Get help, begin rescue-breathing and chest compressions, make sure the crash team is called and get them out of the inner controlled area.
The resuscitation equipment used in the event of cardiac arrest can be extremely dangerous in an MR environment, and should never be taken into the scanning room. In the event of an cardiac arrest, resuscitation should begin immediately, and the crash team should be called as usual, but priority then moves to getting the patient out of the scan room so that resuscitation can occur without risk of further disaster due to unscreened medical personnel or equipment entering the scan room. An MR compatible patient trolley should be kept in the scanning room to enable swift evacuation of the patient.
Further reading on this topic:
Books: MHRA Guidelines p93-94