On obliques, the main goals are to visualize the intervertebral foramina, as well as the intervertebral joint spaces while including C1 - C7. Let's look at controlling factors for each, as well as a couple of films to refer to.
Intervertebral foramina - these are those round-looking things that you should be able to drive a truck through on the lateral side. You can visualize these if you have the appropriate amount of rotation. 45 degrees may not always work for everyone, depending on minor variances in bone structure, and definitely if there is any sort of lordisis or kyphosis. The first picture displays them beautifully, and the second gets a little bit closed off toward the T-spine. My guess at the error on #2 was that the shoulders were not rotated 45 degrees, while the head was turned completely lateral. This produces a different appearance in the foramina from proximal to distal, and this is probably why the newer versions of the atlas require no lateral rotation of the cranium. You know you're over-rotated if you are closing off the foramina and see spinous processes elongated laterally to them. You are under-rotated if you are closing off the foramina and don't see spinous processes.
Intervertebral joint spaces - this is not controlled by rotation, because you should see these in every view of the C-spine. Tube angulation is the key factor here. The second film above shows much better intervertebral joint spaces than the first. While the first excels in foramina demonstration, it would seem that the angle of the tube is too shallow to see a clear separation between the vertebral bodies. If you ever wondered why positioning atlases say "10-15 degrees, or 15-20 degrees," this is exactly why. No patient has exactly the same bone structure... this allows for the "majority" of patients. Someone with a normal curve on the c-spine might be able to employ a 12 degree angle on the oblique, while a kyhpotic patient may require 25 degrees. The key is in looking at your lateral view. I suggest performing your lateral first.
The patient in the first lateral will definitely need a steeper angle than the second. Difficult to see? Take a pen, or even your hand and do the following: Place the tip of your pen/finger at the dens of film #1. Angle your pen/finger so that the tip remains on the dens and the shaft of the pen runs through the vertebral body of C-7. Now maintain that same angle and slide the tip of your pen/finger to the dens of film #2. You should see a definite difference in position of the vertebral bodies here. Film #2 may require a 10-12 degree angle, and film #1 may take a 15 degree or more.
