RAD 254 Chapter 14
Control of Scatter
Break down into: Those that reduce
patient dose and those that are
geometrical in nature and those not
3 factors affecting scatter (primary)
• Increased kVp
• Increased field size
• Increased patient thickness
Spatial Resolution & Contrast
Resolution
• Spatial resolution may be thought of as
geometric in nature (focal spot size,
emission spectrum, etc. dealing with
geometric image formation
• Contrast resolution – driven by scatter and
other sources of “noise”
Scatter
• Increased field sizes = more scatter –
collimation is the most readily available
and easiest thing to lower the amount of
scatter
• Patient thickness also increases scatter –
compression may be used to help avoid
this (IVP’s and mammo are examples)
Beam restricting devices
limit the radiation to the patient
• Aperature diaphram (size and resultant
field size are a direct proportion – draw the
damn picture and figure the problems)
• Cones and cylinders – great for absorbing
scatter, but are circular shaped = great for
improving contrast and removing scatter
BUT requires much more mAs as a result
Variable aperture diaphram
• Mandated in 1974 by the US Dept of Food
and Drug Administration (mandate
removed later)
– Positive Beam Limitation Devices (PBL’s)
• Automatically collimate to the size of
cassette/receptor in the bucky and CANNOT be a
bigger size than the film/receptor
Filtration
• Filtration also will decrease the low energy
rays and limit patient dose and some
scatter
The GRID
Only “FORWARD” scatter is of any
benefit to the radiographic image – all
other scatter degrades the image!
Scatter = LOWER contrast
• Using a grid (alternating strips of fine
leaded strips with alternating radiolucent
interspace material) can effectively reduce
the amount of ANGELED scatter from
reaching the film/recepter
Grid terms
• Grid ratio = height of the lead lines divided
by the interspace WIDTH
• Grid frequency/lines per inch = the more
lines per inch, the more clean up
• Grid clean up = scatter w/o a grid vs
scatter reaching film with a grid AKA
“Contrast Improvement Factor”
• Grid function = improve image contrast
Bucky Factor
• Refers to the AMOUNT of radiation to the
patient with a grid vs W/O a grid.
– Higher the grid ratio the higher the “bucky
factor”
– The higher the kVp, the higher the “bucky
factor”
• Grid weight refers to how heavy it is – duh
– the more lead, the heavier it is
Grid Types
• Parallel
• Crossed (cross-hatch)
• Focused
– Focused - crossed
Grid Problems
• Grid cut-off = short SID’s result in the vertical,
parallel strips absorbing the “diverging” beam at
the outer margins of the grid/film/rescepter
MOST pronounced at short SID’s
• Most grid problems are “positioning” related
–
–
–
–
Uneven grid/off level grid
Off centered (lateral decentering)
Off focus grid
Upside down focused grid
Focused Grid Misalignment
• Off level = grid cutoff across image;
underexposed image (light)
• Off center = ditto
• Off focus = CR centered to one side or the
other of a focused grid
• Upside down grid = severe grid cut-off (no
density) at both sides of the image
Grid Ratio Selection
• 8:1 grid is the most widely used
• Grid ratio is kVp driven
– Higher kVp’s warrant higher grid ratios
– Higher grid ratios = higher patient dose (more
radiation needed to produce an image)
– As kVp increases past MAXIMUM OPTIMUM
kVp, patient dose INCREASES
mAs – Grid considerations
As grid ratio increases, so much
mAs
•
•
•
•
5:1 grid = 2 X mAs
8:1 grid = 4 X mAs
12:1 grid = 5 X mAs
16:1 grid = 6 X mAs
Air gap technique
• By allowing the scatter radiation to
“diffuse” in the atmosphere after the
patient but BEFORE the film results in a
higher contrast image as the scatter
diffuses and does NOT reach the film
– C-spine lateral is a good example of this