Ch 11 Beam Restricting Devices Flashcards
Why do we need to employ beam restricting devices?
- To reduce patient dose
- Reduce Scatter
- Achieve optimal radiographs within technique range
What are the two principle factors that the scatter produces?
- Patient size
2. Field size (we have control over)
As field size increases, so does ______ radiation.
Scatter
Reducing the beam, ______ scatter which also results in lower OD
Reduces
What do beam restrictors do?
Limit the size of the x-ray field to only the anatomic structure of interest
The bigger the field size, the _____ scatter radiation.
More
When you collimate, what do you have to do to your technique?
Increase it
What are the (3) types of beam restricting devices?
- Variable aperture diaphragm
- Aperture Collimator
- Cones & cylinders
Scatter radiation _____ image contrast.
Reduces
What are aperture diaphragm beam restrictors?
Lead lined metal diaphragm attached to the x-ray tube head
What does the aperture diaphragm beam restrictor do?
Projects the image 1cm smaller on all sides, leaving a border around the edges.
What are cones and cylinders?
Extended metal structures that restricts the beam to the required field size
Do cones reduce SID?
No
Cones and cylinders ____ image contrast.
Improve
What is a variable aperture collimator?
- Restricts pt dose by restricting the volume of tissue irradiated
- Decreases scatter radiation
What is the most common beam restricting device?
Variable aperture collimator
When was PBL mandated, then removed?
1974 removed in 1994
What is PBL?
Positive beam limitation
What is penumbra?
Fuzzy border outside the image
How can penumbra be minimized?
By using the correct SID
What is off focus radiation?
X-rays that are produced when the projectile electrons stray and interact at positions on the anode other than the focal spot
Off focus radiation _____ image blur.
Increases
Off focus radiation is absorbed by the collimator shutters made of ___ lead.
3mm
What does the bottom shutters do?
Reduce penumbra
What does the upper shutters do?
Reduce off focus radiation
The light field you see is projected off a mirror on the path of the x-ray beam at a _____ degree angle .
40-45
The light source and the x-ray source should be _______ from each other to ensure their light fields are the same size.
Equidistant
What are lead blockers?
Anything that is impregnated with lead (ex: Lead aprons and lead strips)
What are lead masks?
Lead that is cut to a particular size or shape for a specific patient and exam
