eMedicine Specialties > Pediatrics: Surgery > Urology

Radiographic Evaluation of the Pediatric Urinary Tract

Author: Hsi-Yang Wu, MD, Assistant Professor of Urology, Department of Urology, Children's Hospital of Pittsburgh
Coauthor(s): Richard Bellah, MD, Associate Professor, Departments of Radiology and Pediatrics, The Children's Hospital of Philadelphia and University of Pennsylvania School of Medicine; Howard M Snyder III, MD, Professor, Department of Surgery, Division of Pediatric Urology, University of Pennsylvania School of Medicine
Contributor Information and Disclosures

Updated: Sep 4, 2008

Introduction

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 a physician with a patient in the office. Imaging should be tailored to coincide with an expected change in therapy.

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.

Common Problems

Prenatal hydronephrosis

Dilatation of the upper urinary tract is often detected using prenatal ultrasonography performed at or after 20 weeks' gestation (£ 10% 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. Renal and bladder ultrasonography 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 ultrasonography findings. If the findings are normal at this point, ultrasonography should be repeated 4-6 weeks later.

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

Prophylactic antibiotic administration is maintained until voiding cystourethrography (VCUG) can be performed, if persistent dilation of the upper urinary tract is documented. Approximately 11% of patients with a history of prenatal hydronephrosis have vesicoureteral reflux (VUR) findings on postnatal VCUG. Whether all patients should be screened using VCUG remains controversial. The purpose of the neonatal evaluation of hydronephrosis is to determine if the hydronephrosis is due to VUR, an obstructive uropathy, or both.

Parents often ask whether sedation is used in VCUG. The authors do not recommend the use of sedation because it may result in less-reliable findings. Sedation may affect the force with which the bladder contracts and the propensity for reflux to occur. The authors also do not recommend the use of intravenous urography (IVU) as part of the initial examination, although specific indications for IVU are discussed below.

Because ultrasonography reveals varying degrees of renal pelvis dilation, one of 3 imaging scenarios may be encountered. These scenarios are as follows: 

• VCUG reveals reflux, and radionuclide renal scanning may or may not be performed, depending on whether associated or secondary ureteropelvic junction (UPJ) obstruction is a consideration. 
• VCUG reveals no reflux, and renal scanning reveals partial obstruction.
• VCUG reveals no reflux, and a renal scan reveals no obstruction.

Vesicoureteral reflux

If VCUG reveals VUR, counsel the parents on the options of maintaining antibiotic prophylaxis versus surgical repair. The degree and laterality of the reflux influences the decision. 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

Repeat nuclear cystography and renal and bladder ultrasonography annually to assess for resolution of reflux and to evaluate renal growth if nonsurgical management is used, which is often the case in patients with grade I-III reflux; however, grade IV and V reflux can resolve in newborn boys because voiding pressures dramatically decrease after the first year of life.

Ureteropelvic junction obstruction

If postnatal ultrasonography reveals only pelviectasis or mild-to-minimal dilatation of the renal collecting system, renal scanning is not usually performed because obstruction is unlikely. If postnatal ultrasonography reveals significant hydronephrosis (caliectasis, not just pelviectasis) and parenchymal thinning, renal scanning should be considered. The glomerular filtration rate (GFR) is sufficiently increased, allowing renal scanning 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 pyeloplasty is indicated. When bilateral renal obstruction is discovered, assessment of the extraction factor (activity at 2-3 min after injection) may be helpful because it correlates well with the absolute GFR of each kidney.

Lasix renal scanning 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, is affected by too many variables to be a factor in good decision-making. If observation is chosen because the relative renal function is preserved (>35%), the timing of the next renal scan depends on the severity of the hydronephrosis. A patient with unilateral 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, the length of time between follow-up studies may be increased to 6 months.

Three normal anatomic variants (extrarenal pelvis, wide infundibuli, compound calyces) can 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. The diagnosis can also be made with IVU or contrast-enhanced CT scanning.

Ureterovesical junction obstruction

Congenital megaureter is another antenatally detected abnormality. Hydroureteronephrosis is observed to the level of the bladder on initial ultrasonography findings. 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, it is likely to be benign and rarely requires surgery. With careful observation, the dilatation slowly resolves, and the renal function is usually preserved. The diameter of the ureter has been shown to have prognostic value, with a diameter greater then 1 cm usually associated with persistent dilation.

Urinary tract infection

Renal and bladder ultrasonography and VCUG should be performed in children younger than 5 years.¹ Ultrasonography reveals any upper tract abnormalities (hydronephrosis, dilated ureter) that may predispose a patient to bacterial colonization. VCUG is used to evaluate the presence of reflux, a risk factor for upper tract scarring when coexistent infection is present. The study is normally performed near the end of the hospital stay, if the patient has been admitted for intravenous antibiotic therapy. Further work-up with VCUG and ultrasonography may be necessary in older children if abnormalities of the urinary tract are revealed.

The authors recommend screening all febrile children with a documented urinary tract infection (UTI). The usefulness of ultrasonography in patients with acute pyelonephritis has been questioned. Although this remains controversial, the authors recommend performing ultrasonography because the chance of finding an obstructive lesion is 4%. Many patients have a single UTI in the neonatal period, normal renal and bladder ultrasonography findings, and no evidence of reflux on VCUG. Generally, the authors administer prophylaxis to these patients for 6 months while the immune system gains competence and the kidneys grow. Antibiotic administration can then be discontinued, and these patients can be clinically monitored without further imaging studies.

A failure to respond to standard intravenous antibiotic therapy within 48 hours should be further investigated with renal and bladder ultrasonography to determine the presence of an obstructive process or renal abscess formation. 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 (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 patients, gallium scanning reveals when inflammation has resolved.

Some centers routinely use dimercaptosuccinic acid (DMSA) scanning to diagnose pyelonephritis; however, areas of diminished perfusion on DMSA scanning during an acute infection do not necessarily correspond with areas that develop a pyelonephritic scar later. If previous abnormalities are revealed on DMSA scanning, 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, patients with continent reconstruction who rely on clean intermittent catheterization to empty their reservoirs) in whom a febrile episode develops.

The finding of global renal parenchymal abnormality on DMSA scanning is associated with grade IV-V VUR in male infants. These patient may have abnormal renal development or renal dysplasia. Reflux in patients with high-grade reflux and associated renal abnormalities is unlikely to resolve by age 16 months.

Incontinence

Primary nocturnal enuresis with no 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, consider VCUG to assess 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 MRI. MRI is especially helpful in defining the pelvic course of the ectopic ureter.

Mass lesions

In children with abdominal masses, either ultrasonography and CT scanning or CT scanning alone 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.

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 treatment in 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. Treat angiomyolipomas 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. 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 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 microscopy. 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 is not 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 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 exact. 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.

Less Common Problems

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 findings. 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 (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.

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 and VCUG reveals lower-pole reflux while the upper-pole system is hydronephrotic. If the upper pole is nonfunctional on IVU, too few calyces are observed (only those of the lower pole), 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. Vesicoureteral reflux (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 used to evaluate storage function of the bladder.

Posterior urethral valves

Bilateral hydroureteronephrosis, a thick-walled bladder, and a dilated posterior urethra in males are usually observed on prenatal ultrasonography. 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 posterior urethral valves 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 often detected on 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. Ureteroceles can be endoscopically punctured, and prophylactic antibiotics are maintained. VCUG is performed 6 months postincision. The ureterocele is excised, and the ureter is reimplanted in the second year of life (if needed) 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.

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.

Imaging Modalities

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. Consider a primary nephrectomy in such patients.

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
• Prior to proceeding to ureteroscopy or percutaneous nephrolithotomy for renal stones

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.

Glomerular filtration rate (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. 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 (MAG3), are used in Lasix renal scanning. Some centers believe that the use of MAG3 results in better-quality scanning.

The extraction factor is calculated by measuring the activity between 2 and 3 minutes after injection and expressing the activity as a percentage of the injected radionuclide dose. The extraction factor (EF) correlates with single-kidney GFR calculated by DTPA with a correlation coefficient of 0.92. A newborn's kidney has an EF of 1.5% that increases to 2.5% by age 1 year. It allows assessment of absolute renal function via renal scanning, which is useful in patients with bilateral renal pathology, making the relative renal function an unreliable parameter for determining operative management. However, only a few nuclear medicine units perform this calculation because it has not gained worldwide acceptance.

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 condition, determining when the renal pelvis is full is difficult. In the second condition, 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.

Voiding cystourethrography

A small feeding tube (8F for newborns, 10F for infants) is passed via the urethra into the bladder. 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.

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 voiding cystourethrography (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. Instill the age-appropriate volume to adequately test for reflux. Nuclear VCUG is a good choice for follow-up studies in patients with urinary tract infection (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 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.² 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.³ 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. 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.

Postoperative Imaging

Accessing the preoperative films and informing the radiologist of the details of the procedure are helpful when performing postoperative imaging so an appropriately tailored study can be performed. Whether persistent reflux is present and whether hydronephrosis is persistent should also be elucidated.

Ureteral reimplantation or endoscopic treatment of reflux

Perform renal and bladder ultrasonography one month postsurgery to evaluate for silent obstruction. Perform conventional or nuclear voiding cystourethrography (VCUG) at 3 months postsurgery and discontinue prophylactic antibiotics after 3 months if surgery is successful. Some feel that the rate of successful correction of vesicoureteral reflux (VUR) with open surgery is high enough that postoperative VCUG is not necessary unless the patient has a subsequent urinary tract infection (UTI). Follow-up ultrasonography should 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, leave a nephrostomy tube 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 scanning one month after surgery to ensure that the renal function is stable or improved. Perform IVU 6 months after surgery to define the anatomy. If the renal function is good, IVU provides a baseline for later ultrasound follow-up. A follow-up renal scan one year after surgery depicts the degree of ultimate renal improvement. Finally, if the patient presented with classic flank pain and nausea, IVU is usually performed before surgery, revealing symmetric function and well-defined anatomy. Perform ultrasonography one month after surgery to check for hydronephrosis, perform IVU 6 months after surgery to evaluate anatomy, and perform renal scanning one year later 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 one month after surgery and perform VCUG 6 months after surgery. If persistent urethral obstruction is suspected, perform the studies earlier. Renal scans are useful to evaluate relative and absolute renal function.

Ureterocele incision

Perform renal and bladder ultrasonography one month after surgery (earlier if the patient presented with sepsis) to ensure adequate decompressions have begun. VCUG is performed at 6 months. Maintain prophylactic antibiotics.

Keywords

radiographic evaluation of the pediatric urinary tract, imaging of the pediatric urinary tract, uroradiology, prenatal hydronephrosis, vesicoureteral reflux, VUR, ureteropelvic junction obstruction, ureterovesical junction obstruction, urinary tract infection, UTI, incontinence, mass lesions, abdominal mass, flank pain, gross hematuria, microhematuria, complex adrenal mass, cloaca, cystic kidney disease, CKD, duplication of collecting system, exstrophic anomalies, nephromegaly, neurogenic bladder, posterior urethral valves, PUV, prune belly syndrome, PBS, ureterocele, urethral stricture, wet umbilicus, voiding cystourethrogram, VCUG, obstructive uropathy, intravenous urogram, IVU, ultrasound, ureteropelvic junction obstruction, UPJ obstruction, ectopic ureter, magnetic resonance imaging, MRI, magnetic resonance urography, MRU, CT scan, computed tomography, genitography, retrograde urethrogram, RUG, renal and bladder ultrasound, Lasix renogram

• Search for CME/CE on This Topic »

• Tracheobronchomalacia in Children