Ch. 12 Scatter Control Flashcards
scatter in diagnostic x-ray is primarily due to this type of interaction
Compton effect
what are the 2 major factors that affect scatter production
- tissue volume
- kVp
what is tissue volume dependent on
part thickness and field size
scatter adds unwanted radiation to the image, what do we call this
fog
beam restrictions do these two things
- limits patient dose
- reduces scatter production
relationship between collimation and field size
inverse
increasing collimation does what to field size
decrease field size
increasing collimation does what to scatter production
decreases scatter production
significant changes in collimation will require this
adjusted mAs
going from a 14x17 field size to a 4x4 field size may require what
more mAs
essentially a flat piece of lead with a small hole in it, attaches to the collimator
aperture diaphragms
long tube/cone that attaches to the collimator, most often used for dental imaging today, can also be used for small parts such as nasal bones and fingers. These devices help to limit both scatter and beam divergence
cones/cylinders
most used form beam restriction, this is the part of the x-ray tube that we use to set field size. Typically consists of 2-3 lead shutters that can be adjusted using controls on the collimator
collimators
also called positive beam-limiting devices (PBL). these are essentially the same as standard collimators, the difference is that these automatically set selected field size depending on what exam is being completed. a lot of collimators today are a form of PBL
automatic collimators
who invented the radiographic grid and when
Gustave Bucky - 1913
number of lead strips per unit length
grid frequency
the ratio of the height of the lead strips and the distance between them
grid ratio
what are the 2 types of grid focus
- parallel grid
- focused grid
what are the 4 types of grid
- wafer grid
- grid cassette
- grid cap
- reciprocating grid
stationary, same size as IR, usually taped to secure it to IR
wafer grid
stationary, IR with a permanent grid mounted to the front surface
grid cassette
stationary, very similar to the wafer, but is designed to hold the IR, so no tape is required to secure it. Most common grid type
grid cap
dynamic, they slightly move while the image is being taken, so as to blur out grid lines. these are built into wall stands and table buckys
reciprocating grid
the decrease in the number of transmitted photons that reach the IR due to grid misalignment
grid cutoff
because focused grids are designed to match beam divergence, if they are place upside down, then they will cutoff most of the radiation before it can reach the IR
upside down focused
most common type of cutoff, happens when the IR with grid is tilted and the CR is not adjusted to match. also the only type that can occur with both focused and parallel grids
off-level
happens with focused grids when the CR is not aligned to the center of the IR, once again, because focused grids are designed to work with beam divergence
off-center
occurs when SID is outside of the focal range of the focused grid, meaning the tube is either too close or too far away from the IR at the time of exposure. This results in images with not enough exposure along the edges of the image
off focused
zebra stripe pattern that occurs when the grid frequency of a stationary grid used in CR imaging is similar to the laser scanning frequency of the CR reader
moire effect
what can help eliminate moire effect
- using higher frequency grids
- using moving grids with CR
method for limiting the scatter reaching the image receptor. scatter will miss the image receptor if there is increased distance between the patient and the image receptor (increased OID)
air gap technique
the simplest type of beam-restricting device, constructed of a flat piece of lead that has a hole in it
aperture diaphragm
automatically limits the size and shape of the primary beam to the size and shape of the image receptor
automatic collimator
changes the shape and size of the primary beam; located just below the x-ray tube housing
beam-restricting device
refers to a decrease in the size of the projected radiation field also known as collimation
beam restriction
the potter-bucky diaphragm located directly below the radiographic tabletop, which contains the grid and holds the image receptor
bucky
can be used to determine the adjustment in milliampere/second needed when changing from using a grid to non-grid (or vice versa) or for changing to grids with different grid ratios
bucky factor
refers to a decrease in the size of the projected radiation field, also known as beam restriction
collimation
located immediately below the tube window where the entrance shutters limit the x-ray beam field size
collimator
essentially an aperture diaphragm that has an extended flange attached
cone
an imaginary line if points were connected along the length of a linear focused grid
convergent line
an imaginary point, if imaginary lines were drawn from each of the lead lines in a linear focused grid
convergent point
has lead line that run at a right angle to one another
cross-hatched grid; crossed grid
essentially an aperture diaphragm that has an extended flange attached to it
cylinder
the distance between the grid and the convergent line or point; also known as the grid radius
focal distance
the recommended range of SID measurements that can be used with a focused grid
focal range
has lead line that are angled, or canted, to approximately match the angle of divergence of the primary beam
focused grid
a device that has very thin lead strips with radiolucent interspaces; intended to absorb scatter radiation emitted from the patient before it strikes the IR
grid
contains a permanently mounted grid and allows the IR to slide in behind it
grid cap
an IR that has a grid permanently mounted to its front surface
grid cassette
can be used to determine the adjustment in milliampere/second needed when changing from using a grid to non-grid (or vice versa) or for changing to grids with different grid ratios; also called the bucky factor
grid conversion factor (GCF)
a decrease in the number of transmitted photons that reach the image receptor because of some misalignment of the grid
grid cutoff
the orientation of a grid’s lead lines to one another
grid focus
expresses the number of lead lines per unit length in inches, centimeters, or both
grid frequency
refers to the linear pattern of the lead lines of a grid
grid pattern
the ratio of the height of the lead strips to the distance between them
grid ratio
radiolucent strips between the lead lines of a grid, generally made of aluminum
interspace material
changes the shape and size of the projected x-ray field; similar to an aperture diaphragm
lead mask
has lead lines that run in one direction only
linear grid
a zebra pattern artifact that can occur during computed radiography imaging if the grid frequency is similar to the laser scanning frequency or if a grid cassette is placed in a bucky
Moire effect
has lead lines that run parallel to one another; also called a parallel grid
non-focused grid
has lead lines that run parallel to one another; also called a non-focused grid
parallel grid
automatically limits the size and shape of the primary beam to the size and shape of the image receptor; also called automatic collimator
positive beam-limiting (PBL) device
a stationary grid placed on top of the image receptor
wafer grid
