What is a beam restricting device?
A device that restricts the size of the x-ray field to only the anatomic structure of interest
What is cone cutting?
Misalignment of cones that may interfere with the x-ray beam
What is collimation?
the restriction of the useful beam to reduce patient dose and improve image contrast
What are image-forming x-rays?
X-rays that travel through the patient and enter the IR
What is off-focus radiation?
X-rays produced in the anode but interact at positions on the anode other than the focal spot
What is a positive beam limitation device?
A feature of radiographic collimators that automatically adjusts the radiation field to the size of the IR
What is scatter radiation?
X-rays scattered back in the direction of the incident x-ray beam. They go in a direction other than straight. Produced by compton interactions.
List the two principle characteristics of any image
Spatial resolution (ability to image small objects that have high subject contrast), and contrast resolution (ability to distinguish between and to image similar tissues)
List the two principle tools used to control scatter
Beam restrictors and grids
Factors that contribute to scatter radiation
kVp, field size, and patient thickness
How does each factor contribute to scatter radiation?
kVp affects penetrability of beam--an increase causes fewer x-rays to interact with matter, but more compton interactions ; Field size is controlled by the tech and as it increases the scatter increases ; Patient thickness is a factor because the thicker the body part, the more scatter it yields
How do we reduce the effect of the factors contributing to scatter?
kVp should be used at the lowest reasonable setting ; Field size should be decreased, but to keep the same OD exposure factors should be increased ; Using compression is a way to reduce scatter for thicker patient parts
Three types of beam restrictors
Aperture diaphragm, cones and cylinders, variable aperture collimator
What is the physical makeup of each beam restrictor?
Aperture diaphragm=flad sheet of lead w/ hole in center. It attaches to the tube port ; Cones and cylinders=circular diaphragms with extensions. These extensions are added to the tube port. Cones flare or diverge and cylinders do not. Cylinder more frequently used ; Variable aperture collimators=light localizing and can be automatic (PBL)
When is each beam restrictor used?
Aperture diaphragm: trauma, chest rooms, dental radiography ; Cones and cylinders: sinus series, nasal, orbits, dental, and mammography; Variable aperture collimator: most commonly used
What are the advantages of each beam restrictor?
Aperture diaphragm: simple in design, low cost, easy to use; Cones and cylinders: inexpensive, simple to use, can be used with other beam restricting devices; Variable aperture collimator: reduce patient dose and increase contrast
What are the disadvantages of each beam restrictor?
Aperture diaphragm: increase in penumbra, increase in off-focus radiation, fixed field size; Cones and cylinders: increase in penumbra (cones), alignment sensitive (cone cutting), and fixed field size; Variable aperture collimator: costly, not all x-rays are emitted precisely from the focal spot of the x-ray tube
Why do we use beam restrictors?
Because they limit the area being irradiated, produce less scatter by reducing number of compton interactions taking place which in turn reduces patient dose and increases contrast
What determines spatial resolution?
Focal spot size
What determines contrast resolution?
What is the result of restricting field size?
What two things are improved by compression and the one thing that is lowered by it?
Spatial resolution and contrast are improved; Patient dose is lowered
What is increased when scatter fog is decreased?
Number of shades of gray in image (increased contrast)
What determines size of the aperture diaphragm?
Desired exposure field, SID, and distance from the focal spot
What are two ancillary devices?
Lead blockers and lead masks
What is a sheet of lead?
How are lead blockers used?
Placed on tabletop to absorb scatter
What is penumbra?
Image blur that results from the size of the focal spot; geometric unsharpness