Beam Restriction Flashcards
Compton scatter is dependent on:
the thickness and density of the part
(more matter, more scatter)
(more available electrons)
Increasing kVp increases the amount of…
scatter relative to the photoelctric effect
What does the use of grids do?
(2)
-absorb scatter radiation within the patient
-improves image contrast at the expense of patient dose
Technique must be increased to compensate for the use of a grid because grids remove part of the primary beam
Note
What’s something we can do to reduce patient dose and reduce scatter radiation?
restrict the beam or collimate
What is collimation?
(2)
-decreases the amount of radiation that interacts with the patient
-reduces the patient dose
By properly collimating we..
(2)
-reduce the amount of scatter produced
- reduced the amount of matter that the photons can interact with
Collimation should be limited to
-1” beyond the anatomy of interest on all 4 sides on tabletop exams
Collimation should not exceed…
the recommended field size for a particular part
When we collmate properly we are…
(4)
-reducing scatter radiation
-reducing unnecessary exposure by the primary beam
-increasing contrast
-reducing patient dose
According to Bushong, the atomic number of material has…
no effect on likehood of scattering
Atomic number does not directly…
affect scatter
What is the device collimator?
the foremost method of restricting the primary beam field size
What is the most commonly employed beam restrictor in radiography?
Collimator
Most collimators now equipped with an accessory known as a…
Positive beam limitation (PBL) device
What does the PBL do?
it automatically collimates the beam to the size of the image receptor
What is the collimator function?
(3)
-Permits an infinite number of field sizes
-Provides a light source for the radiographers to properly placing the tube and central ray
-Reduces penumbra
What do entrance shutters do?
help remove off-focus radiation
Off focus radiation?
Xrays created in other parts of the tube not the anode
How many sets do the collimator shutters have?
2 sets of shutters
(sometimes 3)
The second set collimator shutter?
is the one the technologist adjusts
Length and width of field are independently…
controlled by technologist
The light field?
uses a light reflected off mirror to project the area of the x-ray beam
Proper adjustment of mirror is necessary because…
to accurately display location of exposure field
The light field/xray beam coincidence testing should be part of the…
QC program
-Needs to be accurate within +/- 2% of the SID
The use of a collimator results in some filtration of the xray beam because…
the primary beam is passing through the mirror in the collimator
-About 1mm Al/Eq
Features of collimator housing?
-Central ray must be marked
-Some units project location of photo timer sensors in light field
-An alignment light helps to center beam with the image receptor
What is penumbra?
a geometric unsharpness around the periphery of the image also known as edge unsharpness
X-rays photons are being created in all areas of the focal spot rather than at just a single point
note
Xrays originating from slightly different spots on the anode take slightly different angles to hit the same point on the detector
note
Primary photons diverge from the tube at varying angles and intersects with…
structures of interest at varying angles when creating image
What does the shutter reduce?
the penumbra along the periphery of the beam because of their greater distance from the focal spot
Closer to the light source it looks blurry and the father away from the light source…
you have sharper edges
What are other beam restriction devices?
Aperture diaphragms and cones/cylinders
The cones/cylinders are devices that are historically…
used to restrict the beam but are not in common use today
What is the aperture diaphragm?
(Simplest of all beam-restricting devices)
is a flat sheet of metal, usually lead with an opening cut in the center and attached to the x-ray tube head
Disadvantages of aperture diaphragm?
Greater amount of penumbra and off focus radiation due to the proximity to the tube port
Aperture diaphragm allows…
-field to cover area just smaller than image receptor
-can be supplemented with cones and cylinders
Cones and cylinders are essentially…
circular aperture diaphragms with metal extensions
A cone has an…
extension that flares or diverges with the upper diameter smaller than the bottom flared end
A cylinder does not..
flare
What are cones and cylinders used for?
-for greater detail visibility in small area
-reduction in scatter which make images appear more sharper
Some applications of cones and cylinders are?
-Head work: sinus or skull
-Spine work: spot films
-Cross: table hip
-Dental
Cylinders reduce the amount of irradiated material and therefore…
-reduce patient dose
-reduce scatter
-increase contrast
Ideally the beam restrictors decrease field size to anatomy of interest and also?
-unnecessary tissue exposure decreases
-Scatter decreases
-Contrast increases (Scale of contrast shortens)
-Visibility of detail increases
What is ancillary devices?
-Devices tailored for a specific use during given procedure
-designed to restrict the beam to a specific shape for particular examination
Examples of ancillary of devices
Lead blockers and lead masks
What is a lead blocker?
Will help absorb scatter that is produce in the soft tissue of the patient’s back
its placement of it on a radiography table during radiography of a lower spine in lateral position
Lead mask is usually cut to correspond…
to the field size desired and is then secured to the end of the collimator.
Lead mask were sometimes used during cerebral angiography to help improve…
image quality by reducing scatter
