A renal abscess is a necrotic cavity filled with purulent material that corresponds to a nephritis area that ultimately liquefies. Large abscesses require prompt percutaneous or surgical drainage. Most of abscesses are intra-parenchymal, and appear as a well-limited hypoechoic mass with or without thick walls (Fig. 18.5). It may contain septations and mobile debris and rarely highly echogenic foci corresponding to gas bubbles. There is no internal flow at Doppler imaging.

Pyohydronephrosis corresponds to the accumulation of purulent urine in a dilated kidney. It can be due to an obstructive uropathy, or less often secondary to calculi or stricture. This is a urologic emergency and requires prompt percutaneous or surgical drainage. It appears as mobile hypoechogenic debris echoes in the urine (Fig. 18.6), with or without a fluid–fluid level. Care should be taken when describing echogenic urine as false positive cases of pyohydronephrosis are commonly encountered using this sonographic description only. The term pyohydronephrosis should be used in case of severe clinical state at presentation or insufficient response to antibiotics after 48 h of antibiotics.

The main interest of US is to identify an underlying uropathy that favors UTI; it corresponds generally to a pelvicalyceal dilatation or ureteral dilatation. Bladder diverticulum, ureterocele (Fig. 18.7), urachal diverticulum (Fig. 18.8), and dilatation of the upper part of the urethra should be screened as well. Urinary stones can be a risk factor for UTI (and a complication), especially in case of Proteus mirabilis infection. In the setting of UTI, upper tract dilatation in children is much more frequently related to a uropathy than an infected colitis nephritis.

Noteworthy, the ability of US to detect Grades I-V VUR is poor and varies according to different studies, with a sensitivity of 16–40% and negative predictive value of 25–86% [28–31].

It is well known that ureteral and pelvicalyceal dilatation may often be absent in case of high grade VUR (III–V). Also US is a poor predictor of high grade reflux with a sensitivity of 68–77% and a negative predictive value of 88–94% [32–34]. This high negative predictive value of normal US makes the diagnosis of high grade VUR unlikely for children with a first febrile UTI.

^99mTc- dimercaptosuccinic acid (DMSA) is still considered as the gold standard imaging method in detecting renal parenchymal involvement in patients diagnosed with acute pyelonephritis [35]. Approximately 40% of the administered dose accumulates in the distal tubular cells, providing a cortical study. Images are acquired 2–3 h after tracer injection, and APN areas appear as photopenic defects or exhibit a striated pattern. Due to its limited spatial resolution, ^99mTc-DMSA scintigraphy cannot readily differentiate APN from chronic scarring or renal cyst. Moreover, scintigraphy has negative aspects including the need for intravenous access, high cost, and exposure to radiation: approximately 1 mSv regardless of the age of the child [36, 37]. As a result, its use is no longer recommended in daily practice as in case of APN [5]. However, it remains an excellent method to assess the renal parenchyma after recurrent APN to screen for renal scarring and to quantify the differential renal function.

MR imaging can show pyelonephritis without contrast using diffusion-weighted imaging (DWI) [38]. Nephritis appears hyperintense with restricted diffusion on apparent diffusion coefficient maps (Fig. 18.9). Moreover, the use of T2-weighted sequences provides an anatomic study that makes MR imaging an attractive imaging modality in this setting. The combination of axial and coronal oblique views of both sequences is optimal to ensure that diffusion anomalies are real and do not correspond to artifacts. T2-weighted images allow kidney length and renal pelvis measurements. Much better than scintigraphy, T2-weighted images can show abscesses (marked hyperintensities with hypointense rim) but also renal scarring or cortical cysts. It is noteworthy that areas of nephritis are often not visible on T2-weighted images, and when an anomaly is visible it appears as an hypo or hyperintense area [39]. The use of these different sequences makes this examination shorter than 15 min, without the need for intravenous puncture and contrast infusion. However, sedation remains necessary.

Indications for MR imaging can include a doubtful urine culture, antibiotic prescription before urinalysis or unfavorable clinical response after 48 h of antibiotics. Limitations for MR imaging is an age younger than 6–8 months as diffusion is known to be spontaneously reduced in this age group [40]. Another limitation is that the duration of the hypersignal on DWI images is unknown for the moment, and it is not possible to ensure that DWI anomalies do not correspond to a previous pyelonephritis in case of history of multiple UTI.

Computed tomography (CT) is sometimes used in case of doubtful or complicated pyelonephritis due to its easy access, fast acquisition time, and reduced need for sedation. It requires iodinated-contrast infusion (1–2 mL/kg) at the arterial or nephrographic phase. As US should be performed systematically before performing a CT, a single phase study is generally sufficient. Moreover, an unenhanced acquisition has a poor additional value and should be avoided. An abdominal exploration without irradiating the pelvis and gonads should be favored as much as possible, although the protocol should be tailored to each patient. Dedicated age-specific, weight-specific pediatric CT-protocols must be used. Imaging features include a round or wedge-shaped area of diminished attenuation (Fig. 18.10). Alternating pathological and normal areas can generate a striated pattern. Abscesses appear as well-marginated hypodense cavities, with a lower attenuation than the adjacent nephritic area with or without a thick rim (Fig. 18.11). Ideally, CT should be performed only when MR imaging is not available or is not diagnostic enough.

The use of VCUG remains controversial. This examination is relatively invasive with potential side effects including dysuria, urinary retention, hematuria, and infection [41, 42]. Moreover, it exposes the patients to radiations. The radiation burden decreases regularly, thanks to the progressive replacement of screen film radiography by computed radiography and now by digital radiography. Sulieman et al. reported in 2016 measured doses in children explored by VCUG [43]. The mean entrance surface air kerma (ESAK) and range (mGy) corresponded to 2.2 ± 0.5 (0.8–9.2) in infants and young children (≤5 years) to 3.90 ± 0.6 [1–10] in adolescents.

A general trend has been to decrease dramatically the use of VCUG which was historically almost systematic after the first UTI in children. Its main goal is to diagnose vesicoureteric reflux. Thirty to forty percent of children investigated for UTI have VUR [28].

Historically, VUR was treated almost exclusively by open surgical reimplantation of the ureter. With the recognition that VUR often resolves spontaneously with time, the concept of antibiotic prophylaxis was introduced to prevent UTI while waiting for VUR resolution.

The efficiency of antibiotic prophylaxis has been a long-standing debate with inconsistent results. That is why the guideline issued by the American Academy of Pediatrics in 2011, based on a meta-analysis, did not recommend antibiotic prophylaxis in children without reflux or with grade I to grade IV reflux [5]. As a result, VCUG was not recommended systematically after the first UTI, but only in case of hydronephrosis, scarring or other findings that would suggest either high grade VUR or obstructive uropathy or in other atypical or complex clinical circumstances. VCUG should also be performed in case of febrile UTI recurrence.

Since this guideline, two prospective studies have confirmed that prophylaxis has a real impact. The Randomized Intervention for Children with Vesicoureteral Reflux (RIVUR) study, a large multicenter, randomized placebo-controlled trial, involving 607 children between the ages of 2–71 months with grade I to grade IV reflux, showed that prophylaxis reduced the risk of recurrence of UTI by almost 50% [44]. However, the number of new renal scars was not different in this study.

Another large, multicenter clinical trial, the Swedish Reflux Trial, involved 203 children aged 1 to younger than 2 years with grade III–IV reflux also showed a reduction in recurrent UTIs in girls on antibiotic prophylaxis [45]. Based on these studies, the guidelines issued in 2015 by the European Association of Urology (EAU) and the European Society for Paediatric Urology (ESPU) recommended a more frequent use of VCUG than the American Guideline. The European guidelines recommend to perform VCUG in non-toilet trained children systematically in girls after the first UTI even in case of normal ultrasound and in boys younger than 1 year.

VUR depicted by VCUG is classically graded according to The International Reflux Study Group from grade I to V (Fig. 18.12).

If VUR is the most common anomaly in the setting of UTI, VCUG can show a range of other urinary tract abnormalities, including ureterocele (Fig. 18.13) or posterior urethral valves (Fig. 18.14).