I have CT scans that contain the spine and am trying for an easy way to get the Cobb Angle using them.
Cobb angle is measured on 2D x-rays, which is essentially AP projection of CT image. You can create the projection image using this script and then use angle markups angle widget (in recent Slicer Preview Release). We don’t have 4-point angle measurement yet (we’ll probably add it at some point, it would be very easy), so if angle widget is too inconvenient then you can draw 2 lines and compute the angle using a short Python script.
You might also find Scoliosis extension useful, which measures vertebral angles from 3D points defined directly on a volume-rendered CT.