On completion of this chapter, you should be able to:
Discuss the sonographic approach to imaging neonatal/pediatric kidneys and adrenal glands
Distinguish normal anatomy and sonographic findings from abnormal findings
List and discuss the pathologic conditions covered in this chapter
Sonography is the diagnostic imaging method of choice when a urinary tract or adrenal abnormality is suspected in the neonate or pediatric patient. There are numerous indications for the renal sonographic examination in the newborn period. One major indication is a renal abnormality detected during prenatal sonography, most commonly congenital hydronephrosis. Some conditions or findings in the newborn associated with renal abnormalities are palpable mass, abdominal distention, anuria, oliguria, hematuria, sepsis or urinary tract infection, myelomeningocele, chromosomal or VACTERL anomalies, abnormal external genitalia, and prune-belly syndrome. Still other indicators include skin tags (usually near the ear and associated with cardiac anomalies) or a two-vessel umbilical cord. These conditions usually indicate that the renal study in the neonate is for screening the kidneys with no particular renal symptoms present.
In the older infant and child, indications often are for screening of known congenital anomalies and/or for acquired conditions presenting with symptoms, such as flank pain and hematuria.
The urinary bladder is considered an important part of the renal sonographic examination, and therefore any child who has bladder control should come to the examination with a full bladder. This preparation and general aspects of the ultrasound examination of the neonate and pediatric patient are described in Chapter 25 . Visualization of the bladder includes assessment for distal ureteral dilation ( Figure 26-1 ) or pelvic abnormalities ( Figure 26-2 ). Abnormalities in the urinary tract can often be related to abnormities of the reproductive system, hence the genitourinary system.
A 10-MHz curved array (slightly more curved than an adult footprint) is ideal for scanning neonates and infants, whereas a 7.5-MHz transducer with a similar footprint can provide excellent visualization in a young child (toddler/preschooler) or thin, older (school-age) child. It may be best to use a 9- to 12-MHz linear array in a premature infant or to provide better detail resolution when pathology is suspected. A 3- to 5-MHz curved array is typical for an adolescent.
Scanning is gently initiated over the suprapubic region due to the infant’s tendency to urinate spontaneously and a young child’s inability to hold the bladder for extended periods of time. If the urinary bladder is not distended at this time, or if voiding occurs before adequate detail can be obtained, this area can be examined after imaging of the kidneys and perirenal areas. Refilling of the urinary bladder is usually relatively rapid if the infant is fed or when parenteral fluids are being administered. A prevoid bladder wall thickness measurement should ideally be obtained with a full bladder.
With the bladder still full, a preliminary view of the kidneys and perirenal areas are scanned via the anterior abdomen or flanks. Longitudinal images of the kidneys are then obtained to document any fluid in the kidney with the bladder full, as well as document the echogenicity comparative to the liver and spleen. In the potty-trained child or older child, the patient is instructed to urinate fully, which may require a short waiting period. The bladder is then reexamined in longitudinal and transverse views, including a postvoid bladder wall thickness. Next, longitudinal and transverse views of the kidneys are documented. Additionally, in both planes, zoomed-up (write zoom) images of the renal superior, mid, and inferior poles are encouraged. In the infant and young child the dedicated renal views are often obtained from a prone position, whereas older children and adolescents will be scanned in the decubitus position. Sonographers should keep in mind that renal length may be slightly shorter from a prone position, and position-dependent pelviectasis may occur. Renal scanning before and after the infant or child voids can provide useful information. Vesicoureteral reflux, for example, may demonstrate an increase in hydronephrosis as the urinary bladder volume increases. Therefore a prevoid image of the kidney is important in its detection.
Normal anatomy and sonographic findings
The pediatric renal anatomy and sonographic appearance of the kidneys varies widely, depending on the age of the child. This is especially apparent in the young infant. Keep in mind that premature infants are adjusted according to their gestational age in medicine until 2 years of age. Developmentally, in the second trimester the kidney grows from small renunculi that are composed of a central large pyramid with a thin peripheral rim of cortex. As the renunculi fuse progressively, their adjoining cortices form columns of Bertin. The former renunculi are at that point called “lobes.” Remnants of these lobes with somewhat incomplete fusion, often termed fetal or renal lobulation, should not be confused with renal abnormalities or scars when imaging the kidney. Renal lobulation is most prominent at birth and in the neonatal period, often disappearing completely by 6 years of age. The pyramids remain large even after birth in comparison with the thin rim of cortex that surrounds them. The glomerular filtration rate is low right after term birth but increases rapidly thereafter. The cortex continues to grow throughout childhood, whereas the pyramids become smaller. The larger amount of sinus fat is not present in the neonate and pediatric patient, as is often seen in adults.
The normal kidney in the neonate and infant is characterized by a distinct demarcation of the cortex and medullary pyramids or corticomedullary differentiation, owed to a larger medullary volume. The medullary pyramids are prominent and hypoechoic and should not be mistaken for dilated calyces or cysts. They are typically less prominent by 1 year of age. The surrounding cortex is quite thin at birth and echogenicity varies by age. Echogenicity in the full-term neonate is essentially similar to or slightly greater than that of normal liver and splenic parenchyma ( Figure 26-3 ). Premature infants tend to have more echogenic kidneys, owing to underdevelopment of the renal structures. Renal cortical echogenicity normally decreases to less than that of liver parenchyma by 4 to 6 months of age. Increased cortical echogenicity, however, is only seen in the neonatal period (up to 1 month of age). The increased cortical echogenicity may result from glomeruli occupying a larger proportion of cortical volume and the location of 20% of the loops of Henle within the cortex (versus 9% in adults) as opposed to the medulla. Due to a paucity of fat in the renal sinus of the neonate and infant, this area is generally hypoechoic and therefore indistinct.
Children and adolescents have a sonographic renal anatomy similar to adult anatomy. The normal cortex is thick and produces low-level, back-scattered echoes. The medullary pyramids are relatively hypoechoic and arranged around the central, echo-producing renal sinus.
The arcuate artery vessels may be seen as intense specular echoes at the corticomedullary junction. Color Doppler is often used to document renal artery and vein in the pediatric renal examination and is best taken in the zoomed-up midtransverse view. This also aids in differentiating a prominent vein from pyelectasis. Pulsed Doppler also aids in detecting cases of renal vein thrombosis, hypertension, or other suspected vascular disease. A dedicated renal duplex examination reports the renal artery resistive index (RI) (along with acceleration time) and the normal RI value varies greatly within the first year: preterm infant, up to 0.9; neonate, 0.6 to 0.8; and by the end of the first year show similar values as adults, 0.5 to 0.7. It is important to keep in mind that cardiac disease, coarctation of the aorta, and patent ductus arteriosus may affect the RI as well.
The normal renal length varies with the age of the neonate or pediatric patient ( Figure 26-4 ). Accurate and consistent measurements are important to document normal growth and to help detect abnormalities. Although the left kidney is somewhat longer, a kidney measurement greater than 1 cm side to side should be monitored closely, and may indicate infection, scarring, or congenital abnormalities, such as hypotrophy or duplicated renal system. A number of renal anomalies may be encountered and include renal agenesis and supernumerary kidney. Anomalies of position, form, and orientation include pelvic kidney, horseshoe kidney (the most common anomaly), crossed ectopy (see Figure 26-6 ), and renal duplication (see Figures 26-11 and 26-12 ). See Chapter 15 for further discussion of these anomalies, as the pediatric sonogram may be the first detection of such anomalies, which become increasingly difficult to scan later in life.
The normal adrenal glands are larger and more easily identified in the neonate than in the older infant or young child. In fact, the prominent size of the adrenal gland at birth will normally decrease rapidly within the first 10 days, and then slowly atrophy over the next few weeks. Each gland lies immediately superior to the upper pole of the kidney. The left adrenal gland extends slightly more medial than does the right. Sonographically the gland has an inverted “V” or “Y” shape in the longitudinal plane ( Figure 26-5 , A ). In the transverse plane, the portion of the gland delineated has a linear or curvilinear outline ( Figure 26-5 , B ). The central adrenal medulla in the neonate is relatively thin, appearing as a distinctly echogenic stripe, surrounded by the more prominent and less echogenic adrenal cortex. When the kidney is absent or ectopic, the ipsilateral adrenal gland remains in the renal fossa, but as a result it may have an altered configuration ( Figure 26-6 ).
The normal urinary bladder is thin walled in the distended state and should measure less than 3 mm (with a mean of 1.5 mm) in anterior-posterior dimension. When empty, the wall thickness increases but remains less than 5 mm. Often in pediatrics, the posterior bladder wall is measured, and the posterolateral wall has been suggested to avoid the thickened posterior region of the trigone, which has different characteristics than the detrusor muscle. Likewise, the anterior bladder wall is avoided due to ring-down artifact and the prominent urachal remnant in children. The bladder should normally empty completely or at least 90% of the bladder capacity. Infants are an exception to this, as they often retain urine in the bladder ( Figure 26-7 ). The use of color Doppler may aid in the visualization of the distal ureter as it enters the posterior wall of the bladder.
Renal, adrenal, and bladder pathology
The more common congenital anomalies, hereditary renal cystic disease, acquired pathologies, and malignant tumors of the urinary tract and adrenals are presented here. Although discussed separately, it should be understood that congenital or genetic abnormities might contribute to an increased risk of acquired or malignant processes. It should be noted that most of these conditions present as a palpable mass in the neonate. The reasons for both renal and adrenal enlargement, including malignant tumors, are summed up in Tables 26-1 and 26-2 .
|Clinical Findings||Sonographic Findings||Differential Considerations|
|Flank pain||Pelvocalyceal dilation |
Ureter is dilated when vesicoureteral reflux or primary megaureter is present
|Parapelvic cyst |
Multiple renal cysts
Multicystic renal disease
Duplicated collecting system
|Posterior Urethral Valves|
|Decreased urine output||Thickened bladder wall (“keyhole”) |
|Multicystic Dysplastic Kidney|
|Unilateral multicystic mass in kidney |
Contralateral ureteropelvic junction
|Polycystic Renal Disease|
|Pulmonary hypoplasia |
|Enlarged kidneys |
|Absence of abdominal muscle |
Dilated bladder, ureter, prostatic urethra
|Kidney is normal, hydronephrotic, or dysplastic |
Dysplastic enlarged kidneys
|Posterior urethral valves|
|Congenital Mesoblastic Nephroma|
|Found in children <1 yr||Hyperechoic or hypoechoic or mixed||Adrenal tumor |
Benign renal tumor
|Abdominal mass |
|Ovoid enlargement of the gland |
Anechoic to hyperechoic
|Adrenal neuroblastoma |
|Clinical Findings||Sonographic Findings||Differential Considerations|
|Mass usually in adrenal gland |
Look for metastasis to liver
|Adrenal hemorrhage |
Benign renal tumor
Palpable abdominal or flank mass
Weight loss, fever, anemia, pain
|Complex mass in kidney |
Isoechoic to echogenic
May have calcification
Look for tumor extension into renal vein or inferior vena cava
|Mesoblastic nephroma |
Renal cell carcinoma
Adrenal cortical carcinoma
Multicystic renal hamartoma
Congenital urinary tract anomalies
In addition to the many renal anomalies, which may be present in the pediatric patient, hydronephrosis, patent urachus, and multicystic dysplastic kidney disease (MCDK) are other congenital anomalies of the urinary tract found in the neonate and child. Congenital hydronephrosis is a common reason for the pediatric renal sonographic examination, and plays a vital role in the detection and differentiation of the many possible underlying conditions.
Hydronephrosis describes the dilation of the urinary collecting system and is the most common urinary tract anomaly in children, accounting for 50% of congenital malformations. It is also the most common cause of a palpable mass in the neonate. There are many causes of dilation of the collecting system, the most common being obstruction, reflux, or abnormal muscle development. Sonography is sensitive in detecting small amounts of fluid in the renal pelvis. The sonographer is able to determine the severity of the hydronephrosis, whether the condition is unilateral or bilateral, if the ureters and bladder are dilated, and the status of the renal parenchyma.
Sonographic features found in hydronephrosis include visible renal parenchyma surrounding a central cystic component, small peripheral cysts (dilated calyces) budding off a large central cyst (renal pelvis), and visualization of a dilated ureter. This must be distinguished from the noncommunicating cysts of multicystic dysplastic kidneys. To date, there is no consensus on reporting urinary tract dilation, with roughly two thirds of radiologists using descriptive terminology, one third using the Society for Urology (SFU) grading system, and one third measuring the anteroposterior (AP) diameter of the renal pelvis, leaving a third using two of the above reporting methods. Renal pelvis dilation may be normal up to 10 mm in the AP dimension.
Vesicoureteral reflux (VUR) is a common nonobstructive cause of hydronephrosis and is indicated in up to 33% of prenatally diagnosed hydronephrosis. VUR is the abnormal refluxing of urine from the urinary bladder through the ureters and into the kidney. It has five different grades reflecting severity from I = least severe (reflux limited to ureters only) to V = most severe (severe dilation of ureters and kidney with loss of papillary impressions). It is often treated conservatively because it is nonobstructive. Many cases, often males and an appreciable number of grades IV and V, resolve on their own within the first 2 years. Unilateral or bilateral hydronephrosis may occur and different sides may have different levels of reflux. Although ultrasound can often detect the higher levels of VUR, it may be helpful to detect lower grades by using both prevoid and postvoid renal imaging. Additionally, waiting 7 to 10 days to perform a postnatal ultrasound is recommended, as low urine output is typical at birth. It is definitively diagnosed by a voiding cystourethrogram.
Sonographic findings for VUR are often nonspecific and may or may not include hydronephrosis, pelvic or ureteral wall thickening (epithelial thickening), intermittent dilation of the collecting system, or displaced ureteral jet in the bladder.
Ureteropelvic junction obstruction.
Ureteropelvic junction obstruction is the most common type of obstruction causing hydronephrosis of the upper urinary tract and occurs in 1 in 2000 children with a male prevalence (3:1), accounting for about 10% of prenatally diagnosed hydronephrosis. It most often results from intrinsic narrowing or extrinsic vascular compression at the level of the ureteropelvic junction. The obstruction produces proximal dilation of the collecting system; however, the ureter is often normal in caliber. There is an increased incidence of abnormalities of the contralateral kidney, such as multicystic dysplastic kidney or vesicoureteral reflux.
Sonographically, there is pelvocalyceal dilation without ureteral dilation ( Figure 26-8 , A ). When the obstruction is pronounced, the dilated renal pelvis extends inferiorly and medially ( Figure 26-8 , B ). If vesicoureteral reflux or primary megaureter is present, the ureter may be dilated. The best way to demonstrate the dilated ureters at the ureteropelvic junction is with a longitudinal scan plane.