RAD 350 Chapter 11 Control of Scatter Radiation
Scatter radiation – while it is our enemy, is also responsible for about 50% of the overall
blackening of the image
Key factors (bullets) regarding scatter, scatter production, control of, and energy levels, etc.:
-Scatter radiation is INCREASED as each of the following INCREASE: kVp, field size, part
thickness (area being imaged) – Thick body parts may be compressed to improve spatial
resolution and increase contrast and radiographic detail – i.e. mammography
-Increased scatter = DECREASED CONTRAST
-Increased kVp = increased COMPTON AND DECREASE P.E.
-Increased kVp = increased energy of scatter reaching the film
-Increased field size = increased scatter
Beam Restricting devices:
-aperature diaphragm (lead sheet with a whole in it)
-cone – like a “cheerleader megaphone”
-extension cylinder
-positive beam limiting device (PBL) automatically adjusts the size of the collimated
size to that of the cassette/film/image receptor size (can cone in more than the size
of the receptor size, but NOT larger
Collimation reduces patient dose and improves contrast resolution
Filtration- as previously discussed two types of filtration exist: inherent (inherent in the tube’s
window) and added. The added filtration is in the collimator assembly
Also of note: the variable collimator and PBL units have “first stage and second stage
(trimmer) shutters/collimators. Also note the position of the collimator light bulb and the
mirror to reflect the light.
A light beam/radiation beam congruency test may be done to assess whether the light beam is
actually where the radiation beam is.
The GRID
General thoughts before beginning: Increased scatter = increased fog and DECREASED
contrast!
Amount of scatter is DIRECTLY affected by kVp, field size, thickness of irradiated field.
A grid is made of radiopaque material (lead) and a radiolucent material )aluminum or other
“spacer” material
-high quality grids can absorb up to 90% of scatter (remember though – as scatter is
absorbed, density is DECREASED so MORE mAs must be used to make up for the scatter
absorbed by the grid).
Grid ratio= height of lead strips
To the space BETWEEN the lead strips
-increased grid ratio = INCREASED “clean up” of scatter
-Grid FREQUENCY = lines per inch – usually as frequency goes up, so does “clean up”
-General grid ratios are 8:1 or 10:1 (the higher kVp’s used – like in a dedicated chest
room, the HIGHER the grid ratio that is used – mammo uses low kVps thus lower grid ratios =
2:1 or 4:1)
CONTRAST IMPROVEMENT FACTOR = compares contrast with a grid to without a grid
BUCKY FACTOR = attempt to measure penetration of BOTH scatter and PRIMARY radiation
through the grid
-the HIGHER the grid ratio, the HIGHER the bucky factor
-bucky factor INCREASES with INCREASING kVp
GRID SELECTIVITY = ratio of TRANSMITTED primary radiation to TRANSMITTED SCATTER
-basically said, the HEAVIER the grid, the HIGHER it’s selectiviey and the MORE efficient
in cleaning up scatter.
Types of grids:
-Parallel – simplest of all the grid types; parallel grid lines separated by interspace
material; lead strips are true VERTICAL while the x-ray beam is a diverging (angled) beam with
only the CENTRAL BEAM vertical. This produces GRID CUT OFF – especially at short SID’s. Grid
cut off prohibits radiation from reaching the film. In Potter-Bucky mechanisms, the grid lines
MUST run with the LONG direction of the table. Only tube tilts WITH the direction of the grid
lines can be used. The moving Potter-Bucky diaphragm will remove the grid lines from
happening as the grid moves from side to side during exposures.
-Crossed (sometimes called cross hatch) grid – made of two parallel grids superimposed
over one another with the strips of one perpendicular to the other
THE MAIN DISADVANTAGE OF PARALLEL AND CROSED GRIDS IS GRID CUT OFF!
-Focused grid: a focused grid is designed to be used at ONE SID only! The lead strips are
slanted to match the angle of the beam at that “said SID” ANY SID other than the “said SID”
will result in grid cut off.
IMPROPER POSITIONING OF THE GRID with relation to the divergent beam is the MOST
common grid problem
Focused grid problems:
-off level : grid cut off across image; underexposed LIGHT image
-off center: grid cut off across image; underexposed LIGHT image
-off focus: grid cut off toward EDGE of image
-upside down grid: (always a board question!!!) BIG cut off toward BOTH edges of the
image with GOOD image in the center.
Grid selection is based upon: kVp’s to be used (dedicated CSR room uses all high kVp’s – thus a
14:1 or 16:1 grid ratio); degree of clean up desired, patient dose (remember when grids are
used, pt. dose goes up).
“Air-gap technique” certain body parts like a lateral C-spine will permit the image
receptor/film to be placed several inches from the body part (due to the shoulders, the c-spine
is actually several inches away from the film/receptor). The space results in scatter diffusing
in all directions away from the patient and NOT hitting the film/receptor – thus no need for a
grid!