Radiographic Evaluation of the 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, followed by a brief description of common pitfalls encountered with each imaging modality. A simplified review of postoperative imaging is provided in the conclusion.

Prenatal hydronephrosis

Dilatation of the upper urinary tract is often detected using prenatal ultrasonography performed at or after 20 weeks' gestation (approximately 1% of pregnancies). Such findings may require sequential ultrasonography of the fetus. Depending on the degree of dilation, bilaterality, and presence of other anomalies, prenatal consultation by a pediatric urologist may be indicated. Follow-up renal and bladder ultrasonography should be performed after the first day of life if anomalies are detected prenatally. Newborns have a physiologic oliguria on the first day of life that can lead to false-negative ultrasonography findings. If the findings are normal at this point, ultrasonography should be repeated 4-6 weeks later. The Society for Fetal Urology scale for hydronephrosis allows for more precise discussion of hydronephrosis.^[1]

• Grade I - Splitting of the renal pelvis
• Grade II - Renal pelvis dilatation; 1 or 2 calyces seen; preserved parenchyma
• Grade III - Diffuse caliceal dilation; preserved renal parenchyma
• Grade IV - Diffuse caliceal dilation; parenchyma thinned to less than half the thickness of the contralateral kidney

For practical purposes, ultrasonography 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.

Prophylactic antibiotic administration has been recommended until voiding cystourethrography (VCUG) can be performed if persistent dilation of the upper urinary tract is documented. Approximately 12% of patients with a history of prenatal hydronephrosis have vesicoureteral reflux (VUR) on postnatal VCUG. Whether all patients should be screened using 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 ultrasonography is required for grade I or II hydronephrosis, and the option of performing VCUG should be discussed with the parents. Grade III or IV hydronephrosis most likely requires 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 ureteropelvic or ureterovesical junction obstruction.

Vesicoureteral reflux

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

• 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
• 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 renal and bladder ultrasonography are performed to assess for resolution of reflux and to evaluate renal growth. This is often the case in patients with grade I-III reflux; however, grade IV and V reflux can self-resolve in newborn boys because voiding pressures dramatically decrease after the first year of life.^[5]

Ureteropelvic junction obstruction

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

When bilateral renal obstruction is discovered, assessment of the extraction factor (a measure of the uptake of radionuclide in the renal cortex 2-3 min after injection) may be helpful because it correlates well with the absolute GFR of each kidney.

Lasix renal scanning (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 (>35% of cases). 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 symmetrical function on renal scanning should undergo repeat renal scanning within 3 months of the initial evaluation.

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

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

Ureterovesical junction obstruction

Congenital megaureter is another antenatally detected abnormality. Hydroureteronephrosis is observed to the level of the bladder on initial ultrasonography. VCUG and renal scanning are necessary to differentiate obstructed, refluxing, and nonrefluxing megaureters. If the megaureter is associated with a kidney that has near equal function to the contralateral kidney and the diameter of the dilated ureter is 1 cm or less, the process is likely to be benign and rarely requires surgery. With careful observation, the dilatation slowly resolves, and the renal function is usually preserved.

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 renal and bladder ultrasonography 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.^[6] The RIVUR trial, which is comparing daily antibiotic prophylaxis with placebo in patients with VUR, is attempting to determine whether the current standard of care (ie, daily prophylaxis) is effective. This has led some physicians to suggest that imaging be performed only after the second febrile UTI^[4] or to use a “top-down” imaging approach, reserving VCUG for patients with demonstrated renal abnormalities.^[7]

Since renal and bladder ultrasonography is noninvasive, it is a useful initial test. Ultrasonography 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.^[8] If there is no clinical response to antibiotic therapy within 48 hours of initiation, renal and bladder ultrasonography is useful to look for obstruction or renal abscess formation.^[9] Repeat imaging with renal and bladder ultrasonography 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 renal and bladder ultrasonography, and no evidence of reflux on VCUG, some clinicians recommend 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 positive VCUG findings, compared with 33% of patients older than 1 year.^[10] Of patients younger than 1 year with positive VCUG findings, 50% will have evidence of renal lesions on dimercaptosuccinic acid (DMSA) scan; patients older than 1 year old with a positive VCUG finding have a 33% chance of having renal scarring.^[7] 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.^[11]

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.^[12] 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).

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.^[13] Ten percent of these patients will have a poorly functioning kidney on the side of the reflux.^[5]

Incontinence

Primary nocturnal enuresis without a history of infection does not require imaging. Daytime wetting may require ultrasonography 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, a VCUG should be considered to look for a late presentation of PUVs. Constant dribbling in a female who appears to void normally suggests ureteral ectopy, with an opening below the urinary sphincter.

Although renal and bladder ultrasonography may be initially performed to assess for evidence of a duplex kidney, IVU and CT scanning are regarded as more definitive studies. Both are helpful in planning surgery by delineating the anatomy of the ectopic ureter. In cases in which IVU or CT scanning findings fail to be diagnostic and the clinical history is compelling, consider magnetic resonance urography (MRU). MRU is especially helpful in defining the pelvic course of the ectopic ureter.

Abdominal masses

In children with abdominal masses, CT scanning with or without ultrasonography may be used in the initial evaluation. Renal ultrasonography with Doppler is used to determine whether the mass is solid or cystic and reveals if tumor invasion of the inferior vena cava is present. 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 can be due to hydronephrosis, multicystic dysplastic kidney, or cystic renal disease. If the cysts are suspected to be due to hydronephrosis or multicystic dysplasia, VCUG should be performed because VUR is found in approximately 10%-20% of cases (see Cystic Kidney Diseases). Simple renal cysts that are detected incidentally with ultrasonography can be monitored for one year to ensure that the cysts are not enlarging and that no new cysts are appearing, suggestive of autosomal dominant polycystic kidney disease. If ultrasonography reveals no changes, it does not need to be repeated unless the patient has pain or infection.

Cysts are classified using 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. Unless ultrasonography and CT scanning can be used to clearly classify the cystic renal lesion as benign, surgical exploration becomes necessary.

Solid masses are more ominous and may represent Wilms tumor, neuroblastoma, renal cell carcinoma, congenital mesoblastic nephroma, or other less common tumors, such as 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, weight loss). Wilms tumor commonly displaces and compresses vessels. Neuroblastoma also displaces and compresses vessels but is more often infiltrative.

Ultrasonography with Doppler can reveal inferior vena cava (IVC) invasion. CT scanning plays an important role in the staging of Wilms tumor and 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. Renal cell carcinoma is unusual in young children and is more common in adolescents. Renal cell carcinoma 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 treated if they cause pain or bleeding. 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. Ultrasonography is performed at regular intervals in these children, approximately every 6 months.

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 ultrasonography to look for infection associated with significant obstructive uropathy that may require placement of a percutaneous nephrostomy. Renal and ureteral calculi are best imaged with noncontrast spiral CT scanning.

Symptomatic UPJ obstruction is becoming rarer because of prenatal detection (>50% of cases) using ultrasonography 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 (ultrasonography 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 based on the patient’s history. Evaluate hematuria associated with blunt injury with CT scanning 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 should be evaluated with noncontrast spiral CT scanning of the abdomen and pelvis. In children, initially evaluate painless gross hematuria with ultrasonography to exclude a renal anomaly or mass and the possibility of bladder tumor, such as 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 renal and bladder ultrasonography 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 renal and bladder ultrasonography. First, confirm the diagnosis of microhematuria based on the presence of red cells on microscopic examination of a freshly voided urine sample. Second, examine the urine for proteinuria and RBC and WBC casts. If hypertension, proteinuria, and casts are present upon microscopic examination of the urine, the patient has glomerulonephritis and ultrasonography may not be required. Otherwise, an ultrasonography is usually needed. Perform renal and bladder ultrasonography in the setting of blunt trauma with no other associated injury, stable vital signs, and microhematuria because 10% of cases have an underlying congenital renal anomaly revealed by minor trauma. If the mechanism of injury is significant, perform CT scanning with contrast 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 the neonate

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

Cloaca

Retrograde genitography and voiding cystourethrography (VCUG) are helpful in outlining the level of confluence of the urethra, vagina, and colon when planning surgery. Baseline renal ultrasonography is also necessary because renal agenesis, reflux, and megaureter are common associated findings.

Cystic kidney disease

Multicystic dysplastic kidneys can be confused with dilated hydronephrotic kidneys. On ultrasonography findings, 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 ultrasonography 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. Autosomal recessive polycystic kidney disease (ARPKD) tends to present in children, whereas autosomal dominant polycystic disease (ADPKD) tends to present in adults. Both reveal function on renal scanning.

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. Differentiation is aided by careful ultrasonography of the liver and a thorough family history.

Duplication of collecting system

The overall renal length may be greater on ultrasonography. Intravenous urography (IVU) reveals a drooping lily sign. The VCUG may reveal lower-pole reflux, and the upper-pole system may be hydronephrotic. 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.

Exstrophy-epispadias complex

Baseline renal ultrasonography is usually performed, although 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 ultrasonography findings are inconclusive, differential diagnoses can be established or eliminated by obtaining a CBC count and performing CT scanning, urinalysis, and, in some cases, a percutaneous renal biopsy.

Neurogenic bladder

Baseline renal ultrasonography 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 ultrasonography at age 6 months, 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, 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 prenatal ultrasonography in males with PUVs. Postnatal renal ultrasonography is used to evaluate the renal parenchyma because the recognition of corticomedullary differentiation correlates well with better renal function. VCUG is key in determining if this is the cause of bilateral hydronephrosis in a newborn boy. Postoperatively, the distension of the anterior urethra is improved, and the degree of distension of the posterior urethra should decrease.

Prune belly syndrome

On prenatal ultrasonography, the bladder should not be as thick-walled as in patients with PUV; however, the bladder may be massively dilated. Bilateral hydroureteronephrosis is found in patients with both PUV and prune belly syndrome (PBS). VCUG characteristically reveals a funneling, dilated prostatic urethra. The bladder is usually large and is sometimes associated with a patent urachus. The distal ureters are also dilated and tortuous. The anterior urethra varies and can reveal enlargement in the form of a scaphoid megalourethra or can be completely atretic. Because most patients with PBS reflux, 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. Monitor patients using renal ultrasonography to assess hydronephrosis and renal scanning to assess function.

Ureterocele

Ureterocele is now often diagnosed with prenatal ultrasonography. 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 assessing 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 postincision. If necessary, excision of the ureterocele and ureteral reimplantation are performed in the second year of life for associated reflux. A pseudoureterocele is due to a dilated ureter behind the bladder. A pseudoureterocele 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 can be 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 skin very excoriated by digestive enzymes), and (3) urachal pathology. Ultrasonography 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

Bladder views (both prevoid and postvoid) should be included when evaluating 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. This is the only way to differentiate between primary ureterovesical junction obstruction due to secondary upper tract dilatation and primary ureterovesical junction obstruction due to a bladder cause.

The average newborn kidney is approximately 4.5 cm in length. Although prenatal 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 planning a pyeloplasty in a minimally functioning kidney. Nephrectomy should be considered in such patients.

Intravenous urography

Cross-sectional imaging has decreased the use of intravenous urography (IVU) in the pediatric population. Ultrasonography can initially reveal most of the anatomic detail necessary for management decisions, without the use of radiation or the risk of intravenous 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
• Ureteropelvic junction obstruction (UPJ) obstruction if dilation of the ureter is observed on ultrasonography to document that the ureter is normal distal to the UPJ
• A possible intermittent UPJ obstruction with normal renal scan findings

IVU is still the best imaging study for vertically integrating the anatomy of the urinary tract, thus permitting distinction of anatomic variants that mimic UPJ obstruction (extrarenal pelvis, wide infundibulum, compound calyces) from true UPJ obstruction by revealing the anatomy of the UPJ.

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 radio-opaque 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. Adjust the overall number of films, 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 and within collecting ducts and is a good sign of obstruction (Dunbar crescents). In these patients, continue monitoring the excretion with delayed films, sometimes up to 24 hours later, to show full filling of the obstructed renal unit.

Lasix renography

Lasix renography can be difficult to interpret for many technical reasons. The relative function is assessed by the uptake of radionuclide 2-3 minutes postinjection. High-volume early reflux can cause erroneous readings, so an indwelling catheter should be placed prior to initiating the study. Relative function is observed 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 mercaptoacetyl triglycine (MAG-3), are used in Lasix renal scanning. Some centers believe that the use of MAG-3 results in better-quality scanning.

The rate of excretion is measured by following the washout of radionuclide through the UPJ after the pelvis has been filled. Lasix is given to cause diuresis. Washout curves are generated, and a t½ (time for one-half of the isotope to wash out) of less than 10 minutes represents no obstruction, whereas a t½ of longer than 20 minutes indicates obstruction, and times between 10 and 20 minutes are indeterminate.

Conditions that complicate interpretation of the Lasix 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. In the second situation, the timing of the Lasix 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 intravenous fluids prior to the study. The test is also operator dependent, as the Lasix should be administered when the renal pelvis is believed to be full. A full bladder also delays washout of isotope. Assess upper tract drainage after the patient voids or perform the test with a catheter in the bladder.

Dimercaptosuccinic acid scanning

Dimercaptosuccinic acid (DMSA) is bound the cortex of the kidney after injection, revealing relative renal function. It is the criterion standard test for renal scarring.^[14] A bladder catheter is not required for the DMSA scan, since it 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 urinary tract infection (UTI), since it selects for patients with renal injury.^[7] Limitations of DMSA scanning include intermittent unavailability and the occasional need for sedation in young children in order to obtain high-quality images.

Voiding cystourethrography

A small feeding tube (8F for newborns, 10F 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 voiding cystourethrography (VCUG). The authors do not use sedation, as it may result in a less-reliable VCUG. In addition, sedation 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 a reason why the poles are more susceptible to infection and scarring.

Cyclic VCUG, with 2 or more cycles of filling and emptying, is necessary to reveal reflux into an obstructed and refluxing system, such as an ectopic ureter 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, as 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 completely resolve, but good distension 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 scanning is mainly used in children for evaluation of blunt abdominal trauma, and the diagnosis and follow-up of renal and ureteral calculi. The use of noncontrast spiral CT is increasing in patients with urolithiasis due to its higher sensitivity at detecting calculi and as a second-line study in selected patients with spina bifida whose body habitus prevents good visualization with ultrasonography. Because children may be more susceptible to the long-term effects of radiation, the use of ultrasonography and plain radiography in the follow-up of renal and ureteral calculi should be encouraged, unless the calculi are only detectable via CT.

Starting with renal ultrasonography and plain radiography is reasonable if the diagnosis of urolithiasis is considered in a child with minimal symptoms before proceeding 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.^[15] However, concerns remain regarding the relatively high radiation levels in CT scanning.

Magnetic resonance urography

The use of magnetic resonance urography (MRU) is increasing in children.^[16] The benefits of MRU are that both anatomic and functional information can be obtained from one study. MRU correlates well with Lasix renal scanning in terms of assessing renal function and may be superior in terms of distinguishing nonobstructive hydronephrosis from obstructive hydronephrosis.^[17] MRU is currently limited by the need for sedation, cost, and the need for specialized protocols, but it may become widely used in the future. The other area in which MRI gives superior anatomic information compared with ultrasonography is in fetal diagnosis.^[18]

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 in order to perform an appropriately tailored study.

Ureteral reimplantation or endoscopic treatment of reflux

Perform renal and bladder ultrasonography one month after surgery to evaluate for silent obstruction. Perform conventional or nuclear voiding cystourethrography (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 urinary tract infection (UTI). Follow-up ultrasonography 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 3 variables to be evaluated include adequate drainage, renal function, and degree of hydronephrosis. A combination of ultrasonography, intravenous urography (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 renal dilation always appears 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 nephrostomy tune (nephrostography) can be performed 2 weeks after surgery, with the patient in the prone position.

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 ureteropelvic junction (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 ultrasonography can be performed 6 months after surgery to evaluate the hydronephrosis, and renal scanning is performed one year after surgery to determine the final relative function.

If the pyeloplasty was performed to correct a decrease in relative renal function, perform renal and bladder ultrasonography one month after surgery and renal scanning 3 months after surgery to ensure that the renal function is stable or improved. An IVU can be obtained 6 months after surgery to define the anatomy. If the renal function is good, IVU provides a baseline for later ultrasonographic follow-up. A follow-up renal scan one 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 ultrasonography 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 levels decreasing appropriately), perform renal and bladder ultrasonography 1 month after surgery. Timing of the VCUG is controversial. Some authors prefer performing VCUG shortly after valve ablation to ensure that no obstruction persists, while 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 renal and bladder ultrasonography 1 month after surgery (earlier if the patient presented with sepsis) to ensure adequate decompression has occurred. VCUG and renal and bladder ultrasonography can be performed 3-6 months later. Subsequent imaging will depend on the presence of residual VUR.