Patients will be blindly randomized by computer to either placebo (0.45% saline) or high dose (6000 mg) iv n-acétylcystéine (flumicil) iv diluted in 0.45% saline. All patients will receive intravenous hydration (250 ml NaCl 0.45% at least) before the CT scan and 1000 ml after the examination depending on clinical possibilities.