Beam Restricting Devices

Scatter Radiation

change in direction of the x-ray photon after interaction with the atoms of the patient 

Off-focus, Extra-focal

–Refers to photons not produced at the focal spot of the anode

•tube housing

•evaporate tungsten on envelope

Field Light Congruency

Light field (light bulb and mirror) represents the X-ray field

Cross-hairs represent the CR

How does scatter affect an image?

Increase density, decrease contrast and resolution

This increase in density from scatter is called…

fog. Radiograph will have an overall gray appearance

Primary radiation

–radiation emitted from the x-ray tube

Remnant (exit) radiation

unabsorbed and scattered x-rays that interact with the IR, it carries the aerial image

Factors that influence the degree of scatter radiation:

Field size

Thickness of the object

Density (composition) of the object

Tube potential (kV)

Field size-

area of the x-ray beam. Field size is altered by the use of a beam limiting device. Increase field size and scatter

Thickness of the object

more atoms are present in larger object to cause more interactions.

Bigger object, more scatter

Tube potential (kV)

as kV is increased, the energy of scattered radiation also increases, so scatter radiation has a better chance of reaching the IR.

More kV, more scatter

Density (composition) of the object 

the more dense the object is, the more scatter is produced. 

More density, more scatter

All increased given factors ALSO increase scatter reaching IR

Field Size 

Thickness  

Density (composition) 

Tube Potential  (kV)

Beam Limitation

• Reduce patient exposure

• Increase image quality by decreasing scatter to the IR

• Increases visibility of detail

Density on the radiograph is the result of how much scatter radiation?

50-90%

Penumbra greater at the…

edges of the beam

Types of Beam Limiting Devices

1.Aperture Diaphragm

2.Cones

3.Variable Aperture

constructed of lead or lead-lined because of its characteristic to attenuate

Aperture Diaphragms

• Simplest, least expensive

• Change for each size

• Large amount of blur

• Dedicated units

2 types of cones

1.Flared

2.Cylinder

flared cone

Flared Cone

• Matches the divergence of beam

• Flare is usually larger than beam

Cylinder Cone

Blur is reduced because it truly limits the field size

Circular field

cylinder cone

Disadvantages of Cones

Excessive radiation for larger body parts, unneeded patient exposure

VARIABLE APERTURES 

•Collimator

•Automatic Collimator

•Positive Beam Limiting Device (PBL) – assures the size of field does not exceed the size of the I

Automatic Collimator Housing

•Two sets of horizontal lead shutters

•Adjustable. Allows for infinite number of field sizes

•Adds to “added” filtration

•Provides a light field

2 types of shutter

–Upper shutters control stem radiation (off-focus)

–Lower shutters reduce blur

Stem Radiation refers to…

Off-focus, Extra-focal

Off-focus, Extra-focal means…

–Refers to photons not produced at the focal spot of the anode

•tube housing

evaporate tungsten on envelope

Off-focus Radiation

Field Light Congruency

Light field (light bulb and mirror) represents the X-ray field

Cross-hairs represent the CR

Poor field-to-light congruency may lead to

parts of the anatomy being cut off