The widespread practice of prenatal ultrasonography imaging has made ultrasonographic findings the primary presentation of congenital ureteropelvic junction (UPJ) obstruction. Routine prenatal assessment typically occurs at 16-20 weeks' gestation. On the basis of those findings, an appropriate series of in utero and postnatal studies is performed. [1, 2, 3, 4] (See the images below.)
By definition, the diagnosis of UPJ obstruction signifies functionally impaired transport of urine from the renal pelvis into the ureter. Because the increased renal pelvic pressure from obstruction may lead to progressive renal injury and embarrassment, correct diagnosis is clinically important. The impairment may be primary or secondary in nature. This, along with the chronicity and severity of the condition, dictates the course of management. [1, 5, 6, 7]
Current radiologic standards define hydronephrotic kidneys as those with an anteroposterior (AP) diameter at the renal pelvis of greater than 4 mm at a gestational age of less than 33 weeks and an AP diameter of greater than 7 mm at a gestational age of 33 weeks or older. An abnormal initial ultrasonogram should be followed up with another ultrasonogram after 4 weeks in severe cases or after 33-34 weeks in mild to moderate cases.
Criteria for fetal hydronephrosis
The most common criteria used to grade fetal hydronephrosis are the Society of Fetal Urology (SFU) consensus guidelines, which are based on pelvic dilation and caliectasis. They include the following:
Grade 0 — Normal kidney
Grade 1 — Minimal pelvic dilation
Grade 2 — Greater pelvic dilation without caliectasis
Grade 3 — Pelviectasis and caliectasis without cortical thinning
Grade 4 — Hydronephrosis with cortical thinning
Grade 3-4 hydronephrosis is 88% sensitive and 95% specific for obstruction on diuretic renograms. However, because of interobserver and intraobserver variability in assessing the degree of dilatation in ultrasonography, many centers continue to rely on the AP diameter of the renal pelvis to diagnose hydronephrosis. The hydration status of patients naturally has a bearing on the measurement and assessment of dilatation in the collecting system, and any extreme outlying states should be duly noted.
Follow-up postnatal monitoring
Much controversy exists over the optimum timing of follow-up postnatal imaging. Some suggest delaying imaging at least 48 hours after birth to minimize false-negative findings, owing to the infant's relative state of dehydration and decreased glomerular filtration rate (GFR). Others have found no difference between early and delayed ultrasonography. This issue is of some practical significance because an early ultrasonogram may be obtained before the infant is discharged home from the nursery, which helps prevent noncompliance with follow-up.
Typically, disease of the more severe grades requires routine serial monitoring every 3-4 months for the first year of life; hydronephrosis of grades 1 and 2 may be monitored with less frequency (every 6-9 months). For hydronephrosis of grades 3 and 4, administering a diuretic nuclear renogram to assess renal function is usually delayed until 1 month of age to allow for the physiologic increase and stabilization of GFR. The need for repeat nuclear renography is controversial; the decision is made large on the basis of physician preference, ultrasonographic findings, or both. Definitive treatment is initiated with the finding of reduced or worsening renal function, worsening hydronephrosis, or both. Because reflux may cause hydronephrosis and coexists in 13-42% cases of congenital UPJ obstruction, cystourethrography is performed for all patients with these findings to assess vesicoureteral reflux (VUR).
Asymptomatic UPJ in older children and adults
Not uncommonly, asymptomatic UPJ obstruction is discovered in older children or adults when radiologic studies, such as ultrasonography, intravenous pyelography (IVP), computed tomography (CT) scanning, or magnetic resonance imaging (MRI), are performed for other reasons. Often, the initial examination is a CT scan, because CT has become the study of choice at many institutions for evaluating acute flank or abdominal pain. The use of intravenous contrast material—with nephrogenic and delayed excretion phases—during CT scanning may provide qualitative information regarding obstruction, but in general, CT should be avoided because of the inherent high radiation dose. When hydronephrosis is seen when UPJ obstruction is suspected, diuretic renography is more accurate than CT.
Intravenous pyelography (IVP) has historically been the primary study used to diagnose UPJ obstruction in adults, because it also provides anatomic and functional information. However, ultrasonography is preferred as the initial study in children because of its nonionizing and noninvasive nature. Once an obstruction is suspected on the basis of anatomic studies, the diagnosis of UPJ obstruction is made by means of diuretic renography.
Limitations of techniques
To diagnose ureteropelvic junction obstruction, anatomic and functional studies are necessary. As a stand-alone study, ultrasonography cannot definitely depict UPJ obstruction. Ultrasonography provides no information regarding physiologic or functional status, and in many cases, despite the presence of hydronephrosis, urine passage is unobstructed. Obstruction must be demonstrated by means of functional studies, such as diuretic nuclear renography. Even with nuclear renography, the definition of true obstruction is a subject of debate; some proponents rely on decreased differential renal function as an indication of obstruction, whereas others rely on a decrease in washout time. Many features of the infant kidney, including its immature state and its variable appearance (depending on volume status), may lead to inaccurate ultrasonographic findings.
Intravenous pyelography (IVP) has traditionally been the primary study for evaluating hydronephrosis in older children and adults. It provides functional and anatomic detail about the renal parenchyma and collecting system. IVP findings suggestive of ureteropelvic junction obstruction include marked dilatation of the renal calyces and pelvis, funneling down to a narrow beak end, with nonvisualization of the ipsilateral ureter (see the images below).
Delayed imaging is essential to maximize visualization of the urinary tract, because the higher volume of urine in a hydronephrotic pelvis dilutes the contrast medium and delays its passage into the ureter, even in the absence of functional obstruction. The administration of intravenous furosemide (0.5-1.0 mg/kg) during IVP may further assist in differentiating true UPJ obstruction from nonobstructive hydronephrosis.
Although IVP is used regularly in the adult population, the combination of ultrasonography and nuclear renography has replaced it in the pediatric population. This change occurred for 2 reasons: (1) the use of ionizing radiation, (2) the decreased sensitivity of the study in immature kidneys, which do not concentrate and excrete urine optimally. These factors limit the utility of IVP in children.
Retrograde pyelography may provide the greatest degree of anatomic detail in delineating UPJ anatomy when results of antegrade studies (eg, IVP) are inconclusive or equivocal. However, retrograde pyelography is the most invasive study; cystoscopy and ureteral catheterization in children require general anesthesia. In UPJ obstruction, findings may include a kink of the proximal ureter near the UPJ, with a tight jet of contrast passing into the pelvis upon administration or the complete failure of contrast material to flow into the pelvis.
Although cystourethrography is traditionally an unreliable test for diagnosing UPJ obstruction itself, this test is necessary in the workup of UPJ obstruction in children because VUR can cause hydronephrosis. Cystourethrography may demonstrate VUR and other anomalies of the lower urinary tract that may alter renal function. About 10% of patients with UPJ obstruction also have VUR.
Cystographic signs suggestive of UPJ obstruction in the presence of reflux include the following: (1) poorer visualization of the pelvis in comparison with the ureter, which results from the dilution of contrast material by urine in the dilated pelvis, and (2) dilatation of the pelvis and calyces out of proportion to that of the ureter. Contrast-enhanced cystourethrography should be used instead of radionuclide cystography because the latter provides inadequate anatomic detail.
Degree of confidence
IVP can reliably demonstrate existing hydronephrosis and reveal the site of obstruction, provided that function of the ipsilateral kidney is adequate for the excretion of contrast material. IVP is of limited utility in infants, whose kidneys are immature, as well as in patients with poor kidney function. Additionally, IVP is limited in the diagnosis of UPJ obstruction because it cannot provide adequate quantitative assessment of function in the obstructed kidney.
Retrograde pyelography is useful for further delineating the anatomy of the urinary tract when the results of IVP are equivocal. IVP may help in defining the exact site and nature of the obstruction. Often, retrograde pyelography is performed under general anesthesia just before surgery for definitive repair to confirm the pathology.
Cystourethrography is highly sensitive and specific for VUR and is occasionally helpful in diagnosing UPJ obstruction.
False-positive IVP results may occur in cases involving a dilated, nonobstructing collecting system that is slow to drain. False-positive results may also occur as a result of underlying renal insufficiency or when excretion is reduced because of immaturity. An acquired proximal ureteral obstruction, as might occur through the presence of a stone or by extrinsic compression, may also lead to a false-positive finding.
CT may depict ureteropelvic junction (UPJ) obstruction when it is used as a primary study for evaluating common presenting symptoms (see the image below). In older children, the modality is useful for assessing causes of acquired UPJ and ureteral obstruction (eg, stones, tumors, retroperitoneal processes). Often, hydronephrosis is found incidentally on CT scans, and further studies are needed to distinguish UPJ obstruction from nonobstructive hydronephrosis.
Cortical thinning in a hydronephrotic kidney may be seen on CT scans and may be predictive of ipsilateral renal function. The use of intravenous contrast material with nephrogenic and delayed excretory-phase images also may be helpful in determining whether renal function and excretion are impaired.
Degree of confidence
CT provides superior anatomic delineation of the urinary tract system; the use of intravenous contrast material offers some degree of functional information. CT reliably reveals existing hydronephrosis, and the chronicity of obstruction may be correlated with renal cortical thinning. CT is particularly useful for diagnosing acquired UPJ obstructions, such as those caused by stones or extrinsic compression.
False-positive results may occur in cases involving a massively dilated collecting system in the absence of true functional obstruction, as depicted by nuclear renography. False-negative results are unlikely.
Magnetic Resonance Imaging
As with CT, MRI provides excellent details of anatomy in different planes, but it does not offer significant benefit over other, simpler modalities in the evaluation of hydronephrosis. Although MRI is generally not used in the workup of UPJ obstruction, it is as reliable as CT in its ability to depict hydronephrosis and the site of urinary obstruction. False-positive and false-negative findings with MRI are similar to those with CT. [8, 9, 10, 11]
The use of gadolinium-based contrast material may provide some information about renal function. Delayed images may show the presumed site of obstruction with multiplanar imaging in a manner not possible with CT. However, the requirements of MRI—prolonged time and complete stillness—often necessitate the use of sedation in the pediatric patient, making this study less desirable than isotope studies. 
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Ultrasonography is the primary study for evaluating hydronephrosis in children (see the images below). Hydronephrosis appears as an anechoic or hypoechoic cavity that splits the bright, central echo pattern of the renal sinus; this may assume the configuration of the pelvis and calyces. [2, 3]
Although the AP diameter of the renal pelvis may be correlated with the likelihood of obstruction, it is not helpful in diagnosing obstruction. Without functional information, renal ultrasonographic results can only suggest and not confirm ureteropelvic junction (UPJ) obstruction. Furthermore, because ultrasonography is poor for imaging retroperitoneal structures (including the ureters), assessing the site of obstruction is often difficult. In UPJ obstruction, upper-tract dilatation is constant; hydronephrosis that varies intermittently or that increases with voiding suggests the presence of vesicoureteral reflux (VUR).
Degree of confidence
Ultrasonography can depict fluid-filled renal collections with greater than 98% accuracy; therefore, it is reliable in identifying hydronephrosis prenatally and postnatally. Ultrasonography is limited in identifying the site of obstruction and in distinguishing between obstructed and nonobstructed dilated systems. The presence of a dilated ureter suggests a more distal obstruction or high-grade VUR. Therefore, ultrasonography serves as an effective screening and monitoring study for hydronephrosis, but its results cannot confirm the diagnosis of UPJ obstruction.
False-positive results may occur in cases involving a large extrarenal pelvis, a peripelvic renal cyst, nonobstructive hydronephrosis, or VUR. Because of medullary immaturity in patients younger than 3 months of age, the renal pyramids may look sonolucent; this may lead to an erroneous diagnosis of caliectasis. Other anatomic and functional causes of hydronephrosis also may lead to a misdiagnosis of UPJ obstruction.
Patient dehydration may also lead to false-negative results, especially in the newborn. Skin defects, exceptional body habitus, intervening bone, and patterns of bowel gas may limit the transduction of sound waves. These factors, as well as operator experience, may lead to a missed diagnosis of hydronephrosis.
Nuclear medicine scanning may be used to quantitatively assess differential renal function, and it has become a primary study for defining ureteropelvic junction (UPJ) obstruction. In most centers, mercaptoacetyltriglycine (MAG3) has replaced diethylenetriamine pentaacetic acid (DTPA) as the radionuclide of choice (see the images below). Because MAG3 is both filtered and secreted by the renal tubules, it is more useful in immature or chronically insufficient kidneys than is DTPA, which is filtered only by the glomerulus and is not actively secreted. 
Information obtained from a renal scan includes the relative differential renal function; the clearance rate of the radioisotope, with and without a diuretic; and the gross morphology of the collecting system sufficient to distinguish dilatation of the pelvis alone or of both the pelvis and the ureter.
Most centers use the well-tempered renogram,  which was designed to standardize prestudy hydration, as well as the dose and timing of the tracer and diuretic. Its purpose is to allow easy comparison between studies and between institutions. In UPJ obstruction, the clearance rate of a radioisotope, often referred to as the washout half-life, or the time required for the radionuclide activity to be reduced by 50%, is greater than 20 minutes. (Ten minutes is normal; 10-20 minutes is indeterminate.)
The drainage curve of an obstructed kidney is characterized by a prolonged accumulation of radioisotope in the kidney without a subsequent declining slope after the administration of furosemide. As previously mentioned, there is debate as to what defines a true obstruction. Many urologists use only decreased differential renal function (< 40%) as a sign of ureteropelvic junction obstruction, disregarding the half-life and the washout curve. Others perform serial scanning; these practitioners intervene only when the differential renal function declines by greater than 10%, regardless of the initial differential function.
Degree of confidence
In general, the rate of urinary flow (measured by washout half-life) is inversely related to the likelihood of obstruction. However, a more accurate representation of obstruction is the resistance to flow, which takes into consideration the pressure needed to generate the flow rate. The best study for measuring resistance is the Whitaker test, which is an invasive, percutaneous pressure-flow study that allows the measurement of renal pelvic pressures. Although there is no true criterion standard for defining UPJ obstruction, the Whitaker test comes closest. Because of its invasive nature, this test is now rarely performed. Nuclear renography measures only urinary flow; thus, its usefulness is limited to determining the severity of the obstruction in certain cases in which the compliance and capacity of the collecting system is abnormal.
False-positive results may occur in cases involving a large and highly compliant collecting system in which pelvic volume is high but pelvic pressure is not. This condition may lead to a relatively long washout half-life in the absence of significant obstruction. False-positive studies may also occur in patients with poor renal function in whom the washout half-life is prolonged secondary to poor radioisotope excretion. Another cause of false-positive results is a full bladder, which leads to delayed ureteral emptying; therefore, some centers routinely utilize a change in posture, encourage voiding, or employ a bladder catheter.
False-negative results may occur in cases involving an obstructed collecting system of small capacity and low compliance.
Angiography of the renal artery may be performed before surgery to delineate crossing or supernumerary arteries that may be causing extrinsic ureteropelvic junction obstruction. In patients undergoing minimally invasive endoscopic repair, the presence of such vessels should be excluded to ensure a higher rate of success.
Angiography is reliable in identifying supernumerary arteries, but it provides no information as to whether these arteries are causing mechanical obstruction. CT or MR angiography has largely replaced this invasive study.
False-positive and false-negative findings are not an issue, because angiography is not used to diagnose ureteropelvic junction obstruction.