Can I use my 2011 iMac as a Thunderbolt display for my PC? I have an 2011 27" iMac, it has two Thunderbolt ports. I also have an Asus Z87 Deluxe Quad motherboard which has two Thunderbolt ports. I recently got a Thunderbolt to Thunderbolt cable from Apple. 

I've seen another article on here who was able to connect the exact same motherboard to an Apple Thunderbolt Display (not the actual mac computer but the external display version) and it worked, so I know the thunderbolt port can act as a video output. 

I've enabled the iGPU on the motherboards Bios, connected it all up but nothing happens. The mac just loads as normal and so does the Windows pc. 

Am I missing a step or is it just not possible?

Any help would be greatly appreciated.
iMacs can be used in Target Display Mode, but only from another Mac.  

This is a specific support mode, Mac to Mac, it is not the same as connecting an external display to a graphics card.

See OS X Yosemite: Use another Mac as a display