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Published online before print August 18, 2004, 10.1148/radiol.2331031117
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(Radiology 2004;233:41-50.)
© RSNA, 2004

Pediatric Imaging

Renal Transit Time with MR Urography in Children1

Richard A. Jones, PhD, Marcos R. Perez-Brayfield, MD, Andrew J. Kirsch, MD and J. Damien Grattan-Smith, MBBS

1 From the Departments of Radiology (R.A.J., J.D.G.S.) and Pediatric Urology (M.R.P.B., A.J.K.), Emory University School of Medicine, Atlanta, Ga; and Departments of Radiology (R.A.J., J.D.G.S.) and Pediatric Urology (M.R.P.B., A.J.K.), Children’s Healthcare of Atlanta, 1001 Johnson Ferry Rd, Atlanta, GA 30342. Received July 18, 2003; revision requested October 2; final revision received April 6, 2004; accepted April 8. Address correspondence to R.A.J. (e-mail: richard.jones@choa.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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.

© RSNA, 2004

Index terms: Children, genitourinary system • Kidney, MR, 81.121411, 81.121412, 81.121415 • Radionuclide imaging, in infants and children, 81.12171, 81.12172, 81.12174 • Radionuclide imaging, transit studies, 81.12179 • Urography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Obstructive uropathy refers to obstruction of urine flow from the kidney to the bladder (1). In children, this obstruction is usually a result of chronic partial obstruction arising from a variety of causes, including ureteropelvic junction obstruction, ureterovesical junction obstruction, and megaureter. The consequences of the obstruction depend on the degree of obstruction and whether factors such as position or degree of hydration affect the obstruction. The ultimate goal of the treatment of obstruction is the preservation of renal function. Safe and effective interventions to correct urinary tract obstruction due to ureteropelvic junction or ureterovesical junction obstruction exist, but the necessity and timing of these interventions remain controversial (2,3). Currently, to our knowledge, there is no imaging modality that can be used to accurately assess the degree of obstruction and hence to identify which kidneys are at risk for progressive loss of renal function.

Intravenous urography was the first imaging technique used for noninvasive assessment of the kidneys and urinary tract. Intravenous urography combines anatomic imaging with qualitative information about renal function and obstruction but is now rarely performed in children. Currently, ultrasonography (US) is the most commonly performed initial examination, since it provides information on the thickness of the renal cortex, the degree of dilatation of the pelvicalyceal system, and the presence of other anomalies, such as ureteroceles. US, however, does not provide information about renal function, and for this reason, renal scintigraphy is typically used to determine whether substantial obstruction is present and whether surgical intervention may be required. Radionuclide studies can be performed either in a dynamic or a static form; with the latter, technetium 99m (99mTc)dimercaptosuccinic acid typically is used. The dynamic radionuclide studies are usually in the form of diuretic renal scintigraphy, with 99mTc–diethylenetriaminepentaacetic acid as a glomerular tracer (or 99mTc-mercaptoacetyltriglycine as a tubular tracer) and an injection of furosemide to provide a diuretic challenge to the kidney. Standardized protocols for obtaining dynamic renograms have been proposed (4,5); however, in practice, local protocols are often followed, which causes problems in the comparison of results between different centers. The parameters most commonly used to characterize the results of diuretic renal scintigraphic studies are differential renal function (DRF), which compares the areas of the two kidneys; time to peak activity; and half-life (t1/2) of renal signal decay after furosemide administration. Even the details of how these parameters are calculated, however, can affect the classification of the drainage pattern (6). Other workers have used deconvolution techniques to estimate the parenchymal transit time of the tracer (7). Despite widespread use, diuretic renal scintigraphy is not a reference standard for the diagnosis of obstruction, since the presence or absence of obstruction cannot be distinguished with this modality in at least 15% of dilated systems (8,9). A pressure perfusion study (Whitaker test) has been considered to be the reference standard for the diagnosis of obstruction. However, this invasive test provides no information about renal function, lacks objective criteria for pediatric studies, and uses nonphysiologic flow rates. Experimental studies have also indicated a poor correlation between the results of the Whitaker test and the degree of obstruction (10).

MR urography provides anatomic images of the kidneys and ureters, with excellent spatial resolution and contrast. Dynamic contrast-enhanced MR urography with, typically, high temporal resolution typically can be used to follow the passage of a contrast agent through the kidney and can provide useful information on renal function. The interpretation of the images with this modality, however, is not straightforward and often involves extensive postprocessing (11,12). We recently introduced a technique for estimating DRF with MR imaging and showed that the results obtained with this technique correlate well with those obtained by using nuclear medicine studies (13,14). Thus, the purpose of our study was to prospectively evaluate use of dynamic contrast-enhanced MR urography for measurement of the 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
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Patient Population
Between November 2001 and September 2003, 137 studies were performed in 126 children. Repeat studies were performed in nine children at 4–6 months after the initial study, and in two of these nine children, a third study was also performed. Of the 126 children examined, a total of 20 had duplicated kidneys. In these cases, the RTT was separately calculated for each part of the duplicated kidney, resulting in a total of 292 functioning renal units. Parents of children suspected of having hydronephrosis were given the option to have their children participate in the study at the time of the initial diagnosis. The study was approved by the local clinical research committee and the institutional review board. Of the 137 studies, 53 were performed in girls (average age, 4.1 years ± 4.9; range, 0.2–16.5 years) and 84 were performed in boys (average age, 3.5 years ± 4.5; range, 0.02–15.2 years). There was no significant difference in age in the male and female groups (P = .42); 45% of the patients were aged 1 year or younger at the time of the initial study and 67% were younger than 3 years. Children younger than 7 years were sedated by using the department’s standard sedation protocol. The sedative was prescribed by a physician and administered by a trained pediatric sedation nurse. The patient’s blood pressure, respiration, heart rate, and oxygen saturation were continuously monitored throughout MR imaging. Thirty of the patients also underwent diuretic renal scintigraphy at one of two imaging centers.

MR Imaging Procedure and Evaluation
All children were hydrated for 30 minutes prior to the study with an intravenous infusion of 10 mL per kilogram body weight lactated Ringer solution, and a bladder catheter was placed to ensure free drainage of the bladder during the procedure. After induction of sedation (if performed as previously mentioned), the patient was placed in a supine position on the imaging bed of a 1.5-T unit (Symphony; Siemens, Iselin, NJ) fitted with 30 mT/m gradient coils. Signal reception was via the spine coil in conjunction with a two-element phased-array coil placed anteriorly on the abdomen, and all radiofrequency transmission was via the body coil. Once the patient was positioned in the imager, scout images were acquired to determine both the positioning of the kidneys and bladder and the combination of coil elements required to optimize the signal-to-noise ratio for these anatomic structures. After the scout images were obtained, furosemide (Lasix; Aventis Pharmaceuticals, Bridgewater, NJ), 1 mg/kg with a maximum dose of 20 mg, was administered intravenously. Images were acquired with the following: flow-compensated two-dimensional T1-weighted spin-echo, flow-compensated two-dimensional T2-weighted turbo spin-echo, flow-compensated respiratory-triggered three-dimensional (3D) turbo spin-echo, and 3D heavily T1-weighted dynamic gradient-echo sequences. The Table includes parameters for these four imaging techniques. With these techniques, detailed anatomic reference images were obtained. The images obtained with the heavily T2-weighted sequence provided the basis for a precontrast maximum intensity projection (MIP) of the collecting system, ureters, and bladder (15). In order to create the MIP, other structures with long T2 relaxation times, such as cerebrospinal fluid and the gallbladder, were manually edited out of the images.


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  		Parameters for Imaging Used in Current Study

 
The acquisition of 3D dynamic gradient-echo images was started, and a bolus of 0.1 mmol/kg gadopentetate dimeglumine (Magnevist; Berlex laboratories, Wayne, NJ) was hand injected 15 minutes after the injection of furosemide to coincide with the maximal diuretic effect (16). Our protocol is similar to the F-15 protocol (furosemide administered 15 minutes before the contrast or tracer) developed for renal scintigraphy by English et al (17). With the dynamic sequence, an off-axis coronal section orientation was used and the volume was aligned along the long axis of the kidneys. The readout gradient, which was oriented superiorly-inferiorly, and oversampling were used in the two phase-encoding directions to limit wraparound artifacts. Each time point of the dynamic sequence consisted of 36 sections, with the outer three sections on each side (ie, sections 1–3 and 34–36) being discarded. An asymmetric echo position and a high sampling bandwidth were used to minimize the effect of the reduction in the transverse relaxation rate caused by the gadopentetate dimeglumine. The acquisition of each volume required 15 seconds, and for the initial 5 minutes of the dynamic study, the volumes were acquired continuously. For the subsequent 5 minutes, the same sequence was used to acquire images at 1-minute intervals. If both ureters were clearly visualized 10 minutes after the injection of contrast medium, no more dynamic images were acquired. Otherwise, the acquisition of dynamic images was continued for a further 5 minutes, with images being acquired at 1-minute intervals. For the initial studies, a flip angle of 25°, which optimizes signal-to-noise ratio, was used for the dynamic studies. For the last 89 patients, the flip angle was changed to 90°. While this reduces the signal-to-noise ratio, it produces a linear relationship between the signal intensity and the contrast agent concentration over the range of concentrations expected in the kidney (18), which facilitates the calculation of time–intensity curves. MIPs in the coronal plane were automatically generated for each of the dynamic volumes, and these were generally used for the assessment of the RTT. For some of the nonsedated patients, parallel imaging techniques were used to shorten the duration of the dynamic imaging to 8 seconds. The time between acquisition of volumes was maintained at 15 seconds, and patients were instructed to hold their breath during each acquisition and breathe during the intervals between acquisitions. This served to reduce motion artifacts otherwise seen in these images. In these patients, the images obtained at 4–15 minutes after contrast agent administration were also obtained during breath hold.

After dynamic images were obtained, sagittal and coronal 3D images with high spatial resolution were acquired for the purpose of calculating a postcontrast MIP. In cases where poor drainage from the renal pelvis meant that no contrast material was observed in the ureters during the 15 minutes of dynamic imaging, gravity-dependent pooling of the contrast agent was frequently observed. In these cases, the patient was then turned prone, and a second set of coronal and sagittal images were obtained with high spatial resolution. The total imaging time for nonobstructed kidneys was 45 minutes, and that for poorly draining kidneys was typically 1 hour.

All the images from the study were transferred to the picture archiving and communication system of the hospital. For the measurement of the RTT, a pediatric radiologist with 12 years of experience (J.D.G.S.) routinely reviewed the images. The first volume that showed clear enhancement of the cortex and the first volume to clearly show contrast material in the ureter, at or below the level of the lower pole of the kidney, were determined, and the difference in the time of acquisition between these two volumes was considered the RTT. In kidneys with poor drainage, with no contrast material observed in the ureter at the end of 15-minute dynamic imaging, the RTT was set at 900 seconds (15 minutes). While some images were obtained after 15 minutes, in these cases, a specific protocol was not followed, and the contrast material was often seen in the ureter only after the patient had been turned prone to promote mixing of the contrast material in the renal pelvis. For the intraobserver comparison, this radiologist and a urologist with 1 year of experience (M.R.P.B.) independently reviewed the images from 79 studies. The split renal function (SRF) was calculated for all patients by using a previously described method (13).

Nuclear Medicine Studies
At one of two imaging centers, diuretic renal scintigraphy was performed in 30 patients, and one of these patients had a duplicated kidney, which resulted in 61 functioning renal units. These centers used slightly different protocols for their diuretic renal scintigraphic studies, with furosemide administered at either 15 or 30 minutes after the administration of the tracer. Twenty-nine of the patients were examined with 99mTc–diethylenetriaminepentaacetic acid as the radiopharmaceutical, and the remaining patient was examined with 99mTc-mercaptoacetyltriglycine. All children were hydrated intravenously and had a bladder catheter placed for the study. Sedation was rarely required. The standard parameters to characterize the wash-in and excretion of the tracer were calculated by using manufacturer-provided software. The activity and the t1/2 of renal signal decay after furosemide administration of each kidney was categorized as being normal, equivocal, or obstructed, with normal kidneys having t1/2 of renal signal decay after furosemide administration of 10 minutes or less, or close to full excretion of the tracer prior to the administration of furosemide (19). Equivocal kidneys had a t1/2 of renal signal decay after furosemide administration of longer than 10 minutes but shorter than 20 minutes, and obstructed kidneys had a t1/2 of renal signal decay after furosemide administration of longer than 20 minutes.

Statistical Analysis
Data were expressed as the mean ± standard deviation. The intraobserver comparison and interobserver comparisons of two observers were performed with Bland-Altman analysis (20). A power analysis was not performed. The sample size for the nuclear medicine study was determined simply by counting the number of children who were referred for both studies within the specified time frame. The nuclear medicine and MR imaging data were compared by using receiver operating characteristic curves. The receiver operating characteristic analysis was complicated by the fact that each patient had two kidneys, by the use of two observers for the MR imaging data, and by the use of three categories for the reference (nuclear medicine) data (21). To simplify the receiver operating characteristic analysis, the equivocal group was excluded. An additional analysis was also performed to take account of the assumption that the two kidneys for each patient were not correlated, which is not necessarily the case. To ensure independence, the data were also sorted according to name of the patient and results with 99mTc–diethylenetriaminepentaacetic acid in order to select the first obstructed kidney for each patient or the first normal kidney for patients without obstruction. To define the normal range for RTTs without obstruction, a histogram was calculated with the RTTs for all kidneys that were clearly normal. The resulting distribution was fitted to a normal curve, and the mean and standard deviation of the distribution, values within 3 standard deviations of the mean value, were considered to define the RTT for nonobstructed kidneys.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The combination of hydration prior to the study, the short echo time of the dynamic sequence, and the administration of furosemide 15 minutes prior to administration of contrast material ensured that the concentration of gadolinium did not reach high enough levels to cause loss of signal due to very short transverse relaxation times. Images of a representative section and MIPs derived from the full volume for four time points in a patient with a ureteropelvic junction obstruction (Fig 1) illustrate the vascular and cortical phase of enhancement and uniform enhancement of the cortex and medulla at the first two time points. Images from the subsequent two time points show excretion into the calyces, renal pelvis, and ureter of the normal right kidney but no filling of the calyces in the left kidney. On subsequent images, the calyces and distended pelvis of the left kidney, both ureters, and the bladder showed enhancement.



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  		Figure 1. A-D, T1-weighted images of a coronal section obtained through anterior region of the kidneys in a 3-year-old girl with obstruction of ureteropelvic junction of left kidney. Each image is a single section from dynamic volumes acquired at four time points. E-H, MIPs derived from full volumes (30 sections) at same time points. Arrows in A and E correspond to intense cortical and vascular enhancement, respectively. B and F were acquired approximately 1 minute after cortical phase of enhancement, and arrows in B indicate that signal from both cortex and medulla was enhanced; this phase was used for calculation of DRF. C and G were obtained approximately 21/2 minutes after cortical phase. Arrows in C highlight enhancement of right collecting system, while in G, the corresponding right ureter (arrow) also is seen. D was obtained approximately 12 minutes after cortical phase. In D, the left collecting system (top arrow) and bladder (bottom arrow) are also seen. In corresponding MIP in H, both ureters (arrows) are visible.

 
With the temporal resolution used in this study, the contrast material was judged to arrive simultaneously in the renal cortex of both kidneys in all children. A probability plot (Fig 2) shows the distribution of the RTTs for all 292 functioning renal units. The spectrum of RTTs reflects the fact that the degree of obstruction can vary from mild partial to complete obstruction. The range of 0–300 seconds was used for the calculation of the histogram of normal RTTs, and values of 129.9 and 38.5 were obtained for mean and standard deviation, respectively, of the fitted normal distribution. Thus, RTTs between 14 and 245 seconds are considered to be those of nonobstructed kidneys. The remaining studies were classified into two groups on an arbitrary basis. The first of these was deemed to represent equivocal studies and the RTT ranged from greater than 245 seconds to 490 seconds or less, and the second group corresponded to patients with unequivocal obstruction (RTT > 490 seconds). The latter category also included all of the kidneys for which the RTT was set at 900 seconds because no contrast material was observed in the ureters during the 15 minutes of postcontrast dynamic imaging. For the equivocal group, the mean and standard deviation were 362 and 70 seconds, respectively. No calculations were performed for the obstructed group because the majority of RTTs in this category were truncated to 900 seconds. Bland-Altman plots summarize the results of the interobserver and intraobserver comparisons of the nontruncated RTTs (Fig 3).



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  		Figure 2. Probability plot of RTTs, as assessed by a pediatric radiologist, for all patients included in study. Curve was divided into three categories as shown with three boxes, and number of patients in each category is indicated.

 


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  		Figure 3a. Bland-Altman plots show (a, b) interobserver (with two observers [J.D.G.S. and M.R.P.B.]) and (c, d) intraobserver (with one observer [J.D.G.S.] who repeated the analysis in a randomly selected group of 30 patients) reproducibility of RTT data assessed for full nontruncated range (0-900 seconds) and normal range (0-245 seconds) of RTTs. (a) Full range of RTTs. Mean interobserver difference was –4.1 seconds, and 95% limits of agreement were ±64 seconds. (b) Normal range of RTTs. Mean interobserver difference was –2.6 seconds, and 95% limits of agreement were ±45 seconds. (c) Full range of RTTs. Mean difference was –4.8 seconds, and 95% limits of agreement were ±70 seconds. (d) Normal range of RTTs. Mean difference was –1.9 seconds, and 95% limits of agreement were ±32 seconds.

 


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  		Figure 3b. Bland-Altman plots show (a, b) interobserver (with two observers [J.D.G.S. and M.R.P.B.]) and (c, d) intraobserver (with one observer [J.D.G.S.] who repeated the analysis in a randomly selected group of 30 patients) reproducibility of RTT data assessed for full nontruncated range (0-900 seconds) and normal range (0-245 seconds) of RTTs. (a) Full range of RTTs. Mean interobserver difference was –4.1 seconds, and 95% limits of agreement were ±64 seconds. (b) Normal range of RTTs. Mean interobserver difference was –2.6 seconds, and 95% limits of agreement were ±45 seconds. (c) Full range of RTTs. Mean difference was –4.8 seconds, and 95% limits of agreement were ±70 seconds. (d) Normal range of RTTs. Mean difference was –1.9 seconds, and 95% limits of agreement were ±32 seconds.

 


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  		Figure 3c. Bland-Altman plots show (a, b) interobserver (with two observers [J.D.G.S. and M.R.P.B.]) and (c, d) intraobserver (with one observer [J.D.G.S.] who repeated the analysis in a randomly selected group of 30 patients) reproducibility of RTT data assessed for full nontruncated range (0-900 seconds) and normal range (0-245 seconds) of RTTs. (a) Full range of RTTs. Mean interobserver difference was –4.1 seconds, and 95% limits of agreement were ±64 seconds. (b) Normal range of RTTs. Mean interobserver difference was –2.6 seconds, and 95% limits of agreement were ±45 seconds. (c) Full range of RTTs. Mean difference was –4.8 seconds, and 95% limits of agreement were ±70 seconds. (d) Normal range of RTTs. Mean difference was –1.9 seconds, and 95% limits of agreement were ±32 seconds.

 


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  		Figure 3d. Bland-Altman plots show (a, b) interobserver (with two observers [J.D.G.S. and M.R.P.B.]) and (c, d) intraobserver (with one observer [J.D.G.S.] who repeated the analysis in a randomly selected group of 30 patients) reproducibility of RTT data assessed for full nontruncated range (0-900 seconds) and normal range (0-245 seconds) of RTTs. (a) Full range of RTTs. Mean interobserver difference was –4.1 seconds, and 95% limits of agreement were ±64 seconds. (b) Normal range of RTTs. Mean interobserver difference was –2.6 seconds, and 95% limits of agreement were ±45 seconds. (c) Full range of RTTs. Mean difference was –4.8 seconds, and 95% limits of agreement were ±70 seconds. (d) Normal range of RTTs. Mean difference was –1.9 seconds, and 95% limits of agreement were ±32 seconds.

 
Fluid levels in the renal pelvis due to gravity-dependent pooling of the contrast agent were observed in 35 studies; in 30 of these, kidneys were classified as obstructed by the pediatric radiologist, and in the other five, kidneys were classified in the equivocal category. On the basis of the RTT classification of obstruction, fluid levels were associated with approximately 50% of all obstructed kidneys.

For the 30 patients who underwent both nuclear medicine and MR imaging studies, one duplex kidney was excluded, leaving a total of 59 kidneys. These were classified by using the nuclear medicine t1/2 of renal signal decay after furosemide administration parameter as the reference standard. On the basis of this parameter, 28 of these kidneys were considered to be normal, 12 were equivocal, and 19 were obstructed. All 28 kidneys identified as normal with nuclear medicine studies were also identified as normal at MR imaging; 11 of the 12 equivocal kidneys were identified as normal at MR imaging by both observers, while 12 of the 19 obstructed kidneys were identified as obstructed at MR imaging. For the receiver operating characteristic analysis (with the equivocal group excluded), the area under the curve was the same for both observers (0.88, P = .41). After modification of the data to ensure independence, the areas under the curve (0.90) were very similar.

The agreement between the two MR imaging observers was perfect for the subgroup with normal results with 99mTc– diethylenetriaminepentaacetic acid; for the subgroup with obstruction, {kappa} was 0.88, which indicates excellent agreement between the two observers. Limiting the data to one record per patient but including the equivocal data and comparing the nuclear medicine data and two MR imaging observers resulted, not surprisingly, in a lower {kappa} value (0.56).

MIPs derived from the high-spatial-resolution 3D images acquired after the completion of the dynamic study are shown (Figs 4, 5) for two patients with megaureter, which would typically be consistent with obstruction. These examples illustrate the difficulties this condition can cause in interpretation of the RTT. For the patient shown in Figure 4, the RTTs were 135 and 180 seconds in right and left kidneys, respectively, and the DRF was normal with left kidney volume–right kidney volume ratios of 46:54. Hereafter, the numeric ratios, as expressed here, all refer to the left kidney volume–right kidney volume ratio. This patient received a diagnosis of dilated but not obstructed kidney. The patient shown in Figure 5 was a girl aged 2.8 years who had RTTs of 180 in the left kidney and 135 in the right kidney and a normal DRF of 45:55. This patient received a diagnosis of a normal but not obstructed kidney at MR imaging. After she was readmitted to the hospital because of pain and vomiting, a Whitaker test was performed and demonstrated an obstruction of the ureterovesical junction.



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  		Figure 4. MIP image derived from postcontrast T1-weighted 3D images in a 5-month-old girl referred for MR imaging after radiographs obtained at outside institution indicated left ureteropelvic junction obstruction. MIPs demonstrate presence of bilateral ureteral dilatation (short arrows) but with no evidence of obstruction in either kidney. Dilated calyces in left kidney (long arrow) are present. RTT and DRF were in the normal range, and overall diagnosis was dilatation without obstruction.

 


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  		Figure 5. MIP image derived from postcontrast T1-weighted 3D images in a 2.8-year-old girl. Images demonstrate presence of megaureter (short arrow), dilated calyces (long arrow), and dilated renal pelvis in the left kidney. RTT and DRF were normal, implying that no significant obstruction was associated with megaureter; subsequent Whitaker test revealed ureterovesical junction obstruction.

 
Figure 6 shows MIPs from a patient who at 3 months of age had bilateral ureteropelvic junction obstructions. Following a left-sided pyeloplasty, the patient underwent a repeat study. The MIP showed a distinct improvement in the excretion of contrast material in the left kidney, which was accompanied by an improvement in the RTT. Similarly, following a subsequent right pyeloplasty, a third image was obtained, and the MIP showed improved excretion from both kidneys.



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  		Figure 6. MIPs derived from dynamic MR urographic 3D data sets acquired approximately 6 minutes after arrival of contrast material in renal cortex in 3-month-old boy. Images correspond to three different studies in same patient. Left: Initial image shows bilateral obstruction with RTTs in excess of 900 seconds in both kidneys, and DRF was 50:50. Middle: Following a left-sided pyeloplasty, a repeat study showed improvement in RTT in left kidney to 165 seconds, with no change in RTT in the right kidney, and DRF was 56:44. Arrow shows left ureter, which was visible. Right: Subsequently, right-sided pyeloplasty was performed, and 4 months after the second study a third study was performed. RTT was 247 and 229 seconds in right and left kidneys, respectively, and DRF was 41:59. Arrows indicate two ureters. Change in the DRF between second and third studies appears to represent decompression of parenchyma following the second pyeloplasty. Deterioration in RTT between the second and third studies and the poor DRF in the left kidney following the third study may be associated with deterioration of function in that kidney. Left: Image obtained in June 2002. Middle: Image obtained in November 2002. Right: Image obtained in February 2003.

 
For a 2-year-old boy in whom the MR imaging examination showed mild dilatation of the left renal pelvis without any evidence of obstruction, results of the nuclear medicine study implied obstruction of the left kidney (Fig 7). The discrepancy between the two modalities may be due to the difference in the protocols used for the administration of furosemide (16). The case presented in Figure 8 is that of an infant who underwent nuclear medicine studies at 7 weeks of age and 7 months of age and an MR imaging examination at 31/2 months of age. The SRF was identical in both nuclear medicine studies, but the t1/2 of renal signal decay after furosemide administration and the diagnosis changed; the SRF measured with MR imaging differed from that measured with the nuclear medicine studies. The difference in the SRF between the two modalities was probably caused by the rapid excretion of contrast material in the right kidney, where the RTT was substantially less than 11/2 minutes. Since the SRF obtained with the nuclear medicine studies is routinely measured at 2 minutes after administration of contrast material, the nuclear medicine study estimate of the SRF probably included some signal from the calyces, which led to an overestimation of the volume of the right kidney. The differences in these results are unlikely to be related to the timing of the injection, since both the nuclear medicine studies were performed with the F+30 protocol (furosemide administered 30 minutes after the tracer), but temporal changes in the degree of obstruction may have occurred. However, the MR imaging data had the advantage of providing a clear depiction of the location of the obstruction.



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  		Figure 7a. Images obtained in 2-year-old boy. (a) Nuclear medicine renogram and delayed scintigram. Renogram shows delayed uptake in left kidney, while the latter depicts accumulation of the radiopharmaceutical in dilated renal pelvis of left kidney. (b) MIP derived from dynamic 3D MR urographic volume acquired 2 minutes after arrival of contrast material in renal cortex. Both ureters are clearly visible on MIP, implying that the contrast material had a rapid transit through the kidney. SRF was 54:46, and the contrast material appeared simultaneously in both ureters (arrows), with both observers recording RTTs of 101 seconds for both kidneys. The child had had an initial nuclear medicine study at 6 months of age and a follow-up study at 2 years of age; both studies were obtained with F+30 protocol. SRF for both studies was 56:44, and both studies exhibited t1/2 of renal signal decay after furosemide administration times of longer than 30 minutes for the left kidney and a normal right kidney. MR imaging was also performed at 2 years of age.

 


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  		Figure 7b. Images obtained in 2-year-old boy. (a) Nuclear medicine renogram and delayed scintigram. Renogram shows delayed uptake in left kidney, while the latter depicts accumulation of the radiopharmaceutical in dilated renal pelvis of left kidney. (b) MIP derived from dynamic 3D MR urographic volume acquired 2 minutes after arrival of contrast material in renal cortex. Both ureters are clearly visible on MIP, implying that the contrast material had a rapid transit through the kidney. SRF was 54:46, and the contrast material appeared simultaneously in both ureters (arrows), with both observers recording RTTs of 101 seconds for both kidneys. The child had had an initial nuclear medicine study at 6 months of age and a follow-up study at 2 years of age; both studies were obtained with F+30 protocol. SRF for both studies was 56:44, and both studies exhibited t1/2 of renal signal decay after furosemide administration times of longer than 30 minutes for the left kidney and a normal right kidney. MR imaging was also performed at 2 years of age.

 


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  		Figure 8a. Nuclear medicine studies were obtained with F+30 protocols at 2 and 7 months of age in this girl. At initial study, SRF was 55:45, and t1/2 of renal signal decay after furosemide administration times were 3.2 and 11.8 minutes for the left and right kidney, respectively. Some stasis of the radiopharmaceutical with the pyelocalyceal system was reported, with the effect more pronounced in the left kidney, but no significant obstruction was present. (a) Renogram from diuretic renal scintigraphy at 2 months of age shows relatively prompt excretion of radiopharmaceutical from both kidneys following the administration of furosemide. In the follow-up nuclear medicine study, the SRF was unchanged, but the t1/2 of renal signal decay after furosemide administration time for the left kidney was longer than 30 minutes, and diagnosis was left ureteropelvic junction obstruction. MR imaging was performed at 3 months of age. SRF at MR imaging was 46:54, the mean RTTs determined by both observers were 73 and 900 seconds, and the diagnosis was left ureteropelvic junction obstruction. (b) MIP derived from a dynamic 3D MR urographic volume acquired 2 minutes after the arrival of contrast material in renal cortex, and only the right ureter (arrow) was seen. (c) MIP acquired approximately 25 minutes after administration of contrast material and after the patient had been turned prone. Both ureters (arrows) were seen.

 


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  		Figure 8b. Nuclear medicine studies were obtained with F+30 protocols at 2 and 7 months of age in this girl. At initial study, SRF was 55:45, and t1/2 of renal signal decay after furosemide administration times were 3.2 and 11.8 minutes for the left and right kidney, respectively. Some stasis of the radiopharmaceutical with the pyelocalyceal system was reported, with the effect more pronounced in the left kidney, but no significant obstruction was present. (a) Renogram from diuretic renal scintigraphy at 2 months of age shows relatively prompt excretion of radiopharmaceutical from both kidneys following the administration of furosemide. In the follow-up nuclear medicine study, the SRF was unchanged, but the t1/2 of renal signal decay after furosemide administration time for the left kidney was longer than 30 minutes, and diagnosis was left ureteropelvic junction obstruction. MR imaging was performed at 3 months of age. SRF at MR imaging was 46:54, the mean RTTs determined by both observers were 73 and 900 seconds, and the diagnosis was left ureteropelvic junction obstruction. (b) MIP derived from a dynamic 3D MR urographic volume acquired 2 minutes after the arrival of contrast material in renal cortex, and only the right ureter (arrow) was seen. (c) MIP acquired approximately 25 minutes after administration of contrast material and after the patient had been turned prone. Both ureters (arrows) were seen.

 


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  		Figure 8c. Nuclear medicine studies were obtained with F+30 protocols at 2 and 7 months of age in this girl. At initial study, SRF was 55:45, and t1/2 of renal signal decay after furosemide administration times were 3.2 and 11.8 minutes for the left and right kidney, respectively. Some stasis of the radiopharmaceutical with the pyelocalyceal system was reported, with the effect more pronounced in the left kidney, but no significant obstruction was present. (a) Renogram from diuretic renal scintigraphy at 2 months of age shows relatively prompt excretion of radiopharmaceutical from both kidneys following the administration of furosemide. In the follow-up nuclear medicine study, the SRF was unchanged, but the t1/2 of renal signal decay after furosemide administration time for the left kidney was longer than 30 minutes, and diagnosis was left ureteropelvic junction obstruction. MR imaging was performed at 3 months of age. SRF at MR imaging was 46:54, the mean RTTs determined by both observers were 73 and 900 seconds, and the diagnosis was left ureteropelvic junction obstruction. (b) MIP derived from a dynamic 3D MR urographic volume acquired 2 minutes after the arrival of contrast material in renal cortex, and only the right ureter (arrow) was seen. (c) MIP acquired approximately 25 minutes after administration of contrast material and after the patient had been turned prone. Both ureters (arrows) were seen.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study extends our technique of MR urography by adding the calculation of RTTs and applying this calculation to the classification of obstructive uropathy in children. The technique can be performed by using clinical MR imagers, and the values for RTTs, when used in conjunction with high-quality morphologic images of the urinary tract obtained in the same study, permit a clear distinction to be made between normal and obstructed kidneys in most cases. The combination of an objective time criterion, the SRF, and morphologic imaging in a single imaging session is a powerful indicator for obstruction and provides a useful tool for follow-up studies.

Because of the excellent spatial and contrast resolution of contrast-enhanced MR urography, identification of excreted contrast medium in the ureters is straightforward. We chose to categorize the excretion patterns as nonobstructed, equivocal, and obstructed to correlate with typical categories described at renal scintigraphy. The RTT was well defined in children without obstruction. Inevitably, there was some overlap between the normal and equivocal categories, but these are often associated with unusual physiology or anatomy. The differentiation between the equivocal and obstructed categories causes more problems, since the term obstruction refers to a range of impairments to urinary flow, which results in a spectrum of RTTs. Children who show no enhancement of the ureter during the 15 minutes of dynamic imaging have clearly obstructed kidneys; however, refining the borderline between the categories will always cause problems.

The agreement between the two observers was very good for RTTs in the normal range, with the Bland-Altman analysis showing 95% limits of agreement of ± 45 seconds and ± 32 seconds for the inter- and intraobserver studies, respectively. For the cortical phase, the selection of the image that showed the cortical phase was straightforward; however, the temporal resolution of these studies is relatively low compared with the time required for the first passage of the blood through the cortex, such that there is some imprecision in defining this time point. At shorter RTTs, it is straightforward to select the image that depicts the initial appearance of contrast material in the ureter; however, at longer RTTs, defining which image represents the arrival of the contrast material in the ureter becomes increasingly subjective. This can decrease the precision of the definition of the RTT and cause poorer agreement between the observers. Despite these limitations, the comparison of the two observers yielded very good results (interobserver 95% limits of agreement of ± 64 seconds when using the full range of RTTs without obstruction), implying that this is not a substantial problem. The comparison of the nuclear medicine parameter, as characterized by the t1/2 of renal signal decay after furosemide administration, with the RTT yielded an excellent correlation, showing that both of these tests yield information on the response to a diuretic challenge.

The depiction of the anatomy is not, in itself, sufficient to differentiate between obstructed and nonobstructed systems (22). While the RTT appears to be a reliable tool for characterization of ureteropelvic junction obstructions, its applicability to diagnosis of ureterovesical junction obstructions is less apparent. In the case of a ureterovesical junction obstruction, the back pressure from the obstruction has to be sufficient to hinder the flow of contrast material into the ureters. In patients with megaureter, the compliance of the megaureter can lead to observance of a normal RTT, as was the case with the patient shown in Figure 5. However, it should be noted that this same mechanism also provides some degree of protection for the kidney and typically allows renal function to be preserved.

Our results imply that the degree of hydronephrosis correlates with the RTTs, with the hydronephrosis decreasing in those patients in whom the RTTs improve. Furthermore, the fact that MR imaging does not involve the use of ionizing radiation means that multiple follow-up studies can be performed to monitor either the evolution of the patient’s hydronephrosis or the effect of surgery. The improvement shown both anatomically and functionally provides a cogent argument that the hydronephrosis is resolving and that continued observation is appropriate.

The quality of functioning renal tissue can affect interpretation of obstructive uropathy with both renal scintigraphy and MR urography (23). Although MR urography is very sensitive to even small amounts of contrast agent, in the presence of both poor function and hydroureteronephrosis, there is only a gradual increase in signal, which makes the calculation of the RTT somewhat more subjective. In the absence of complete obstruction, the hydronephrotic ureter fills with contrast material, allowing visualization of the ureteric anatomy. Similarly, in a poorly functioning system with no hydronephrosis, an accurate calculation of the RTT and delineation of the collecting systems are possible.

We were able to demonstrate that MR imaging and nuclear medicine studies are equally sensitive in detection of obstruction, but in the absence of a reference test to determine whether obstruction is present, we cannot determine which test is superior. As mentioned in the introduction, even the Whitaker test, which is often considered the reference standard for detection of obstruction, has a number of problems (24) and is rarely performed because of the invasive nature of the test. At present, many clinicians think that there is no reference imaging test for the detection of obstruction (9). Rather, the diagnosis is made retrospectively by demonstrating a loss of function, or relief of symptoms, after an intervention. MR imaging is more expensive and requires sedation for a greater percentage of patients, and the frequency of adverse reactions to the contrast agent, while very low, is higher than that encountered for the nuclear medicine tracers. In spite of these problems, we believe that the superior resolution, contrast, and 3D nature of MR imaging provide it with a substantial advantage over nuclear medicine studies. The noninvasive nature of the test and the potential to provide information on renal function are substantial advantages with respect to the Whitaker test. In this article, we concentrated on the RTT and the calculation of the SRF; however, the data acquired could also be processed to yield renographic curves similar to those obtained from diuretic renal scintigraphic studies (25). Furthermore, findings of recent research showed that the data could also be processed to yield an index of glomerular filtration, which was well correlated with the values of radionuclide measurements with 99mTc–diethylenetriaminepentaacetic acid clearance (26,27). This is particularly true in assessment of the anatomy and in the calculation of the DRF, although in the absence of a reference standard, we cannot prove that MR imaging is superior for the calculation of the DRF. Several areas of current research in MR imaging, including diffusion (28,29) and blood oxygenation level–dependent (30,31) imaging of the kidney, promise to provide other methods for assessment of renal function. This diversity of information promises to make MR urography an attractive method for assessment of obstructive uropathy.


   ACKNOWLEDGMENTS
 
The authors thank Kirk A. Easley, MS, MApStat, for help with the statistical tests used in this study.


   FOOTNOTES
 
Abbreviations: DRF = differential renal function, MIP = maximum intensity projection, RTT = renal transit time, SRF = split renal function, t1/2 = half-life, 3D = three-dimensional

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, R.A.J.; study concepts and design, all authors; literature research, R.A.J., J.D.G.S.; clinical studies, all authors; data acquisition, R.A.J., J.D.G.S.; data analysis/interpretation, R.A.J., M.R.P.B., J.D.G.S.; statistical analysis, R.A.J.; manuscript preparation, definition of intellectual content, revision/review, and final version approval, all authors; manuscript editing, R.A.J.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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