SYDNEY, Aug 25 (Reuters) - I-MED Radiology Network, Australia's biggest medical imaging firm, is seeking to raise A$500 million ($360 million) in an initial public offering and has hired investment banks Goldman Sachs (NYSE:GS) GS.N and Macquarie Group Ltd MQG.AX to help manage the sale, a source told Reuters. ($1 = 1.3847 Australian dollars)