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Pictorial Essay |
1 Department of Radiological Sciences, University of Messina, Policlinico G.
Martino, Gazzi, 98100, Messina, Italy.
2 Department of Urology, University of Messina, Policlinico G. Martino, Gazzi,
98100, Messina, Italy.
Received March 4, 2002;
accepted after revision April 17, 2002.
Address correspondence to A. Blandino, Via Fondelle e Canale 26, 98168,
Messina, Italy.
Introduction
 Top Introduction TechniquesClinical ApplicationsCongenital AnomaliesFilling DefectsWall ThickeningUreteral ObstructionReferences |
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Use of these two techniques, either individually or in combination, permits investigation of all relevant aspects in the diagnosis of urinary tract disease [1,2,3,4,5].
The purpose of our pictorial essay, which is based on more than 400 MR urography examinations, is to familiarize the reader with the MR urography features of congenital and acquired ureteral diseases.
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Unenhanced T2-weighted MR urography in our series of patients was performed with two different ultrafast breath-hold sequences: single-shot turbo spin-echo and half-Fourier acquisition single-shot turbo spin-echo (HASTE) [2, 3].
The parameters of the single-shot turbo spin-echo sequence were as follows: TR/effective TE, infinite/1100; echo-train length, 240; interecho spacing, 10.2 msec; matrix size, 240 x 256; number of excitations, 1; slice thickness, 70 mm; field of view, 360 x 360 mm; fat suppression; and acquisition time, 7 sec.
The HASTE sequence parameters were as follows: TR/effective TE, infinite/66; echo-train length, 128; interecho spacing, 8.2 msec; matrix size, 128 x 256; number of excitations, 1; slice thickness, 4 mm; field of view, 360 x 360 mm; fat suppression; and acquisition time, 13 sec.
Three-dimensional urograms were obtained by processing the source images after acquisition in a console using maximum intensity projection.
Excretory MR urography was performed after IV injection of 10 mg of furosemide (Lasix; Aventis, Hoechst, Germany) immediately followed by a standard dose of gadopentetate dimeglumine [4, 5].
Coronal MR urograms were obtained by means of a high-resolution breath-hold T1-weighted spoiled three-dimensional gradient-echo sequence with the following parameters: TR/TE, 4.6/1.8; flip angle, 30°; matrix size, 200 x 512; number of excitations, 1; field of view, 450-490 mm; slab thickness, 90 mm; number of partitions, 30; slice thickness, 1.5 mm; and acquisition time, 18-23 sec.
The first excretory MR urogram was obtained 5 min after injection of gadopentetate dimeglumine and was followed by a series of from two to four sequences acquired every 5 min in the coronal plane and, if necessary, also in the axial and sagittal planes.
Maximum-intensity-projection images were processed from the original source images of each three-dimensional sequence.
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Gadolinium-enhanced excretory MR urography provides both functional information and detailed morphologic information about the ureters and periureteral tissue [4, 5]. In comparison with conventional excretory urography and contrast-enhanced CT, excretory MR urography is a safer examination because gadolinium is not nephrotoxic and is well tolerated by patients who cannot tolerate iodinated contrast material. Consequently, this technique is indicated for assessing patients with a known intolerance of iodinated contrast medium and patients with moderately reduced excretory function [4, 5]. Moreover, excretory MR urography can also be performed to examine children and young adults because it does not require the use of ionizing radiation.
Gadolinium-enhanced MR urography is a promising technique that may be of value in differentiating benign from malignant ureteral diseases and may play a role in the staging of ureteral carcinoma.
The major drawbacks of MR urography are its high cost and its low sensitivity in detecting calcifications.
The combination of hydrographic and excretory MR urography yields a rapid and complete diagnostic test of the ureter [1,2,3,4,5], allowing a patient to be evaluated for the full spectrum of obstructive and nonobstructive ureteral diseases.
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In cases of incomplete duplication, which is caused by branching of the ureteric bud, the two ureters join at any level to form a common ureter with only one ureteral orifice [6] (Figs. 2A and 3A).
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In cases of complete duplication, the presence of two ureteric buds leads to the formation of two totally separate ureters and two separate renal pelves, with the upper pole pelvis generally consisting of only a single minor calix (Fig. 4A). The orifice draining the lower renal pelvis is always situated cranially and laterally to the orifice draining the upper renal pelvis and ureter, which is usually ectopic [6] (Fig. 5A).
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The MR urography features of noncomplicated incomplete or complete duplication are characteristic (Figs. 2A and 6A). On the other hand, both hydrographic and excretory MR urography are useful in depicting the full spectrum of complications often found in patients with complete duplication (Fig. 5A,5B).
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Anomalies of Origin and Termination
Anomalies of origin and termination encompass ectopic ureteral orifice and
ureterocele. An isolated single ectopic ureter is rare, whereas an ectopic
ureteral orifice most commonly occurs in association with a ureterocele and
complete duplication of the ureter
[6,
7]
(Fig. 5A).
An ectopic orifice can be in the bladder as well as in other locations (i.e., urethra, vestibule, vagina, seminal vesicles, or vas deferens).
Ureters that open into an abnormal location in the bladder are predisposed to obstruction, ureteropyelectasis, reflux, infection, and impaired kidney function (Fig. 5A,5B).
A ureterocele is a cystic dilatation of the lower end of the ureter. A simple ureterocele is located where the normal ureteral orifice should be, whereas an ectopic ureterocele arises from a ureter with the orifice situated in an ectopic site, either in the bladder or outside the bladder [7].
MR urography features of ureterocele are characteristic, showing a cobra-head deformity of the distal end of the ureter with a variable degree of ureterohydronephrosis. Both hydrographic and excretory MR urography permit an easy diagnosis of ureterocele (Figs. 4B and 7).
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Anomalies in Position and Form
The most common anomalies in ureteral position and form are retrocaval
ureter and primary megaureter.
Retrocaval ureter is a rare congenital anomaly limited to the right side in which a normal ureter becomes entrapped behind the vena cava because a subcardinal vein persists as the infrarenal segment of the vena cava itself.
Congenital primary megaureter—unilateral or bilateral, obstructive or nonobstructive—derives from a failure of the distal ureter to transmit the normal peristaltic wave without any stricture at the ureterovesical junction [7].
MR urography features are pathognomonic and show various degrees of dilatation of the lower third of the ureter with either no or mild dilatation above and a normal-sized intramural tract (Fig. 8).
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Benign tumors of the ureter are rare and arise from the nonepithelial cells of the ureteral wall. Fibrovascular polyp, composed of a thick dense core of fibrous connective tissue covered with normal transitional epithelium, represents 80% of all benign ureteral tumors [6]. On MR urography, fibrovascular polyp appears as a long smooth hypointense filling defect outlined by a thin rim of hyperintense urine (Fig. 3A,3B). The thin pedicle may be difficult to see. The ureter may show only mild dilatation. These features are highly suggestive of a fibrovascular polyp.
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Other benign ureteral tumors are exceptional. They produce a round or polypoid defect that partially or completely fills the ureteral lumen. No or slight involvement of the ureteral wall is present (Fig. 6A,6B).
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Most malignant tumors are epithelial, with transitional cell carcinoma being more frequent (93%) than squamous cell epithelioma (5-7%) [6]. In approximately half the cases, the tumor involves the lower third of the ureter.
Early stage (stages I and II) transitional cell carcinoma of the ureter usually presents as sessile lesions and, less commonly, as focal or diffuse thickenings of the ureteral walls. Pathologically, early stage carcinomas are limited to the ureteral wall and do not invade periureteric fat.
The MR urography findings of sessile early stage transitional cell carcinoma consist of a vegetating mass in the ureter that expands centrifugally. Both the upper and lower margins of the vegetating tumor as well as the degree of ureteral wall involvement and of urinary tract dilatation are well depicted on the source image of both hydrographic and excretory MR urography (Fig. 9).
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The signal intensity of transitional cell carcinoma usually differs sufficiently from that of other causes of ureteral filling defects, such as stones and blood clots, to suggest an accurate diagnosis. Stones show no signal, and blood clots are usually hyperintense on T1-weighted MR images [8]. Moreover, transitional cell carcinomas are hypovascular, but an area of subtle enhancement is usually apparent on gadolinium-enhanced excretory MR urography (Fig. 9).
Stones are the most common cause of ureteral filling defects. Both in hydrographic and excretory MR urography, a ureteral stone appears as a round or ovoid signal void filling defect that causes a variable degree of dilatation of the urinary tract.
Because small calculi may be obscured by hyperintense urine on maximum-intensity-projection images in both excretory and multislice HASTE MR urography, evaluation of the source images is essential to confirm the presence of stones [3, 5] (Fig. 10A,10B).
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Surgery, diagnostic or therapeutic procedures, or gas-producing infections of the urinary tract can be the source of intraluminal gas bubbles that appear as a single or, more frequently, multiple round or oval sharply delineated signal void filling defects creating a typical string-of-pearls appearance (Fig. 11). Characteristically, the gas bubbles float into the mid ureter, which in a patient who is in the supine position is the uppermost part of the excretory urinary tract.
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Finally, a rare cause of ureteral filling defect is coagulum, which is typically hyperintense on T1-weighted MR images [8].
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Nonspecific infections of the ureter generally result from pyelonephritis or vesicoureteral reflux of infected urine [6]. Ureteral wall thickening that is caused by an infection is usually smooth, and periureteral fat is not or is minimally involved (Fig. 2B).
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The MR urography features that are related to the response of the collecting system are characterized by ureteral dilatation that is reversible, resulting from dynamic factors in active ureteral infections, or irreversible, caused by a chronic infection (Fig. 13).
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Isolated strictures of the ureter can be seen in rare instances, generally in patients with a chronic ureteral infection or with ureteral involvement caused by a chronic renal disease such as xanthogranulomatous pyelonephritis (Fig. 2A,2B).
Among the specific infections of the genitourinary tract, multiple strictures are characteristic of tuberculosis (Fig. 14).
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Ureteral infiltration can result from both intrinsic and extrinsic diseases. Infiltrative ureteral carcinoma is the most common intrinsic cause of ureteral obstruction.
Extrinsic involvement of the ureter can result from a seemingly endless variety of pathologic processes that cause obstruction by direct invasion, pressure, or constriction. Examples include pelvic neoplasms, extrapelvic neoplasms, inflammatory diseases, and miscellaneous disorders.
Although diagnosing the specific obstructing disorder is not possible, MR urography findings may be of value. Determining the site of obstruction—whether involvement is mono- or bilateral and whether displacement of the ureter is medial or lateral—is useful in the differential diagnosis. In these cases, obtaining images in multiple projections and performing multiplanar reconstructions help to better define the spatial relationship between the ureter and the obstructing mass.
Bladder carcinoma, extravesical extension of prostatic carcinoma, all pelvic tumors in women, rectosigmoid carcinoma, and pelvic metastatic adenophaties can be responsible for involvement of the pelvic ureter (Fig. 15A,15B). Mixed tumors with cystic or necrotic areas, such as ovarian carcinoma, are easily seen on ultralong T2-weighted hydrographic MR urography (Fig. 16).
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Pelvic abscess complicating gynecologic surgery and ovarian abscess are the most frequent inflammatory conditions responsible for obstruction of the pelvic ureter [6, 7]. Hydrographic MR urography is a technique that is well suited for displaying both the abscess, which appears as a cystlike lesion, and the ureters to determine the extent of involvement [2, 3] (Fig. 17).
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Large diverticula of the bladder can press on the pelvic ureter (Fig. 18). Endometriosis or huge pelvic hematomas are unusual causes of involvement of the pelvic ureter [8].
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Retroperitoneal tumors such sarcoma, pancreatic carcinoma, lymphoma, and abdominal adenopathies from any malignant tumor can obstruct the ureters in their lumbar tracts.
Retroperitoneal fibrosis—whether idiopathic, caused by a malignancy, or drug-induced—is a chronic inflammatory process that involves the retroperitoneal tissue over the lower lumbar vertebrae and that causes ureteral obstruction [6]. Midline deviation of one or both ureters and a gradual tapering of the ureter in a fusiform manner to the area of obstruction are the MR urography diagnostic clues for this pathologic condition (Fig. 19).
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