RAD 254 Chapter 14 Control of Scatter

1
RAD 254 Chapter 14Control of Scatter

• Break down into Those that reduce patient dose
  and those that are geometrical in nature and
  those not

2
3 factors affecting scatter (primary)

• Increased kVp
• Increased field size
• Increased patient thickness

3
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

4
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 (IVPs
  and mammo are examples)

5
Beam restricting deviceslimit 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

6
Variable aperture diaphram

• Mandated in 1974 by the US Dept of Food and Drug
  Administration (mandate removed later)
• Positive Beam Limitation Devices (PBLs)
• Automatically collimate to the size of
  cassette/receptor in the bucky and CANNOT be a
  bigger size than the film/receptor

7
Filtration

• Filtration also will decrease the low energy rays
  and limit patient dose and some scatter

8
The GRID

• Only FORWARD scatter is of any benefit to the
  radiographic image all other scatter degrades
  the image!

9
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

10
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

11
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

12
Grid Types

• Parallel
• Crossed (cross-hatch)
• Focused
• Focused - crossed

13
Grid Problems

• Grid cut-off short SIDs result in the
  vertical, parallel strips absorbing the
  diverging beam at the outer margins of the
  grid/film/rescepter MOST pronounced at short
  SIDs
• Most grid problems are positioning related
• Uneven grid/off level grid
• Off centered (lateral decentering)
• Off focus grid
• Upside down focused grid

14
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

15
Grid Ratio Selection

• 81 grid is the most widely used
• Grid ratio is kVp driven
• Higher kVps 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

16
mAs Grid considerationsAs grid ratio
increases, so much mAs

• 51 grid 2 X mAs
• 81 grid 4 X mAs
• 121 grid 5 X mAs
• 161 grid 6 X mAs

17
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