Grids
A. Use
1. Reduce the amount of scatter radiation reaching the
image receptor
a. Scatter travels in divergent paths compared with
image-producing rays
b. More likely to be absorbed in the grid
2. Generally used when part thickness is 10 cm or
greater or when greater than 60 kVp is used
B. Construction
1. Lead strips separated by aluminum interspacers
2. Grid ratio
a. Grid ratio is the height of the lead strips divided
by the distance between the lead strips: grid ratio
= H/D
b. Ratios range from 4:1 to 16:13. Grid frequency
a. Number of lead strips per inch (or centimeter)
b. As grid frequency increases, lead strip thickness
decreases and becomes less visible
c. Ranges from 60 to 150 lines per inch
C. Grid types
1. Linear
a. Lead strips are parallel to one another
b. X-ray tube may be angled along the length of the
grid without cutoff
c. Grid cutoff: Decreased density along the periphery
of the image caused by absorption of image-forming
rays
d. Used primarily with large SID or small field
2. Focused grids
a. Lead strips are angled to coincide with divergence
of x-ray beam
b. Used within specific ranges of SID
c. Grid radius: Distance at which focused grid may
be used (also called focal distance or focal range)
d. Focal range is wide for low-ratio grids
e. Focal range is narrow for high-ratio grids
f. Focal range is stated on the front of the grid
3. Crossed grids
a. Also called crosshatch grids
b. Consist of two linear grids placed perpendicular to
each other
c. Superior scatter cleanup
d. Allow no angulation of x-ray beam
e. Require perfect positioning and centering
D. Grid characteristics
1. Contrast improvement factor
a. Measure of ability of a grid to increase contrast
b. Expressed as the ratio of the contrast with a grid to
the contrast without a grid
2. Grid selectivity
a. Expressed as the ratio of primary radiation transmitted
through the grid to secondary radiation
transmitted through the grid
b. The higher the grid frequency and grid ratio, the
more selective the grid
c. High selectivity indicates high efficiency of scatter
cleanup
3. Grid conversion factor (GCF)
a. Also called Bucky factor
b. Amount of exposure increase necessary to compensate
for the absorption of image-forming rays
and scatter in the cleanup process
c. Used to indicate the increase in mAs needed when
converting from nongrid to grid status (multiply
mAs by GCF)
d. Used to indicate the decrease in mAs when converting
from grid to nongrid status (divide mAs by
GCF)
e. Factors used at 120 kVp—5:1 grid, 2; 6:1 grid, 3;
8:1 grid, 4; 12:1 grid, 5; 16:1 grid, 6
E. Grid motion
1. Stationary grids
a. Do not move during the exposure
b. Grid lines may be seen
2. Moving grids
a. Reciprocate (move back and forth) during exposure
b. Eliminate the visibility of grid linesc. Grid must begin moving just before the exposure
and continue until just after the exposure to blur
grid lines.
F. Grid errors: Focused grids
1. Upside down
a. Result is normal density in the middle of the
radiograph with decreased density on the sides
b. Focused grid must be placed with labeled tube side
facing x-ray tube
2. Off-level
a. Result is image-forming rays absorbed all across
the radiographic field, with cutoff (decreased density)
visible over the entire radiograph
b. Grid must be perpendicular to the central ray
3. Lateral decentering
a. Central ray does not strike the grid in the center
b. Cutoff visible, more to one side of the radiograph
4. Grid-focus decentering
a. Violation of the grid radius when a focused grid is
used
b. Normal density in the middle of the radiograph
with cutoff visible on the sides
G. Air gap technique
1. Uses increased OID
2. Increased OID allows scatter (which travels in widely
divergent paths) to exit the patient and miss the
image receptor
3. Example: Lateral cervical spine
a. Distance from spine to shoulder causes gap
b. Eliminates need for grid (gap is similar to using a
10:1 grid)
c. Use of grid with air gap increases patient dose and
is unnecessary
H. Radiographic quality and grids
1. Produce higher contrast by absorbing Compton scatter
rays, which produce fog if they strike the image receptor
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