Fluke Biomedical 07-644 - Manual (Page 8)

Nuclear Associates 07-644
Operators Manual
2-2
Overhead FFD = 100 cm W/Bucky Positioned On Center
Figure 2-2A. Photograph showing example of x-ray with centered grid. Note that the center hole has
maximum optical density and the densities in the other holes taper off in a symmetrical
manner.
Overhead FFD=100 cm W/Bucky Positioned Off Center ~ 0.9”
Figure 2-2B. Photograph showing example of x-ray with centered grid. This photograph shows a
maximum optical density to the left of center.
Process the test film and measure and record the optical density of each of the hole images (Figure 2-2A
and 2-2B). The Models 07-443 or 07-444 Densitometers are very useful for measuring the optical
densities. This data may be recorded on the film itself and/or on a data form such as the sample shown at
the end of this manual, Grid Alignment QC Form.
Review the recorded data and note whether the middle hole has the maximum density. If the grid is
properly aligned, the middle hole should display the maximum density. The density in the other four holes
should fall off symmetrically about the middle hole as the tube is laterally decentered to either side of the
grid center. Should the density pattern on the test film indicate the maximum density occurs in one of the
holes other than the middle, or if the density fall off pattern is not symmetrical a misalignment is
suspected.
The requirement that the middle hole display the highest density translates to a maximum tolerance for
grid misalignment of + ½
inch.
Grid misalignment may result from any of several causes. There may be misadjustment in the x-ray tube
detent mechanism that locks the tube into lateral alignment with the grid. Misalignment may also occur in
the reciprocating action of the grid, in the position of the grid or in its construction. If this test indicates a
possible misalignment problem exists, a service representative should be contacted.
In radiographic units using moving grids be sure to inspect the test film for any evidence of imaging of the
grid lines. Appearance of the grid lines on the film may indicate one of the following problems.
1. The generator was set for tabletop exposure so that the grid motion was not activated. Redo the
test after setting the generator for a grid exposure.
2. The reciprocating action of the grid is impaired or inoperative. There may be a malfunction of the
delay between initiation of the grid motion and beginning of the x-ray exposure.
3. The exposure time is too short for the grid motion to effectively blur the grid lines. In this case the
solution is to use a longer exposure time and reduce the mA and/or kVp setting.
.65 1.14 1.47 1.11 .61
y
y
1.05 1.47 1.17 .73 .21
y
y