If you don't want to use OSL as in [lemon's answer](http://blender.stackexchange.com/a/62141/5705) I have created a method to do this using only Cycles nodes.  (Remember OSL isn't supported on GPU.)  The downside to my method is that it requires the image sequence be entirely contained in one image, strip-style (like below).

[![enter image description here][1]][1]  

It's a little clunky having such a big image, especially considering the resolution of the files you are dealing with.  But it's the best I can come up with right now since the Image Texture node is not very flexible with image sequences.

Here is the result:

[![enter image description here][2]][2]

And the nodes:

[![enter image description here][3]][3]

It works by chopping up the image and using pixelated z-coordinates to offset the x-coordinates in a stepped manner.  The green value node is where you set the number of subimages in the whole image.

Here's the .blend so you can play around with it without having to manually recreate my node tangle. (The voxel scan is a modified version of a CC-0 one I found online somewhere.)

[<img src="http://blend-exchange.giantcowfilms.com/embedImage.png?bid=1889" />](http://blend-exchange.giantcowfilms.com/b/1889/)

  [1]: https://i.sstatic.net/2bWBY.png
  [2]: https://i.sstatic.net/fZNEl.png
  [3]: https://i.sstatic.net/yIMfA.png