Scintigraphic features of duplex kidneys on DMSA renal cortical scans

Abstract

The spectrum of manifestations of duplex kidneys on ^99mTc-dimercaptosuccinic acid (DMSA) renal cortical scans and correlating findings on other imaging modalities are presented. Relevant embryology of the duplex systems and technical aspects of DMSA scintigraphy are reviewed.

Introduction

DMSA (^99mTc-dimercaptosuccinic acid), a renal cortical imaging radiopharmaceutical, is commonly used in the evaluation of children with urinary tract infections. Approximately 60% of the intravenously administered dose is gradually taken up and retained by the proximal tubular cells. The remaining 40% is filtered and excreted at low concentration. Therefore, unlike in other functional imaging modalities such as intravenous urogram (IVU), CT, MRI and renal scans with MAG-3 or DTPA, direct visualization of the duplex pelvicalyceal systems is not possible on DMSA cortical scans. However, there are often changes in the renal cortex that are diagnostic or highly suggestive of duplication as demonstrated in this pictorial essay.

Types of duplication

Duplication of the ureters is the most common congenital urinary tract abnormality. It was present in 0.7% in two large autopsy studies [1–4]. It is seen more frequently in children being evaluated for urinary tract infections (8%) [1]. It is 2–4 times more common in girls compared with boys [4]. The reported incidence of bilateral duplication has varied from 17% to 33% [4].

The most widely accepted nomenclature for kidneys drained by two ureters is the one proposed by the Committee on Terminology, Nomenclature and Classification of the American Academy of Pediatrics Section on Urology. A duplex or duplicated system is defined as a kidney that has two pelvicalyceal systems. In incomplete duplication, the two ureters join before entering the bladder. A bifid pelvis is the mildest form. In complete duplication, two ureters enter the bladder separately [4, 5].

Incomplete duplication is significantly more common than complete duplication by a ratio of 3:1 [2]. Incomplete duplication can be associated with vesicoureteral reflux into both moieties, ureteropelvic junction (UPJ) obstruction of the lower moiety and rarely with ureteroureteral or yo-yo reflux [2].

Complete duplication is present in 1 in 500 individuals [2]. The two ureters have separate ureteral orifices in the bladder. The upper moiety typically consists of only a few calices or a compound calyx drained by a single infundibulum. The most common abnormalities associated with complete duplication are vesicoureteral reflux (lower moiety) and, ectopic ureterocele or ectopic ureteral insertion (upper moiety). A less common associated abnormality is UPJ obstruction of the lower moiety [2].

A ureterocele is the dilated submucosal terminal segment of the upper moiety ureter; it is 4–8 times more common in girls than boys [2]. More commonly, ureteroceles are associated with hydronephrosis and ureterectasis of the upper moiety. Rarely, the ureterocele may be large with a small dysplastic upper moiety, described as ureterocele disproportion [6, 7].

Embryology

Incomplete duplication develops from early bifurcation of a single ureteral bud before it meets the metanephric blastema at approximately 5–6 weeks [2, 4, 7]. Occasionally, one of the bifid ureters is blind ending [4, 6, 7].

Complete duplication results from two separate ureteral buds arising from the mesonephric duct. As the distal portion of the mesonephric duct is being absorbed into the developing bladder, the ureter to the lower segment of the metanephros separates earlier from the mesonephric duct and is absorbed into the bladder. The ureter to the upper part of the metanephros separates later after it has been carried medially and inferiorly by the migrating mesonephric duct and after the two ureters have crossed. This embryological relationship, wherein the upper moiety ureteral orifice is always medial and caudad to that of the lower moiety ureter, is known as the Weigert-Meyer law (Fig. 1) [4, 6]. Mackie and Stephens [8] hypothesized in 1975 that metanephric blastema is an elongated structure, only the mid-portion of which can form normal renal parenchyma. If a ureteral bud meets the more cephalad or the more caudal ends of the metanephric blastema, renal parenchymal abnormality results. The final location of the ureteral orifice correlates with the degree of renal abnormality [8].

Fig. 1

Embryogenesis of ureteral duplication. Two ureteral buds arise from the mesonephric ducts at approximately 4 weeks. The lower pole ureter bud (a) contacts the urogenital sinus by 7 weeks and the upper pole ureter bud (b) does the same by 8 weeks. As the lower pole ureter contacts the urogenital sinus earlier, its orifice achieves a slightly more craniad and lateral position than the upper pole ureter in the course of migration and rotation (Weigert-Meyer law). Figure adapted from Tanagho EA (1976) Embryologic basis for lower ureteral anomalies: a hypothesis. Urology 7:451–464, with permission from Elsevier

DMSA renal cortical scintigraphy

At our center, approximately 1.5 h after intravenous injection of DMSA, a posterior image of both kidneys using high-resolution parallel hole collimator, as well as posterior and posterior oblique images of each kidney using a pinhole collimator (4-mm aperture insert) are obtained (Fig. 2a and b). The administered dose of ^99mTc-DMSA is 0.05 mCi/kg (minimum 0.5 mCi and maximum 3 mCi). Imaging may be done with or without sedation depending on patient age. Differential renal function is calculated using the posterior parallel-hole image, except in cases of spinal deformities where the kidneys may be at different positions. In such cases, simultaneous anterior and posterior images are obtained and the geometric mean of the counts obtained in both projections is used to obtain the differential function. In some cases of complicated duplication, relative function of the individual moieties of the duplex kidney is also calculated. This can often guide in choosing the best surgical option (Fig. 2c and d).

Fig. 2

Region of interest and calculations. a Posterior parallel-hole image of normal kidneys; regions of interest drawn for calculation of differential renal function. b Posterior and posterior oblique pinhole images in the same patient using a 4-mm insert depicting normal cortical uptake with central photopenia corresponding to the normal medullary pyramids and collecting system. c Posterior parallel-hole image depicts duplex left kidney with diffusely decreased tracer uptake in its lower moiety. Regions of interest have been drawn for calculation of differential renal function. The renal differential function is approximately 41% on the left and 59% on the right. d Separate regions of interest have been drawn for upper and lower moieties of the left kidney for the estimation of their relative function. The left lower moiety has approximately 25% of the left renal function. Therefore, the left lower moiety has approximately 10% of total renal function

Radiation considerations

The administered dose should be optimized toward answering the clinical question and at the same time decreasing the radiation risk to the patient. As per the North American consensus guidelines, the recommended administered activity of DMSA in children is 0.05 mCi/kg (1.85 MBq/kg) with a minimum of 0.5 mCi (18.5 MBq) [9]. The estimated radiation dose to the kidneys (target organ) of a 1-year-old child from ^99mTc-DMSA is 0.78 mGy/MBq administered dose. Similarly, for a 5-year old, the target organ dose is 0.45 mGy/MBq [10]. The effective doses for 1- and 5-year-old children are 0.037 and 0.021 mSv/MBq, respectively [11].

These estimates are averages over a wide range of children at each age and may not take into consideration anatomical and physiological differences. A given child’s body type may vary from the standard model with respect to size, weight, shape, organ orientation and distances from other organs. Hence, the absorbed fraction and organ mass vary among children. The existing methods were developed for estimating the average dose to a population and should not be used to estimate the dose to a specific child [12].

Normal DMSA renal scan

In the absence of duplication, a normal kidney on a high-resolution DMSA scan shows tracer uptake in the renal cortex with relative central photopenia corresponding to the complex of the medullary portion of the kidney and the pelvicalyceal system. Prominent cortical columns are often seen. However, they are partial and do not traverse the full thickness of the kidney and do not cause division of the normal central photopenic region into two separate compartments (Fig. 2a and b).

Scintigraphic findings in uncomplicated duplications

In uncomplicated duplications, the renal cortical uptake is normal. Occasionally, the only clue to unilateral renal duplication is asymmetry in renal sizes; the duplex kidney appears longer than the contralateral side. The cortical bar separating the upper and lower moieties may be sometimes hard to appreciate on the parallel-hole images, but is well depicted on the pinhole images (Figs. 3 and 4). In cases of bilateral uncomplicated duplication, the renal sizes may be symmetrical, but cortical bands separating the upper and lower moieties are seen. The relative position of the moieties of duplex kidneys is usually superior and inferior, separated by a transverse cortical band (Fig. 3). Occasionally, the upper moiety is small with normal cortex, located superomedially and separated by an oblique cortical band from the larger lower moiety, which is located inferolaterally (Fig. 4).

Fig. 3

An 11-year-old girl with recurrent febrile urinary tract infections (UTIs). a Duplex left kidney is longer than the right on the parallel-hole image. b The pinhole image is very helpful in such a case as it demonstrates well the cortical band separating the two moieties of the duplex kidney (arrow)

Fig. 4

A 5-month-old girl with a urinary infection. a Note the prominent oblique parenchymal bar (arrows) separating the superomedial upper and inferolateral lower moieties on the posterior pinhole image of duplex right kidney. b Corresponding US image also depicts the cortical bar (arrows) separating the two moieties

Scintigraphic findings in complicated duplications

The scintigraphic findings in complicated duplications are dependent on the nature and severity of the associated developmental or acquired abnormalities such as dysplasia, vesicoureteral reflux, ectopic ureteral insertion, UPJ or ureterovesical junction (UVJ) obstruction, acute pyelonephritis and post-pyelonephritic scars.

The scintigraphic pattern of duplex kidneys with abnormal function of one of the moieties and normal function of the other moiety may vary, depending on the relative size of the two moieties.

A duplex system may appear as a small kidney located either at the level of the upper pole of the contralateral normal kidney, indicating a nonfunctioning lower moiety, or at the level of the lower pole of the contralateral kidney in case of a nonfunctioning upper moiety. High-resolution pinhole image may show minimal tracer uptake in the abnormal moiety (Figs. 5 and 6).

Fig. 5

A 2-month-old girl with prenatal diagnosis of left hydronephrosis and hydroureter. a The posterior parallel-hole image demonstrates a small left kidney at the level of the lower portion of the right kidney. This is consistent with the presence of left duplication with a poorly functioning upper moiety (thick white arrow). The right kidney is also duplicated, with a prominent cortical bar separating the moieties (thin white arrow). b On the pinhole image of the left kidney, there is again faint visualization of the upper moiety indicating poor function (asterisk). The cortical band separating the poorly functioning upper moiety and the lower moiety is noted (arrow). c, d US images reveal duplex left kidney with hydronephrosis of the upper moiety with thinned parenchyma and a dilated tortuous upper moiety ureter inserting ectopically (arrow) below the bladder (asterisk) base

Fig. 6

A 3-year-old boy with UTI and left vesicoureteral reflux. a, b The left kidney is smaller in size with only minimal activity seen in the area of the lower pole (arrow). The upper poles of both kidneys are at the same level. Renal sizes should be compared on parallel hole (a) and not on pinhole images (b). c On correlation with the voiding cystogram, DMSA findings are due to a nonfunctioning lower pole moiety of a duplex left collecting system associated with severe vesicoureteral reflux, and a normally functioning upper pole moiety

A small dysplastic upper moiety, which is usually associated with ectopic ureteral insertion or vesicoureteral reflux, may be seen as an indentation or flattening of the medial aspect of the upper pole if it is nonfunctioning (Fig. 7) or as focal decreased uptake if it functions poorly (Fig. 8). These scintigraphic findings may mimic focal cortical scar or acute pyelonephritis. The only clue to duplication is normal uptake and thickness of the cortex at the site of indentation or underneath the area of focal decreased uptake. This normal cortex underneath corresponds to the upper pole of the normal lower moiety. Ectopic insertion of the ureter of a small nonfunctioning or poorly functioning upper moiety into the vagina clinically presents as diurnal enuresis [7]. Occasionally, the abnormality on the DMSA scan of the small upper moiety with ectopic ureteral insertion is subtle such as mild cortical thinning and/or prominent central photopenia (Fig. 9). Therefore, in interpretation of DMSA scans particularly in girls with diurnal enuresis, careful attention should be made to the medial aspects of the upper poles of the kidneys. Correlation with renal sonogram carefully directed to finding a suspected duplex collecting system is often helpful (Figs. 8 and 9).

Fig. 7

A 3-year-old child with recurrent UTI. a Note the indentation along the superomedial aspect of the left kidney on the pinhole images with good cortex underneath it (superior surface of the normal lower moiety), indicating a non-functioning upper moiety of a duplex system rather than a scar (arrow). b Corresponding voiding cystourethrogram (VCUG) image shows reflux into a small dysplastic upper moiety of the left duplex kidney. c Companion case: DMSA pinhole images of another child demonstrate abnormal cortex underneath the defect along the upper pole, indicating a scar rather than a duplex system (arrow)

Fig. 8

6-year-old girl with day and night wetting. a The DMSA images reveal focal decreased uptake in the medial aspect of the upper pole of the left kidney (white arrow) with good cortex underneath (black arrow) indicating a dysplastic upper moiety of a duplex left kidney rather than focal pyelonephritis. b Note the duplex left kidney on correlating sagittal US image This child had an ectopic left ureter (figure not shown)

Fig. 9

Another girl (8-year-old girl) with enuresis. a On the DMSA images, there is evidence of slight photon deficiency in the upper pole (asterisk) with thin cortex and a prominent cortical bar between the two moieties (arrow). b Corresponding US image demonstrates a duplex left kidney with a prominent cortical bar separating an abnormal echogenic upper moiety from the normal lower moiety (arrow). c Image at the bladder shows an ectopically inserting distal ureter (arrow), the cause of dribbling incontinence in this child

Sometimes the lower moiety may be developmentally small related to high-grade reflux (Fig. 10).

Fig. 10

A 6-year-old girl with bilateral vesicoureteral reflux and breakthrough UTI. a On the DMSA images, there is evidence of bilateral duplex kidneys (white arrows depicting the cortical bars). Diffuse volume loss of the left lower moiety (asterisks) is probably developmental (reflux nephropathy). A small scar is also seen in the upper moiety of the right kidney (arrowhead). b The VCUG reveals reflux into an incomplete duplication on the right. Reflux is also seen on the left, into the lower moiety of a completely duplicated collecting system. In a single system, the upper calyces are normally situated medial in relation to the lower calyces. When the upper calyx is lateral to the lower calyx and a full complement of calyces is not seen (dotted axis), it suggests reflux into the lower moiety of a complete duplication, as seen in this case. Another cause for the upper calyx being lateral to the lower calyx is displacement by a suprarenal mass. In such a case, however, the full complement of calyces is present

In hydronephrotic kidneys without duplication, cortical thickness of the upper and lower poles is usually similar. But a duplex kidney with hydronephrosis of the upper or lower moiety may be manifested as preserved normal cortical thickness in the lower or upper pole, respectively, with reduced cortical thickness in the affected pole (Fig. 11). In addition, the hydronephrosis will be eccentric in the kidney, related to the dilated upper or lower moiety, rather than central as is seen with a single collecting system. Hydronephrosis can be caused by reflux and/or obstruction in either of the upper and lower moieties (Figs. 11 and 12).

Fig. 11

A 2.5-month-old girl with febrile urinary tract infection. a The left kidney is enlarged with prominent collecting system photopenia in the upper and mid zones (arrow). The cortex in the lower pole region is thick and normally functioning in contrast to the cortical thinning seen in the rest of the kidney. This is consistent with a left duplex system with a normally functioning lower moiety (asterisk) and a poorly functioning hydronephrotic upper moiety (arrow). b The pinhole image also shows the normally functioning lower pole parenchyma (asterisk). Note also the large patchy areas of decreased DMSA uptake without volume loss in the upper moiety consistent with acute pyelonephritis (arrows).c Sagittal US image of the left kidney reveals a duplicated system with markedly hydronephrotic upper moiety (arrow) and preserved parenchyma in the lower moiety (asterisk). These findings correlate well with the DMSA scan. d The dilated tortuous upper moiety ureter terminates in a ureterocele (arrows). Note the debris within the ureter; the child had acute urinary infection at this time

Fig. 12

A 4-month-old girl with one episode of febrile UTI. a Two areas of photopenia in the lower pole cortex of the left kidney with no associated volume loss indicate acute pyelonephritis (arrows). There is an area of photopenia more centrally within the lower pole, corresponding to the dilated lower moiety of a duplicated collecting system (asterisk). b Corresponding US image depicts urothelial thickening and pelviectasis of the lower moiety collecting system (asterisk).c VCUG image reveals high-grade reflux in the lower moiety. Note intraparenchyma reflux

Conclusion

Diagnosis of duplex kidney on a DMSA scan requires a careful systematic review of the images. The findings can be subtle and it is important for the radiologist to recognize them. Correlation with clinical history and other imaging modalities such as US and voiding cystogram can be complementary.