7. Beam Restriction Flashcards
is produced during Compton interaction
Scatter radiation
Beam-Restricting Devices
Aperture Diaphragm,
Cones or Cylinders,
Variable aperture collimator
This can be accomplished by restricting the x-ray beam and by using a grid.
Reducing scatter
As the beam is _____, fewer primary photons are emitted from the tube and collimator and fewer
scattered photons are created
restricted
The principal factors that affect the amount of scatter produced are:
kilovoltage and irradiated material
As kVp _____, fewer photons undergo interaction with matter and more pass through the patient to interact with the image receptor
increases
As kilovoltage increases, the percentage of x-rays that undergo Compton interaction _____ and the percentage of photons that undergo photoelectric
absorption ______
increases,
decreases
If kVp is increased with no other change in the technical factors selected for a given exposure, the end result will be an ______ in the transmission of photons & therefore an ______ in the exit dose
increase,
increase
In radiography, the ______ is selected based predominantly on the size of the part being examined
and the radiographic contrast desired for the image
kilovoltage level
Increase kVp and reduce mAs to maintain the same exit dose, the overall result will be a ______ in the amount of exposure produced
decrease
The amount of scatter created during an interaction is
affected by the ______ and ______ of the
material being irradiated
volume,
atomic number
The volume of irradiated material is controlled by ____ and
field size,
patient thickness
As the volume of irradiated tissue ______, the amount of scatter ______
increases,
increases
Volume ______ as the field size increases or as the patient thickness increases
increases,
The atomic number of the irradiated material also has
an impact on the amount of scatter produced because _____ atomic number materials have a greater number
of electrons within each atom and photons have a
greater chance of striking an electron creating an
absorption interaction.
higher
High atomic number materials (iodine, barium, lead)
absorb ______ radiation through photoelectric interaction
than low atomic number materials do
more
Of the factors affecting the amount of scatter produced,
the _____ and the _____ are under the direct control of the radiographer
kilovoltage,
field size
When the beam is ______, less scatter radiation will
reach the image receptor and as a result, technical
factors may need to be increased to compensate for the
reduction in the overall image receptor exposure.
restricted
Is a flat sheet of metal, usually
lead, with a hole cut in the center and attached to am
xray tube port
Aperture Diaphragm
depends on the desired exposure field, the source to image receptor distance (SID), and
the distance from the aperture to the focal spot.
The size of the opening
Aperture Diaphragm
Principal disadvantage:
Increase in penumbra
geometric unsharpness around the periphery of the image
penumbra
is the result of x-ray photons being created
in all areas of the focal spot rather than at just a single
point.
Penumbra
While the use of different types of restrictors does not alter the sharpness of the internal image details, the ______ the beam restrictor is from the port, the ______
the edges of the exposed area will be
farther,
sharper
To avoid exposing an area larger than necessary,
selection of the correct diaphragm and proper SID is
critical
are essentially circular aperture
diaphragms with metal extensions
Cones/Cylinders
has extension which flares or diverges with the
upper diameter smaller than the bottom flared end
Cone
does not flare and has the same diameter at
the bottom of the extension as it has at the top
Cylinder
May be equipped with an extension sleeve which can expand or collapse to vary the degree of beam
restriction.
Cylinder
Most commonly employed in radiography of the skull,
spine, gallbladder and breast
Cylinder
The _______ of a cone sometimes makes it
difficult to handle and when used with a horizontal
beam, the weight of the cone may cause the tube to angle slightly (causes one cutting if central ray is not checked before exposure)
physical weight
Unlike cones and aperture diaphragms, _____ do
reduce penumbra and off-focus radiation because they provide better beam restriction at a greater distance from the focal spot
Cylinders
Determining the _______ –
Involves a similar
method to that used with an aperture diaphragm.
Cone field size
most commonly employed beam
restrictor in radiography.
Collimator
It consists of sets of lead shutters at right angles to one another which move in opposing pairs. Each set moves symmetrically from the center of the field
Collimator
reduces penumbra along the
periphery of the beam because of their greater distance
from the focal spot.
Bottom shutters
help in reducing the amount of off-focus (stem) radiation reaching the image receptor by
absorbing this radiation before it exits the tube
Upper shutters
Some units provide an outline of the _________ chambers’ size and location
automatic exposure control (AEC)
AEC
automatic exposure control
devices are automatic
collimators
Positive Beam Limitation (PBL)
PBL
Positive Beam Limitation
The use of a collimator results in some _____ of the xray beam because the primary beam is passing through
the mirror in the collimator
filtration
are generally designed with a special
need in mind.
Ex: lead blockers and lead masks.
Ancillary devices
is simply a sheet of lead-impregnated
rubber that can be cut to any size or shape
Lead blocker
usually is cut to correspond to the
particular field size desired and is then secured to the
end of the collimator
Lead mask
