A high res 2D X-ray is preferable for a PCB, which is nearly a 2D rectangle itself.

That'd be an MRI.

A CT is, simplifying, an x-ray machine that takes lots of images in slices, then analyze them with certain algorithms to reconstruct 2D and 3D images of the interior of the 'subject'.

Well, not really in slices, but from all angles. So with a computer you can reconstruct the density of whatever you're imaging and also do the slices

Yeah true, I was still half asleep and couldn't explain further. It's more like you image from lots of different viewpoints, then calculate a slice, then join the slices to form a volume.

It's better explained visually (at least to me) :P