From the last paragraph of the article:
"MRI machines in the new hospital could affect the compass of helicopters landing at the helipad, he conceded, but by the time this occurred pilots would be flying visually and not relying on their compass. The nearest MRI is over 30 metres away from a flight path, while best practice suggests a minimum distance of 15 metres.".
If you need further explanation I'm sure there will be others who will be able to give you a full reasoning.