Please explain X-ray projections to me (PA vs AP)?

Question

What is the difference, and which is the most common?

How does it work?

Cheers

Answer 1

AP means the rays pass anterior to posterior, and vice versa. Normall you want the side of the body you are interested in closest to the film. If you wanted a chest Xray, this would be PA as the heart is situated anteriorly. However, portable CXR's are always AP. Abdominal films would be PA, where as a spinal Xray would be AP.

Answer 2

The previous comment is correct.  PA is posteroanterior, meaning back to front, and AP is anteroposterior, meaning front to back.

The most common situation where this is important is a chest xray.  In the radiology department, they shoot the film PA, with the person facing the film and the camera 5-6 feet away.  This minimizes the size of the heart and allows you to see the lungs clearly.  It also uses xrays (the radiation waves) that are nearly parallel due to the distance of the source from the film.

AP films are generally shot by a portable xray generator and for patient who cannot be moved, like in the ER or ICU or anyone who is too sick to go to the radiology department.  The heart is distorted and enlarged for two reasons.  The distance of the xray source to the film is less, usually about 3 feet, leading to more spread of the xrays as they travel through the patient.  Also, the heart is closer to the source than the film.

Think of it this way--if you hold your finger right in front of your eye, you may not be able to see much around it, but someone looking at you may not be able to see your eye, but they can see the rest of you.

If a person is healthy enough, they should always get PA chest films to allow for a better picture.