Some initial thoughts:

For the CAT scan analogy:

Making a distinction between the background image slice, and the layer on which
the 2D object is created.

Each image is a sequential slice, of the 3D object,
separated by an increment of ____ mm, which determines the layer of the traced
2d object. (The example is using the front frame, x,z.)

Each image slice is loaded with the same x,z (or x,y?) position and (non)rotation.
The image is "below" created objects? (or with objects or above objects?)
The transparency setting is ___?

A slice is loaded, and the user traces a 2D object on the new layer.
The previous 2D objects, (layers), need to be hidden?

The user can flip back and forth between 2d object layers, or background slices,
to glean 3D help for the 2D trace.

...
- Brian