Imaging of urinary tract in children in different clinical scenarios: a guide for general radiologists

Children are frequently referred to the pediatric radiology department due to complaints related to the urinary tract. We aimed to describe common clinical scenarios in paediatric urology practice and their recommended imaging diagnostic algorithms. Practical imaging approach to following common clinical scenarios and the common radiological findings are discussed: (A) perinatal urinary tract dilatation and other congenital anomalies; (B) recurrent urinary tract infection; (C) enuresis and daytime urinary incontinence; (D) abdominal masses; (E) flank pain; (F) hematuria; (G) trauma; (H) nonpalpable testis, ambiguous genitalia and common urogenital sinus anomalies; and I) renovascular hypertension Imaging investigations should be tailored according to the clinical presentation in a stepwise approach aiming for optimum patients’ care.


Background
Children are frequently referred to the pediatric radiology department due to complaints related to the urinary tract. Perinatal urinary tract dilatation, recurrent urinary tract infection and enuresis are frequently encountered. Hematuria, loin pain or abdominal masses are also among the presenting symptoms of urologic diseases. Children may be referred following trauma or to investigate hypertension. Establishing diagnostic approaches and imaging protocols is of paramount importance for the evaluation and management of pediatric urinary tract disorders entailing proper selection of appropriate imaging modality for a given clinical scenario [1]. Pediatric imaging recommendations have been elaborated by European society of pediatric radiology Uroradiology Task Force and ESUR Paediatric Working Group [2][3][4][5][6][7][8]. The current review intends to outline practical imaging approach to common clinical presentations in paediatric urology practice. Common radiological findings in different scenarios are also demonstrated.

Main text
Practical imaging approach to following common clinical scenarios and the common radiological findings are discussed (Table 1) Perinatal urinary tract dilatation (UTD) is commonly encountered among children with complaints related to urinary tract. It can be detected on antenatal screening or in the work up of children with recurrent urinary tract infection. Being available, devoid of ionizing radiation and a bed side examination tool, ultrasound is the ideal initial imaging modality. Ultrasound check list has been developed to objectively assess the urinary system in children. It comprises assessment of renal parenchymal echogenicity and thickness; presence of parenchymal cysts; determination of anteroposterior renal pelvic diameter; accurate estimation of renal calyceal dilatation (central or peripheral); assessment of the ureters (dilated, not dilated, duplication); urinary bladder wall thickness, trabeculations and diverticulae, ureterocele and if the posterior urethra is dilated [9]. In this initial examination, various urinary tract anomalies can be diagnosed guiding for further diagnostic procedures (Fig. 1). Ultrasound also enables cine display during filling and emptying of ureterocele (Additional file 1). Recently, urinary tract dilatation grading system has been developed aiming for standardization of the grading system which has important prognostic and therapeutic implications (Table 2) [9].
Semiautomatic quantification of urinary tract dilatation severity can be achieved by newly emerged two-dimensional ultrasound technologies. Size of the collecting system, depth of the calyces and renal parenchymal thickness can be calculated. These measurements are subsequently used in identifying those patients who will benefit from diuretic scintigraphy [10]. Novel three-dimensional (3D) ultrasound technologies provide higher spatial and temporal resolution with the potential to demonstrate complex anomalies. Moreover, 3D ultrasound can provide parenchymal volume calculations which were found to be comparable to those from scintigraphy and MR urography [11,12]. After sonographic detection of UTD, micturating cystourethrography (MCUG) may be required for detection of vesicoureteric reflux (VUR) (especially in UTD 2 and 3). MCUG is the ideal imaging modality for detection and grading of VUR. In addition, posterior urethral valve can be readily diagnosed (Fig. 2) [2,13]. However, radiation exposure, discomfort, relatively high cost and the risk of inducing urinary tract infection are main disadvantages of this technique [13]. However, contrast enhanced ultrasound voiding cystourethrography was found to be more sensitive in detecting VUR with good ability in evaluation of the urethra without the use of ionizing radiation [14].
If reflux is found, statistic cortical renal scintigraphy (Tc-99m-DMSA scintigraphy) is the investigation of choice for functional evaluation and determination of split renal function and detection of possible renal  scarring. If VUR is not encountered, so obstruction rather than reflux is the probable cause of UTD and dynamic renal scintigraphy (MAG-3 or DTPA scintigraphy) is the examination of choice to assess urodynamics of tracer excretion and determination of split renal function as well ( Fig. 3) [15]. Caution should be taken when interpreting the slope of the time intensity curve of tracer excretion, as prolonged T-half fails to differentiate poor drainage due to obstruction from that due to dilated nonobstructed collecting system [15]. MRI also can be requested especially in complex renal anomalies providing robust anatomical resolution with three-dimensional capabilities ( Fig. 4) with proper detection of associated spinal anomalies (Fig. 5). MRI not only proving anatomical evaluation of the urinary tract, but also functional evaluation can be achieved if dynamic post-contrast study is performed (Figs. 6,7,8) [16]. In addition, fetal MRI is the appropriate next step after detection of fetal urinary tract dilatation specially if fetal intervention is available ( Fig. 9) [17].
By using dynamic MRI urography, calculation of renal transit time (RTT) can be achieved. RTT helps estimate the presence of obstruction. RTT is defined as the time it takes for the contrast agent to pass from the renal cortex to the ureter below the lower pole of the kidney. If the transit time is less than 245 s, the system is considered non-obstructive, while RTT greater than 490 s means probably obstructed system. If RTT times are between 245 and 490 s, the obstruction is considered equivocal and is managed conservatively with close follow-up to ensure that renal function is stable (Figs. 6, 7, 8; Additional file 2: video 2) [16].
Hydronephrotic systems are classified as either compensated or decompensated based on the contrast dynamics and parenchymal changes. Signs of decompensated system include: signs of an acute-on-chronic obstruction with edema in the renal parenchyma seen as T2 hyperintensity; delay in excretion of contrast on the affected side with a delayed calyceal transit time; dense and persistent nephrogram; functionally, there is a 4% discrepancy between the Patlak deferential renal function (pDRF) and the volumetric DRF (vDRF); and signal intensity versus time curves shows a persistent increase in signal intensity within the decompensated system (Fig. 7). However, signs of compensated hydronephrotic system that can accommodate the fluid challenge ( Fig. 8) include symmetric calyceal transit time (time it takes for the contrast agent to pass from the renal cortex to the calyces); symmetric nephrograms and signal intensity versus time curves; and the pDRF and vDRF are within 4% points. In addition, MRI can detect signs of uropathy which include small subcortical cysts (Figs. 5,9); poor cortico-medullary differentiation; decreased cortical signal intensity; and poorly defined or patchy nephrogram indicating impaired concentration [18,19].
In resource constraint environment with limited access to MRI, intravenous pylography (IVP) might be requested to assess excretory renal function and areas of blockage of the urinary tract (Fig. 8). IVP can be requested in evaluation of post-operative urinary tract especially in cases of pelvi-ureteric junction obstruction [3].

(B) Recurrent urinary tract infection
Urinary tract infection (UTI) is the most common bacterial infection in children below two years of age. UTI is mostly attributed to congenital anomaly of the kidneys and urinary tract and probably vesicoureteral reflux. The diagnostic algorithm varies among different guidelines [20]. Ultrasound is indicated in all patients less than two years of age; or present with atypical or recurrent In patients with recurrent UTI, DMSA scintigraphy should be performed 4 to 6 months after UTI [20].

(C) Enuresis and daytime urinary incontinence
Nocturnal enuresis (i.e., Enuresis) is defined as intermittent incontinence that occurs exclusively while asleep. According to the International Children's Continence Society, initial assessment of children with enuresis starts with thorough clinical examination searching for warning signs. Management of enuresis depends upon whether it is monosymptomatic or nonmonosymptomatic. Associated symptoms in nonmonosymptomatic type can be daytime incontinence, urgency, voiding difficulties, abnormally low or high diurnal voiding frequency (less than four or more than seven times per day) [21]. Screening ultrasound should be done for all children with lower urinary tract dysfunction. The bladder should be scanned when full (pre-void scan) and immediately after emptying (post-void scan). This contributes to assessment of the bladder capacity, wall thickness, bladder neck appearance, evaluation of distal ureters; and post-void residual urine, respectively. In addition, transverse rectal diameter can be measured behind the bladder. Transverse rectal diameter more than 30 mm could indicate fecal impaction and constipation. Children with dysfunctional voiding have high prevalence of constipation serving as an etiological factor for incontinence [22].
MRI of the spine is reserved for children with suspected abnormalities of the lumbosacral spine (Fig. 5). MR urography can be of value in detection of structural anomalies particularly ectopic ureter [16]. IVP may be of value if the access to MRI is limited (Fig. 10).

(D) Abdominal masses
Palpable abdominal mass is the most common presenting sign of pediatric renal malignancy necessitating ultrasound examination. Following ultrasound, crosssectional imaging with CT or MRI is mandatory for further evaluation and initial staging. Because of its wider availability and its shorter scan time, CT is the most used imaging modality. CT also enables simultaneous evaluation of both lungs. MRI lacks ionizing radiation with better soft tissue resolution. Children's oncology group found similar diagnostic performance of CT and MRI in terms of detection of lymph node metastasis and capsular invasion. However, MRI was found to be more sensitive in detecting contralateral synchronous masses (Fig. 11) [23]. Being patient specific, 3-D printed models have been found to be valuable surgical planning tool especially for complex kidney tumors [24].

(E) Flank pain
Acute abdominal pain is a common complaint of patients presenting to the emergency department. It is defined as pain with a maximum duration of 5 days with no evidence of trauma. When acute abdominal pain is associated with normal appetite, renal tenderness and/ or hematuria (more than 10 erythrocytes), acute renal colic should be considered [25]. While typical renal colic presents in school-aged children and adolescence, inconsolable crying and irritability are typical signs in infancy.
Ultrasound and radiography are the imaging modalities of choice. Ultrasound can demonstrate the presence, position, and dimensions of renal calculi. Most stones are usually found in the pelvicalyceal system, proximal or distal ureter. Ultrasound evidence of lithiasis includes shadowing echogenic foci, urinary tract dilatation, and increased renal parenchymal echogenicity (Figs. 12,13). Colour Doppler ultrasound can elicit twinkle artifact with increased resistive index on Duplex Doppler in cases with acute severe obstruction. Due to radiation burden, low-dose non-contrast CT should be reserved when ultrasound is non-diagnostic or further anatomical details are needed for surgical intervention (Figs. 12, 13, 14) [26]. Hematuria is relatively a common and alarming complaints in children. It comes into attention as incidental finding of urine analysis (microscopic hematuria), or when gross hematuria is evident (macroscopic hematuria). The management of hematuria starts with identifying the underlying etiology [27]. A stepwise approach for managing cases with hematuria has been developed [2]. After through clinical examination and performing necessary laboratory investigations (urine analysis and culture, serum creatinine and blood urea nitrogen), ultrasound is the first recommended imaging modality searching for structural urinary abnormalities (renal echogenity, urolithiasis, renal vein thrombosis or masses) [2,27]. Subsequent imaging depends on the initial ultrasound findings. While urinary bladder inflammatory process or glomerulonephritis are the most common causes of gross hematuria, it can be caused by lithiasis or by the rare urinary bladder rhabdomyosarcoma (Fig. 15).

(G) Trauma
Imaging plays a pivotal role in managing patients who have experienced blunt or penetrating genitourinary injury. Being fast and bedside imaging modality, Ultrasound is the first modality for trauma surveillance especially in patients with minimal symptoms. In cases of urinary tract injury four-phase post-contrast CT of the urinary tract is recommended. Pre-and post-contrast phases in arterial, nephrographic and pyelographic phases are acquired (Fig. 16) [4].
(H) Nonpalpable testis, ambiguous genitalia and common urogenital sinus anomalies (UGS) Nonpalpable testis represents failure of normal testicular descent. The nonpalpable testis is located either in intraabdominal or extrabdominal location. Ultrasound is initially employed in imaging pathway being able to detect 97% of viable inguinal testes. However, ultrasound can't reliably detect intraabdominal testes which represent 20% of nonpalpable testes. Moreover, MRI can't diagnose monorchidism. Consequently, both the American Urologic Association and European Association of Urology Guidelines recommend against imaging for the routine management of patients with nonpalpable testis [28,29]. Nevertheless, cases with nonpalpable testes associating ambiguous genitalia or hypospadias have a great likelihood of having disorder of sexual development. For this instance, ultrasound or MRI is recommended to look for internal female pelvic organs especially the uterus [30] Common UGS is a term used to describe an anomaly in the female when the urethra and vagina unite in a common urogenital channel that opens into the perineum via a single opening. Common UGS may be broadly classified into two main types: those associated with abnormal virilization, and the other non-virilized type (cloaca) [31,32].
Virilized common urogenital sinus anomalies is one of disorders of sexual development associating congenital adrenal hyperplasia. The diagnosis depends on clinical presentation, chromosomal analysis, and specific hormonal profile. Non-virilized common urogenital sinus anomalies are essentially associated with anorectal anomalies (cloaca malformation) which further adds to the complexity and diversity of the spectrum. The genitogram represents the main preoperative radiological Fig. 12 8-year-old girl with multiple stones a renal ultrasound reveals multiple stones filling the calyces and renal pelvis appears as dense echogenic foci with acoustic shadowing. b longitudinal ultrasound through the pelvis shows multiple stones at the distal ureter as well. Owing to complexity of the case, c non-contrast CT of the urinary tract is requested prior to surgical intervention and reveals multiple bilateral renal and left ureteric stones  (Figs. 17, 18). This is important for preoperative counselling when deciding about the type and extent of reconstructive surgery. Moreover, ultrasound of the upper urinary tract is of paramount importance to detect and follow-up associated urinary tract dilatation in cases of cloaca [32,33].  Renovascular hypertension is suspected when hypertension is severe or refractory to multiple drugs. Ultrasound is the initial imaging modality. Abnormally low resistive index less than 0.5 and tardus parvus pattern of spectral Doppler waveform with slow systolic acceleration and decreased peak systelic velocity have been used either isolated or in combination as an indicator for diagnosis of renal artery stenosis (Fig. 18). Ultrasound can also detect renal parenchymal abnormality and non-renal causes of hypertension as pheochromocytoma [34]. However, positive, or negative ultrasound can't preclude further imaging investigation especially in cases with moderate to severe suspicion of renovascular hypertension. CT or MR angiography is the recommended further imaging modalities (Fig. 19). Children are referred for catheter renal angiography after failure of medical therapy for hypertension or when ultrasound reveals size discrepancy between both kidneys, and in children with neurofibromatosis. Catheter angiography is more