within the patient through Compton interaction. X-rays that
exit from the patient are remnant x-rays and those that exit and interact with
the image receptor are called image-forming x-rays
Proper collimation of the x-ray beam has the primary effect of reducing patient
dose by restricting the volume of irradiated tissue. Proper collimation also
improves image contrast. Ideally, only those x-rays that do not interact with the
patient should reach the image receptor.
As scatter radiation increases, the radiograph loses contrast and appears gray
and dull. Three primary factors influence the relative intensity of scatter
radiation that reaches the image receptor:
1. kVp
2. Field size
3. Patient thickness
kVp
As x-ray energy is increased, the absolute number of Compton interactions
decreases, but the number of photoelectric interactions decreases much more
rapidly. Therefore, the relative number of x-rays that undergo Compton
interaction increases.
Approximately 1% of x-rays incident on the patient reach
the image receptor.
Field Size
Another factor that affects the level of scatter radiation and is controlled by the
radiologic technologist is x-ray beam field size. As field size is increased, scatter
radiation also increases
Extremity radiographs appear sharp because of less tissue and, hence,
less scatter radiation.
Beam Restrictors
Basically, three types of beam-restricting devices are used:
1. The aperture diaphragm
2. Cones or cylinders
3. The variable-aperture collimator
Aperture diaphragm is a fixed lead opening designed for a fixed image
receptor size and constant source-to-image receptor distance
The most familiar clinical example of aperture diaphragms may be radiographic
imaging systems for trauma. The typical trauma system has a fixed source-toimage receptor distance (SID) and is equipped with diaphragms designed to
accommodate film sizes of 13×18 cm, 20×25 cm, and 25×30 cm. Radiographic
imaging systems for trauma can be positioned to image all parts of the body
Cones and cylinders are shaped differently, but they have many of the same
attributes. A cone or cylinder is essentially an aperture diaphragm that has an
extended flange attached to it. The flange can vary in length and can be shaped
as either a cone or a cylinder. The flange can also be made to telescope,
thereby increasing its total length . Like aperture diaphragms, cones and
cylinders are easy to use. They slide onto the tube directly below the window
Cones and cylinders limit unsharpness surrounding the radiographic image
more than aperture diaphragms do, with cylinders accomplishing this task
slightly better than cones. However, they are limited in terms of the sizes that
are available, and they are not necessarily interchangeable among tube
housings
Images with and without a Cylinder. Radiograph of the frontal and maxillary
sinuses. A, Not using a cylinder. B, Using a cylinder.
Variable Aperture Collimator
The light-localizing variable-aperture collimator is the most commonly used beamrestricting device in radiography.
Increased Factor
Result
Collimation
Patient dose decreases.
Scatter radiation decreases.
Radiographic contrast increases.
Film-screen: Radiographic density
decreases.
Digital: Quantum noise increases.
Field Size
Patient dose increases.
Scatter radiation increases.
Radiographic contrast decreases.
Film-screen: Radiographic density
increases.
Digital: Quantum noise decreases.
Patient Thickness
Imaging thick parts of the body results in more scatter radiation than imaging thin
parts does. Compare a radiograph of the bony structures in an extremity with a
radiograph of the bony structures of the chest or pelvis. Even when the two are
taken with the same screen-film combination, the extremity radiograph will be
much sharper because of the reduced amount of scatter radiation
Normally, patient thickness is not controlled by the radiologic technologist. If you
recognize that more x-rays are scattered with increasing patient thickness, you
can produce a high-quality radiograph by choosing the proper technique factors,
and by using devices that reduce scatter radiation to the image receptor, such as
a compression paddle