Renal transit time with MR urography in children.

PURPOSE To prospectively evaluate use of dynamic contrast material-enhanced magnetic resonance (MR) urography for measurement of renal transit time (RTT) of a contrast agent through the kidney and collecting system so as to identify obstructive uropathy in children. MATERIALS AND METHODS One hundred twenty-six children suspected of having hydronephrosis were hydrated prior to undergoing both conventional and dynamic contrast-enhanced MR urography of the kidneys and urinary tract. A three-dimensional sequence was used to track passage of contrast agent through the kidneys. Time between the appearance of contrast material in the kidney and its appearance in the ureter at or below the level of the lower pole of the kidney was defined as RTT. Bland-Altman plots were used to quantify intra- and interobserver performance. In 30 children, a nuclear medicine renogram was also obtained, and the half-life of renal signal decay after furosemide administration was derived and compared with the MR imaging RTT by using receiver operating characteristic curves. RESULTS On the basis of RTT, kidneys were classified as normal (RTT </= 245 seconds), equivocal (245 seconds > RTT </= 490 seconds), or obstructed (RTT > 490 seconds). Inter- and intraobserver agreement indicated that the technique is both robust and reproducible. Receiver operating characteristic analysis for comparison of results of MR imaging and diuretic renal scintigraphy showed good agreement between the modalities, with a mean area under the curve of 0.90. CONCLUSION When used in conjunction with morphologic images obtained in the same examination, RTT generally allowed normal kidneys to be differentiated from obstructed and partially obstructed kidneys.