RAD 354 Chapt. 11 Control of Scatter
• Break down into: Those that reduce pt. dose
and those that are geometrical in nature and
those NOT!
• 3 factors affecting scatter (primary)
– Increased kVp
– Increased field size
– Increased pt. thickness
Spatial Resolution & Contrast
Resolution
• Spatial resolution may be thought of as
GEPMETRIC in nature (F.S. 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 scattercompression may be used to help avaid this
(IVP’s and mammo are examples)
Beam restricting devices limit the
radiation to the patient
• Aperature diaphram (size and resultant filed
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
require MUCH MORE mAs as a result!
Variable aperature diaphram
• Mandated in 1974 by the US Dept. of Food
and Drug Admin. (mandate later removed)
• Positive Beam Limitation Devices (PBL’s)
– Automatically collimate to the size of the
cassette/receptor in the bucky and CANNOT be a
bigger size than the cassette/receptor
Filtration/Grid
• Filtration also will decrease the low energy
rays and limit patient dose and some scatter
• Grids remove “angled scatter” so 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/thin
leaded strips with alternating radiolucent
interspace material) can effectively reduce the
amount of ANGLED 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
• 5:1 grid most widely used PORTABLE grid
• 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 must mAs!
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•
•
•
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/receptor as the scatter diffuses and
does NOT reach the film/receptor
– C-spine lateral is a good example of this