Imaging of Pediatric Urinary Tract

This article provides a practical guide to the appropriate imaging of the pediatric urinary tract. The pathophysiologies of the various diseases and the broad spectrum of normal variants are omitted because they are covered elsewhere (see Antenatal Hydronephrosis, Vesicoureteral Reflux, Ureteropelvic Junction Obstruction, Urinary Tract Infection, Constipation and Bowel Management, Hematuria, Cloacal Malformation, Chronic Kidney Disease, and Ureteral Duplication, Ureteral Ectopia, and Ureterocele).

This article contains the basic guidelines for pediatric imaging of the urinary tract in children. Imaging should be tailored to confirm clinical impressions and should either guide in the initial management of urologic conditions or help to modify the therapeutic plan.

Appropriate imaging for a given presentation is described, accompanied by a brief description of common pitfalls encountered with each imaging modality. A simplified review of postoperative imaging is provided in the conclusion.

Antenatal hydronephrosis

Dilatation of the upper urinary tract (antenatal hydronephrosis) is often detected by means of antenatal ultrasonography (US) performed at or after 20 weeks' gestation (~1% of pregnancies). Such findings may warrant sequential US of the fetus. Depending on the degree of dilation, bilaterality, and presence of other anomalies, antenatal consultation by a pediatric urologist may be indicated.

If anomalies are detected antenatally, follow-up renal and bladder US (RBUS) should be performed after the first day of life. Newborns have a physiologic oliguria on the first day of life that can lead to false-negative findings on US. If the findings are normal at this point, US should be repeated 4-6 weeks later.

The Society for Fetal Urology (SFU) scale for hydronephrosis facilitates more precise discussion of this condition by specifying the following four grades ^[1] :

• Grade I - Splitting of the renal pelvis
• Grade II - Renal pelvis dilatation; one or two calyces seen; preserved parenchyma
• Grade III - Diffuse calyceal dilation; preserved renal parenchyma
• Grade IV - Diffuse calyceal dilation; parenchyma thinned to less than half the thickness of the contralateral kidney

For practical purposes, US is often performed just before discharge from the hospital to ensure that the contralateral kidney is normal and, in males, to ensure that the bladder wall looks normal, making posterior urethral valves (PUVs) unlikely.

If persistent dilation of the upper urinary tract is documented, prophylactic antibiotic administration has been recommended until voiding cystourethrography (VCUG) can be performed. Approximately 12% of patients with a history of antenatal hydronephrosis have vesicoureteral reflux (VUR) on postnatal VCUG. Whether all patients should be screened via VCUG remains controversial. ^{[2, 3, 4]} The purpose of the neonatal evaluation of hydronephrosis is to determine if the hydronephrosis is caused by VUR or obstruction.

Follow-up US is required for grade I or II hydronephrosis, and the option of performing VCUG should be discussed with the parents. Given that more than half of the population with antenatal hydronephrosis has grade I or II hydronephrosis, ^[5] which is usually caused by normal variants of renal pelvic anatomy without obstruction, prolonged follow-up is not necessary if there is no progression of hydronephrosis. ^[4]

Grade III or IV hydronephrosis most likely calls for a renal scan, and it would be useful to obtain a VCUG first. Further imaging depends on the resolution of hydronephrosis or demonstration of clinically significant obstruction of the ureteropelvic junction (UPJ) or the ureterovesical junction (UVJ).

Vesicoureteral reflux

The clinical course of VUR depends significantly on the age and sex of the patient and on whether it is detected owing to antenatal hydronephrosis or to a symptomatic urinary tract infection (UTI). The management of VUR and UTI are fully discussed in separate articles. Primary reflux is graded on a scale of I through V as follows:

• Grade I - Ureteral reflux that does not reach the renal collecting system
• Grade II - Reflux into the ureter and pelvis with crisp calyceal impressions
• Grade III - Mild dilation of the ureter and pelvis with mild caliectasis and blunting of the fornices
• Grade IV - Moderate dilation of the ureter and pelvis with ureteral tortuosity and major blunting of calyces but maintenance of papillary impressions
• Grade V - Massive ureteral tortuosity and caliectasis with loss of papillary impressions

If the management plan is to maintain antibiotic prophylaxis and to repeat imaging in 12-18 months, either nuclear or fluoroscopic VCUG and RBUS are performed to assess for resolution of reflux and to evaluate renal growth. ^{[6, 7, 8]} This is often the case in patients with grade I-III reflux; however, grade IV and V reflux can also resolve in newborn boys because voiding pressures dramatically decrease after the first year of life. ^[9]

Secondary reflux occurs as a result of other conditions, such as neurogenic bladder and PUVs. Although secondary reflux can be described, it is not necessarily graded, since the prognosis for spontaneous resolution by grade is different from that for primary reflux.

Ureteropelvic junction obstruction

If postnatal US reveals only grade I-II hydronephrosis, mercaptoacetyl triglycine (MAG-3) renal scanning is not usually performed, because UPJ obstruction is unlikely.

If postnatal US reveals grade III-IV hydronephrosis, renal scanning should be considered after the first month of life. By this time, the glomerular filtration rate (GFR) will have increased sufficiently for the renal scan to be more accurate.However, patients with a solitary kidney or bilateral hydronephrosis have no normal kidney to compare with the dilated kidney. In these patients, radionuclide renal scanning before the first month of life may be necessary to assess whether relief of an obstructive process is indicated.

Diuretic (furosemide) renal scanning (with MAG-3) is used to measure relative renal function and to assess the degree of obstruction by measuring the length of time the radionuclide takes to wash out of the renal pelvis. A greater emphasis is placed on the relative function of the hydronephrotic kidney than on the washout time. The obstructive washout time, typically defined as longer than 20 minutes, may be an indication for intervention.

However, the timing of surgical intervention is controversial, especially if the relative renal function is preserved at greater than 35%. In this case, the timing of the next renal scan depends on the severity of the hydronephrosis. A patient with unilateral severe hydronephrosis and near-symmetric function on renal scanning should undergo repeat renal scanning within 3 months of the initial evaluation.

If follow-up renal scanning continues to reveal symmetric function between the normal kidney and the hydronephrotic one, the interval between follow-up studies may be increased to 6 months. Alternating between RBUS and renal scanning during follow-up can also be useful.

Three normal anatomic variants (extrarenal pelvis, wide infundibula, compound calyces) appear as a dilated renal collecting system on US and thus mimic obstructive uropathy. If symmetric function is observed and the renal pelvis is full with a rapid washout, these variants should be suspected. Monitoring with US may be sufficient.

Magnetic resonance urography (MRU) is becoming more widely used for the evaluation of UPJ obstruction because it provides both anatomic and functional information with a single examination. The need for general anesthesia in young children and the cost of MRU have limited its use to date. In UPJ obstruction, MRU can accurately define the site of ureteral narrowing and assess for a possible crossing vessel. ^[10] With functional MRU, differential renal function is also calculated.

Ureterovesical junction obstruction

Congenital megaureter is another antenatally detected abnormality. Hydroureteronephrosis is observed to the level of the bladder on initial US. VCUG and renal scanning or MRU are necessary to differentiate obstructed, refluxing, and nonrefluxing megaureters. If the megaureter is associated with a kidney whose function is nearly equal to that of the contralateral kidney and if the diameter of the dilated ureter is 1 cm or less, the process is likely to be benign and rarely necessitates surgery.

As with the UPJ, relative renal function should be weighed more than an obstructive drainage curve or washout time. With careful observation, the dilatation slowly resolves, and the renal function is usually preserved. In a study by Di Renzo et al, 80% of patients with preserved renal function and less than grade III hydroureteronephrosis avoided surgery at long-term follow-up (7 years). ^[11]

Urinary tract infection

The indications for radiographic evaluation of UTIs are rapidly changing. Previously, all patients younger than 5 years with a febrile UTI underwent RBUS and VCUG after the first UTI. These recommendations were based on the Birmingham Reflux Trial, which showed that renal scarring in children older than 5 years is unusual. ^[12]

The RIVUR trial assessed daily antibiotic prophylaxis against placebo in patients with VUR to determine the efficacy of the current standard of care (ie, daily prophylaxis). ^{[13, 14]} Questioning of the usefulness of antibiotic prophylaxis has led to the increased popularity of performing a VCUG only after the second febrile UTI ^[4] —the “top-down” imaging approach—with VCUG reserved for patients with demonstrated renal abnormalities. ^[15]

These changes were reflected in subsequent guidelines for the management of UTI. The American College of Radiology (ACR) Appropriateness Criteria from 2008 provided imaging guidelines for various scenarios of patients with UTI, which were similar to those provided by the American Urological Association (AUA). In 2010, the AUA ^[16] recommended that RBUS be performed after the first febrile UTI and that a discussion on the rationale for diagnosing VUR be conducted with parents before a VCUG is obtained.

In 2011, the American Academy of Pediatrics (AAP) ^[17] recommended that RBUS be performed after the first febrile UTI but that a VCUG should be obtained only if there are renal abnormalities, or after a second febrile UTI. Significant controversy surrounded this recommendation, and many pediatric urologists disagreed with the AAP guidelines. ^{[18, 19]}

Because RBUS is noninvasive, it is a useful initial test. US reveals any upper-tract abnormalities (hydronephrosis, dilated ureter, ureterocele) that would predispose to bacterial colonization. In most cases, it does not affect management of the acute infection. ^[20] If there is no clinical response to antibiotic therapy within 48 hours of initiation, RBUS is useful to look for obstruction or renal abscess formation. ^[21]

Repeat imaging with RBUS is necessary in patients with known obstructive urologic abnormalities who develop a febrile UTI to ensure that no new process (eg, an obstructing stone) has developed.

Knowing when to discontinue antibiotic therapy is sometimes useful in patients who have required long-term treatment for pyelonephritis. In such cases, gallium scanning reveals when inflammation has resolved. For patients who have a single UTI in the neonatal period, normal findings on RBUS, and no evidence of reflux on VCUG, some clinicians have recommended prophylaxis for 6 months while the immune system gains competence and the kidneys grow.

If the decision is made to proceed with VCUG, it can be safely performed in patients with acute pyelonephritis when they are afebrile and clinically improved, near the end of their hospital stay. Approximately 50% of patients younger than 1 year who present with a febrile UTI have VUR, compared with 33% of patients older than 1 year. ^[22]

Of patients younger than 1 year with VUR, 50% will have evidence of renal lesions on dimercaptosuccinic acid (DMSA) scan; patients older than 1 year old with VUR have a 33% chance of having renal scarring. ^[15] This has led some centers to use DMSA scanning as the initial test after a child has a febrile UTI. It is proposed that 50% of VCUGs could be avoided by reserving the study for patients with demonstrated renal injury. ^[23]

One practical consideration that decreases the utilization of DMSA scanning is the need for significant sedation or general anesthesia in young children in order to obtain adequate images; another is the radiation exposure involved.

Areas of diminished perfusion on a DMSA scan during an acute infection do not necessarily correspond to areas that will develop a pyelonephritic scar later. In fact, only 40% of patients with acute scarring develop long-term renal scars. ^[24] If the DMSA scan reveals previous abnormalities, the ability to distinguish new scars is diminished.

DMSA scanning can be very useful in patients with a possible infection above the bladder level who have chronic bacteriuria (eg, a patient with continent reconstruction who relies on clean intermittent catheterization for reservoir emptying). MRU can demonstrate pyelonephritis and may be more successful than DMSA in distinguishing acute pyelonephritis from scarring .

The finding of global renal parenchymal abnormality on DMSA scanning is associated with grade IV-V VUR in male infants. These patients may have abnormal renal development or renal dysplasia, which should not be confused with postpyelonephritic renal scarring. Reflux in patients with high-grade reflux and associated renal abnormalities is unlikely to resolve by age 16 months. ^[25] About 10% of these patients will have a poorly functioning kidney on the side of the reflux. ^[9]

Diffusion weighted imaging (DWI) ^[26] and diffusion tensor imaging (DTI) ^[27] are promising noninvasive MRI modalities for identification and characterization of UTI and pyelonephritis.

Incontinence

For primary nocturnal enuresis without a history of infection, imaging is not required. Daytime wetting may necessitate US to look for upper-tract and bladder changes due to dysfunctional voiding. In older boys who present with new daytime wetting and who do not improve with the usual behavioral modifications for dysfunctional voiding, VCUG should be considered to look for a late presentation of PUV. Constant dribbling in a female who appears to void normally suggests ureteral ectopy, with an opening below the urinary sphincter to the urethra, perineum, or vagina.

Although RBUS may be initially performed to assess for evidence of a duplex kidney, intravenous urography (IVU), computed tomography (CT), and MRU are regarded as more definitive studies. All can facilitate the planning of surgery by delineating the anatomy of the ectopic ureter, but MRU is especially helpful in defining the pelvic course of the ectopic ureter, and it does not expose the child to radiation.

Abdominal masses

In children with abdominal masses, US is used for the initial evaluation, followed by magnetic resonance imaging (MRI) or CT. Doppler US of the kidney is performed to determine whether the mass is solid or cystic and reveals whether the tumor has invaded the inferior vena cava (IVC). Although the differential diagnoses of urologic causes of an abdominal mass are beyond the scope of this article, all lesions can be approached by determining whether they are solid or cystic and by determining the organ from which they arose.

Cystic lesions may be due to hydronephrosis, multicystic dysplastic kidney, or cystic renal disease. If they are suspected to be due to hydronephrosis or multicystic dysplasia, VCUG may be performed because VUR is found in approximately 10-20% of cases (see Cystic Diseases of the Kidney). Simple renal cysts that are detected incidentally on US can be monitored for 1 year to ensure that the cysts are not enlarging and that no new cysts are appearing, suggestive of autosomal dominant polycystic kidney disease. If US reveals no changes, it need not be repeated unless the patient has pain or infection.

Cysts are classified according to the Bosniak criteria, which take into account enhancement of cyst walls, septations, calcifications within cysts, solid components, and thickness of the cyst wall. Renal tumors that can be cystic include necrotic Wilms tumor, multilocular cystic nephroma, and renal cell carcinoma (RCC). Unless US, CT, or MRU can clearly classify the cystic renal lesion as benign, surgical exploration becomes necessary.

Solid masses are more ominous and may represent Wilms tumor, neuroblastoma, RCC, congenital mesoblastic nephroma, or other less common tumors (eg, malignant rhabdoid tumors). Wilms tumor arises from the kidney and is most often detected as a large asymptomatic abdominal mass. Neuroblastoma, which commonly arises from the adrenal gland, often manifests as a mass and constitutional symptoms (eg, fever or weight loss). Wilms tumor commonly displaces and compresses vessels. Neuroblastoma also displaces and compresses vessels but is more often infiltrative and encapsulating.

Doppler US can reveal IVC invasion. CT and MRI play an important role in the staging of Wilms tumor and neuroblastoma. Iodine-123 (¹²³I) MIBG is commonly used to identify sites of primary and metastatic neuroblastoma. Unilateral Wilms tumors are managed with surgery followed by chemotherapy, whereas bilateral tumors are managed with chemotherapy followed by nephron-sparing surgery to each kidney.

Congenital mesoblastic nephroma, a rare nonencapsulated benign tumor, is the most common solid renal lesion in neonates. RCC is unusual in young children and is more common in adolescents. It can occur in young patients with tuberous sclerosis and von Hippel-Lindau syndromes. von Hippel-Lindau disease causes recurrent small renal cell adenomas that should be excised when they approach 3 cm in size because they can exhibit malignant behavior with metastases. Angiomyolipomas are common in patients with tuberous sclerosis and should be embolized when they approach 4 cm in size because spontaneous bleeding and pain are likely.

Other conditions that predispose children to the development of renal tumors (nephroblastomatosis, Wilms) include Beckwith-Wiedemann syndrome, hemihypertrophy, sporadic aniridia, Denys-Drash syndrome, and trisomy 18. In these children, US is performed at regular intervals (approximately every 4 months) until age 7 years. ^[28]

Flank pain

A thorough history and physical examination are necessary to differentiate renal or ureteral pain from musculoskeletal pain. Pain that is altered by positional change is likely to originate in the muscles of the torso or the flank. In patients with flank pain, fever, and pyuria, pyelonephritis can be diagnosed and treated without immediate imaging. Failure to respond to therapy within the usual 48 hours should prompt renal US to look for infection associated with significant obstructive uropathy, which may necessitate placement of a percutaneous nephrostomy. Renal and ureteral calculi are best imaged with noncontrast spiral CT.

Symptomatic UPJ obstruction is becoming rarer because of antenatal detection (>50% of cases) on US during pregnancy; however, some patients with UPJ obstruction present later in life with intermittent flank pain, often with vomiting. If intermittent UPJ obstruction is considered but imaging findings do not support the diagnosis, the patients should be sent home with instructions to return immediately for imaging (US or IVU) when they are symptomatic. Once symptomatic UPJ obstruction is diagnosed, surgery is indicated. Postoperative renal scanning or IVU should reveal good drainage from the repaired kidney.

Gross hematuria

The pattern of hematuria can suggest its source. Initial hematuria or terminal hematuria is usually associated with urethral pathology. Total hematuria is usually due to renal or bladder pathology. The etiology of the gross hematuria is sometimes apparent from the patient’s history.

Hematuria associated with blunt injury is best evaluated with CT of the abdomen and pelvis. If blood at the meatus is observed in a male with a history of pelvic trauma, perform retrograde urethrography (RUG) before placement of a urethral catheter. Hematuria, gross or microscopic, and colicky pain suggestive of obstructive ureteral calculi can be initially evaluated with RBUS and an abdominal plain film. If the symptoms are severe or progressive and the imaging is inconclusive, noncontrast spiral CT of the abdomen and pelvis is the definitive test for renal or ureteral calculi.

In children, initially evaluate painless gross hematuria with US to exclude a renal anomaly or mass, as well as the possibility of bladder tumor (eg, rhabdomyosarcoma or, more rarely, transitional cell carcinoma). Transitional cell carcinoma is rare in children and is almost always low-grade. Cystitis may produce bullous lesions and may mimic a bladder tumor, as is the case with nephrogenic adenoma, a benign inflammatory lesion.

In an adolescent male with painless terminal hematuria, normal RBUS findings favor the diagnosis of urethrorrhagia or benign urethritis, a benign condition due to self-limited nonbacterial urethral inflammation.

Microhematuria

Not all patients with asymptomatic microhematuria require RBUS. First, confirm the diagnosis of microhematuria on the basis of the presence of red blood cells (RBCs) on microscopic examination of a freshly voided urine sample. Second, examine the urine for proteinuria and RBC and white blood cell (WBC) casts. If hypertension is present and if proteinuria and casts are found on microscopic examination of the urine, the patient has glomerulonephritis, and US may not be required; otherwise, US is usually necessary.

Perform RBUS in the setting of blunt trauma with no other associated injury, stable vital signs, and microhematuria; 10% of cases have an underlying congenital renal anomaly revealed by minor trauma. If the mechanism of injury is significant, perform contrast CT of the abdomen and pelvis. Delayed CT images are useful to look for urinary extravasation.

Microhematuria is commonly associated with hypercalciuria in children. Some authors recommend a spot calcium-to-creatinine ratio to screen for hypercalciuria in children with microhematuria. A 24-hour urine collection that shows more than 4 mg/kg of calcium is more accurate. The long-term significance of hypercalciuria and microhematuria in children is unclear, but patients with these conditions may be at increased risk for future nephrolithiasis.

Complex adrenal mass in neonate

About 70% of adrenal hemorrhages occur in the right adrenal gland. Adrenal hemorrhage presents as a flank mass or appears as a complex cystic collection on US. Over time, the hemorrhage shrinks, but it may persist as a small speckled calcification. Neuroblastoma can also have calcification and is occasionally cystic. Unlike an adrenal hemorrhage, however, neuroblastoma does not usually regress over 6 weeks. If neuroblastoma is suspected, perform the appropriate urine studies (homovanillic acid [HVA] and vanillylmandelic acid [VMA]).

Cloaca

Retrograde genitography and VCUG are helpful in outlining the level of confluence of the urethra, vagina, and colon when surgery is being planned. Baseline renal US is also necessary because renal agenesis, reflux, and megaureter are common associated findings. MRI of the pelvis is an important ancillary study in complex cloacal cases.

Cystic kidney disease

Multicystic dysplastic kidneys can be confused with dilated hydronephrotic kidneys. On US, multicystic kidneys generally tend not to have a reniform shape, show no connection between cysts, and do not show a dilated ureter. Renal scanning reveals that the multicystic kidney has no function. If renal US reveals classic findings of a multicystic dysplastic kidney and a contralateral hypertrophied kidney, a renal scan is not necessary for diagnosis.

Simple renal cysts are rare in children. The differential diagnosis includes calyceal diverticulum, an early manifestation of autosomal dominant polycystic disease (ADPKD). Autosomal recessive polycystic kidney disease (ARPKD) tends to present in children in a bimodal distribution, either (a) in infancy with enlarged kidneys, coexistent liver and biliary disease, and renal tubular ectasia or (b) during adolescence with enlarged kidneys, hepatic fibrosis, and portal hypertension.

Many syndromes are associated with cystic kidney disease, such as tuberous sclerosis, Meckel-Gruber, and von Hippel-Lindau. Autosomal-dominant polycystic kidneys are typically associated with multiple cysts of different sizes in older children and are rarely seen in infancy. It can also be asymmetric in its presentation. Differentiation is aided by careful US of the liver and a thorough family history.

Duplication of collecting system

The overall renal length may be greater on US, and multiple abnormalities may be seen in the upper and lower poles. IVU reveals a drooping lily sign. VCUG may reveal lower-pole reflux, and the upper-pole system may be normal, dilated, cystic, or dysplastic. If the upper pole is nonfunctional on IVU, too few calyces (only those of the lower pole) are observed, and the normally long upper-pole infundibulum is absent. The lower portion of a duplicated kidney usually accounts for two thirds of the function of a given kidney. Careful US examination of the bladder helps determine whether ureteral insertion is normal or ectopic and whether a ureterocele is present.

Exstrophy-epispadias complex

Baseline renal US is usually performed, though upper urinary tract abnormalities are rare in patients with classic bladder exstrophy. Renal abnormalities are much more common in patients with cloacal exstrophy. VUR is present in nearly all patients with exstrophic anomalies. After closure of the bladder, VCUG is useful to assess bladder capacity and to evaluate the competency of the bladder neck.

Nephromegaly

Renal vein thrombosis, uric acid nephropathy, acute tubular necrosis, infection, and nephroblastomatosis are all causes of large kidneys. Diffuse enlargement may also suggest infiltrative processes, such as leukemia and lymphoma. If US findings are inconclusive, differential diagnoses can be evaluated by obtaining a complete blood count (CBC) and performing CT, urinalysis, and, in some cases, a percutaneous renal biopsy.

Neurogenic bladder

Baseline renal US is necessary to assess for hydronephrosis, stone disease, and bladder-wall thickening. It should be performed at birth, if possible. Upper-tract dilation due to high bladder storage pressure initially manifests as a dilated ureter behind the bladder. The ability to classify patients into low-risk and high-risk groups depends on urodynamic findings.

Some centers routinely perform baseline cystography. The authors’ routine has been to perform follow-up US at age 6 months, at age 12 months, and annually thereafter. Urodynamic studies are performed in the newborn period and at age 1 year. If a child with a neurogenic bladder develops an infection or hydronephrosis, the storage function of the bladder may have changed. Cystography and urodynamic studies are then repeated to evaluate bladder storage function.

Posterior urethral valves

Bilateral hydroureteronephrosis, a thick-walled bladder, and a dilated posterior urethra are observed on antenatal US in males with PUVs. Postnatal renal US is used to evaluate the renal parenchyma because recognition of corticomedullary differentiation correlates with better renal function. Varying degress of renal cystic dysplasia may be seen. VCUG is key in determining if valves are the cause of bilateral hydronephrosis in a newborn boy. Postoperatively, the distention of the anterior urethra is improved, and the degree of distention of the posterior urethra should decrease.

Prune belly syndrome

On antenatal US, the bladder should not be as thick-walled in patients with prune belly syndrome (PBS) as in patients with PUV; however, the bladder may be massively dilated. Bilateral hydroureteronephrosis is found in patients with both PUV and PBS. The kidneys have a variable appearance, from dysplasia to hydronephrosis. VCUG characteristically reveals a funneling, dilated prostatic urethra. The bladder is usually large and is sometimes associated with a patent urachus or urachal diverticulum. The distal ureters are also dilated and tortuous. The anterior urethra varies and can be enlarged in the form of a scaphoid megalourethra or can be completely atretic.

Because most patients with PBS have VUR, the authors maintain indefinite antibiotic prophylaxis and try to avoid performing VCUG or other instrumentation of the lower urinary tract to avoid infecting the system. Patients should be monitored with renal US to assess hydronephrosis and renal scanning to assess function.

Ureterocele

Ureterocele is now often diagnosed with antenatal US. If the patient presents with sepsis, urgent puncture of the ureterocele may be indicated to decompress and drain the infected system. If the patient is healthy, VCUG is helpful in determining whether the ureterocele is intravesical or ectopic and whether associated reflux is present. These are all prognostic factors that determine whether the patient requires surgery beyond an endoscopic puncture of the ureterocele.

The authors have usually endoscopically punctured ureteroceles and maintained patients on prophylactic antibiotics. VCUG is performed 3-6 months post incision. If necessary, excision of the ureterocele and ureteral reimplantation are performed in the second year of life for associated reflux. A pseudoureterocele, due to a dilated ureter behind the bladder, mimics a ureterocele and is most commonly caused by an ectopic ureter inserting at or below the bladder neck.

Urethral stricture

RUG reveals the location of the stricture but often does not outline the entire length of the stricture. A thin catheter can sometimes be passed and VCUG performed to show the proximal extent of the stricture. If the stricture is too narrow, the patient may require a suprapubic tube to achieve bladder drainage, after which VCUG can be performed.

Wet umbilicus

The differential diagnoses include (1) weeping from an umbilical granuloma (most common), (2) patent omphalomesenteric duct (prolapse of bowel and very excoriated skin due to digestive enzymes), and (3) urachal pathology. US can often demonstrate urachal anomalies, such as a patent urachus, urachal cyst, or sinus. VCUG may determine if patency is present with the bladder dome.

Renal and bladder ultrasonography

RBUS is a useful tool in this setting. Bladder views (both prevoid and postvoid) should be included in the evaluation of the kidneys in children.

In neonates, the renal cortex is isoechoic or hyperechoic relative to the liver. In the immediate newborn period, the renal pyramids may be echogenic with transient stasis nephropathy due to Tamm-Horsfall protein. In children, the renal pyramids are hypoechoic, which allows for clear observation of the corticomedullary junction. Upper-tract hydronephrosis should always be reevaluated when the bladder is empty to determine the degree to which a full bladder affects the dilation.

Doppler US provides additional information by showing renal perfusion and vascularity. Areas of decreased blood flow with color-flow Doppler may indicate focal areas of acute pyelonephritis. The "twinkling artifact" improves the sensitivity of US for detecting small renal calculi.

The average newborn kidney is approximately 4.5 cm in length. Although antenatal compensatory hypertrophy was not previously thought to occur, it has been observed in patients with solitary or multicystic dysplastic kidney, in whom the newborn contralateral kidney is larger than normal. The presence of cortical cysts and increased echogenicity, indicators of dysplasia and poor function, are useful signs when a pyeloplasty in a minimally functioning kidney is being planned. Nephrectomy should be considered in such patients.

Advanced US techniques include the following:

• Harmonic imaging
• Three-dimensional (3D) US
• Voiding urosonography (VUS)

Harmonic imaging provides images that are clearer and sharper than those obtained with conventional US, particularly for larger patients. 3D US provides multiplanar capabilities similar to those of CT and MRI. Contrast-enhanced VUS, which uses an intravesical US contrast agent, can be used in the evaluation of UTI and in the follow-up of known VUR. ^[29]  Follow-up US is required for grade I or II hydronephrosis, and the option of performing VCUG should be discussed with the parents, as well as renal transplantation evaluation. ^[30]

VUS has come to be widely used in Europe as a radiation-free alternative to fluoroscopic or nuclear VCUG. ^[31] Attempts have been made to introduce VUS in Canada, where appropriate contrast agents are available.

In the United States, lack of a suitable FDA-approved contrast agent for children delayed the introduction of VUS. As of December 2016, however, sulfur hexafluoride lipid-type A microspheres (Lumason; Bracco Diagnostics, Milan, Italy) is approved by the FDA for use in contrast-enhanced VUS of the urinary tract in children.

An in-vitro study by Back et al, aimed at assessing optimization of dose and imaging parameters for the second-generation US contrast agent Optison (GE Healthcare, Princeton, NJ) in the setting of VUS, found that a very low dose would suffice for intravesical application. ^[32]

Intravenous urography

Cross-sectional imaging has decreased the use of IVU in the pediatric population. Initial US can reveal most of the anatomic detail necessary for management decisions, without the use of radiation or the risk of intravenous (IV) contrast. IVU is a functional study that depends on contrast uptake by the renal parenchyma and excretion to show the collecting system. Conditions in which IVU may be beneficial include the following:

• Evaluation of possible ectopic ureter or megaureters that are dilated to the level of the bladder
• UPJ obstruction if dilation of the ureter is observed on US to document that the ureter is normal distal to the UPJ
• A possible intermittent UPJ obstruction with normal renal scan findings

IVU is an excellent modality for vertically integrating the anatomy of the urinary tract, thus permitting distinction of anatomic variants that mimic UPJ obstruction (eg, extrarenal pelvis, wide infundibulum, or compound calyces) from true UPJ obstruction by revealing the anatomy of the UPJ.

The GFR approaches adult levels by age 6 months and is fully at adult levels by age 12-24 months. Therefore, IVU is most helpful after age 6 months because the concentrating ability of the renal tubules improves.

The initial scout film of IVU reveals radiopaque stones and the bowel gas pattern. Limited IVU is usually all that is necessary. The nephrography phase of IVU reveals contrast uptake, whereas the excretion phase shows the collecting system. Frontal films that are taken at 3 minutes and at 15 minutes may be all that is necessary to evaluate the upper urinary tract. The overall number of films should be adjusted (and kept to a minimum) according to each patient's physiology and indication.

On the early IVU films, contrast pooling may be observed at the edge of calyces or within collecting ducts, and is a good sign of obstruction (Dunbar crescents). In these patients, excretion should continue to be monitored with delayed films, sometimes up to 24 hours later, to show full filling of the obstructed renal unit.

Diuretic renography

Diuretic (furosemide) renography can be difficult to interpret for many technical reasons. The relative function is assessed on the basis of the uptake of radionuclide 2-3 minutes post injection. Because high-volume early reflux can cause erroneous readings, an indwelling catheter should be placed beforehand.

Relative function is measured by drawing a region of interest around each kidney and comparing the region with a background area. Significant errors are possible if the region of interest is drawn around the liver or spleen. Debate continues regarding whether supranormal function (split function >50% in the obstructed kidney) is a real physiologic entity or just a technical problem.

Two agents, diethylenetriamine-pentaacetic acid (DTPA) and MAG-3, are used in diuretic renography. Some centers believe that the use of MAG-3 yields better scan quality.

The rate of excretion is measured by following the washout of radionuclide through the UPJ after the pelvis has been filled. Furosemide is given to cause diuresis. Washout curves are then generated. A t_1/2 (time for one half of the isotope to wash out) shorter than 10 minutes represents no obstruction; a t_1/2 longer than 20 minutes indicates obstruction; and a t_1/2 between 10 and 20 minutes is indeterminate.

Conditions that complicate interpretation of the furosemide washout curve include a megaureter or pelvis that accepts a large bolus of urine and poor renal function. In the first situation, determining when the renal pelvis is full is difficult. If the renal pelvis is not monitored, the longer time required to fill the renal pelvis will cause an artefactual prolongation of the t_1/2, thereby making it difficult to diagnose obstruction. In the second situation, the timing of the furosemide administration is prolonged.

In order to overcome the problem of poor renal function or relative hypovolemia in a patient who has been fasting, the patient should be well hydrated with IV fluids before the study. The test is also operator-dependent; the furosemide should be administered when the renal pelvis is believed to be full. A full bladder also delays washout of isotope. Upper-tract drainage should be assessed after the patient voids, or the test should be performed with a catheter in the bladder.

Dimercaptosuccinic acid scanning

DMSA is bound in the cortex of the kidney after injection, revealing relative renal function. It is the criterion test for renal scarring. ^[33] A bladder catheter is not required for DMSA scanning, because DMSA is not excreted into the urine. The "top-down" approach has suggested that the DMSA scan be used as the primary test in patients with UTI, in that it selects for patients with renal injury. ^[15] Limitations of DMSA scanning include intermittent unavailability and the need for prolonged sedation in young children in order to obtain high-quality images.

Voiding cystourethrography

A small feeding tube (8 French for newborns, 10 French for infants) is passed via the urethra into the bladder. Contrast material is then dripped into the bladder under gravity. Serial radiographs of the pelvis and abdomen are then taken. The first film obtained is important for the diagnosis of ureterocele. It is observed as a round filling defect at this point but may be compressed with further filling. The bladder is then filled until the expected capacity is reached.

Tapping on the bladder or gentle massaging it is sometimes necessary to encourage the patient to void. Views of the kidneys and an oblique view of the male urethra are obtained once voiding has started. Ensuring that this is a voiding study is important because 20% of reflux occurs only upon voiding.

Parents often ask whether sedation will be used for VCUG. The authors do not routinely use sedation, the reason being that sedation may render VCUG less reliable because it may affect the force with which the bladder contracts and the propensity for reflux to occur.

The contrast in the bladder is usually as opaque or as bright as the contrast that refluxes. Dilution of contrast suggests stasis of urine (ie, urine mixing with the refluxed contrast) and suggests coexisting obstruction at the ureteral or renal pelvis level. Intrarenal reflux or reflux into compound papillae typically occurs at the poles of the kidney, which is why the poles are more susceptible to infection and scarring.

Cyclic VCUG, with two or more cycles of filling and emptying, is necessary to reveal reflux into an obstructed and refluxing system, such as an ectopic ureter draining into the bladder neck or urethra. During the first void, the urine in the pressurized obstructed system drains, and on the subsequent void, contrast may be observed to reflux. Cyclic VCUG has also been demonstrated to increase the sensitivity of the test.

Nuclear VCUG is a good choice for follow-up studies in patients with UTI or in screening for siblings of patients with reflux, in that the radiation exposure is decreased; however, in patients with an abnormal ureteral anatomy (eg, duplication) that would influence a surgical approach, contrast VCUG is indicated.

When VCUG is performed after valve ablation, the posterior urethral dilation may not resolve completely, but good distention of the anterior urethra should be present, as well as a decrease in the previous size gradient in the urethra caused by the valve.

Computed tomography

CT is mainly used in children for evaluation of blunt abdominal trauma, as well as the diagnosis and follow-up of renal and ureteral calculi. The use of noncontrast spiral CT is growing in patients with urolithiasis because of its higher sensitivity at detecting calculi, and this modality is becoming increasingly popular as a second-line study in selected patients with spina bifida whose body habitus prevents good visualization with US.

If the diagnosis of urolithiasis is considered in a child with minimal symptoms, it is reasonable to start with renal US and plain radiography and then proceed to CT if initial findings are negative and clinical symptoms persist. Pediatric radiologists adjust the CT study so that children are exposed to much lower radiation doses than adults undergoing the same study. ^[34] However, concerns remain regarding the relatively high radiation levels in CT. The use of US and plain radiography in the follow-up of renal and ureteral calculi should be encouraged, unless the calculi can only be detected via CT.

State-of-the-art CT includes two imaging modalities, 3D CT angiography (CTA) and 3D CT urography (CTU). Each of these methods requires precise synchronization between contrast media delivery and scan acquisition. Each should be considered if US and MRI yield indeterminate results, if higher spatial resolution is required, or if a more rapid scan time is desired. CTA can evaluate renal hypertension, vasculitis, vascular malformations, and traumatic injury. CTU is used less often and can be applied in assessing congenital anomalies, obstruction, and traumatic collecting system injury.

Magnetic resonance urography

The use of MRU is increasing in children. ^[35] The benefits of MRU are that both anatomic and functional information can be obtained from one study. Applications of MRU include identification of congenital anomalies, ^[36] urinary tract obstruction, and hematuria; differentiation of infection from scarring; and posttreatment assessment of the urinary tract.

MRU correlates well with diuretic renal scanning in terms of assessing renal function and may be superior in terms of distinguishing nonobstructive hydronephrosis from obstructive hydronephrosis. ^[37] Its use is currently limited by the need for sedation, cost, and the need for specialized protocols but is nonetheless becoming more widespread. The other area where MRU yields better anatomic information than US is in fetal diagnosis. ^[38]

Functional MRU for hydronephrosis requires prehydration and a bladder catheter. Furosemide is given 10 minutes before the patient is placed on the table. Because the contrast agent is heavier than urine, the patient should be positioned prone so that contrast collects at the UPJ. The following three sets of data are generated during functional MRU:

• Time-to-peak (TTP), which is the time for maximal concentration of contrast in the parenchyma
• Calyceal transit time (CTT), which is the time for contrast to first appear in the calyces
• Renal transit time (RTT), which is the time when contrast first reaches the proximal ureter

Functional data are determined by calculating renal parenchymal volume to determine relative function, and the Patlak number serves as an index for the GFR. ^[39]

Having the preoperative images, informing the radiologist of the details of the procedure, and having the question to be answered (persistent reflux or hydronephrosis) are helpful when performing postoperative imaging, so that the study can be appropriately tailored to the situation.

Ureteral reimplantation or endoscopic treatment of reflux

Perform RBUS 1 month after surgery to evaluate for silent obstruction. Perform conventional or nuclear VCUG at 3 months after surgery, and discontinue prophylactic antibiotics after the VCUG if surgery is successful. Some feel that the rate of successful correction of VUR with open surgery (95-98%) is high enough that postoperative VCUG is unnecessary unless the patient has a subsequent febrile UTI. Follow-up US can be performed at 1 year and 5 years after surgery.

Pyeloplasty

The follow-up protocol depends on the function of the kidney and the original indication for surgery. Postoperative problems usually present within 18 months, and prolonged follow-up is not useful. The three variables to be evaluated are as follows:

• Adequate drainage
• Renal function
• Degree of hydronephrosis

A combination of US, IVU, and renal scanning is selected to assess if the surgery was successful.

If the kidney had poor function or if significant intrarenal hydronephrosis was present, leaving a nephrostomy tube should be considered because the renal dilation may always appear significant. Allowing contrast to cross the anastomosis may be the only way to assess patency. If a nephrostomy tube has been placed intraoperatively, low-pressure injection through the tube (nephrostography) can be performed 2 weeks after surgery, with the patient in the prone position, with the contrast hung at a height of 40 cm.

If nephrostography reveals good drainage into the bladder, the nephrostomy tube can be clamped. If the patient remains asymptomatic, the tube is removed a few days later. Occasionally, the nephrostomy tube is placed too close to the repair and causes edema, obstructing the UPJ. This is remedied by pulling the tube back into the renal pelvis and repeating nephrostography a week later. This step ensures that the anastomosis is patent and that adequate drainage has been achieved.

Renal US can be performed 6 months after surgery to evaluate the hydronephrosis, and renal scanning is performed 1 year after surgery to determine the final relative function. Dynamic MRU can be used to assess the results of pyeloplasty by comparing preoperative and postoperative parameters.

If the pyeloplasty was performed to correct a decrease in relative renal function, perform RBUS 1 month after surgery and renal scanning 3 months after surgery to ensure that renal function is stable or improved. IVU can be performed 6 months after surgery to define the anatomy. If the renal function is good, IVU provides a baseline for later follow-up with US. A follow-up renal scan 1 year after surgery shows the degree of ultimate renal improvement.

Some patients present with classic flank pain and nausea and have symmetric function on renal scanning. Perform US 1 month after surgery to check for hydronephrosis, and consider a renal scan 3 months after surgery to check drainage and function. For these patients, the important outcome is that they no longer have symptomatic UPJ obstruction.

Posterior urethral valve ablation

If the patient is improving clinically (creatinine level decreasing appropriately), perform RBUS 1 month after surgery. The timing of VCUG is controversial. Some authors prefer performing VCUG shortly after PUV ablation to ensure that no obstruction persists, whereas others advocate waiting 6 months. If persistent urethral obstruction is suspected, perform the studies earlier. Renal scans are useful to evaluate relative renal function.

Ureterocele incision

Perform RBUS 1 month after surgery (earlier if the patient presented with sepsis) to ensure that adequate decompression has occurred. VCUG and RBUS can be performed 3-6 months later. Subsequent imaging will depend on the presence of residual VUR.