Mr Urography Bjr

PICTORIAL REVIEW

Magnetic resonance urography: a pictorial overview

1R GARCIA-VALTUILLE, MD, 2A I GARCIA-VALTUILLE, MD, 1F ABASCAL, MD, 1L CEREZAL, MD and3M C ARGUELLO, MD

1Instituto Radiologico Cantabro, Clınica Mompıa, Avenida de los Condes, s/n. 39108 Santa Cruz deBezana (Cantabria), 2Department of Pathology, Clınica Mompıa, Santa Cruz de Bezana (Cantabria)and 3Department of Oncology, Clınica Mompıa, Santa Cruz de Bezana (Cantabria), Spain

ABSTRACT. Magnetic resonance urography (MRU) can be performed on the basis oftwo different imaging strategies: static-fluid MRU, based on heavily T2 weighted turbospin echo (TSE) sequences, and gadolinium-enhanced excretory MRU. Both MRurographic techniques in combination with standard MRI permit a comprehensiveexamination of the entire urinary tract. This pictorial review illustrates the MRUfeatures of the a wide spectrum of pathological conditions affecting the urinary tract.

Received 6 February 2005Revised 10 May 2005Accepted 23 May 2005

DOI: 10.1259/bjr/21075982

’ 2006 The British Institute of

Radiology

Introduction

Magnetic resonance urography (MRU) is an emergingtechnique of MRI which provides a non-invasivevisualization of urinary tract. Most of previous studieshave used the unenhanced, heavily T2 weighted pulsesequences to obtain images in which static fluid exhibitsa higher signal intensity relative to background (staticMRU) [1–4]. Clinical urography requires both morpho-logical and functional information about the kidneys andthe collecting system. However, these sequences do notprovide information about the renal excretory function.MRU performed with contrast material can meet alldemands of clinical urography and, in some cases, couldreplace conventional X-ray urography [1, 4, 5].

This pictorial essay reviews the MRU features of themajor urinary tract disorders in which static or excretoryMRU provides information of diagnostic value.

Technique

The images were acquired by a 1 T superconductingmagnet (New technology [NT] Gyroscan; PhilipsMedical Systems, Best, The Netherlands) using a bodycoil. 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 time3 min 30 s) and T2 weighted turbo spin echo (TSE)sequence (TR 4200 ms/TE 100 ms; flip angle 90 ; scantime 3 min 12 s) in axial and coronal planes.

In static MRU, heavily T2 weighted TSE pulsesequences are used to obtain water images of the urinarytract (three-dimensional; respiratory-triggering; TR2000 ms/TE 700 ms; flip angle 90 ; TSE-factor 101;matrix size 2566256; number of excitations 2; field ofview (FOV) 360–390 mm; number of slices 40–50; slicethickness 2 mm; scan time 3 min 30 s to 4 min 20 s).

Before the acquisition of the excretory MR urographicsequences, the patients received an intravenous dose of0.1 mg kg21 of furosemide and 0.1 mmol kg21 ofGdDTPA-BMA (Gadodiamide). A delay of 1–5 minbetween the administration of both drugs is necessaryfor achieving optimal contrast enhancement of theurinary tract. Excretory MRU was performed at ourinstitution using a respiratory gating, three-dimensional,T1 weighted gradient-echo sequence (TR 15 ms/TE 5 ms;flip angle 70 ; matrix size 2566256; number of excita-tions 2; FOV 360–390 mm; scan time 3 min) with ananteriorly 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 materialinjection. In selected cases, additional transverse planeswere performed to optimize visualization of anatomicstructures.

For the examination of children, we reduce the FOV ofthe sequences and adjust furosemide and gadoliniumdosages (0.05 mg kg21 of furosemide and 0.05 mmol kg21

of gadolinium).The source images of static and excretory MRU were

then post-processed by the use of a maximum intensityprojection (MIP) algorithm.

When no dilatation of the urinary tract is visible onthe initial T2 weighted images we use excretoryMRU. With the use of a diuretic in MRU within thedose range of 4–10 mg of furosemide, the induceddistention of the urinary tract was mild and did notresult in false-positive diagnosis of substantialdilatation. In patients with mild dilatation, both techni-ques (static and excretory MRU) are employed. Incases of marked dilatation of the urinary tract andimpaired excretory function, static MRU is used. StaticMRU is also used for the visualization of urinary tractdisorders in women during pregnancy (Figures 1 and 2)[5–7].

The British Journal of Radiology, 79 (2006), 614–626

614 The British Journal of Radiology, July 2006

Normal variants and congenital anomalies

The main indications in children of MRU are con-genital anomalies of the kidneys and collecting system[6]. Normal variants and congenital anomalies of thecollecting system can be accurately identified with thistechnique [5]. Knowledge of the myriad appearances ofcongenital renal and collecting system anomalies andminor anatomic variants is essential for the correctinterpretation of urograms.

Congenital ureteropelvic junction (UPJ) obstruction issharply defined UPJ narrowing with dilatation of thepelvocalyceal system, which persists even when patientis placed in a position favouring gravity drainage of thepelvis (Figure 3) [1]. Large extrarenal pelves maysimulate hydronephrosis when they are stressed bydiuresis.

MRU can also accurately detect complete and incom-plete ureteral duplication by locating the level of fusion.

In cases of complete duplication, the insertion of thesuperior collecting system is usually ectopic [8].

Common congenital anomalies of the fusion varietyhave characteristic MR appearances. True congenitalhypoplasia is distinctly rare or very difficult to docu-ment. Hypoplastic kidneys usually are caused bytrauma, infection or ischaemic or obstructive insultduring the growth phase. Renal agenesis with contra-lateral solitary kidney usually associates with Mullerianduct abnormalities (Figure 4) [1].

Filling defects in the ureter or in thepelvocalyceal system

Filling defects are demonstrated on MRU as signal-void areas outlined by the hyperintense surroundingurine, except when they are impacted or filling the entirelumen of ureter. We sometimes perform complementary

(a) (b)

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 ureteralobstruction (arrow). Note the changes of chronic hydronephrosis and hydroureter. (b) The axial standard T2 weighted turbo spinecho (TSE) image visualizes a soft tissue mass with heterogeneous signal intensity surrounding the ureter (arrowheads).

Pictorial review: MR urography

The British Journal of Radiology, July 2006 615

(a) (b)

Figure 2. Staghorn calculus and chronic hydronephrosis in a 32-year-old pregnant patient. (a) Coronal T2 weighted turbo spinecho (TSE) image and (b) urogram from static MR urography show diffuse cortical atrophy, pyelocaliectasis and a voluminouspyelocaliceal filling defect (arrows). Note also the gestational sac (arrowheads) and a left corpus luteum cyst (white arrow).

R Garcıa-Valtuille, A I Garcıa-Valtuille, F Abascal et al


axial images because the small filling defects are bettervisualized in this plane. Instead of MIP images, thesource images must always be reviewed because smalldefects may be obscured by the surrounding urine onMIP projections [5].

The acute stone colic should not be a primaryindication for MRU. However, it is important to beaware of the findings of stones in MRU because mostcommon filling defects are the calculi (Figures 2 and 5);round or oval filling defects that tend to becomeimpacted in areas of normal anatomic narrowing –ureteropelvic and ureterovesical junctions, and the sitewhere the ureter crosses the sacrum and the iliac vessels– and cause a variable degree of dilatation of the urinarytract [1].

Blood clots are single or multiple filling defects ofvarious sizes and shapes that may cause temporaryureteral obstruction (Figure 6). They are usually hyper-intense on T1 weighted MR images, do not enhance withgadolinium and become much smaller or disappearwithin several weeks [8].

Transitional cell carcinoma appears as smooth orirregular shaggy filling defects (Figure 7). The signalintensity of transitional cell carcinoma usually differssufficiently from that of other causes of ureteral fillingdefects, on conventional T1 and T2 weighted images, tosuggest the diagnosis. There is often localized dilatationof the ureter below the level of the expanding intralum-inal tumour, in contrast to ureteral collapse distal to an

obstructing stone. The ‘‘globet sign’’ and the ‘‘sipplesign’’ are also useful in differentiation with other entities.However, the morphological differentiation between asmall calculus and a small early intrinsic tumour isdifficult in some cases, especially if the clinical symp-toms are non-specific [1, 5, 8].

Mimickers of filling defects are flow artefacts (usuallywith central location within the ureter) [9], vessels thatcan cause an extrinsic impression on the ureter(Figure 8), and ureteral spasm and peristalsis.

Obstruction of the ureter

The differential diagnosis of ureteral obstructioninclude intraluminal (calculi, blood clots, papillarynecrosis with sloughed papilla), intramural (tumour,infection diseases, post-surgery/instrumentationtrauma, lesions after radiotherapy, ureterocele, mega-ureter) and extrinsic abnormalities (retroperitonealfibrosis, invasion or compression by extrinsic malig-nancy, lymphadenopathy, inflammatory diseases) [1, 3,8].

MRU allows the precise depiction of the site of theobstruction and the degree of ureterectasis, and maydemonstrate the underlying pathology with the help ofconventional T1 and T2 weighted sequences (Figures 1, 6and 9).

Filling defects in the urinary bladder

MRU provides a non-invasive mean to detect fillingdefects in the urinary bladder – calculus, blood clot, airbubble, neoplasm, prostatic enlargement, ureterocele orforeign body [5].

The transitional cell carcinoma of the urinary bladderis a single or multiple polypoid defect that arises fromthe bladder wall and is fixed in position – unlike acalculus, blood clot or air. Sometimes they may produceonly focal bladder wall thickening and rigidity(Figure 10).

Prostatic enlargement causes a smooth or irregularextrinsic filling defect of varying size at the base of thebladder (Figure 11). If a chronic process, there istrabeculation of the bladder wall and diverticula forma-tion. The distal ureters often have a fishhook deformitydue to elevation of the trigone.

Post-operative changes

The role of MRU in patients undergoing urinarydiversion (ureteroileal by-passes, ureterosigmoidostomy,skin ureterostomy, orthotopic neobladder reconstruc-tion) or after renal transplantation is emerging. MRUallows visualization of anastomoses, as well as ofassociated complications such as strictures (Figure 12),ureteral compression by lymphocele or haematoma,urine leaks, fistulae (Figure 13), stones or signs ofinfection. Signal-void within urinary tract in post-operative patients does not always correspond to stones,but may be due to air bubbles or susceptibility artefactscaused by surgical material [1, 10, 11].

Figure 3. A 20-year-old man with ureteropelvic junctionnarrowing (arrow). Coronal maximum intensity projection(MIP) excretory MR urography. The renal pelvis shows typicaldilatation and convex inferior border.



(a) (b)

(c)

Figure 4. Left renal agenesia in a 14-year-old woman with didelphic uterus and a vaginal septum. Maximum intensityprojection (MIP) image from (a) excretory MR urography demonstrates a normal right kidney with no evidence of left renaltissue. (b) Axial and (c) coronal T2 weighted turbo spin echo (TSE) images show a bicornuate uterus (arrows) with two cervix(arrowheads).



(a) (b)

Figure 5. A 55-year-old man with left-sided ureteral stone. (a) Coronal maximum intensity projection (MIP) excretory MRurograph shows filling defect (arrow) in left distal ureter that is causing mild pyeloureterectasis. (b) Enhanced axial T1 weightedgradient-echo image shows a round dependent filling defect (arrow) in left ureter.



(a) (b)

(c) (d)

Figure 6. A 52-year-old woman with temporary ureteral obstruction caused by blood clots. (a) Coronal maximum intensityprojection (MIP) excretory MR urography and (b) complementary retrograde pyelography show complete proximal ureteralobstruction (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). (Continued)



(e)

Figure 6. (Cont.) (e) After several days, excretory urogram from conventional intravenous pyelography demonstrates patency ofpreviously occluded ureter (arrowheads).



(a) (b)

(c)

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 ureteraland pelvocalyceal dilatation. (c) The axial standard T2 weighted turbo spin echo (TSE) sequence confirms the diagnosis bydemonstrating a soft-tissue mass (arrowhead) with heterogeneous signal intensity.



Conclusions

Static and excretory MRU are complementary methodsfor morphological and functional evaluation of the urinarysystem, which can be alternatively employed according tothe degree of urinary tract dilatation and renal function.These techniques have some advantages over ultrasound,conventional urography and CT urography in the diag-nosis of urological diseases. The three-dimensional natureof the data permits reformation into any plane, and thusvirtually eliminates the potential of projection relatederrors in the diagnosis of different pathological conditions.There are also the safety advantages of eliminatingionizing radiation and the risk of medical complicationsdue to iodinated contrast agents, and is even suitable forassessing transplanted kidneys because of the lownephrotoxicity of gadolinium.

MRU, due to its non-use of ionizing radiation, is themost important tool in the diagnostic work-up of

genitourinary pathologies in infants, small children andin women during pregnancy.

The major drawback of MRU is its low sensitivity indetecting calcifications and subtle urothelial lesions, thelatter due to the reduced spatial resolution comparedwith conventional excretory urography. However, MRUcan be offered as an alternative to conventional urogra-phy and CT urography to avoid repetitive radiationexposure in patients with chronic urolithiasis.

In conclusion, static and contrast-enhanced excretoryMRU provide high-quality imaging of the urinary tractand are an accurate and safe diagnostic alternative toother urological diagnostic procedures. These techni-ques, combined with conventional MR images, func-tional MR sequences or MR angiography, in a singlesession yields a rapid and complete diagnostic evalua-tion of the entire urinary tract, and have the potential toprovide the same information as can be obtained withmultiple separate diagnostic studies.

(a) (b)

Figure 8. A 73-year-old woman with mild narrowing of the midureter (arrow) caused by left common iliac artery. (a) Coronalmaximum intensity projection (MIP) image from excretory MR urography and (b) composite coronal MIPs of both urogram anMR angiography.



(a) (b)

(c) (d)

Figure 9. A 77-year-old man with transitional cell carcinoma of the right ureter. (a) Maximum intensity projection (MIP) imagefrom 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.



Figure 12. A 56-year-old woman with ileal loop urinarydiversion. Maximum intensity projection (MIP) image fromexcretory MR urography shows the post-operative urinarytract anatomy. Both sides are dilated because of stenosis(arrowheads) close to the ureteroenteric implantation site.

(a) (b)

Figure 10. A 61-year-old man with transitional cell carcinoma of the bladder. (a) Axial T2 weighted turbo spin echo (TSE) imageshows an irregular wall thickening at the left-side of the bladder (arrows). (b) Maximum intensity projection (MIP) image fromexcretory MR urography confirms large irregular filling defect (arrows) on the floor and left-sided wall of the bladder. Thetumour does not produce obstruction at the ureterovesical junction.

Figure 11. An excretory MR urograph in a 78-year-old manwith benign prostatic hypertrophy. Large, smooth fillingdefect at the base of the bladder (arrowheads).



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Figure 13. Vesicovaginal fistula (arrowheads) formationcaused by inadvertent injury to the bladder during surgeryin a 48-year-old woman. Sagittal maximum intensity projec-tion (MIP) excretory MR urography.



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