Grids, Errors, and Air Gap Technique

Raylei Pettit, Taylor Hosley, and Charlea Britt

RADS 3300

Grids and Errors

Grids:

First, we should know what grids are. Grids are devices used to improve image quality by reducing scattered radiation. They help with removing the scatter and only allowing primary radiation to reach the image receptor which will allow for the image to be clear and capture great contrast. Grids are typically used when the thickness of the tissue measures more than 10-13cm. But the major problem with grids is that they can be very helpful but often create errors when not used properly.

Hypothesis:

When using grids, we should see an increase in image quality, but when used improperly you will see grid cut off depending on the error.

The bottom image depicts why we have to compensate by increasing our technique, because the grid will also absorb some of the useful beam (Clover Learning).

Grid frequency and ratio:

Grid frequency is the number of lead strips to the interspace material. Grid ratio is the height of the lead strips divided by the thickness of the interspace material (Bushong, 2021). Both important factors when it comes to controlling patient exposure while impoving image quality. When you have a higher grid ratio you have to use a higher technique in order to penetrate through the grid and produce a better image. You will use the grid conversion factor formula to find your new technique.

Grid Conversion Formula

mAs1/mAs2 = GCG1/GCF2

Grid Ratios

No Grid = 15:1 = 26:1 = 38:1 =310:1/12:1 = 416:1 = 5

Image from Fauber.

Focal Distance:

The focal distance is the distance from the convergent point to the grid and is determined by the type of grid being used so when you have a focused grid you have canted lines that go with the diverging beam producing a better image. With non focused grids the pattern is not set up to go with the beam. With that being said, a focused grid has a certain SID range that you have to abide by in order to prevent grid cut off from occurring.

EXPERIMENT:
In this experiment, we will demonstrate the grid errors.
We accomplished this by setting our technical factors to a 41 kVp and .1 mAs.
Each error was demonstrated with a 12:1 grid.
This is done in order to demonstrate the grid lines while using the grid. Now let's go through the errors.

Image receptor without grid

We took this image in order to demonstrate how the image receptor looked with the same technical factors as to what the image receptor looked like without the gird.

Image receptor with grid

This image is going to be used in reference in order to compare where the grid cut offs will occur. Therefore, this image can be referenced to identify where the image degraded.

Off level Error

This error occurs when the grid is not parallel with the image receptor. As you can tell this will cause cut off across the entire image because of the uneven exposure the grid received. This was achieved by placing several wedges under the image receptor.

Off focus Error

As discussed, we have a focal range that must be followed for each focused grid in order to prevent cut off. The grid we used was a 12:1 grid and it had a focal range of 40-47 inch SID.

In this image we used a 56 in SID in order to exceed the focal range and achieve the grid cut off on the edges of the image.

Off center Error

This grid error occurs when the beam is not properly centered to the grid. This error causes degradation across the entire image.

The image to the right is how we set up the room to perform this error.

Upside-down Error

This error occurs when the grid is placed upside down on the image receptor, which is how we performed this procedure. This error causes cut off along the edges, because the angle of the lead strips are going against the beam divergence and only passing through the center to the IR.

Overview:

In conclusion, we see that grids can cause a lot of degrading to an image and affect overall image quality. This would cause you to double your patient dose when an error occurs, so it is important to educate yourself as a technologist and know how to correctly use grids to prevent these errors from occurring.

Air Gap Technique

What is Air Gap Technique? When is it used?

Air Gap is a useful technique utilized when a grid is not available. It is simply when you increase the OID to allow scatter to miss the image receptor. Essentially, the "gap" causes the scatter to go outside the parameters of the IR, resulting in an increase in image contrast.

Hypothesis:

Using the air gap technique, if OID is increased, then scatter radiation will decrease, resulting in better image contrast because the scattered photons will miss the image receptor.

See this youtube video explaining scatter radiation and ways to reduce it!

(includes grids and AGT)

https://www.youtube.com/watch?v=5sMEAlvN598

Experiment:

Set the room up & technical factors

(75 kVp, 5 mAs)

Position patient in front of the IR (phantom)

(Lateral C-Spine)

Take exposure

Set the room up & technical factors

(75 kVp, 10 mAs)

For every cm of OID, I increased the mAs 10% - there was 9cm of OID added

Position patient in front of the IR WITH increased OID (phantom)

(Lateral C-Spine)

Take exposure

Natural OID

Low Contrast

Air Gap Technique/Increased OID

High Contrast

In the first image, there is less distinct differences in the shades of grey. There are so many shades that it makes it harder to visualize the details of the spine. In the second image, with increased OID, there is vivid difference in the contrast. The increased contrast makes it easier to depict the details of the spine, and ultimately easier to identify any issues that could be occurring in the patient. This technique also results in magnification, which is evident in the second image as less of the frontal part of the skull is visible.

A lateral c-spine is an example of unintentional use of the air gap technique. As the shoulder prevents the patient from being able to be completely against the IR, creating OID, and ultimately using the AGT. This results in a decrease in scatter radiation and overall higher/improved contrast.

Additional Example/Experiment:

Set the room up & technical factors

(75 kVp, 15 mAs)

Position patient on the IR (phantom)

(AP Pelvis)

Take exposure

Set the room up & technical factors

(75 kVp, 28 mAs)

For every cm of OID, I increased the mAs 10% - there was 9cm of OID added

Position patient in front on the IR with OID (phantom)

(AP Pelvis)

Take exposure

No OID

Low Contrast

9cm of OID

High Contrast

In the first image there is no OID, and no grid, resulting in a decreased amount of contrast. Looking closely at the spine, wings, and rami, there is a distinct difference. The second image reveals the flaws in the bones of the phantom, and you can see the details of the spine more clearly, meaning there was an increase in contrast. Magnification is also evident in the second image as the anatomy appears larger.

Overview/Limitations/Disadvantage

Overall, the air gap technique is a useful tool when a grid is not available. By increasing OID, scatter can miss the image receptor, resulting in an increase in contrast, and ultimately a better image. There is, however, one major disadvantage. As seen above, the AGT calls for an increase in mAs, which would result in an increased patient dose. One limitation that should also be noted is that with the advanced digital imaging systems that we have now, the air gap technique is not as likely to be seen. Virtual grids and software often correct the issues on an image before we can physically do anything about it.

Reference:

Bushong, S. C. (2021). Chapter 22 Scatter Radiation. In Radiologic Science for Technologists: Physics, Biology, and protection (pp. 300–300). essay, Elsevier.

Fauber, T. L., & Griswold, R. (2025). Chapter 7 Scatter Control. In Fauber’s Radiographic imaging and exposure. essay, Elsevier.

Radiography Image Production: Advanced Exposure Factors: Grids (Errors). Clover learning. (n.d.). https://app.cloverlearning.com/learn/courses/radiography-image-production/advanced-exposure-factors/grids-errors

Gaillard, F. (n.d.). Air gap technique. Radiopaedia. https://radiopaedia.org/articles/air-gap-technique

McEntee, M. F., Brennan, P. C., Rainford, L., & Foley, S. J. (2021). An RCT investigating the use of the intelligent cardiac imaging system (IC-ARM) to reduce patient radiation dose (Final flight report). Ulster University. https://pure.ulster.ac.uk/ws/portalfiles/portal/129404509/RCT_IC_ARM_final_flight_1_.pdf

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