MR urography: examination techniques and clinical applications

Modern MR urography is performed on the basis of two different imaging stra tegies, which can be used complementarily to cover almost all aspects in th e diagnosis of upper urinary tract diseases. The first technique utilizes u nenhanced, heavily T2-weighted pulse sequences to obtain static-fluid image s of the urinary tract, T2-weighted MR urograms have proved to be excellent in the visualization of the markedly dilated urinary tract, even if the re nal excretory function is quiescent. Static-fluid MR urography is less suit able for imaging of disorders that occur in the nondilated collecting syste m. The second MR urography technique is analogous to the methodology of con ventional intravenous pyelography and is, therefore, designated as excretor y MR urography For this purpose, a non-nephrotoxic gadolinium chelate is in travenously administered and after its renal excretion, the gadolinium-enha nced urine is visualized using fast T1-weighted gradient-echo sequences. Th e combination of gadolinium and low-dose furosemide (5-10 mg) is the key fo r achieving a uniform distribution of the contrast material inside the enti re urinary tract and, secondly, to avoid high endoluminal gadolinium concen trations, which cause signal loss of the urine due to T2* effects. Gadolini um excretory MR urography allows to obtain high-quality images of both nond ilated and obstructed urinary tracts in patients with normal or moderately impaired renal function. This article reviews the principles of T2- and T1- weighted MR urography in detail and informs how to use these techniques saf ely in potential clinical applications such as chronic urolithiasis, intrin sic and extrinsic tumor diseases, and congenital anomalies. Magnetic resona nce urography performed in combination with standard MR imaging offers a po tential to reduce the need for invasive retrograde pyelography. Although th e economic aspect is still problematic, it is obvious that MR urography wil l continue to increase its role in clinical uroradiology.