Test 3 Flashcards
beam restricting devices
tools an RT can use to limit the amount of scatter radiation reaching the IR
scatter
x ray photons that have changed direction after interacting with matter
factors contributing to scatter
increase kvp
increased xray field size
increased pt. thickness
beam restricting devices reduce scatter by:
decreasing xray beam size
decreasing amount of tissue radiated
fog
an unintended optical density on a radiograph that reduces contrast because of light or chemical contamination
effects of scatter
degrades visibility or image detail
degrades contrast resolution
degrades spacial resolution
spatial resolution
controlled by focal spot size
contrast resolution
affected by scatter radiation
increased kvp =
increased scatter and decreased contrast
high kvp is preferred to:
low kvp
as field size increases:
scatter radiation is increased
collimation reduces
scatter
we must raise our technique when we collimate
TRUE
thick parts of the body results in more scatter
TRUE
Compression devices
improve spatial resolution by reducing the thickness bringing the object closer to the IR
types of beam restricting devices
aperture diaphragm
cones
cylinders
collimators
aperture diaphragm
simplest
makes beam a bit smaller
low in cost
easy to use
aperture diaphragm disadvantages
field size is not adjustable
edges of image blurry aka penumbra
penumbra/image blur/edge gradient
area of unsharpness surrounding the image
penumbra is reduced when beam restrictor is further away from the tube port
TRUE
cones and cylinders
modifications of the aperture diaphragm
extended metal structure
distal end determines field size
has a circular field
cones
attach to bottom of collimator
limit the penumbra better than appertures
disadvantages of cones
if angle of cone is greater than divergent angle of primary beam tehn beam is not being restricted
cone cutting can occur
formula to determine field size
SID*diameter of lower opening / distance from focal spot to bottom of aperture or cone
most common beam restricting device
collimator box
off focus radiation
when xrays are produced at a spot on the anode other than the focal spot
collimators control this
off focus radiation results in
images similar to shadows of the pt.
PBL
positive beam limiting devices
Positive beam limiting devices
manual collimation is still necessary with these in order to more tightly cone down image and reduce pt. exposure
mandated in 1974
removed in 1994
the xray beam should never exceed the size of the IR
TRUE
ancillary devices
lead blocker
lead mask
cone cutting
when the cone and IR are not aligned one side of the radiograph may not be exposed
when was the grid invented and by who
Gustave bucky 1913
grids
consist of sections of radiopaque material AKA the grid strips and sections of radiolucent material AKA interspace
the material used in the interspace can be
plastic or aluminum
plastic is preferred
the surface of the grid is called
the face
information about the grids construction consists of
type of interspace material
grid frequency and ratio
grid size and range of SID’s that can be used
grids can attenuate
80-90% of scatter radiation
Grid ratio
relationship between the height of the lead strips and the distance between the strips
h/d
high ratio grids are more effective in cleaning up scatter
TRUE
as grid ratio increases
radiographic density decreases
as grid ratio decreases
radiographic density increases
as grid ratio increases patient dose
increases
the higher the grid ratio the more spot on you have to be with
positioning
the lower the grid ratio the less accurate you have to be with
positioning
grid frequency
the number of grid strips or grid lines per inch
the higher the grid frequency the thinner
the led strips
contrast improvement factor
the ratio of radiographic contrast with a grid to that without a grid
contrast is approximately doubled when using a grid
TRUE
bucky factor
a number that can be used to determine the adjustment in mAs when changing from one grid to another or not using a grid at all
bucky factor =
mAs with grid / mAs without grid
no grid
bucky factor = 1
5:1 GRID
BUCKY FACTOR = 2
6:1 GRID
BUCKY FACTOR = 3
8:1 GRID
BUCKY FACTOR = 4
12:1 GRID
BUCKY FACTOR = 5
16:1 GRID
BUCKY FACTOR = 6
GRID CONVERSION FORMULA
mAs1/mAs2 = bucky factor 1/bucky factor 2
selectivity
the ratio of primary radiation transmitted through the grid to the amount of scatter radiation transmitted through the grid
the higher the selectivity the more efficient the grid is at cleaning up radiation
TRUE
types of grids
linear crossed focused parallel moving stationary short/long dimension
1st grid made was what pattern
criss cross
focused grid
lead strips are angled to work with the diverging beam
parallel grid
lead strips not angled and doesnt work as well with the shape of the beam
cutoff is more pronounced
linear grid
most popular
allow angulation of xray tube
grid cutoff
undesirable absorption of primary xrays by the grid
convergent point
imaginary point in space above the grid where the focused lead lines would meet or converge if they were extended
focal distance
the distance between the grid and the convergent line
how you determine SID
focal range
the recommended range of SIDs that can be used with a focused grid
the higher the grid ratio the more the grid cutoff
TRUE
four types of grid cutoff errors
upside down focused grid
off level grid
off center grid
off focus
upside down focused grid
happens when a focused grid is placed upside down on the IR which results in the grid lines going opposite the angle of the diverging xray beam
appears as a loss of density along edges of image
off level grid
occurs when the xray beam is angled across the lead strips
most common type of cutoff
happens a lot with portables
entire image turns out lighter
off center grid
occurs when the CR is not aligned from side to side with the center of the focused grid
appears as an overall loss of density
off focus grid
occurs when the SID is outside of the recommended focal range
if the SID is less than or greater than the focal range
loss of density in the outer edges
off focus/off center grids
appears dark on one side and light on the other
stationary grid
ppossible to see the grid lines when using this type
wafer
matches size of cassette
grid cassette
IR that has a grid permanently mounted to its front surface
grid cap
contains permanently mounted grid
IR slides in behind it
moving grids
blurs out the grid lines
part of the potter-bucky diaphragm AKA bucky
2 kinds of moving grids
reciprocating - moves side to side
oscillating - moves in a circular motion
disadvantages of moving grid
increased OID
motion
pronounced grid lines
minimum exposure time
RTs should select a grid according to
size of body part
kvp being used
grid pattern
pt. dose
grids should be used when the kvp is set above
70
Long dimension grid
Lead strips run parallel to long axis horizontal
Short dimension grids
Lead strips run vertical/ perpendicular
Air gap technique
By increasing distance between the pt and the IR the scatter will miss the IR
mAs must be increased
Greater the gap greater reduction of scatter
Moire effect
Creates a wavy pattern on the image
