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In order to prevent the shadows cast
onto the film by the grid from interfering with visualization of diagnostic
detail, certain principles must be followed:
 | For one, the lead should be as thin as possible
to be consistent with adequate absorption of scattered radiation. The
thinner the lead, the narrower the shadow it will produce on the film and
the less visible it will be to the eye. |
| Also, the thinner it is the less absorption of
primary radiation will be in the grid. However, it must be noted that
adequate absorption of scattered radiation is the function of the grid and
lead must be thick enough to provide this function. |
| Another factor is the relative fineness of the
grid. This quality is represented by the number of lines per inch. In
general, the greater the number of lines per inch, the less visible will
the individual lines be, but this is subject to certain practical
considerations which modify it in actual use. |
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Pratical Considerations in
Grid Selection |
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The selection of a grid to be used for a particular
radiograph will be primarily dependent on the following considerations:
| Relative quantity of scattered radiation
produced by subject being radiographed. |
| Kilovoltage technique used.
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| Capacity of x-ray generator. |
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The quantity of scattered radiation produced is
dependent on the thickness and relative density of the body being
radiographed. A non-grid exposure of the chest will consist of about one
half scattered radiation, while a non-grid exposure of the abdomen may
consist of more than 90% scattered radiation.
From this, it is apparent that for dense body sections the more effective
removal of scattered radiation will provide the most striking improvement in
the radiograph. This suggests the use of a high ratio grid or a crossed
grid. The choice between these two grids depends on the ease of aligning the
grid correctly relative to the x-ray tube, and whether a high or low voltage
techniques are in use.
If there are questions about the proper centering or leveling, or if low
kilovoltages are in use, a low ratio grid will present much greater
advantage from the point of view of positioning latitude and cleanup. For
high voltage techniques, if the grid can be accurately aligned (see effect
of misalignment in Figures 1 & 2 below), greater advantages will result from
the use of an 8:1 ratio crossed grid or high ratio linear grid. |
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At kilovoltages of the order of 100 KVP or more, comparable radiographic
effect requires low milliampere-second values than at low kilovoltages, thus
reducing the radiation dosage to the patient.
However, in order to maintain the same contrast range of the higher
kilovoltage, it is necessary to use a higher ratio grid. The exposure
factors are not the same for all ratios, and the increased exposure required
for a high ratio grid may to some extent reduce the patient-dosage advantage
gained by going to higher kilovoltage techniques. In general, in spite of
the higher exposure factors involved, the use of high kilovoltage and high
ratio grids will result in somewhat lower radiation dosage to the patient.
All radiographers must work within the limitations of the physical
characteristics of the x-ray equipment at their disposal. While this may not
be as important a consideration in the selection of a grid as some others,
it is a factor to be considered. For instance, the maximum benefits to be
derived from a 12:1 ratio grid will not be realized with a unit whose top
limit is 90 KVP, although there will be some advantage over a lower ratio
grid. In general, a 16:1 ratio grid will do the most good with equipment
which can be used at kilovoltages above 100 KVP.
This applies also, to a lesser extent, to the 10:1 ratio grid. With a
bedside or portable unit, where the likelihood of near-perfect alignment of
the grid relative to the primary beam is poor, the use of the high ratio
grids is practically impossible, and difficulties may be encountered even
with the 8:1 ratio grids. For such use, where wide latitude in distance,
centering, and leveling is necessary, the 5:1 ratio grid is advisable, and
for maximum cleanup under these conditions the 5:1 crossed grid is ideal.
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Choosing
the correct grid for your application may be a difficult task. MXE provides
technical advice to assist you in selecting the proper grids and evaluating
their performance.
1) X-ray Grid Selection Based on Clean-up Requirements:
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|
Cleanup |
Ratio/Type |
Positioning
Latitude |
Recommended
Up To |
Remarks |
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SUPERLATIVE |
8:1 criss-cross |
Distance fair; centering and
leveling-slight |
120 KVP |
Not recommended for tilted tube
technique |
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EXCELLENT |
12:1 linear |
Very slight |
110 KVP (Suitable for highr KV) |
Extra care required for proper
alignment; usually used in fixed mount |
|
EXCELLENT |
6:1 criss-cross |
Good |
100 KVP |
Tube tilt limited to five degrees |
|
GOOD |
8:1 linear |
Distance fair; centering and
leveling-slight |
100 KVP |
For general stationary grid use |
|
GOOD (CR) |
4:1,6:1 |
Good |
120 KVP |
CR grid |
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MODERATE |
6:1 linear |
Good |
80 KVP |
Least expensive of stationary grids |
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2)Basic Guidelines:
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ANATOMY |
LINE |
RATIO |
DISTANCE |
|
SKULL |
103 |
10:1 |
36-40" |
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CHEST |
103-150 |
10:1-12:1 |
60-72" |
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CR
CHEST |
150,178 |
4:1,6:1 |
Parallel |
|
ABDOMINAL |
103 |
8:1 |
34-44" |
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CR
ABDOMINAL |
150,178 |
4:1,6:1 |
Parallel |
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SCOLIOSIS STUDIES |
85-103 |
8:1 |
48-72" |
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SPECIAL PROCEDURES |
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MOST STUDIES |
103 |
10:1 |
36-40" |
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BI-PLANE |
103
criss-cross |
10:1 |
36-40" |
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SURGICAL ROOM |
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ORTHOPEDICS |
85 |
8:1 |
34-44" |
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CHOLANGIOGRAMS VENOUS STUDIES |
103 |
8:1,10:1 |
34-44,36-40" |
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EMERGENCY ROOM |
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TRANS LATERAL SKULL, SPINES, HIPS |
103-85 |
6:1-8:1 |
34-44" |
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Simply call 888-459-9729 to request your pricing.
E-Mail your request to:
[email protected].
Fax your request to: Your purchase order or
Download our Quote Request form
(type right in the PDF
file or write in it), print it and then fax it to 440-942-1388
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