Nuclear Associates 07-644
Grid Alignment Test Tool
March 2005
Manual No. 07-644-1 Rev. 2
©2004, 2005 Fluke Corporation, All rights reserved. Printed in U.S.A.
All product names are trademarks of their respective companies
Users Manual
Fluke Biomedical
Radiation Management Services
6045 Cochran Road
Cleveland, Ohio 44139
440.498.2564
www.flukebiomedical.com/rms
Table of Contents
Section 1: Introduction................................................................................................ 1-1
1.1 Introduction .................................................................................................. 1-1
1.2 Description................................................................................................... 1-1
Section 2: Operation.................................................................................................... 2-1
2.1 Operation ..................................................................................................... 2-1
i
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Introduction
Introduction
Section 1
Introduction
1.1 Introduction
The Model 07-644 Grid Alignment Test Tool is designed to test alignment of the radiograph grid with
respect to the central ray of the x-ray tube.
An upside-down focused grid or an incorrect tube to grid distance can be easily detected by exposing a
cassette in its tray and observing the density fall off toward opposite edges of the processed film (grid
cutoff). , However, lateral decentering or tilting of the grid cannot be detected in this manner: the grid
cutoff in these cases causes a uniform reduction of density across the film. The result of such a
misalignment is increased patient dose and reduced image contrast. It is in detecting this type of
"unobvious" grid cutoff that the Grid Alignment Test Tool is very useful.
1.2 Description
1
The test tool is an easily used device consisting of a plastic covered lead blocker 1/16" x 3 ½ x 9-1/8 (0.2
cm x 9 cm x 23 cm) containing several holes. The five larger holes have centers spaced 1" (2.5 cm) apart
and allow test exposures to a film cassette while the smaller holes provide marking exposures on the film
to show the tool orientation. When an exposure is made through one of the large holes, two small lead
blockers are used to block out the other four large holes. For longer exposure times copper strips (0.040"
thickness is recommended) can be placed
over the holes.
Figure 1-1. Model 07-644 Grid Alignment Test Tool, with Lead Blockers
1-1
Nuclear Associates 07-644
Operators Manual
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Operation
Operation
2
Section 2
Operation
2.1 Operation
Center the x-ray tube with respect to the image receptor both longitudinally and transversely and adjust
the height of the tube to the proper source to image (SID) distance for the grid employed on the system.
Lock the tube in place. Position an 8" x 10" (20 x 25 cm) or larger cassette in the tray so that the long
dimension of the cassette is perpendicular to the long dimension of the table.
Position the Grid Alignment Test Tool on top of the table so that its long dimension is perpendicular to the
grid lines (long dimension of the tube). Center the middle hole of the tool in the optical crosshairs of the
collimator light field. The three small marking holes should be pointed toward the front of the table (Figure
2-1). Tape the Grid Alignment Tool to the table so it does not move for the duration of the test.
Collimate to approximately a square just smaller than the width of the test tool. Place the small lead
blockers on the tools so only the middle (and the two small holes on either side of it) are irradiated and all
other large holes are covered. Make an exposure at approximately 60 kVp and 2-4/mAs using the lowest
available mA settings to obtain an optical density in the middle hole of between 1. 0 and 2.0 on the
developed film. For longer exposures copper strips (0.040" thick are available from Fluke Biomedical,
Radiation Management Services) can be placed over the holes. Repeat the exposure so that by moving
the tube laterally each of the five large holes is irradiated while centered in the crosshairs. It is not critical
to set the middle hole of the test tool under the exact center of the optical crosshairs for the initial
positioning. However, it is important that the hole is in a uniform part of the x-ray beam and not in the
edge of the field falloff. It is also important that whatever collimator marking is initially aligned over the
middle hole, all hole exposures are made with that same collimator marking aligned over the hole
irradiated. This insures that the x-ray tube is actually moved laterally one inch between adjacent hole
exposures. Be sure to use the same technique settings at each position and do not forget to move the
small lead blockers each time to cover the four large holes not being irradiated. Also be sure to irradiate
the small marking holes on the test tool to insure the tool orientation is recorded on the film.
Figure 2-1. Diagram Showing Arrangement of Grid Alignment Test Tool for Checking Grid Alignment
2-1
Nuclear Associates 07-644
Operators Manual
.65 1.14 1.47 1.11 .61
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.
1.05 1.47 1.17 .73 .21
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.
y
y
y
y
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
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.
inch.
2-2
Operation
Operation
Sample Grid Alignment QC Form
Grid
Room 103 Unit GE II
Line
Grid Ratio
SID 40” kVp 60 mA 5 mAs Time N/A
12:1
No.
103 lines/in.
Tested Radiographic Table
Focal
Distance
40”
2
Date 2 1
1-15-94 .61 1.11 1.47 1.14 .65 Alignment OK MR
5-18-94 .31 .73 1.17 1.47 1.05 Check Alignment MR
5-19-94 .63 1.13 1.51 1.15 .66 OK After Service MR
Measured Optical Density
: : Reference Location
(3 small marking holes toward
the front of the table)
0
Center
1 2 Comments
Checker’s
Initials
2-3
Fluke Biomedical
Radiation Management Services
6045 Cochran Road
Cleveland, Ohio 44139
440.498.2564
www.flukebiomedical.com/rms
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