A guy I used to work for sent me the following about iVent
"You should also check out the iVent from Versamed. ICU features (PC, VC, PSV) and function, transport capability, non-invasive ventilation with battery backup, and MRI compatible. Would handle patients who had higher FiO2 and flow requirements than the pNeuton/ParaPac."...
http://www.versamed.com/products.cfmI've heard good things from other clinicians about both ParaPac and pNeuton. I've personally "played" with a pNeuton at some conferences and have been impressed. That unit operates off pneumatics, no electrical power or battery needed.
http://www.pneuton.com/I would consider if you have piped in O2 in the MRI vs. tanks, what your range of expectations for using an MRI vent in case of disaster would be. There seems to be a trade-off in that devices which use electricity use less gas, and vice versa.
If the electriciy is out but gas flow in abundance then the pNeutons and Vortrans will rule the disaster day. But if we're running off tanks, or having to ventilate mass casualties at an out of hospital disaster site (Sarin attack at a conference for instance) then hand bagging with self-inflating bags and Eagles will rule.