MR Colonography With Fecal Tagging. Barium vs. BariumFerumoxsil

The purpose of this study is to determine whether Barium or BariumFerumoxsil is better for fecal tagging in MR colonography. Patients referred to colonoscopy are offered MR colonography before colonoscopy. Two days before colonography, patients ingest either a contrast agent A (200 ml Barium sulphate solution 1g/ml)) four times a day or a 200 ml contrast agent B (Barium sulfate(25%) and Ferumoxil(75%)) four times a day, which will render fecal masses "invisible" on the following MR colonography. The patients are randomised to either contrast agent A or B. The examinations are evaluated by two independent blinded readers, who will rate the tagging quality of the contrast agents. The quality of tagging will be rated by a Visual Analog Scale (VAS) and Relative contrast (ReCon = Iwall - Ilumen/ Iwall + Ilumen).