Gadopentetic acid as an oral negative gastrointestinal contrast agent for magnetic resonance urography Study on MR urography and intravenous urography in diagnosis of urologic obstructive diseases 口服钆喷酸葡胺溶液行磁共振泌尿系水成像的临床应用磁共振尿路水成像与静脉尿路造影在泌尿系梗阻性疾病的对比研究