Mru For Non Dilated Duplex Sysyem

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Figure 9 A nine-month-old girl with a left duplex kidney with upper pole ureterocele. (A) A coronal T2-weighted image shows a duplicated left collecting system with severe upper pole pelvocaliectasis and mildly dilated lower pole calyces. Portions of the left upper pole megaureter are visible in the pelvis (u). (B) A more anterior coronal T2-weighted image shows the markedly dilated left upper pole, multiple segments of the left upper pole megaureter, and a large left ureterocele in the bladder (uc).

suspected or because the requested imaging studies are not those that are usually capable of making the correct diagnosis (76).

In the past, diagnosis of extravesical upper pole ureteral ectopia has traditionally been made on intravenous pyelography, although the findings can be very subtle (75,76,79). Renal US will occasionally reveal a dilated upper pole collecting system and ureter; however, very often, US findings are not diagnostic. In many cases, the US examination will be normal. With VCUG, the only two potentially contributory findings are lower pole reflux, which might suggest the presence of a duplication or, much more rarely, reflux directly into an ectopic ureter that inserts into the urethra. Contrast-enhanced CT performed with thin (5 mm) sections through the kidneys is more reliable than either excretory urography or US but requires administration of intravenous contrast material as well as the use of ionizing radiation (75,76). The diagnosis of extravesical upper pole ureteral ectopia is readily made on MRU (4,67,80). In addition to demonstrating the presence of the duplication anomaly and dysplastic upper pole moiety, MRU frequently also allows for identification of the precise site of insertion of the distal upper pole orifice (4,14,24,67,80). Although extravesical ectopic ureters also rarely occur in boys, incontinence does not result because in boys, the ectopic ureter always inserts proximal to the external urethral sphincter (36,37,81).

Complications in the lower poles of duplex collecting systems are generally the same as the more commonly observed childhood abnormalities seen in nondupli-cated collecting systems and ureters: vesicoureteral reflux, UPJ obstruction, and primary megaureter (82). Lower pole ureteroceles are extremely rare (83).

Figure 10 A 10-year-old girl with persistent incontinence. (A) A coronal postcontrast T1-weighted, excretory phase MIP image shows a complete left duplex kidney with a dilated upper calyx with minimal overlying cortex. Proximally, the upper pole megaureter lies medial to the nondilated lower pole ureter. Distally, the upper pole megaureter extends caudally below the bladder. On the right there is a partial duplication anomaly with the right upper and lower pole ureters joining near the pelvic brim to form a single nondilated distal right ureter. (B) An axial T2-weighted image of the pelvis confirms the ectopic insertion of the left upper pole ureter posterior to the bladder base into the vagina (arrow). Abbreviation: MIP, maximum intensity projection.

Figure 10 A 10-year-old girl with persistent incontinence. (A) A coronal postcontrast T1-weighted, excretory phase MIP image shows a complete left duplex kidney with a dilated upper calyx with minimal overlying cortex. Proximally, the upper pole megaureter lies medial to the nondilated lower pole ureter. Distally, the upper pole megaureter extends caudally below the bladder. On the right there is a partial duplication anomaly with the right upper and lower pole ureters joining near the pelvic brim to form a single nondilated distal right ureter. (B) An axial T2-weighted image of the pelvis confirms the ectopic insertion of the left upper pole ureter posterior to the bladder base into the vagina (arrow). Abbreviation: MIP, maximum intensity projection.

Calyceal Diverticulum (''Congenital Calyceal Cyst'')

A calyceal diverticulum is a congenital saccular outpouching of the collecting system (36,37,84-86). The neck of the diverticulum usually communicates with the calyceal fornix but can arise from any site within the calyces, infundibula, renal pelves, or even ureters. On US, calyceal diverticula appear as solitary or multiple cystic lesions within the renal parenchyma and are usually relatively centrally located within the kidney, adjacent to the renal pyramids and collecting system. Although many caly-ceal diverticula are less than 1 cm in diameter, they can occasionally be quite large. Calyceal diverticula can also vary in size when measured from one imaging study to the next, depending upon the degree of distention of the diverticulum.

Most calyceal diverticula communicate freely with the renal collecting systems and will become opacified during antegrade or retrograde urography, as well as during excretory urography and contrast-enhanced CT (on images obtained after renal excretion has occurred). Occasionally, however, the neck of the diverticulum is stenotic, in which case the diverticulum will become only faintly opacified or, in some cases, will remain completely unopacified. Detection of faint opacification in a calyceal diverticulum on CT can be assisted by comparing the attenuation of the fluid in the "cyst" on delayed imaging sequences when the patient lies supine and prone.

The ability of MRI to specifically identify calyceal diverticula is similar to that of CT. Typically, the diverticulum is indistinguishable from a simple renal cyst on unenhanced MR images or on enhanced MR images obtained prior to excretion of gadolinium into the renal collecting systems (Fig. 11). The signal characteristics of the fluid within the diverticulum are usually the same as those of the urine in the adjacent collecting system, although these can vary depending on the content of the diverticulum, such as when it contains debris, milk of calcium, or pus (87). Following gadolinium administration, enhancement of the fluid in the diverticulum provides proof of communication between the diverticulum and the collecting system and establishes the diagnosis. Calculi within a calyceal diverticulum are often more easily visualized on US (where they appear echogenic and demonstrate posterior shadowing) or nonenhanced CT (where they are of high attenuation) than on MRI (where they appear as signal-void filling defects).

UPJ Obstruction

Unilateral or bilateral pelvocaliectasis without ureteral dilatation is the most common sonographic appearance in the fetus and neonate with a dilated urinary tract. In some cases, the diagnosis of UPJ obstruction is straightforward and can be made with the combination of US and nuclear scintigraphy (the latter demonstrating asymmetric washout). However, in the neonate, imaging differentiation between obstruction and transient hydronephrosis of the neonate is often difficult (31,88-93). Numerous sonographic grading systems have been proposed to aid in more rapidly identifying those patients who would benefit from surgical intervention, although none is foolproof. Consensus on criteria for obstruction is still elusive (91,92,94,95). Progression of renal collecting system and ureteral dilatation and deterioration in renal function are uncommon in neonates and infants with milder dilatation. When the anteropos-terior renal pelvic diameter exceeds 10 mm, the risk of developing classical scinti-graphic findings of obstruction or deterioration in renal function increases with the severity and extent of the infundibular and calyceal dilatation. However, even moderately severe dilatation will sometimes regress spontaneously (91,95).

In older children with UPJ obstruction who present with flank pain, vomiting, or hematuria, the diagnosis is generally straightforward (13,36,37,54,96). Pelvoca-liectasis is often severe, and cortical atrophy is frequently present. As a result, functional evaluation with excretory urography or diuretic renal scintigraphy is typically diagnostic of obstruction. The diagnosis can be more difficult in those patients who have intermittent UPJ obstruction secondary to extrinsic compression by a crossing vessel or band because the kidney can appear nearly or entirely normal when the patient is asymptomatic (96-98). In these patients, timing the examination to coincide with the patient's symptoms is more important in arriving at the correct diagnosis than is the choice of imaging modality.

T2-weighted MRU is particularly suited for imaging congenital anomalies that cause urinary tract obstruction and dilatation, such as UPJ or UVJ obstruction, because the urine in the dilated collecting system and ureter produces a very high signal with excellent signal-to-noise ratio (4,8,9,14-16,22-29). Although it is also possible to get excellent quality images with T1-weighted spin-echo-imaging sequences following administration of gadolinium-based contrast material (Figs. 12 and 13) (15,16,22-26), T2-weighted MRU has the advantage of not requiring the use of MR contrast agents. With T2-weighted MRU, the urographic effect is based solely on the presence of water (i.e., urine) in the renal collecting systems and ureters and is independent of renal excretory function. For this reason, T2-weighted MRU can be used to visualize a dilated obstructed collecting system of a poorly or even nonfunctioning kidney. T2-weighted MRU has also been used to visualize congenital obstructive uropathy affecting the fetal urinary tract. The major limitations of T2-weighted MRU are that it provides little information regarding renal function and that it is less useful in patients with obstructed, but nondilated, urinary tracts because these nondilated tracts are not well seen. Gadolinium-enhanced T1-weighted spin-echo MRU images are generally of much better quality in all patients with nondilated renal collecting systems and ureters.

Thus, T2-weighted (noncontrast) and T1-weighted (gadolinium-enhanced) MRU examinations have different advantages and disadvantages. The choice of which type of study to perform should be tailored to each individual patient. MRU can also be utilized to follow patients with UPJ obstructions after they have been treated (Fig. 14).

Ureteral and UVJ Obstruction

As is the case in patients with UPJ obstructions, T2-weighted MRU is particularly helpful in patients with ureterectasis, because the urine in the dilated ureter will be

Figure 11 (Figure on facing page) An eight-year-old boy with right calyceal diverticulum containing several small calculi that could be identified on CT but not MRI. (A) An axial T1-weighted image shows a rounded, low-signal intensity, centrally located lesion in the lateral aspect of the mid-right kidney, consistent with either a cyst or a calyceal diverticulum. (B) An axial T2-weighted image shows a rounded, high-signal intensity, centrally located lesion in the lateral aspect of the mid-right kidney, consistent with either a cyst or a calyceal diverticulum. (C) An axial noncontrast-enhanced CT image shows a rounded, low attenuation, centrally located lesion in the lateral aspect of the mid-right kidney. The location of the lesion between two calyces and the presence of two small calculi within the lesion suggest it is a calyceal diverticulum, rather than a renal cyst. Abbreviations: CT, computed tomography; MRI, magnetic resonance imaging.

Figure 12 A 15-month-old boy with left UPJ obstruction. Coronal postcontrast T1-weighted images during the cortical (left), excretory (middle), and delayed (right) phases show normal left cortical perfusion with delayed medullary enhancement and prolongation of the left nephrogram. In the delayed phase, there is marked left pelvocaliectasis with no visualization of the left ureter. The pattern matches the classic obstructive urogram appearance on excretory urography. Abbreviation: UPJ, ureteropelvic junction.

Figure 12 A 15-month-old boy with left UPJ obstruction. Coronal postcontrast T1-weighted images during the cortical (left), excretory (middle), and delayed (right) phases show normal left cortical perfusion with delayed medullary enhancement and prolongation of the left nephrogram. In the delayed phase, there is marked left pelvocaliectasis with no visualization of the left ureter. The pattern matches the classic obstructive urogram appearance on excretory urography. Abbreviation: UPJ, ureteropelvic junction.

Figure 13 A six-month-old boy with right UPJ obstruction. A series of six coronal postcontrast T1-weighted images show normal cortical perfusion bilaterally, with the dilated right calyces appearing as low-signal intensity filling defects on the early images. During the excretion phase, the calyces have a high signal intensity bilaterally, with severe right pelvocaliectasis and diffuse right cortical thinning. Visualization of the right renal pelvis is delayed and the right ureter is never seen. The left kidney appears normal. Abbreviation: UPJ, ureteropelvic junction.

Figure 13 A six-month-old boy with right UPJ obstruction. A series of six coronal postcontrast T1-weighted images show normal cortical perfusion bilaterally, with the dilated right calyces appearing as low-signal intensity filling defects on the early images. During the excretion phase, the calyces have a high signal intensity bilaterally, with severe right pelvocaliectasis and diffuse right cortical thinning. Visualization of the right renal pelvis is delayed and the right ureter is never seen. The left kidney appears normal. Abbreviation: UPJ, ureteropelvic junction.

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