First published online April 26, 2006
British Journal of Radiology (2006) 79, 614-626
© 2006 British Institute of Radiology
doi: 10.1259/bjr/21075982
Magnetic resonance urography: a pictorial overview
R García-Valtuille, MD
1
A I García-Valtuille, MD
2
F Abascal, MD
1
L Cerezal, MD
1 and
M C Argüello, MD
3
1 Instituto Radiológico Cántabro, Clínica Mompía, Avenida de los Condes, s/n. 39108 Santa Cruz de Bezana (Cantabria), 2 Department of Pathology, Clínica Mompía, Santa Cruz de Bezana (Cantabria), 3 Department of Oncology, Clínica Mompía, Santa Cruz de Bezana (Cantabria), Spain
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Abstract
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Magnetic resonance urography (MRU) can be performed on the basis of two different imaging strategies: static-fluid MRU, based on heavily T2 weighted turbo spin echo (TSE) sequences, and gadolinium-enhanced excretory MRU. Both MR urographic techniques in combination with standard MRI permit a comprehensive examination of the entire urinary tract. This pictorial review illustrates the MRU features of the a wide spectrum of pathological conditions affecting the urinary tract.
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Introduction
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Magnetic resonance urography (MRU) is an emerging technique of MRI which provides a non-invasive visualization of urinary tract. Most of previous studies have used the unenhanced, heavily T2 weighted pulse sequences to obtain images in which static fluid exhibits a higher signal intensity relative to background (static MRU) [1–4]. Clinical urography requires both morphological and functional information about the kidneys and the collecting system. However, these sequences do not provide information about the renal excretory function. MRU performed with contrast material can meet all demands of clinical urography and, in some cases, could replace conventional X-ray urography [1, 4, 5].
This pictorial essay reviews the MRU features of the major urinary tract disorders in which static or excretory MRU provides information of diagnostic value.
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Technique
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The images were acquired by a 1 T superconducting magnet (New technology [NT] Gyroscan; Philips Medical Systems, Best, The Netherlands) using a body coil. The MR sequence protocol started with localizing T1 weighted gradient-echo sequence (repetition time (TR) 18 ms/echo time (TE) 6.9 ms; flip angle 30°; scan time 3 min 30 s) and T2 weighted turbo spin echo (TSE) sequence (TR 4200 ms/TE 100 ms; flip angle 90°; scan time 3 min 12 s) in axial and coronal planes.
In static MRU, heavily T2 weighted TSE pulse sequences are used to obtain water images of the urinary tract (three-dimensional; respiratory-triggering; TR 2000 ms/TE 700 ms; flip angle 90°; TSE-factor 101; matrix size 256x256; number of excitations 2; field of view (FOV) 360–390 mm; number of slices 40–50; slice thickness 2 mm; scan time 3 min 30 s to 4 min 20 s).
Before the acquisition of the excretory MR urographic sequences, the patients received an intravenous dose of 0.1 mg kg–1 of furosemide and 0.1 mmol kg–1 of GdDTPA-BMA (Gadodiamide). A delay of 1–5 min between the administration of both drugs is necessary for achieving optimal contrast enhancement of the urinary tract. Excretory MRU was performed at our institution using a respiratory gating, three-dimensional, T1 weighted gradient-echo sequence (TR 15 ms/TE 5 ms; flip angle 70°; matrix size 256x256; number of excitations 2; FOV 360–390 mm; scan time 3 min) with an anteriorly located pre-saturation slab. 60 sections, 2.2 mm thick, were obtained in coronal plane 5 min, 10 min and 20 min after diuretic and contrast material injection. In selected cases, additional transverse planes were performed to optimize visualization of anatomic structures.
For the examination of children, we reduce the FOV of the sequences and adjust furosemide and gadolinium dosages (0.05 mg kg–1 of furosemide and 0.05 mmol kg–1 of gadolinium).
The source images of static and excretory MRU were then post-processed by the use of a maximum intensity projection (MIP) algorithm.
When no dilatation of the urinary tract is visible on the initial T2 weighted images we use excretory MRU. With the use of a diuretic in MRU within the dose range of 4–10 mg of furosemide, the induced distention of the urinary tract was mild and did not result in false-positive diagnosis of substantial dilatation. In patients with mild dilatation, both techniques (static and excretory MRU) are employed. In cases of marked dilatation of the urinary tract and impaired excretory function, static MRU is used. Static MRU is also used for the visualization of urinary tract disorders in women during pregnancy (Figures 1
and 2) [5–7].
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Figure 1. A 52-year-old woman with an extrinsic ureteral obstruction caused by a metastasis of an ovarian carcinoma. (a) Maximum intensity projection (MIP) image from an unenhanced T2 weighted MR urograph (MRU) shows a left ureteral obstruction (arrow). Note the changes of chronic hydronephrosis and hydroureter. (b) The axial standard T2 weighted turbo spin echo (TSE) image visualizes a soft tissue mass with heterogeneous signal intensity surrounding the ureter (arrowheads).
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Figure 2. Staghorn calculus and chronic hydronephrosis in a 32-year-old pregnant patient. (a) Coronal T2 weighted turbo spin echo (TSE) image and (b) urogram from static MR urography show diffuse cortical atrophy, pyelocaliectasis and a voluminous pyelocaliceal filling defect (arrows). Note also the gestational sac (arrowheads) and a left corpus luteum cyst (white arrow).
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Normal variants and congenital anomalies
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The main indications in children of MRU are congenital anomalies of the kidneys and collecting system [6]. Normal variants and congenital anomalies of the collecting system can be accurately identified with this technique [5]. Knowledge of the myriad appearances of congenital renal and collecting system anomalies and minor anatomic variants is essential for the correct interpretation of urograms.
Congenital ureteropelvic junction (UPJ) obstruction is sharply defined UPJ narrowing with dilatation of the pelvocalyceal system, which persists even when patient is placed in a position favouring gravity drainage of the pelvis (Figure 3) [1]. Large extrarenal pelves may simulate hydronephrosis when they are stressed by diuresis.
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Figure 3. A 20-year-old man with ureteropelvic junction narrowing (arrow). Coronal maximum intensity projection (MIP) excretory MR urography. The renal pelvis shows typical dilatation and convex inferior border.
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MRU can also accurately detect complete and incomplete ureteral duplication by locating the level of fusion. In cases of complete duplication, the insertion of the superior collecting system is usually ectopic [8].
Common congenital anomalies of the fusion variety have characteristic MR appearances. True congenital hypoplasia is distinctly rare or very difficult to document. Hypoplastic kidneys usually are caused by trauma, infection or ischaemic or obstructive insult during the growth phase. Renal agenesis with contralateral solitary kidney usually associates with Müllerian duct abnormalities (Figure 4) [1].
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Figure 4. Left renal agenesia in a 14-year-old woman with didelphic uterus and a vaginal septum. Maximum intensity projection (MIP) image from (a) excretory MR urography demonstrates a normal right kidney with no evidence of left renal tissue. (b) Axial and (c) coronal T2 weighted turbo spin echo (TSE) images show a bicornuate uterus (arrows) with two cervix (arrowheads).
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Filling defects in the ureter or in the pelvocalyceal system
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Filling defects are demonstrated on MRU as signal-void areas outlined by the hyperintense surrounding urine, except when they are impacted or filling the entire lumen of ureter. We sometimes perform complementary axial images because the small filling defects are better visualized in this plane. Instead of MIP images, the source images must always be reviewed because small defects may be obscured by the surrounding urine on MIP projections [5].
The acute stone colic should not be a primary indication for MRU. However, it is important to be aware of the findings of stones in MRU because most common filling defects are the calculi (Figures 2
and 5); round or oval filling defects that tend to become impacted in areas of normal anatomic narrowing – ureteropelvic and ureterovesical junctions, and the site where the ureter crosses the sacrum and the iliac vessels – and cause a variable degree of dilatation of the urinary tract [1].
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Figure 5. A 55-year-old man with left-sided ureteral stone. (a) Coronal maximum intensity projection (MIP) excretory MR urograph shows filling defect (arrow) in left distal ureter that is causing mild pyeloureterectasis. (b) Enhanced axial T1 weighted gradient-echo image shows a round dependent filling defect (arrow) in left ureter.
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Blood clots are single or multiple filling defects of various sizes and shapes that may cause temporary ureteral obstruction (Figure 6). They are usually hyperintense on T1 weighted MR images, do not enhance with gadolinium and become much smaller or disappear within several weeks [8].
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Figure 6. A 52-year-old woman with temporary ureteral obstruction caused by blood clots. (a) Coronal maximum intensity projection (MIP) excretory MR urography and (b) complementary retrograde pyelography show complete proximal ureteral obstruction (arrow) and mild dilatation of the collecting system. (c) Axial gadolinium-enhanced T1 weighted gradient-echo and (d) T2-weighted turbo spin echo (TSE) images demonstrate hypointense tissue filling completely a mildly dilated ureter (arrowhead). (e) After several days, excretory urogram from conventional intravenous pyelography demonstrates patency of previously occluded ureter (arrowheads).
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Transitional cell carcinoma appears as smooth or irregular shaggy filling defects (Figure 7). The signal intensity of transitional cell carcinoma usually differs sufficiently from that of other causes of ureteral filling defects, on conventional T1 and T2 weighted images, to suggest the diagnosis. There is often localized dilatation of the ureter below the level of the expanding intraluminal tumour, in contrast to ureteral collapse distal to an obstructing stone. The "globet sign" and the "sipple sign" are also useful in differentiation with other entities. However, the morphological differentiation between a small calculus and a small early intrinsic tumour is difficult in some cases, especially if the clinical symptoms are non-specific [1, 5, 8].
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Figure 7. Transitional cell carcinoma of the midureter in a 68-year-old man. (a) Coronal maximum intensity projection (MIP) and (b) source images from excretory MR urography demonstrate a large mass inside the midureter (arrows) with proximal ureteral and pelvocalyceal dilatation. (c) The axial standard T2 weighted turbo spin echo (TSE) sequence confirms the diagnosis by demonstrating a soft-tissue mass (arrowhead) with heterogeneous signal intensity.
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Mimickers of filling defects are flow artefacts (usually with central location within the ureter) [9], vessels that can cause an extrinsic impression on the ureter (Figure 8), and ureteral spasm and peristalsis.
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Figure 8. A 73-year-old woman with mild narrowing of the midureter (arrow) caused by left common iliac artery. (a) Coronal maximum intensity projection (MIP) image from excretory MR urography and (b) composite coronal MIPs of both urogram an MR angiography.
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Obstruction of the ureter
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The differential diagnosis of ureteral obstruction include intraluminal (calculi, blood clots, papillary necrosis with sloughed papilla), intramural (tumour, infection diseases, post-surgery/instrumentation trauma, lesions after radiotherapy, ureterocele, megaureter) and extrinsic abnormalities (retroperitoneal fibrosis, invasion or compression by extrinsic malignancy, lymphadenopathy, inflammatory diseases) [1, 3, 8].
MRU allows the precise depiction of the site of the obstruction and the degree of ureterectasis, and may demonstrate the underlying pathology with the help of conventional T1 and T2 weighted sequences (Figures 1, 6 and 9).
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Figure 9. A 77-year-old man with transitional cell carcinoma of the right ureter. (a) Maximum intensity projection (MIP) image from excretory MR urography (MRU) demonstrates right ureteral obstruction (arrow), hydronephrosis and hydroureter. (b) Original source image from excretory MRU shows a large hypointense filling defect inside distal ureter (arrows). (c) Axial T1 weighted image shows a hypointense soft-tissue mass (arrowhead) in the pelvis. (d) An area of subtle enhancement (arrowhead) is demonstrated on the axial section of a contrast-enhanced T1 weighted sequence.
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Filling defects in the urinary bladder
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MRU provides a non-invasive mean to detect filling defects in the urinary bladder – calculus, blood clot, air bubble, neoplasm, prostatic enlargement, ureterocele or foreign body [5].
The transitional cell carcinoma of the urinary bladder is a single or multiple polypoid defect that arises from the bladder wall and is fixed in position – unlike a calculus, blood clot or air. Sometimes they may produce only focal bladder wall thickening and rigidity (Figure 10).
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Figure 10. A 61-year-old man with transitional cell carcinoma of the bladder. (a) Axial T2 weighted turbo spin echo (TSE) image shows an irregular wall thickening at the left-side of the bladder (arrows). (b) Maximum intensity projection (MIP) image from excretory MR urography confirms large irregular filling defect (arrows) on the floor and left-sided wall of the bladder. The tumour does not produce obstruction at the ureterovesical junction.
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Prostatic enlargement causes a smooth or irregular extrinsic filling defect of varying size at the base of the bladder (Figure 11). If a chronic process, there is trabeculation of the bladder wall and diverticula formation. The distal ureters often have a fishhook deformity due to elevation of the trigone.
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Figure 11. An excretory MR urograph in a 78-year-old man with benign prostatic hypertrophy. Large, smooth filling defect at the base of the bladder (arrowheads).
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Post-operative changes
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The role of MRU in patients undergoing urinary diversion (ureteroileal by-passes, ureterosigmoidostomy, skin ureterostomy, orthotopic neobladder reconstruction) or after renal transplantation is emerging. MRU allows visualization of anastomoses, as well as of associated complications such as strictures (Figure 12), ureteral compression by lymphocele or haematoma, urine leaks, fistulae (Figure 13), stones or signs of infection. Signal-void within urinary tract in post-operative patients does not always correspond to stones, but may be due to air bubbles or susceptibility artefacts caused by surgical material [1, 10, 11].
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Figure 12. A 56-year-old woman with ileal loop urinary diversion. Maximum intensity projection (MIP) image from excretory MR urography shows the post-operative urinary tract anatomy. Both sides are dilated because of stenosis (arrowheads) close to the ureteroenteric implantation site.
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Figure 13. Vesicovaginal fistula(arrowheads) formation caused by inadvertent injury to the bladder during surgery in a 48-year-old woman. Sagittal maximum intensity projection (MIP) excretory MR urography.
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Conclusions
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Static and excretory MRU are complementary methods for morphological and functional evaluation of the urinary system, which can be alternatively employed according to the degree of urinary tract dilatation and renal function. These techniques have some advantages over ultrasound, conventional urography and CT urography in the diagnosis of urological diseases. The three-dimensional nature of the data permits reformation into any plane, and thus virtually eliminates the potential of projection related errors in the diagnosis of different pathological conditions. There are also the safety advantages of eliminating ionizing radiation and the risk of medical complications due to iodinated contrast agents, and is even suitable for assessing transplanted kidneys because of the low nephrotoxicity of gadolinium.
MRU, due to its non-use of ionizing radiation, is the most important tool in the diagnostic work-up of genitourinary pathologies in infants, small children and in women during pregnancy.
The major drawback of MRU is its low sensitivity in detecting calcifications and subtle urothelial lesions, the latter due to the reduced spatial resolution compared with conventional excretory urography. However, MRU can be offered as an alternative to conventional urography and CT urography to avoid repetitive radiation exposure in patients with chronic urolithiasis.
In conclusion, static and contrast-enhanced excretory MRU provide high-quality imaging of the urinary tract and are an accurate and safe diagnostic alternative to other urological diagnostic procedures. These techniques, combined with conventional MR images, functional MR sequences or MR angiography, in a single session yields a rapid and complete diagnostic evaluation of the entire urinary tract, and have the potential to provide the same information as can be obtained with multiple separate diagnostic studies.
Received for publication February 6, 2005.
Revision received May 10, 2005.
Accepted for publication May 23, 2005.
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Abstract
Introduction
