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Pediatric Genitourinary Radiology
1
What is the role of the radiologist in pediatric urinary tract infection (UTI)?
UTI that occurs and recurs in some children is the result of many interrelated factors, most of which the radiologist cannot assess. Generally, UTI results when bacterial virulence outweighs host resistance. One important factor affecting host resistance that the radiologist can assess is whether there is impairment of unidirectional flow of urine out of the urinary tract, resulting in urinary stasis and predisposing infants and children to UTI.
2
What conditions can be detected radiologically that impair normal urinary flow?
Urinary tract obstruction (congenital), vesicoureteral reflux (VUR), and dysfunctional voiding/neurogenic bladder dysfunction all can be detected radiologically ( Figure 88-1 ).
3
Which imaging tests are used to diagnose these conditions?
Renal and bladder ultrasonography (US) and voiding cystourethrography (VCUG) are used to diagnose the above-mentioned conditions.
4
In a child with history of UTI, what are the indications for renal and bladder US?
Renal and bladder US is used to detect hydronephrosis and distal ureterectasis secondary to obstruction or VUR, to detect bladder abnormalities (including diverticula or wall thickening secondary to obstruction), and to assess for any renal parenchymal damage that may have been caused by prior infections.
5
When is renal and bladder US performed?
US is performed within several weeks after an initial UTI is diagnosed, but sooner if the child fails to respond to conventional antibiotic therapy. (Also, as a result of the routine use of prenatal US, hydronephrosis is often detected in utero, and UTI can be prevented by prophylactically administering antibiotics to the infant.)
6
How is VCUG performed?
The bladder is catheterized using sterile technique, and under fluoroscopy, the bladder is filled to capacity with iodinated contrast material. An approximation of the expected normal bladder capacity (mL) in a child is calculated by (age [in years] + 2) × 30. The catheter is removed, and the child voids while on the fluoroscopy table. During the procedure, the anatomy of the lower urinary tract and bladder function are examined, a series of radiographic images is obtained, and the presence or absence of reflux of contrast material (i.e., VUR) into the ureters is determined.
7
How is VCUG modified in infants?
A cyclic study is typically performed. Infants usually void around the small catheter that is placed through the urethra into the bladder; multiple cycles of voiding and bladder filling are studied. This method also increases the probability that reflux, if present, will be elicited and detectable.
8
When should VCUG be performed?
According to recent American Academy of Pediatrics guidelines (from the Subcommittee on Urinary Tract Infection, Steering Committee on Quality Improvement and Management), a VCUG should be performed after a second well-documented UTI in both boys and girls. VCUG is indicated if renal and bladder US demonstrates hydronephrosis, scarring, or other indications of high-grade VUR or obstructive uropathy.
9
What is voiding urosonography?
Pediatric imagers are currently looking to offer voiding urosonography as an alternative to fluoroscopic or radionuclide voiding cystography. With this technique, which avoids the use of ionizing radiation, fluid containing microbubbles is instilled into the urinary bladder; VUR reaching the kidneys can be readily detected utilizing US.
10
What are the pros and cons of a radionuclide cystography or voiding urosonography compared with fluoroscopic VCUG?
Fluoroscopic VCUG provides anatomic detail but at a much higher radiation dose. Voiding urosonography does not require radiation and may also detect low-grade VUR not clearly identifiable fluoroscopically. Radionuclide cystography is highly sensitive for detecting reflux because the child is continuously monitored by the gamma camera but at the expense of good anatomic detail. Radionuclide cystography is particularly useful for follow-up studies that assess for resolution of previously detected VUR and is useful for screening siblings of patients with known VUR.
11
What is primary VUR?
VUR occurs either primarily or secondarily. Primary VUR is due to “immaturity” or abnormality of the ureterovesical junction (UVJ), which allows urine to ascend into the ureters during bladder filling or voiding. Generally, reflux is related to ureteral orifice size and the length of the ureter as it tunnels into the bladder.
12
What is secondary VUR?
Secondary VUR occurs as the result of an abnormality of the UVJ, such as with presence of a distal paraureteral diverticulum or ureterocele (an outpouching of the ureter that extends into the bladder). Secondary VUR may also occur as a result of bladder outlet obstruction or secondary to a neurogenic bladder.
13
How is primary VUR graded?
Primary VUR is graded on a scale of 1 to 5, based on the degree of ureteral and pyelocalyceal (renal collecting system) filling and dilation ( Figure 88-2 ). In addition to providing the referring physician with a visual description of the degree of reflux, assigning a grade also gives the clinician an idea of the likelihood of spontaneous resolution (e.g., 80% of cases of grade 2 reflux resolve within 3 years).
14
What is dysfunctional voiding?
Dysfunctional voiding may be due to a neurogenic bladder or the so-called pediatric unstable bladder. The bladder may show variable degrees of wall thickening, trabeculation, and alteration in contour and storage capacity (see Figure 88-1 ). Bladder-sphincter dyssynergia may be indicated by the presence of a dilated posterior urethra caused by the external sphincter failing to relax as the bladder neck opens. This condition often leads to high-pressure voiding, incomplete emptying, and urine retention.
15
What diagnostic tests are useful for the detection of acute and chronic pyelonephritis in children?
Renal cortical scintigraphy, contrast-enhanced computed tomography (CT), and renal US are used to detect acute and chronic pyelonephritis in children. Pyelonephritis may be difficult to diagnose accurately, especially in infants and young children who cannot give a history. It may be difficult to distinguish between uncomplicated UTI and pyelonephritis (with or without subsequent complications such as renal abscess) on the basis of physical examination, history, and laboratory studies. Imaging plays an important role in the workup of children with known or suspected UTI.
16
What are the findings on a renal cortical scintigraphic study in acute and chronic infection?
A renal cortical scintigraphic study reveals parenchymal defects in children with acute infection secondary to focal areas of edema and inflammation. These defects revert to normal if the process completely resolves. In chronic infection or chronic pyelonephritis secondary to reflux nephropathy, residual areas of scar appear as persistent cortical defects on follow-up examinations.
17
What are the CT findings associated with pyelonephritis?
Contrast-enhanced CT is less sensitive than renal cortical scintigraphy for the detection of acute infection. Acute infection is seen as wedge-shaped areas of decreased attenuation in the kidney. Areas of scar related to prior infection can also be identified. CT is particularly helpful when abscess formation is suspected.
18
What is the “top-down approach” in the evaluation of the child with a history of UTI?
The “top-down approach” refers to a protocol that more selectively identifies children with history of UTI who may require voiding cystography. In these patients, rather than having a VCUG performed initially, VCUG is obtained based on whether an initial renal cortical scintigraphic study is normal or abnormal.
19
How useful is renal US in the workup of suspected pyelonephritis?
Gray-scale US is limited in its ability to depict acute pyelonephritis. The sensitivity of US is increased, however, when power Doppler US is used ( Figure 88-3 ). The latter technique is an advance in Doppler US that depicts normal and abnormal blood flow in a manner that is independent of the direction of the flow; power Doppler is more sensitive than routine color Doppler. Renal US may also be used to assess for significant renal scarring secondary to chronic pyelonephritis. Small focal scars may be difficult to detect with US.
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