Imaging infrared doesn't mean that you take a snap of plane and feed it to your missile and ask it "Go buddy, find this guy in the sky and kill it". R-73 being an early example with substantial off-bore capability and later examples following which can be used with HMDs, When you look off-bore at the target missile's seeker too looks in that direction and locks-on and after launch due to extreme manueverability can turn highly off-bore and go to other direction than launching platform. Aim-9x is essentially this type, its fire-and-forget like the ones described above. It has to see the target before launch. MICA IR on the other hand is more advanced with Lock-on before Launch and Lock-on after launch capability with one way data-link.When used in conjunction with FSO, it can be devastating. Until recently, Integrated IRSTs could only be seen on MiG-29 and Su-27 with main reason being the passive detection of target without using radar, hence not being detected easily. MICA has a data-link, you can launch it and then provide targeting data from FSO.