Look at the x-ray image to the left. Notice that the carina is not visible (not an uncommon occurrance). What now? A previous chest ray can be used to estimate the position of the carina, or the carina can be assumed to be at the T4-T5 interspace, given that 95% of patients' carinas project over the T5, T6, or T7 vertebral bodies.

Alternatively, the Dee method for approximating the position of the carina can be used. This involves defining the aortic arch and then drawing a line inferomedially through the middle of the arch at a 45 degree angle to the midline. The intersection of the midline and the diagonal line is the most likely position of the carina.

Click to see Dee Method