Radiographic Equipment Testing (Part 2)
When you adjust the collimation on the tube housing, the only visual indicator of the
actual field size is produced by a light being reflected off of a mirror in the tube housing
assembly.
You may have noticed a variance in your clinical assignments in some x-ray rooms between the
light field and the actual x-ray field produced with an exposure.
This is due to misalignment of the lead shutters compared to the light projected during collimation.
We can test for this a few different ways.
The test tools on the right have multiple purposes, one of them being collimator field
alignment.
The light field should be set to the square on the face of the test tool, and once exposed,
the image can be interpreted by measuring the distance between that line on the radiograph
and the actual border of the radiation field.
Acceptance limits are plus or minus 2% of the SID.
Most of these test tools are created for testing at a 40-inch SID, but it metric, that’s
100 cm – a nice, round number to know the value of 2% for acceptance limits.
If the SID is 100 cm, then the radiation field on the image should be within 2 cm of the
line that you collimated to.
If you don’t have this test tool available, you can perform what’s called a “9-penny
collimator test” seen on the left.
Using a clean image receptor, you can collimate to a square smaller than the image receptor
size.
For each of the four borders, place one penny inside the light field and one penny outside
the light field, so that they touch one another at the light field border.
The ninth penny is simply used for orientation in case your image is flipped… you can also
use a radiopaque marker.
The idea is that since the penny measures slightly more than 1.9 cm, if you can visualize
the entire penny on either side of the collimated field on your radiographic image, you’ll
be outside of acceptance limits using a 100cm SID.
The collimator field alignment test should be performed semi-annually.
As previously mentioned, this tool can be used for additional tests like Vertical Beam
Alignment.
When you set your x-ray tube to zero degrees, pointing straight down at the x-ray table,
it should be perfectly perpendicular.
This test, which is sometimes called the “beam perpendicularity test” in some textbooks,
determines if zero degrees at the tube controls truly represents a beam with no angulation
at the central ray.
A plastic cylinder is used, which is enclosed at both ends.
Each end contains a small ball bearing that has been specifically placed in the center
of the circle on each end.
The cylinder is placed on the center of the collimator test tool, and an exposure is made.
If the bbs are not superimposed on the radiograph, then your x-ray tube is not exactly positioned
perpendicular to the image receptor and adjustments need to be made.
This should be tested semi-annually, and acceptance limits are +/- 2 degrees from perpendicular.
This test tool allows for collimator alignment, vertical beam alignment, and a third test,
central ray alignment, all in one exposure.
The central ray should simply be pointed at the crosshairs on the center of the tool,
where the bbs would be lined up, and the exposure should be accurate within +/- 2 cm from the
light field’s central ray.
When a new x-ray tube is installed, the actual Focal Spot Sizes for large and small focal
spots are rated nominally and typically recorded on the outside of the x-ray tube housing.
There is a lot of lenience for accuracy here due to the complicated nature of the tube
construction and multiple factors affecting beam geometry.
A baseline reading should be taken and archived when every new x-ray tube is installed for
comparison to annual tests and/or if testing is needed between annual recommendations when
a loss in resolution may be visibly noticed suddenly.
Each focal spot size can be as much as 50% larger (for focal spots smaller than 0.8 mm)
or 40% larger (for focal spots larger than 0.8 mm) than the rated nominal size referenced
by the manufacturer.
As x-ray tubes age and experience normal wear and tear, changes in the filament and the
anode will reduce resolution as the focal spot size increases over time.
This should be tested annually, and though there are many tools to test for resolution,
the preferred tool is the slit camera, which looks similar to the cylinder for the beam
perpendicularity test.
Instead of bbs, there is a thin metal template with finely cut slits in groups of three which
run crosswise and lengthwise.
We’ve also experimented with this tool in your exposure principles lab.
Once the exposure is made, you should examine each set of vertical and horizontal lines
from largest to smallest.
The last set of observed lines that are not blurred together should be determined, which
will serve as the highest resolved pair.
The blur points may vary between horizontal and vertical slits, but care should be taken
when aligning the tool prior to exposure so that one set of lines is parallel to the long
axis of the x-ray table and the other parallel to the short axis.
Automatic Exposure Control should be monitored for repeatability.
Your textbook suggests 5 exposures should be taken using the ion chamber to measure
the output.
The same test should be performed at various density settings when using AEC as well.
This should be checked annually with acceptance limits plus or minus 10% within each radiographic
room, but with a noted 20% variance is acceptable when comparing different rooms within the
same department.