I just received an email today asking for an explanation on obliques... I'm fairly certain that it was about ribs, but I thought I would go ahead and post this because it can be difficult to understand why we position oblique ribs the way we do. The rule of thumb for most obliques is to simply place the anatomy of interest closest to the image receptor to visualize the most detail possible. This does not apply with every oblique view of the ribs, however.

Reasons vary for why we position most body parts in certain obliques, and ribs are very different as well. Typically, if an area of interest is anterior, an anterior oblique is performed (RAO or LAO), and if an area of interest is posterior, a posterior oblique is performed (RPO or LPO).

Posterior rib pain: If your patient is complaining of right posterior oblique pain, you would perform RPO. If the pain is left posterior, perform LPO.

Anterior rib pain (this is where it gets tricky): If there is right anterior rib pain, perform an LAO. This seems like the opposite of what should be done, but if a RAO is performed, the thoracic spine will superimpose the area of interest. You need to move the spine out of the way with an LAO. This position still places the anterior ribs closer than a posterior oblique would, providing better detail by getting the anatomy closer to the image receptor, and still prevents the superimposition of the spine. If there's left anterior rib pain, perform an RAO for the same reason - get the part of interest closer to the image receptor, and move the spine out of the way.

It's always a good idea to mark the area of interest with a bb marker or annotate some arrows on the image just to help out the Physicians viewing the images as well.