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Published online before print July 19, 2002, 10.1148/radiol.2243011207
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(Radiology 2002;224:683-694.)
© RSNA, 2002

Pediatric Imaging

Functional and Morphologic Evaluation of Congenital Urinary Tract Dilatation by Using Combined Static-Dynamic MR Urography: Findings in Kidneys with a Single Collecting System1

Wiltrud K. Rohrschneider, MD, PhD, Sabine Haufe, MD, Manfred Wiesel, MD, PhD, Burkhard Tönshoff, MD, PhD, Rainer Wunsch, MD, Kassa Darge, MD, PhD, John H. Clorius, MD, PhD and Jochen Tröger, MD, PhD

1 From the Dept of Pediatric Radiology, Radiological Clinic, Univ of Heidelberg, Im Neuenheimer Feld 153, D-69120 Heidelberg, Germany (W.K.R., R.W., K.D., J.T.); Dept of Nuclear Medicine, Radiological Univ Clinic Heidelberg, Germany (S.H.); Div of Pediatric Urology, Dept of Urology, Univ Hospital Heidelberg, Germany (M.W.); Div of Pediatric Nephrology, Univ Children’s Hospital Heidelberg, Germany (B.T.); and Dept of Nuclear Medicine, German Cancer Research Center Heidelberg, Germany (J.H.C.). From the 2000 RSNA scientific assembly. Received Jul 16, 2001; revision requested Sept 10; final revision received Mar 7, 2002; accepted Mar 25. Address correspondence to W.K.R. (e-mail: wiltrud_rohrschneider@med.uni-heidelberg.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
REFERENCES
 
PURPOSE: To assess combined static-dynamic magnetic resonance (MR) urography in the evaluation of congenital urinary tract dilatation in infants and children.

MATERIALS AND METHODS: Sixty-two patients with urinary tract dilatation underwent prospective examination with combined static-dynamic MR urography. A combination examination involved use of a static T2-weighted three-dimensional inversion-recovery fast spin-echo sequence and a dynamic T1-weighted two-dimensional fast field-echo sequence with gadopentetate dimeglumine-DTPA and furosemide application. Twelve additional patients underwent examination with only static MR urography. Thus, both image quality and morphologic features were assessed in 74 patients with the use of MR urography. The results were compared with those of ultrasonography and, when available, conventional urography or surgery. In 62 patients, the dynamic sequence was used to calculate split renal function from renograms generated from parenchymal regions of interest and to assess urinary excretion from whole-kidney renograms. Results were compared with those of diuretic renal scintigraphy (DRS) for split function (Spearman rank correlation coefficient) and urinary excretion ({kappa} coefficient).

RESULTS: Stenoses at the ureteropelvic (n = 33) and ureterovesical (n = 31) junctions and within the ureter (n = 3) and nonstenotic dilatation (n = 23) were clearly depicted, while the normal urinary tract (n = 51) was depicted in its entirety in 47 of 51 examinations. Image quality was considered good or excellent in 95% of the kidney-ureter units. For split renal function, dynamic MR urography and DRS showed significant correlation (r = 0.92, P < .001). For urinary excretion, MR urography and DRS showed strong agreement ({kappa} = 0.67), with concordant classification of urinary excretion in 59 (81%) of 73 abnormal kidney-ureter units and in all 47 (100%) normal kidney-ureter units.

CONCLUSION: Combined static-dynamic MR urography provides high-quality depiction of the urinary tract in infants and children, while allowing accurate determination of single-kidney function and reliable evaluation of urinary excretion.

© RSNA, 2002

Index terms: Children, genitourinary system, 821.842, 825.845 • Genitourinary system, MR, 80.121413, 80.12143, 82.121413, 82.12143 • Infants, genitourinary system, 821.842, 825.845 • Kidney, function, 80.8422, 80.8432, 80.8452


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
DISCUSSION
REFERENCES
 
Patients with congenital dilatation of the urinary system need both functional and anatomic data available for management decisions. The conventional diagnostic methods for assessing dilatation of the urinary system are ultrasonography (US), diuretic renal scintigraphy (DRS), excretory urography, and voiding cystoureterography (1–5). Magnetic resonance (MR) imaging offers an alternative approach for imaging the kidneys and urinary tract. Various techniques have been reported that provide primarily morphologic information (6–19), while additional approaches exist to functionally assess the kidney by using dynamic sequences (20–31). Limited experience exists in evaluating urinary tract obstruction in pediatric patients (16,17,32–34). To our knowledge, however, no method has been described up to now that provides both quantitative determination of single-kidney function and assessment of urinary excretion analogous to DRS, in addition to offering detailed morphologic depiction. In animal studies, combined static-dynamic MR urography was found to be superior to excretory urography for the evaluation of urinary tract morphology and equal to DRS for the determination of single-kidney function and urinary excretion (35,36).

The purpose of our study was to assess combined static-dynamic MR urography in the evaluation of congenital urinary tract dilatation in infants and children.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
A prospective comparative study was undertaken over a 2-year period. Inclusion criteria were (a) unilateral or bilateral urinary tract dilatation depicted at US that required further evaluation with DRS and (b) age more than 6 weeks. Exclusion criteria were (a) unequivocal US diagnosis of a duplicated collecting system (nonduplicated kidneys permit a more reliable assessment of right-to-left organ function and urinary excretion than do duplex kidneys, in which regional differences in function and excretion may complicate interpretation of the results), (b) current urinary tract infection, and (c) no written informed consent obtained from the parents or guardians after detailed explanation of the procedure. The study protocol was approved by the ethics committee of the University of Heidelberg, and informed consent was obtained from the parents or guardians.

Of 119 consecutive patients screened with US, 29 with collecting system duplication and 16 in whom consent was not given were excluded. Seventy-four patients (47 male and 27 female patients) with a median age of 12 months (age range, 6 weeks to 15 years) were recruited. Forty-eight (65%) patients were asymptomatic, and dilatation had been detected during pre- or postnatal screening US. The remaining patients underwent US because of urinary tract infection, flank pain, urine dribbling, or postoperative follow-up. Written consent was obtained for both combined static-dynamic MR urography in 62 patients and for only static MR urography in 12 patients (thus, no dynamic imaging was performed). In the group of 62 patients (38 male and 24 female patients; median age, 11 months; age range, 6 weeks to 12 years), DRS findings served as the reference standard for functional comparison. MR urography and DRS were performed no more than 2 months apart.

In all 74 patients, a detailed evaluation of MR urography with regard to urinary tract morphology was undertaken. Since results of experimental studies demonstrated that MR urography was superior to excretory urography for depicting morphologic details (35,36), it was decided that excretory urography should be omitted in the clinical study for ethical reasons. In three patients, excretory urography had been performed previously in another hospital, and in two patients, antegrade urography had been performed previously in another hospital. Surgery was later performed in 17 patients. In 50 children 5 years of age or younger, MR urography was performed during intravenous (IV) sedation. One hour prior to MR urography, 10 mg of phenobarbital (Luminal; Desitin Arzneimittel, Hamburg, Germany) per kilogram of body weight was given as a slow IV injection. Twenty minutes prior to imaging, a loading dose of 40 mg/kg of gamma-hydroxybutyric acid (Somsanit; Franz Köhler Chemie, Alsbach-Hähnlein, Germany) was given as an IV infusion over a period of 20 minutes, followed by a maintenance dose of 10–15 mg/kg per hour during MR urography. Pulse rate and oxygen saturation were monitored by means of peripheral pulse oximetry during the entire period of sedation. The remaining patients cooperated well (12 patients between the ages of 6 and 8 years and 12 patients older than 8 years).

MR Urography
A Gyroscan T5-NT (Philips Medical Systems, Eindhoven, the Netherlands) imager with a magnetic field strength of 0.5 T was used in the examinations. The actual technique was slightly modified from the method initially developed and evaluated in experimental studies (35,36). When the size of the patient allowed it, a neck coil (high-spatial-resolution quadrature coil) was used; otherwise, we used a wrap-around coil. On the basis of experimental results (35,36), IV administration of furosemide (Lasix; Aventis Pharma, Bad Soden, Germany) (0.3 mg/kg) 20 minutes before the examination was considered useful to promote urinary tract distention to allow detailed depiction of nondilated structures. Infusion of an electrolyte solution was started with a 15 mL/kg fluid load 30 minutes prior to imaging and continued with 8 mL/kg per hour during the examination. The examination started with localizing T1-weighted images in three orthogonal planes. This was followed by a respiratory-triggered, heavily T2-weighted three-dimensional (3D) fast spin-echo (3,500/600 [repetition time msec/echo time msec], 90° flip angle) sequence (turbo factor, 100), performed in a slightly angulated coronal orientation over the kidneys, ureters, and bladder. For fat suppression, the short inversion time inversion-recovery, or STIR, method was used (inversion time, 250 msec). A 3D volume-acquisition technique was employed (56–84-mm thickness, 1.4-mm partitions). The selected field of view was 250 or 350 mm, and the rectangular field of view was 80%. Two signals were acquired, and the effective imaging time was 10–15 minutes. This was followed by a dynamic T1-weighted fast field-echo (17/4.2, 90° flip angle) sequence, performed in an angulated coronal plane of 10-mm thickness that was placed exactly through the long axis of the kidneys. Both kidneys are thus depicted in their maximal longitudinal extension, with a rim of parenchyma surrounding the central collecting system. The selected field of view was 250 or 350 mm, the rectangular field of view was 90%, and four signals were acquired. For dynamic MR urography we employed established scintigraphic techniques, including the imaging duration and the method of furosemide application (3,4). After the IV application of a bolus of 0.1 mmol/kg of gadopentetate dimeglumine-DTPA (Magnevist; Schering, Berlin, Germany), images were obtained every 10 seconds during continuous breathing over a period of 40 minutes, with additional administration of 0.5 mg/kg of furosemide 20 minutes after contrast material injection. The 40-minute period for the dynamic sequence was chosen with respect to the rate at which glomerularly filtered contrast material is excreted. Our former experimental results showed that this sequence, with a flip angle of 90°, offers an approximately proportional relationship between the gadopentetate dimeglumine-DTPA concentration in tissue and the resulting signal intensity (36). For both sequences, the image matrix was 256 x 256. The duration of the entire examination was approximately 75 minutes.

Postprocessing included reformatting of 3D volume images from the static T2-weighted sequence. This was performed by using a maximum intensity projection algorithm and a vector of interest editing technique that permits removal of superimposing structures that contain static fluids. These images depict the fluid-filled urinary tract and, less intensely, the medullary pyramids. The method allows 3D rotations, which generally consist of 12–15 volume images being processed around the longitudinal axis. In selected instances, additional transverse or sagittal planes were reformatted to optimize visualization of anatomic structures. The dynamic sequence was used to generate time-intensity curves from two different manually selected regions of interest (ROIs), copied automatically onto every image of the sequence. One ROI exactly enclosed the entire parenchyma, including the cortex and medulla (range, 123–2,943 pixels); the other ROI circumscribed the entire kidney, including tissue and the collecting system (range, 619–4,688 pixels). ROIs were placed by one author (W.K.R.). The mean signal intensity within a particular ROI for every image was plotted against time, resulting in one parenchymal renogram for the determination of single-kidney function and one whole-kidney renogram for the assessment of urinary excretion. With our system, ROI positioning and copying and renogram generation required 15–20 minutes per examination.

Diuretic Renal Scintigraphy
DRS was performed and results evaluated according to current recommendations (3,4). Scintigraphy followed IV injection of 12 µCi/kg (0.44 mBq/kg) technetium-99m MAG-3 (mercaptoacetyltriglycine), with a minimum activity of 150 µCi (5.6 MBq). A large-field-of-view gamma camera (Starcam; GE Medical Systems, United States) equipped with a low-energy all-purpose collimator was used. The window was placed over the photopeak of the tracer and was opened by 20%. A 128 x 128 image matrix was used. Data were collected in 12-second time frames. The scintigraphic examination lasted 40 minutes, and furosemide was administered 20 minutes after IV injection of the radiotracer. ROIs were manually placed around each kidney and included the tissue and the collecting system. The ROIs were placed by an experienced technician who prepares the imaging material for medical evaluation. Rectangular background ROIs near the upper and lower pole were automatically selected by the system software and manually corrected, if necessary. The area was, on average, one-fourth that of the kidney ROI and was prorated to correspond to the area of the kidney ROI. Time-activity curves were generated from the background-corrected count rates. For visual analysis, the information from five 12-second frames was summarized into 1-minute images. Children up to the age of 5 years were sedated by using rectal application of 10–15 mg/kg chloral hydrate (Chloraldurat; G. Pohl-Boskamp, Hohenlockstedt, Germany).

Morphologic Evaluation
MR urography was performed for a total of 90 abnormal and 51 normal kidney-ureter units (51 patients with unilateral dilatation, 16 patients with bilateral dilatation, and seven patients with only one kidney). The maximum intensity projection images were evaluated first. If any doubt remained and further clarification was needed, the single coronal and/or reformatted transverse and sagittal images were considered, as well. The parenchyma was assessed by using the images from the dynamic sequence. Different morphologic features were assessed with MR urography, including entirety of urinary tract depiction, localization of the level of the stenosis, and definition of the ureterovesical junction.

In addition, image quality was assessed according to the following scoring system: 1 = excellent discrimination of morphologic details, with sharp delineation of the urinary tract; 2 = good discrimination of morphologic details; 3 = sufficient discrimination of morphologic details, with faint delineation of the urinary tract; 4 = poor discrimination of morphologic details; and 5 = no depiction of the urinary tract.

In all kidney-ureter units, the morphologic findings of MR urography were correlated with the results of US. When available, additional comparison was performed with conventional urographic or surgical findings. The morphologic evaluation was performed independently by two experienced pediatric radiologists (W.K.R., J.T.) who were blinded to patient history and to the results of conventional imaging examinations and surgery.

Evaluation of Function and Urinary Excretion
In the dynamic MR urographic series, the following imaging sequence was as follows: After the vascular transit of contrast material lasted a few seconds, a peripheral cortical rim of high signal intensity was seen when the contrast material bolus arrived in the glomeruli. The subsequent homogenous increase and decrease in signal intensity within the tissue reflected the contrast material uptake from the blood and, later, its transport into the collecting system. A morphologic image of the collecting system and ureter was obtained during the excretion of contrast material. In the nonobstructed kidney, the contrast material was washed out by the time the study was completed.

The determination of split renal function was based on time-intensity curves generated from ROIs placed only over the tissue. In normal kidneys, the MR urographic renograms exhibited three typical phases, similar to scintigraphic time-activity curves: The first segment increases steeply, reflecting contrast material bolus delivery to the kidney by means of blood circulation. The second segment shows a slower, almost linear increase to a peak maximum that is normally reached at 2–4 minutes (37,38). This segment represents parenchymal transfer and continues to increase while more contrast material is extracted from the blood into the kidney than is excreted by the tissue into the collecting system. This segment is used to calculate single-kidney function with DRS (4,37) and dynamic MR urography (35,36). The third segment is characterized by a prompt decline and reflects contrast material elimination from the parenchyma into the collecting system.

Calculation of differential renal function was based on the right-to-left ratio of two parameters: (a) the second-segment slope and (b) the volume of functioning tissue. The latter parameter had to be included for the following reason: The MR urographic renograms are generated from mean values of the signal intensity of the tissue, contrary to DRS renograms, in which every point represents the sum of counts within the tissue at a given time. In MR urography, a function calculation based purely on the right-to-left ratio of the second-segment slope (as usual with scintigraphy) would provide the function per unit of tissue but would not take the mass of functioning tissue into account. For MR urographic renograms of the right and left kidney, the areas under the second curve segments were measured. Usually, the beginning and the end of the second segment occur simultaneously on both sides. When the time to the maximum differs for each kidney, the earlier maximum is used for both curves. To approximate the right-to-left ratio of the tissue volume, the parenchymal rim is measured in the midcoronal plane with a computerized planimeter. The products of the area under the second curve segment (in square millimeters) and the tissue area (in pixels or square millimeters) are calculated separately for the right and left kidneys. For each side, the ratio of this product to the sum of both products represents the single-kidney function in percentage of total renal function. An Excel spreadsheet (Microsoft, Redmond, Wash) is available for a computerized calculation of split renal function, in which width and amplitude of the area under the second curve segment and the tissue area is inserted for each organ. The spreadsheet will be provided by the author by e-mail if requested.

The scintigraphic results were evaluated for split renal function in the following manner: Single-kidney function was determined as the right-to-left ratio of the integrals under the renograms from the 24th to 120th seconds postinjection (4). Additionally, the contribution of each organ to total function was visually assessed from the serial images obtained during the first 120 seconds of the examination.

The assessment of urinary excretion with MR urography and DRS was based on time-intensity or time-activity curves (39), respectively, generated from ROIs placed over the whole kidney. The third segment of the whole-kidney renogram reflects contrast material elimination from the kidney into the urinary bladder. In normal renal drainage, the third segment of the MR urographic renogram decreases quickly and concavely to, or slightly below, the level from which the second segment starts (P level). This level therefore serves as the baseline to assess pelvic washout. Urinary excretion was assessed from the course of the excretory curve of the whole-kidney renogram, as well as from the visually determined contrast material, or tracer, washout. A kidney was classified as having relevant obstruction when (a) an accumulation curve was present or (b) the third curve segment of the renogram was not concave after furosemide application and a substantial amount of contrast material or tracer (>50%) remained within the collecting system after 20 minutes (40) and (c) the visually determined outflow from the renal pelvis was minimal or lacking. The kidney was classified as being without relevant obstruction if (a) a concave decline of at least 50% from curve maximum was present before or after furosemide application and (b) good washout was seen on the images. Kidneys without contrast material in the collecting system because of markedly reduced concentration ability were classified as having "poor or no function."

Single-kidney function and urinary excretion were determined separately by two radiologists (W.K.R., R.W.) and two nuclear medicine physicians (J.H.C., S.H.), respectively, who were blinded to the results of each other and to the results from the other modality. For each modality, a consensus diagnosis was reached between the two readers in cases of discrepancy.

Statistical Evaluation
For comparison of the two modalities with regard to kidney function, the single-kidney function of 62 abnormal kidneys (in 62 patients) as assessed with dynamic MR urography was correlated with the corresponding results from DRS (Spearman rank correlation coefficient). We defined a level of significance of P less than .01 as the rejection criterion. In patients with bilateral dilatation, only one side was randomly selected to avoid consideration of dependent values. Urinary excretion as assessed with dynamic MR urography and DRS was classified according to the above-mentioned criteria into three diagnostic categories. These data were arranged in a 3 x 3 cross table. The agreement between both methods was estimated by using the {kappa} coefficient of agreement. A {kappa} value of more than 0.6 was considered to indicate strong agreement. Seventy-seven abnormal and 47 normal kidney-ureter units (in 62 patients, 15 of whom had bilateral dilatation) were evaluated. In four abnormal kidney-ureter units, inappropriate furosemide administration or data loss compromised the evaluation of the second half of the study. Accordingly, 73 abnormal kidney-ureter units remained for the evaluation of excretion.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
REFERENCES
 
Morphologic Evaluation
With MR urography, the urinary tract was depicted entirely, including the complete ureters and ureterovesical junctions, in 44 (86%) of 51 normal kidney-ureter units and in 67 (74%) of 90 abnormal kidney-ureter units (Fig 1). Seventeen of the 23 abnormal kidney-ureter units that lacked complete depiction had ureteropelvic junction stenosis, in which only the poststenotic ureter was not seen. The main diagnoses and additional findings obtained with MR urography and the comparative results obtained with US, conventional urography, and surgery are presented in Table 1. In 75 abnormal kidney-ureter units, MR urographic findings were in accordance with US findings. In 15 abnormal kidney-ureter units, MR urographic findings were superior to US findings in the evaluation of ureteric and parenchymal abnormalities (Fig 2). In all patients in whom urography or surgery was performed, the MR urographic findings could be confirmed. Regarding the MR urographic findings, the two readers agreed in all cases. Intraluminal structures (eg, calculus and ureterocele) were delineated more easily on the sectional images (original coronal sections or other reformatted planes) than on the volume image, on which they tended to be obscured by surrounding high-intensity fluid. The image quality was scored by the two readers as being excellent in 62 (reader 1) and 65 (reader 2) of the 90 abnormal kidney-ureter units and in 36 (reader 1) and 38 (reader 2) of the 51 normal kidney-ureter units, good in 25 (reader 1) and 22 (reader 2) of the 90 abnormal kidney-ureter units and in 12 (reader 1) and nine (reader 2) of the 51 normal kidney-ureter units, sufficient in three (readers 1 and 2) of the 90 abnormal kidney-ureter units and in one (reader 1) and two (reader 2) of the 51 normal kidney-ureter units, and poor in only two (readers 1 and 2) of the 51 normal kidney-ureter units. Overall agreement was 95.7%. For interreader reliability, we calculated a {kappa} coefficient of agreement of 0.902 ± 0.039 (standard error).



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  		Figure 1. 3D volume maximum intensity projection images, reformatted from the static T2-weighted 3D inversion-recovery fast spin-echo (3,500/600/250 [inversion time msec], 90° flip angle) images obtained in a 1-year-old girl with right-sided hydroureteronephrosis due to ureterovesical junction stenosis, are displayed in three projections (left oblique, posteroanterior, and right oblique), rotated around the longitudinal axis. Bilateral orthotopic ureteral insertions are demonstrated with the oblique projections. Note depiction of the entire normal left side of the urinary tract. The medullary pyramids are also shown (arrowheads) because of their high water content.

 

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  		TABLE 1. Main MR Urographic Diagnoses and Additional Findings in 90 Abnormal Kidneys in 74 Patients Compared with Results of US, Conventional Urography, and Surgery

 


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  		Figure 2. Images obtained in an 8-month-old female infant. A, US image might suggest the presence of a duplex kidney with dilated upper collecting system. A dilated ureter is not depicted, however, and morphology remains unclear. B, MR urographic image shows fluid collection without evidence of calyces within the upper pole tissue, seen as an area of high signal intensity (arrows) in the maximum intensity projection image from the static T2-weighted 3D inversion-recovery fast spin-echo (3,500/600/250, 90° flip angle) sequence. C, Fluid collection is also seen without contrast material enhancement on a dynamic coronal T1-weighted fast field-echo (17/4.2, 90° flip angle) image. The lower portion of the fluid is surrounded by normal parenchyma belonging to the well-depicted normal collecting system. Neither any separate upper pole parenchyma nor a second ureter is demonstrated. These MR urographic findings disprove duplication but suggest a cyst. L = left.

 
Evaluation of Differential Renal Function
In all patients, the dynamic MR urographic image series was good in quality and suitable for visual image analysis (Fig 3) and resulted in the generation of good-quality renograms useful for evaluation (Fig 4). Concerning interreader reliability for the evaluation of single-kidney function, we calculated an intraclass correlation coefficient of 0.932. The single-kidney function of the abnormal kidney calculated from MR urographic images showed a close correlation with the corresponding values determined from DRS (Figs 5, 6), with a correlation coefficient of 0.92, which was highly significant (P < .001). MR urographic and DRS results diverged in only one kidney, when the MR urographic calculated function was 17% of total function, compared with 40% determined with DRS. The collecting system of this kidney was massively dilated. We later learned that the patient had experienced an upper urinary tract infection at the time of MR urographic examination.



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  		Figure 3. Left-sided hydroureteronephrosis in a 5-year-old girl. Nine MR images selected from the dynamic T1-weighted fast field-echo (17/4.2, 90° flip angle) sequence demonstrate the characteristic phases. On the normal right side, a peripheral cortical rim with high signal intensity (arrowheads in top middle image) is seen initially (glomerular arrival of the contrast material bolus). Note the subsequent homogenous signal intensity increase and decrease in the tissue, enhancement of the collecting system and ureter, and final washout. Conversely, the tissue on the left side is narrow and shows faint enhancement. Contrast material transport is delayed into the dilated collecting system and ureter, but furosemide application (two images on bottom right) leads to timely washout.

 


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  		Figure 4. Time-intensity curves generated from the dynamic T1-weighted fast field-echo (17/4.2, 90° flip angle) sequence used to obtain the images in Figure 3. A, Parenchymal renograms show three typical segments (steep increase, reflecting perfusion; slower increase to a peak maximum, reflecting kidney function; and prompt decline, reflecting contrast material elimination). To improve differentiation of the first and second curve segments, the horizontal axis is spread to 600 msec. The right-to-left-ratio of the product of the area under the second curve segment (in square millimeters) and the tissue area (in pixels) represents the relative distribution of renal function. B, Renograms obtained from ROIs placed over the whole kidney. Urinary excretion is assessed from the decline of the third curve segment. On the normal right side, this decreases quickly and concavely to baseline, from which the second segment starts (P level), indicating good washout. On the left side, the decline is delayed. After furosemide (F) administration, excretion is prompt and complete, while the third segment is concave, indicating that there is no relevant obstruction.

 


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  		Figure 5. Plot shows correlation of single-kidney function of the abnormal kidney in 62 corresponding dynamic MR urographic and DRS examinations (symbols vary according to the number of kidneys with identical values).

 


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  		Figure 6. Right-sided hydronephrosis due to ureteropelvic junction stenosis in a 12-month-old boy. A, Dynamic T1-weighted fast field-echo (17/4.2, 90° flip angle) MR urographic images depict an area of irregular tissue narrowing and reduced enhancement at the right upper pole, indicative of scarring (arrows), and reduction of single-kidney function. B, DRS images confirm the zone of functional compromise at the right upper pole (arrows), as well as the distribution of single-kidney function. The morphology is poorly visualized when compared with the MR urographic images.

 
Evaluation of Urinary Excretion
With regard to interreader reliability for the assessment of urinary excretion, we calculated a {kappa} coefficient of 0.928 ± 0.041. The evaluation of urinary excretion showed a strong agreement between MR urographic and DRS findings, with a coefficient of agreement of {kappa} = 0.67 and a standard error of 0.07. Both methods provided a concordant classification of 59 (81%) of 73 abnormal kidney-ureter units (Table 2, Fig 7), while MR urographic findings overestimated the excretion disturbance in 14 (19%) of 73 abnormal kidney-ureter units. MR urographic findings never indicated that urinary drainage was good or sufficient, while DRS findings indicated relevant obstruction (Table 2). In all 47 normal kidney-ureter units, MR urographic and DRS results indicated good urinary drainage, with a complete decline of the third segment of the whole-kidney renogram (Fig 7).


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  		TABLE 2. Comparison of DRS and MR Urography regarding Urinary Excretion from 73 Abnormal Kidneys in 62 Patients

 


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  		Figure 7. Right-sided hydroureteronephrosis due to ureterovesical junction stenosis in a 6-month-old female infant. A, 3D volume image processed from the static T2-weighted 3D inversion-recovery fast spin-echo (3,500/600/250, 90° flip angle) MR sequence, which also depicts a cyst in the normal left kidney (arrow). B, Renograms generated from the dynamic T1-weighted fast field-echo (17/4.2, 90° flip angle) MR urographic images with ROIs placed over the whole kidney demonstrate accumulation curves for the right kidney with MR urography and scintigraphy, the latter showing minimal convex curve decrease after furosemide application. This was judged as relevant obstruction with both modalities. The left kidney has prompt and complete washout.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
REFERENCES
 
Quality of Morphologic Depiction
Different T2-weighted methods for depiction of urinary tract morphology have been reported, showing various degrees of spatial resolution and image quality (6–9,11–16,18,32,33). In general, the single-shot rapid acquisition with relaxation enhancement, or RARE, and half-Fourier RARE techniques (6,8,12,13,15,16,18) offer rapid image acquisition, but both have a limited spatial resolution. Other techniques are based on gadopentetate dimeglumine-DTPA excretion and T1-weighted imaging (10,17,19). However, adequate renal function is a requirement for imaging the urinary tract with contrast material. Moreover, image quality is dependent on contrast material concentration within the urine. Massively dilated urinary systems are faintly depicted because of contrast material dilution. These disadvantages apply to excretory urography and to static T1-weighted contrast material–enhanced MR urography. This is in contrast to T2-weighted methods in which depiction of the urinary tract is influenced neither by kidney function nor by the degree of dilatation. Our technique, a 3D inversion-recovery fast spin-echo sequence, provided objective operator-independent volume images of the complete urinary tract with high spatial resolution, which is particularly important when examining infants and children. The advantages of the proposed 3D volume-acquisition technique are that the sections within the volume are contiguous and that the detected signal intensity is based on the total volume rather than on individual sections. This allows high morphologic resolution and a good signal-to-noise ratio. The possibility of using 3D rotation and multiplanar reconstruction offers additional diagnostic information. By using easily applicable software, we always remove superimposing fluid-containing structures from the images, which further improves image quality. In this study, image quality was rated as excellent or good in 95% of the kidney-ureter units, and MR urography depicted various congenital abnormalities. The visualization of the dilated urinary tract does not present a problem for most T2-weighted MR urographic techniques, but depiction of nondilated urinary pathways remains a challenge (7,15,16). In our previous experimental study (36), we were able to show that good-quality images are obtained only when distention of the urinary tract is promoted. We found stimulated diuresis, initiated with a low-dose IV furosemide application 20 minutes prior to examination, to be a practicable approach that leads to good results. In the present study, the urinary tract was depicted in its entirety, including all parts of the ureters and the ureterovesical junctions in 86% of the normal kidney-ureter units and in 74% of the abnormal kidney-ureter units. Most abnormal kidney-ureter units in which the urinary tract was not entirely depicted had ureteropelvic junction stenosis, without or with only partial depiction of the poststenotic ureter. This is probably a reflection of minimal or lacking fluid in the poststenotic ureter in high grades of obstruction. Conventional methods are clearly inferior in their ability to depict the complete urinary tract. In excretory urography, a nondilated ureter is never delineated completely, particularly when it is located distal to a stenosis. US does not depict nondilated ureters and is unable to provide a comprehensive total urinary tract image.

Accuracy of Dynamic MR Urography for the Determination of Split Renal Function and Urinary Excretion
The presented method of dynamic MR urography permits determination of split renal function in a manner similar to that used in scintigraphy. The calculation of single-kidney function demonstrated high agreement between dynamic MR urography and renal scintigraphy, resulting in a close correlation (r = 0.92, P < .001) between modalities. While several approaches were proposed to assess kidney function by using MR imaging (21,22,24,27–31,41,42), none have been used in infants and children. Most authors used T1-weighted gradient-recalled-echo sequences with short repetition and echo times and a low flip angle. Principally, an increase in signal intensity reflects the local increase of gadopentetate dimeglumine-DTPA concentration. This relationship is generally nonlinear, however, when these sequences are used. Low concentrations of gadopentetate dimeglumine-DTPA shorten T1 and T2 relaxation times (43). With increasing concentrations, T2 shortening dominates (44), which prevents a further linear increase in signal intensity. This effect is less pronounced with high flip angles (22). In addition, magnetic susceptibility effects shorten the T2 relaxation time of tissue (22). Both effects cause a characteristic decrease in signal intensity when high gadopentetate dimeglumine-DTPA concentrations are present in tissue during the medullary phase. In most former approaches, the diagnosis of reduced kidney function was based on a lack or delay of this decrease in signal intensity during contrast material passage through the kidney. Our approach differs fundamentally: For a correct quantitative determination of split renal function, it is essential that a linear relationship exists between gadopentetate dimeglumine-DTPA concentration in tissue and signal intensity. This requirement appears to be fulfilled with the proposed sequence, since we never observed an intermittent decrease in signal intensity during dynamic imaging. Furthermore, the generated renograms were comparable to those of DRS, exhibiting the typical second curve segment with a high peak maximum and concave third-segment decline. These have not been obtained with other previously described sequences. The shortest possible repetition time and a high flip angle of 90° appear necessary to achieve this approximate proportionality (36).

It is important to calculate split function only from those renograms that are generated from parenchymal ROIs, not from whole-kidney renograms. In patients with good kidney function and nondilated collecting systems, the parenchymal renograms are similar to the whole-kidney renograms. However, a dilated collecting system influences the shape of the whole-kidney renogram, even in the absence of any functional compromise for the following technical reasons: The renograms are generated from multiple signal intensity values, in which each value is used to arrive at a mean signal intensity for the selected ROI. In a whole-kidney ROI with a dilated collecting system, the early mean signal intensity values would be calculated by averaging the high tissue signal intensity values with the low signal intensity values from the nonenhanced contrast-material-free collecting system. This would decrease the mean signal intensity values used to generate the renographic curves. A dilated collecting system must therefore result in a less steep rise of the second curve segment, leading to falsely low values in calculated kidney function.

The assessment of urinary excretion with dynamic MR urography was based on the decline of the third segment of whole-kidney MR urographic renograms, according to established scintigraphic protocols. Our study was based on the F +20 technique described by O’Reilly et al (39). In the normal kidney, the third segment of the renogram shows a rapid concave decrease, starting promptly after the curve maximum, which primarily reflects the efficiency with which the agent is excreted from the collecting system (37,38). In MR urography, the curve typically falls to or only slightly below the P level, which is therefore regarded as baseline. This pattern was observed in all morphologically normal kidney-ureter units, with good washout, as confirmed with the use of DRS. In the description of urinary excretion, however, DRS and dynamic MR urography were concordant in only 81% of the abnormal kidney-ureter units. In the remaining organs, MR urographic results overestimated the excretion disturbance. This could have resulted from the different agents used for dynamic MR urography and DRS. Gadopentetate dimeglumine-DTPA is eliminated by means of glomerular filtration (45), while Tc-99m MAG-3 is predominantly excreted by means of tubular secretion (46). In chronic urinary tract obstruction, this difference should not influence the results of calculated single-kidney function (4,5). In assessing urinary excretion, however, different results may be documented because of the kidney’s more efficient extraction of Tc-99m MAG-3 (5). The decline of the third-curve segment of the renogram might be less steep with gadopentetate dimeglumine-DTPA than with Tc-99m MAG-3, particularly in massively dilated collecting systems (4,47). For both dynamic MR urography and DRS, good patient hydration is necessary to prevent false-positive results suggestive of obstruction. Therefore, we adapted our protocol to the current recommendations for DRS (3,4) and hydrated patients with IV infusion.

When compared with earlier studies on the evaluation of kidney function and urinary tract obstruction with MR imaging (24,25,31,42,48,49), the proposed approach permits a quantitative determination of single-kidney function and the differentiation between different grades of obstruction.

Advantages
1. Combined static-dynamic MR urography does not require ionizing radiation. This is particularly advantageous when a method is performed repeatedly in infants and young children. Ureteropelvic and ureterovesical junction obstruction have the potential for resolution at maturation (50–52). Patients with obvious stenosis who are treated conservatively will certainly undergo sequential follow-up examinations of kidney morphology and function, in addition to clinical status, until maturation has occurred or surgery is performed. These patients are followed up with US and scintigraphy at least every year, or even more frequently, depending on the individual situation (50–52). The younger the patient, the closer the follow-up needed. From this point of view, a method that could replace scintigraphy and excretory urography would offer some benefit.

2. Combined static-dynamic MR urography offers the combination of high-quality 3D morphologic imaging of the urinary tract with reliable information about kidney function and urinary excretion available in a single examination.

Limitations
1. The morphologic evaluation of MR urography lacks a reference standard. Most of our patients have not undergone surgery, since the tendency is toward conservative management of kidneys with maintained function. Since results of our earlier experimental studies (35,36) demonstrated that MR urography was superior to excretory urography for the depiction of morphologic details, it was decided that this comparison should not be included in the clinical study for ethical reasons. Therefore, we correlated the MR urographic findings with those of US, even if this US method has some limitations in depicting the entire urinary tract.

2. In the present study, we did not evaluate the ability to obtain reproducible results for calculated split renal function and urinary excretion, either for a single operator or for different operators. Practical experience shows that a learning curve exists for all postprocessing procedures. In particular, selection of the section plane for the dynamic sequence and correct ROI positioning, especially in the presence of motion artifacts, may be a challenge and requires experience.

3. Our MR urographic and DRS examinations were performed without a bladder catheter to minimize invasiveness and enhance acceptance by the patients’ parents. However, an overfilled bladder may inhibit urinary drainage. This was demonstrated with the use of dynamic MR urography in our experimental study (36) and is well known from experience with DRS (53). Furosemide stimulation combined with sedation are high-risk factors for bladder overfilling. Therefore, the use of a bladder catheter would improve the comparison between both methods when assessing urinary excretion.

4. Combined static-dynamic MR urography requires sedation for noncooperative patients, generally for children under 5 years of age. It remains the most expensive imaging modality when compared with US, excretory urography, and DRS. Moreover, the availability of MR imaging remains limited.

5. While the calculation of single-kidney function with DRS is based on the total volume of the kidney, the MR urographic calculation is based on a 1-cm-thick coronal section passing through the center of the kidney. This does not influence the results from calculating split function when the function distribution within the organ is homogeneous. When focal dysfunction occurs in areas outside the image plane, split renal function calculations will be compromised with a greater or lesser error. It would be highly desirable to study kidney function from whole-kidney 3D images. With our system, however, it was not possible to perform dynamic sequences with both a high temporal resolution and an acceptable morphologic resolution, while simultaneously offering a linear relationship between gadopentetate dimeglumine-DTPA concentrations and signal intensity. Moreover, placing an ROI in a 3D manner over the parenchyma of the entire kidney, while excluding the collecting system, would be a major technical problem. These problems remain a challenge for future research.

In conclusion, the proposed method of combined static-dynamic MR urography was shown to be feasible for examining pediatric patients. The normal as well as the dilated pediatric urinary tract was depicted with high-quality 3D rotational images, even when renal function was compromised. Dynamic MR urography was highly accurate in determining individual kidney function. MR urographic findings tended to overestimate urinary excretion disorders when compared with those of DRS. Two major advantages of this technique stand out: The method does not require exposure to radiation, and it offers simultaneous functional and morphologic data. Urologists and nephrologists at the University of Heidelberg have shown a high level of acceptance of combined static-dynamic MR urography.


   ACKNOWLEDGMENTS
 
We express our thanks to Klaus Rohrschneider, MD, PhD, for the extensive help with statistical evaluation, manuscript preparation, and handling of computerized data and images, and to Winfried Koch, PhD, who reviewed the statistical methods. Petra Schüller, together with all other technicians, was very helpful in optimizing the MR urographic studies. The efforts of the technicians were of enormous value for the success of this study.


   FOOTNOTES
 
Abbreviations: DRS = diuretic renal scintigraphy, MAG-3 = mercaptoacetyltriglycine, ROI = region of interest, 3D = three-dimensional

Author contributions: Guarantors of integrity of entire study, W.K.R., J.T., J.H.C.; study concepts, W.K.R., J.T., B.T., M.W.; study design, W.K.R., B.T., M.W.; literature research, W.K.R., B.T., K.D.; clinical studies, W.K.R., K.D., R.W.; data acquisition, W.K.R., K.D., R.W.; data analysis/interpretation, W.K.R., J.T., R.W.; statistical analysis, W.K.R., K.D., R.W., S.H., J.H.C.; manuscript preparation, W.K.R., K.D., B.T., J.H.C.; manuscript definition of intellectual content, W.K.R., K.D., J.T., J.H.C.; manuscript editing, W.K.R., K.D., J.T., B.T.; manuscript revision/review, B.T., W.K.R., K.D., R.W.; manuscript final version approval, all authors.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
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