Chest PA AEC cell selection: use the two outside cells over the lungs
Key takeaways
- PA chest with the lungs as the area of interest: two outside cells.
- Center cell on a PA chest: by our reasoning, the exposure runs longer and the lungs are overexposed.
- Lateral chest: center cell.
- AEC controls exposure time. You still choose kVp, mA, cells and back-up time.
- A digital image can look fine after overexposure, so check the exposure indicator.
Use the two outside cells for a routine PA chest. They sit behind the lungs. The center cell sits behind the mediastinum and spine.
Why would the center cell overexpose the lungs?
AEC ends the exposure once enough radiation has reached the active cell. The mediastinum lets less radiation through than the lungs do. So a cell behind it takes longer to reach its preset level, and the lungs keep receiving radiation the whole time. No source we opened prints this outcome for a PA chest. It is our reasoning from the mechanism Bushong describes, and the ACR-AAPM practice parameter says AEC may miss the desired receptor dose if the incorrect cell is selected.
So by the same reasoning, “the center cell stops the exposure too early” is backwards. Early termination would underexpose the lungs, not overexpose them.
Choosing the center cell isn’t always wrong. Bushong says to use it alone when the mediastinum is the area of interest. On a routine chest, the lungs usually are.
Which cell for other projections?
| Projection | Cells | Basis |
|---|---|---|
| PA chest | Two outside | Bushong, Bontrager diagram |
| Lateral chest | Center | Bontrager diagram |
| AP abdomen (KUB) | Center plus one outside | Bontrager diagram |
Follow your department’s protocol if it differs. See the PA chest and lateral chest pages for positioning.
What can still go wrong?
- Anatomy off the active cell: the cell measures the wrong thing. Center the part to the cell, not only to the table.
- Dense object over the cell, such as a prosthesis: the ACR-AAPM practice parameter says AEC may miss the desired receptor dose, and to use a size-based manual technique if you anticipate this. If such an object covers the AEC, it says the back-up timer must end the exposure before the regulatory maximum.
- Wrong setup hidden by processing: AAPM Report 116 says an overexposed digital image may not look dark. Check the exposure indicator.
Bushong advises a back-up time of 1.5 times the expected exposure time. Many units set this for you. For a very small patient, see pediatric AEC errors.
Is this on the registry?
“Detector selection” and “anatomic alignment” are listed under AEC in both ARRT® outlines, before and after March 1, 2027. The outlines don’t list a cell for each projection, so learn the reasoning.
The Image Production guide puts AEC in context. Practice in the Equipment QA set, or read the Radiographic Quality chapter.
Frequently asked questions
Which AEC cells are used for a PA chest?
The two outside cells, which sit behind the lungs, when the lung fields are the main interest. Bushong says the center cell alone is the choice when the mediastinum is the area of interest.
What happens if you use the center cell on a PA chest?
The cell sits behind dense anatomy, so radiation should reach it more slowly, run the exposure longer and overexpose the lungs. That is our reasoning from how AEC works, and no source we opened prints it for a PA chest. On a digital monitor the image may still look acceptable.
Which AEC cell is used for a lateral chest?
The center cell, as shown on the Bontrager lateral chest diagram. Follow your department's protocol if it differs.
What stops the exposure if AEC doesn't?
The back-up timer. Federal rules also limit AEC exposures at 51 kVp and above to 600 mAs or 60 kWs.
Sources
Found an error? Email [email protected] with the page and the source.
- ACR-AAPM-SIIM-SPR Practice Parameter for Digital Radiography, revised 2022, amended 2023, PDF p.8 Guidance
- 21 CFR 1020.31(a)(3), automatic exposure controls Regulation
- AAPM Report 116, An Exposure Indicator for Digital Radiography, July 2009 Official
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