Renal Ultrasonography

The presence of a dilated urinary tract is the most common characteristic of obstruction. Hydronephrosis is a common ultrasonographic finding (Fig. 534-1). Dilation is not diagnostic of obstruction and can persist after surgical correction of an obstructive lesion. Dilation can result from vesicoureteral reflux, or it may be a manifestation of abnormal development of the urinary tract, even when there is no obstruction. Renal length, degree of caliectasis and parenchymal thickness, and presence or absence of ureteral dilation should be assessed. Ideally, the severity of hydronephrosis should be graded from 1 to 4 using the Society for Fetal Urology grading scale (Table 534-2). The clinician should ascertain that the contralateral kidney is normal, and the bladder should be imaged to see whether the bladder wall is thickened, the lower ureter is dilated, and bladder emptying is complete. In acute or intermittent obstruction, the dilation of the collecting system may be minimal and ultrasonography may be misleading.

Figure 534-1 Ultrasonographic image of the kidney with marked pelvic and calyceal dilatation (grade 4 hydronephrosis) in a newborn with ureteropelvic junction obstruction.

Table 534-2 SOCIETY FOR FETAL UROLOGY GRADING SYSTEM FOR HYDRONEPHROSIS

Voiding Cystourethrogram

In all cases of congenital grade 3 or 4 hydronephrosis and in any child with ureteral dilatation, a contrast voiding cystourethrogram (VCUG) should be obtained, because the dilation is secondary to vesicoureteral reflux in 15% of cases. In boys, the VCUG also is performed to rule out urethral obstruction, particularly in cases of suspected posterior urethral valves. In infravesical obstruction in infants, the bladder may be palpable because of chronic distention and incomplete emptying. In older children, the urinary flow rate can be measured noninvasively with a urinary flowmeter; decreased flow with a normal bladder contraction suggests infravesical obstruction. When the urethra cannot be catheterized to obtain a VCUG, the clinician should suspect a urethral stricture or an obstructive urethral lesion. Retrograde urethrography with contrast medium injected into the urethral meatus helps delineate the anatomy of the urethral obstruction.

Radioisotope Studies

Renal scintigraphy is used to assess renal anatomy and function. The 2 most commonly used radiopharmaceuticals are mercaptoacetyl triglycine (MAG-3) and technetium-99m-labeled dimercaptosuccinic acid (DMSA). MAG-3, which is excreted by renal tubular secretion, is used to assess differential renal function, and when furosemide is administered, drainage also can be measured. An alternative to MAG-3 is diethylene tetrapentaacetic acid (DTPA), which is cleared by glomerular filtration. The background activity of DTPA is much higher than that of MAG-3. DMSA is a renal cortical imaging agent and is used to assess differential renal function and to demonstrate whether renal scarring is present. It is used infrequently in children with obstructive uropathy.

In a MAG-3 diuretic renogram, a small dose of technetium-labeled MAG-3 is injected intravenously (Figs. 534-2 and 534-3). During the first 2-3 min, renal parenchymal uptake is analyzed and compared, allowing computation of differential renal function. Subsequently, excretion is evaluated. After 20-30 min, furosemide 1 mg/kg is injected intravenously, and the rapidity and pattern of drainage from the kidneys to the bladder are analyzed. If no obstruction is present, half of the radionuclide should be cleared from the renal pelvis within 10-15 min, termed the half-time (t_1/2). If there is significant upper tract obstruction, the t_1/2 usually is >20 min. A t_1/2 of 15-20 min is indeterminate. The images generated usually provide an accurate assessment of the site of obstruction. Numerous variables affect the outcome of the diuretic renogram. Newborn kidneys are functionally immature, and, in the first month of life, normal kidneys might not demonstrate normal drainage after diuretic administration. Dehydration prolongs parenchymal transit and can blunt the diuretic response. Giving an insufficient dose of furosemide can result in inadequate drainage. If vesicoureteral reflux is present, continuous bladder drainage is mandatory to prevent the radionuclide from refluxing from the bladder into the dilated upper tract, which would prolong the washout phase.

Figure 534-2 Same patient as in Figure 534-1. 6 wk old, MAG-3 diuretic renogram. The right kidney is on the right side of the image. A, Differential renal function: left kidney 70%, right kidney 30%. B, After administration of furosemide, drainage from the left kidney was normal and drainage from the right kidney was slow, consistent with right ureteropelvic junction obstruction. Pyeloplasty was performed on the right kidney.

Figure 534-3 Same patient as in Figure 534-1. A, MAG-3 diuretic renogram at 14 mo of age shows equal function in the two kidneys. B, Prompt drainage after the administration of furosemide.

The MAG-3 diuretic renogram is considered superior to the excretory urogram in infants and children with hydronephrosis, because bowel gas and immaturity of renal function often cause the intravenous pyelogram (IVP) images to be suboptimal. The diuretic renogram provides an objective assessment of the relative function of each kidney.

Excretory Urogram

Excretory urogram is rarely used in assessing the pediatric urinary tract, although it may be useful in selected cases with indeterminate upper urinary tract obstruction or a suspected duplication anomaly.

Magnetic Resonance Urography

MR urography is also used to evaluate suspected upper urinary tract pathology. The child is hydrated and given intravenous furosemide. Gadolinium-DTPA is injected and routine T1-weighted and fat-suppressed fast spin-echo T2-weighted imaging is performed through the kidneys, ureters, and bladder. This study provides superb images of the pathology, and methodology permits assessment of differential renal function and drainage (Fig. 534-4). There is no radiation exposure; younger children need sedation or anesthesia. It is used primarily when renal sonography and nuclear imaging fail to delineate complex pathology.

Figure 534-4 MR urogram in boy with distal ureterovesical obstruction.

Computed Tomography

In children with a suspected ureteral calculus, noncontrast spiral CT of the abdomen and pelvis is a standard method of demonstrating whether a calculus is present, its location, and whether there is significant proximal hydronephrosis. This study is the initial study of choice in many these patients. The disadvantage of CT is the significant radiation exposure, and it should be used only when the results will direct management decisions.