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(Radiology. 2000;216:237-241.)
© RSNA, 2000

Genitourinary Imaging

Spinal Dysraphism at MR Urography: Initial Experience1

Michael M. Maher, MD, FRCSI, FRCR, Tripuraneni A. S. Prasad, MSc, FFR (RCSI), John M. Fitzpatrick, MCh, FRCSI, John Corr, MCh, FRCSI, David H. Williams, PhD, Joseph T. Ennis, MD, FRCR and John G. Murray, MSc, MRCPI, FRCR

1 From the Departments of Radiology (M.M.M., T.A.S.P., J.T.E., J.G.M.) and Urology (J.M.F., J.C.), Mater Misericordiae Hospital, Eccles St, Dublin 7, Ireland, and the Department of Statistics, University College, Dublin, Ireland (D.H.W.). From the 1998 RSNA scientific assembly. Received April 29, 1999; revision requested June 15; final revision received December 21; accepted November 12. Address correspondence to J.G.M.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To prospectively evaluate the role of magnetic resonance (MR) urography in the radiologic assessment of patients with spinal dysraphism.

MATERIALS AND METHODS: Fourteen patients with spinal dysraphism were referred for MR urography with half-Fourier rapid acquisition with relaxation enhancement (RARE) (repetition time msec/echo time msec = 11.9/95) and RARE (2,800/1,100) sequences on a 1.5-T MR machine. Six patients did not tolerate MR urography owing to claustrophobia (n = 4) or flexion deformities (n = 2), giving a final success rate of 57% (eight patients). Two patients had a single kidney (one after nephrectomy, one with a crossed-fused ectopic kidney). Images were jointly assessed by two radiologists and compared with excretory urographic studies. The signal intensity ratio and contrast-to-noise ratio were also calculated.

RESULTS: Visualization of the kidneys, pelvicaliceal system, and ureters was better with half-Fourier RARE than with RARE imaging, whereas visualization of the bladder was comparable with both sequences. The mean signal intensity ratios for half-Fourier RARE and RARE sequences, respectively, were 16.2 ± 3.65 and 19.2 ± 3.65 (P = .58, factorial analysis of variance model), whereas the mean contrast-to-noise ratios were 55.4 ± 5.16 and 47.8 ± 5.16 (P = .34). Cortical scarring was depicted more clearly at MR urography than at excretory urography, whereas a renal calculus seen at excretory urography was not detected at MR urography.

CONCLUSION: MR urography was feasible in 57% of patients with spinal dysraphism and is a safe, accurate method of evaluating the upper urinary tract.

Index terms: Kidney, MR, 81.121416, 81.121419 • Spine, developmental defect, 30.145 • Urography, 80.1221


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
When early neurologic complications have been treated, ultimate prognosis in patients with spinal dysraphism is dependent on the condition and function of the upper urinary tract (1,2). Thus, the care of patients with spinal dysraphism requires regular and thorough evaluation of the urinary tract to detect complications and avoid a potentially fatal outcome (1,3,4). The optimal imaging modality for regular surveillance of the urinary tract is controversial, and either excretory urography or ultrasonography (US) is used in most centers (4). Technical difficulties are frequently encountered when excretory urography is performed in patients with spinal dysraphism due to severe kyphoscoliosis, poor renal function, chronic constipation, or the presence of spinal rods. These difficulties often result in the need for repeated radiation exposures and administration of additional intravenous contrast material. Furthermore, follow-up studies result in a high cumulative radiation dose and an increased risk of contrast material–induced nephropathy. US is also difficult in spinal dysraphism due to overlying bowel gas, kyphoscoliosis, or an inability to exclude obstruction in a chronically dilated system (4).

Magnetic resonance (MR) urography has been shown to be useful in the evaluation of the urinary tract in a wide variety of clinical settings (5–10). Potentially, MR urography has a number of advantages in spinal dysraphism. MR urography permits multiplanar imaging and avoids radiation dose and the need for the intravenous contrast media associated with excretory urography. The purpose of this study was to prospectively evaluate the role of MR urography in the investigation of the urinary tract of patients with spinal dysraphism.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
This study was approved by the institutional review board of our hospital, and informed consent was obtained from each patient or his or her guardian where appropriate. The computer database of patients attending the urologic outpatient clinic was examined to identify patients with spinal dysraphism who presented with urologic problems. Fourteen patients (seven male and seven female patients; mean age, 20 years; age range, 16–30 years) were identified and were offered MR urography at their next visit. For the whole group, mean serum urea nitrogen and creatinine levels, respectively, were 4.2 mmol/L ± 0.7 (normal, 2.5–7.5 mmol/L) and 67 µmol/L ± 9.5 (normal, 45–115 µmol/L). Serum urea nitrogen and creatinine levels were normal in seven patients, and one patient had a persistent slight rise in serum urea nitrogen level to 8.4 mmol/L and a serum creatinine level of 112 µmol/L.

Six patients did not tolerate MR urography owing to claustrophobia in four and flexion deformities in two, resulting in a successful examination in eight patients (57%). Six of the eight patients had severe kyphoscoliosis, and four of the six had undergone spinal fusion with metallic spinal rods. Six of the eight patients had ventriculoperitoneal shunts. Two patients had a single kidney: One had undergone nephrectomy and one had a crossed-fused ectopic kidney (Fig 1). Thus, a total of 14 kidneys and 15 ureters were assessed in eight patients.



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  		Figure 1a. Recurrent urinary tract infections in a 20-year-old man. (a) Coronal half-Fourier RARE (11.9/95) MR image shows the kidneys and pelvicaliceal system with bilateral caliceal clubbing (arrowheads) and cortical loss (curved arrows). Ileal conduit (open arrow) is seen in the right iliac fossa. (b) Coronal RARE (2,800/1,100) MR image shows the collecting system (straight solid arrows). However, the renal parenchyma is not depicted. The large meningomyelocele (curved arrow) is clearly depicted owing to larger field coverage. Ileal conduit (open arrow) is again demonstrated. (c) Sagittal RARE (2,800/1,100) MR image shows the use of multiplanar imaging to depict the ileal conduit (open arrow) and catheter (long thin straight arrow) to the skin surface. Note also both pelvicaliceal systems (large solid straight arrows) and myelomeningocele (curved arrow).

 


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  		Figure 1b. Recurrent urinary tract infections in a 20-year-old man. (a) Coronal half-Fourier RARE (11.9/95) MR image shows the kidneys and pelvicaliceal system with bilateral caliceal clubbing (arrowheads) and cortical loss (curved arrows). Ileal conduit (open arrow) is seen in the right iliac fossa. (b) Coronal RARE (2,800/1,100) MR image shows the collecting system (straight solid arrows). However, the renal parenchyma is not depicted. The large meningomyelocele (curved arrow) is clearly depicted owing to larger field coverage. Ileal conduit (open arrow) is again demonstrated. (c) Sagittal RARE (2,800/1,100) MR image shows the use of multiplanar imaging to depict the ileal conduit (open arrow) and catheter (long thin straight arrow) to the skin surface. Note also both pelvicaliceal systems (large solid straight arrows) and myelomeningocele (curved arrow).

 


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  		Figure 1c. Recurrent urinary tract infections in a 20-year-old man. (a) Coronal half-Fourier RARE (11.9/95) MR image shows the kidneys and pelvicaliceal system with bilateral caliceal clubbing (arrowheads) and cortical loss (curved arrows). Ileal conduit (open arrow) is seen in the right iliac fossa. (b) Coronal RARE (2,800/1,100) MR image shows the collecting system (straight solid arrows). However, the renal parenchyma is not depicted. The large meningomyelocele (curved arrow) is clearly depicted owing to larger field coverage. Ileal conduit (open arrow) is again demonstrated. (c) Sagittal RARE (2,800/1,100) MR image shows the use of multiplanar imaging to depict the ileal conduit (open arrow) and catheter (long thin straight arrow) to the skin surface. Note also both pelvicaliceal systems (large solid straight arrows) and myelomeningocele (curved arrow).

 
MR Urographic Technique
Sedation was not administered to any patient before or during the MR examination. Patients underwent coronal half-Fourier rapid acquisition with relaxation enhancement (RARE) imaging (HASTE; Siemens, Erlangen, Germany) (repetition time msec/echo time msec of 11.9/95, field of view of 306 x 350, acquisition time of 23 seconds, section thickness of 6 mm) and single-slab RARE imaging (2,800/1,100, field of view of 400 x 400, acquisition time of 7.0 seconds) with a 1.5-T MR machine (Magnetom Vision; Siemens). Sagittal and oblique half-Fourier RARE and RARE images were acquired to visualize the urinary tract in patients with severe kyphoscoliosis or complex urinary tract reconstructions. Typical imaging times were 25–40 minutes, most of which was accounted for by patient positioning and reassurance.

Excretory Urographic Technique
Excretory urography was performed with use of the following protocol. Patients were asked to avoid eating for 6 hours immediately before the examination, but fluid intake was encouraged. A control radiograph was obtained with relatively low kilovoltage (65–75 kV) before the administration of intravenous contrast material. If an opacity was noted in the renal area, further radiographs (renal area in inspiration, oblique or lateral) were obtained to confirm that the opacity maintained its position within the kidney in various projections. A 50-mL bolus injection of iopromide (300 mg/mL) (Ultravist; Schering, Berlin; Germany) was then administered. In patients with poor urinary tract function or in whom image quality was poor, a second bolus was administered. Linear nephrotomography was performed in each patient with an angle of swing of between 25° and 40°. A 1-minute postinjection tomogram of the renal areas was obtained during arrested inspiration to demonstrate the nephrogram phase. Performance of nephrotomography was frequently difficult in these patients, usually owing to severe kyphoscoliosis, and this difficulty resulted in repeated nephrotomography. A mean of 2.5 nephrotomograms per patient (range, 1–4) were obtained. Abdominal compression was used when possible. Five- and 10-minute postinjection radiographs of the renal areas in arrested inspiration were obtained to demonstrate contrast material in the pelvicaliceal system. Fifteen minutes after contrast material administration, compression was released and a full-length 35 x 43-cm radiograph was acquired to demonstrate the entire urinary tract. Prone full-length radiographs were obtained where necessary to aid visualization of the lower ureters.

Image Analysis
MR urographic images were jointly reviewed by two radiologists (T.A.S.P., J.G.M.), and findings were recorded by consensus. MR urographic images were compared with the most recent excretory urograms. Images were assessed for the presence of pathologic conditions including caliceal clubbing, parenchymal scarring, urinary tract obstruction, and urinary tract calculi.

Image quality was assessed subjectively and objectively. Visualization of the renal pelvicaliceal system and the bladder was graded from 1 to 4: grade 1, excellent, entire renal pelvicaliceal system or bladder seen; grade 2, good, renal pelvicaliceal system or bladder partially seen; grade 3, poor, renal pelvicaliceal system or bladder poorly seen; grade 4, not seen, renal pelvicaliceal system or bladder not seen. Visualization of the ureters was graded on the basis of the percentage of the ureteric course visualized: grade A, 75%–100%; grade B, 50%–75%; grade C, 25%–50%; grade D, 25% or less.

Objective assessment of image quality was based on the signal intensity and contrast-to-noise ratios. The two ratios were calculated from region-of-interest measurements in the renal pelvis (SRP), and background signal measurements were calculated from the adjacent psoas muscle (SPM). Region-of-interest measurements were performed jointly by two radiologists (M.M.M., J.G.M.). Noise was defined as the SD of the pixel value of the adjacent psoas muscle. Signal intensity ratio (SIR) and contrast-to-noise ratio (CNR) in the pelvis of each kidney were calculated by placing these values in standard equations: SIR = (SRP – SPM)/SPM, and CNR = (SRP - SPM)/noise.

Data and Statistical Analysis
Data measured on a continuous scale were analyzed by using a factorial model with factors for method (half-Fourier RARE or RARE), patient (eight levels), and side of kidney (left or right). The model terms were method, patient, patient by method interaction, and side nested within patient. Mean least squares values (with SEMs) were calculated from the model and reported as were the standard error of the difference and P value. In cases in which image quality was assessed by grading, the mean grade (in the case of two kidneys) was calculated for each patient, and the differences between the means for half-Fourier RARE and RARE imaging were ranked and analyzed with a rank sum test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Subjective assessment of MR image quality in spinal dysraphism is shown in the Table. There was observer agreement in all cases. Overall, image quality was better for half-Fourier RARE than RARE images in the visualization of the kidneys and ureters but equal for visualization of the bladder. The difference between half-Fourier RARE and RARE images was not significant (P = .25, signed rank test on the difference [between half-Fourier RARE and RARE images] of the mean grade of each patient's kidneys).


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  		Image Quality for Half-Fourier RARE and RARE Images
 
Mean signal intensity ratios for half-Fourier RARE and RARE images, respectively, were 16.2 ± 3.65 and 19.2 ± 3.65 (P = .58; standard error of difference, 5.16, factorial analysis of variance model). The mean contrast-to-noise ratios for half-Fourier RARE and RARE images, respectively, were 55.4 ± 5.16 and 47.8 ± 5.16 (P = .34; standard error of difference, 7.30).

With reference to the pathologic conditions encountered, caliceal clubbing was seen in 12 kidneys at MR urography compared with three kidneys at excretory urography. Likewise, renal parenchymal scarring was detected in four kidneys at MR urography compared with in one kidney at excretory urography. There was no case of urinary tract obstruction. One renal calculus seen at excretory urography was not detected at MR urography. Bladder calculi were seen in two patients at both MR urography and excretory urography.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
To our knowledge, the first reports of MR urography appeared in the literature in 1987 (10). This technique takes advantage of the long T2 of fluid in the urinary tract collecting system. Heavily T2-weighted sequences have been used to investigate urinary tract obstruction in adults, children, and pregnant patients (5,6,10,11). MR imaging also has uses in the examination of patients with poorly functioning kidneys or malpositioned kidneys with ectopic ureters (5), and MR urography can be used in the work-up of patients before renal transplantation at the same examination as MR angiography (7). Initially, most au thors describe the use of the RARE sequence for this purpose (12–15). Recently, studies in the literature report the value of half-Fourier RARE imaging in the performance of MR urography (8,9,16). This is a very fast T2-weighted sequence with extreme sensitivity in fluid detection, and the acquisition time is on the order of 1 second per section

Overall, our experience with MR urography was encouraging. MR urography showed good image quality in the assessment of the renal pelvicaliceal system, ureters, and bladder with use of both half-Fourier RARE and RARE sequences. Subjectively, image quality was better on half-Fourier RARE than RARE images, but objectively there was no difference between the groups. This discrepancy can be explained in part by the fact that subjective assessment was based on visualization of the renal cortex and pelvicaliceal system, whereas objective assessment was based on signal intensity and contrast-to-noise ratios for the pelvicaliceal system alone (Figs 1, 2). MR urography was much more sensitive than excretory urography in the detection of caliceal clubbing and parenchymal scarring, which are important parameters in urologic surveillance. The advantages of multiplanar imaging were most obvious in patients with marked kyphoscoliosis and complex urinary tact anatomy (Fig 1).



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  		Figure 2a. Crossed-fused ectopic kidney and severe kyphoscoliosis in a 24-year-old woman. (a) Anteroposterior excretory urographic image shows poor anatomic detail particularly of the ureters. There is marked pelvicaliceal clubbing (arrows), and the renal parenchyma is poorly depicted. (b) Coronal half-Fourier RARE (11.9/95) MR image shows the anatomy of the crossed-fused ectopic kidney. Both ureters (oblique arrows) are seen entering the bladder. There is diffuse cortical loss (curved arrow) and caliceal clubbing (straight arrows). (c) Coronal RARE (2,800/1,100) MR image shows the pelvicaliceal system (large straight solid arrow), ureter (oblique arrow), and bladder (long thin solid arrow) clearly, but the renal parenchyma is not depicted. Cerebrospinal fluid (open arrow) in the spinal canal is seen owing to larger field coverage.

 


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  		Figure 2b. Crossed-fused ectopic kidney and severe kyphoscoliosis in a 24-year-old woman. (a) Anteroposterior excretory urographic image shows poor anatomic detail particularly of the ureters. There is marked pelvicaliceal clubbing (arrows), and the renal parenchyma is poorly depicted. (b) Coronal half-Fourier RARE (11.9/95) MR image shows the anatomy of the crossed-fused ectopic kidney. Both ureters (oblique arrows) are seen entering the bladder. There is diffuse cortical loss (curved arrow) and caliceal clubbing (straight arrows). (c) Coronal RARE (2,800/1,100) MR image shows the pelvicaliceal system (large straight solid arrow), ureter (oblique arrow), and bladder (long thin solid arrow) clearly, but the renal parenchyma is not depicted. Cerebrospinal fluid (open arrow) in the spinal canal is seen owing to larger field coverage.

 


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  		Figure 2c. Crossed-fused ectopic kidney and severe kyphoscoliosis in a 24-year-old woman. (a) Anteroposterior excretory urographic image shows poor anatomic detail particularly of the ureters. There is marked pelvicaliceal clubbing (arrows), and the renal parenchyma is poorly depicted. (b) Coronal half-Fourier RARE (11.9/95) MR image shows the anatomy of the crossed-fused ectopic kidney. Both ureters (oblique arrows) are seen entering the bladder. There is diffuse cortical loss (curved arrow) and caliceal clubbing (straight arrows). (c) Coronal RARE (2,800/1,100) MR image shows the pelvicaliceal system (large straight solid arrow), ureter (oblique arrow), and bladder (long thin solid arrow) clearly, but the renal parenchyma is not depicted. Cerebrospinal fluid (open arrow) in the spinal canal is seen owing to larger field coverage.

 
There were, however, some disappointing results. First, the fact that MR urography could not be tolerated in 43% of the patients who presented for MR urography was a source of disappointment. This high failure rate is partly explainable by the severe spinal and hip deformities in some patients with spinal dysraphism that prevent entry into the magnet. The high rate of claustrophobia is partially due to the higher than normal rate of mental retardation and increased rates of psychologic morbidity found in these patients (3). The use of open MR imaging machines may improve the success rate. In addition, the use of sedation in the claustrophobic patient would undoubtedly further improve this success rate.

The entire ureter was not routinely seen in this series. We believe, however, that it is probably safe to assume that when a ureter is not visualized, it is of normal caliber and thus excludes obstruction, particularly in this young patient population. The use of intravenous furosemide and compression during T2-weighted fast spin-echo MR urography can result in good visualization of the entire collecting system in as many as 100% of cases even when the system is not obstructed (6). By aiding bladder distention, furosemide would also aid bladder visualization. However, administration of intravenous furosemide has the disadvantage of making the procedure more invasive. In our series, a calculus seen at excretory urography was missed at MR urography. Previous reports have suggested that the capability of MR urography in detecting urinary tract calculi is disappointing; Regan et al reported a success rate of 66% (8). At MR urography, urinary tract stones are seen as foci of low signal intensity on half-Fourier RARE and RARE images, but resolution is not yet sufficient to confidently rule out small nonobstructing calculi (8). We concur with a prior study that recommends abdominal radiography before all MR urographic examinations in an effort to minimize the number of missed calculi (17).

In recent years, helical computed tomography (CT) without intravenous contrast material has replaced excretory urography in many centers as the imaging modality of choice in the evaluation of renal colic. Helical CT is at least as accurate as excretory urography (18–21), can depict most calculi even those that are not opaque on radiographs, obviates intravenous contrast material, and has the ability to help diagnose conditions beyond the urinary tact (21). Also, helical CT can be performed in less than 1 minute (21) and should therefore be faster than MR urography in most patients, and the more open gantry of CT may reduce the failure rate due to claustrophobia and hip deformity. All of these advantages would appear particularly beneficial in patients with spinal dysraphism. Helical CT, however, may have disadvantages in these patients. Severe kyphoscoliosis and artifact from spinal rods would make adequate evaluation of the ureters impossible. Furthermore, in the evaluation of congenital anomalies and complex postoperative cases, coronal images obtained with excretory urography or MR urography may be more beneficial than the standard transverse CT images. Without the administration of contrast material, evaluation of the renal parenchyma and collecting system may not be as effective as with MR urography with half-Fourier RARE sequences. Radiographs, although readily available, depict renal calculi in only 60% of cases (18), and this value would probably decrease in patients with spinal dysraphism. With a clinical presentation of ureteric colic, therefore, helical CT would appear to be a logical first line of investigation in patients without spinal rods. However, the value of helical CT in patients with spinal dysraphism has not been assessed, to our knowledge.

Metallic rods, which also cause problems with urinary tract visualization during excretory urographic studies, resulted in bloom artifact at MR urography. Previous studies examining methods of MR imaging of joints in the presence of orthopedic hardware have suggested that section selection, favorable orientation of frequency-encoding gradients, and application of turbo spin-echo sequences can reduce the frequency of susceptibility artifacts compared with those at conventional spin-echo imaging (22–24). The ability to obtain diagnostic studies in patients with spinal rods was due primarily to the fact that half-Fourier RARE and RARE are both turbo spin-echo sequences and to favorable section selection.

In our study, no patient with a ventriculoperitoneal shunt experienced a complication. In patients with MR-compatible ventriculoperitoneal shunts, MR imaging has been shown to be safe and effective in the investigation of complications of shunt placement particularly when ventriculitis or meningitis is suspected (25).

The use of intravenous gadolinium-based contrast material in MR urography was assessed by Nolte-Ernsting et al (17), who described their experience in 71 patients with high-spatial-resolution, gradient-echo, T1-weighted sequences after injection of a low dose of a diuretic. The results were impressive particularly in visualizing the ureters and bladder. However, this technique has disadvantages including the inability to characterize small filling defects in the ureters, considerably longer imaging times than half-Fourier RARE imaging, and the need for intravenous furosemide (17). Other authors have taken contrast material–enhanced MR urography one step further by performing MR renography to assess renal function (26). Good correlation has been reported between measurements of global glomerular filtration rates based on plasma clearance of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) and those based on conventional isotope renograms (r = 98%) (26).

In conclusion, MR urography is feasible in 57% of patients with spinal dysraphism and is more accurate than excretory urography in detecting caliceal clubbing and renal parenchymal scarring. Although MR urography is normally reserved for patients with iodine allergy and renal impairment, it shows promise in patients with spinal dysraphism.


   FOOTNOTES
 
Abbreviation: RARE = rapid acquisition with relaxation enhancement

Author contributions: Guarantors of integrity of entire study, M.M.M., T.A.S.P., J.G.M.; study concepts, M.M.M., T.A.S.P., J.C., J.M.F., J.T.E., J.G.M.; study design, M.M.M., T.A.S.P., J.G.M.; definition of intellectual content, M.M.M., J.M.F., J.T.E., J.G.M.; literature research, M.M.M., T.A.S.P., J.G.M.; clinical studies, M.M.M., T.A.S.P., J.C., J.M.F., J.T.E., J.G.M.; data acquisition and analysis, M.M.M., T.A.S.P., J.G.M.; statistical analysis, M.M.M., D.H.W.; manuscript preparation and editing, M.M.M., J.G.M., J.M.F.; manuscript review, M.M.M., T.A.S.P., J.C., J.M.F., J.T.E., J.G.M.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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