Current Status of Imaging of the Urinary Tract : Imaging Techniques, Overview of Anatomy and Radiation Issues

Renal

Renal abnormalities may be subclassified into those of number, position, fusion, vasculature and ureteropelvic junction anomalies. Failure of the ureteric bud to join the metanephric blastema causes renal agenesis. This is observed in 1 in 1000 live births and is associated with absence of the ipsilateral ureter or the presence of a ureteric stump (see Figs 27.1 and 27.2 ). An absent ipsilateral epididymis, vas deferens, seminal vesicle or the presence of a seminal vesicle cyst is observed in up to 70% of patients with renal agenesis. Interestingly, anomalies of the urinary tract are almost twice as common among men compared with women; however, congenital anomalies of the genital tract are much more common in women. Female patients with renal agenesis have a 70% incidence of Müllerian anomalies such as vaginal and uterine agenesis, or unicornuate uterus, which are part of Mayer–Rokitansky–Küster–Hauser syndrome. An absent ipsilateral adrenal gland is observed in 10%. Agenesis of the left kidney can be recognised on an abdominal radiograph if the splenic flexure is located medial to the lesser curve of the stomach. Bilateral renal agenesis results in Potter syndrome (1 in 4000 live births) consisting of oligohydramnios, pulmonary hypoplasia and facial anomalies such as low-set ears, broad flat nose and prominent infraorbital skin folds. Supernumerary kidney, on the other hand, refers to the presence of an additional kidney. A supernumerary kidney is uncommon but normally occurs on the left, probably due to duplication of the ureteric bud. The supernumerary kidney is located caudal to the normal kidney drained by either a separate ureter or a branch of a bifid ureter.

An abnormal amount of renal rotation around its long axis can position the ureteropelvic junction anterior, or less commonly, posterior to the kidney. Abnormal caudocranial renal ascent during development results in an ectopic renal location. An ectopic kidney is more commonly inferior to the normal anatomical position or, rarely, in the upper abdomen where it is referred to as a thoracic kidney . Ectopic kidneys can be located in the pelvis ( Fig. 27.10 ), in the iliac fossae or can cross the midline. There is an increased incidence of renal anomalies in both the ectopic kidney and its contralateral counterpart. A pancake kidney is produced when bilateral pelvic kidneys fuse ( Fig. 27.11 ). Pelvic kidneys are more prone to decreased function, ureteropelvic junction obstruction, vesicoureteric reflux, stone formation and trauma ( Fig. 27.12 ).

Fusion anomalies of several types and severity occur in the kidneys. A horseshoe kidney deformity consists of medial-facing lower renal poles connected by an isthmus which rotates the renal axis ( Fig. 27.13 ). This anomaly occurs following metanephric contact and fusion during development. The incidence of horseshoe kidney is approximately 1 in 400 live births and has a 2-to-1 male-to-female preponderance. The isthmus lies anterior to the IVC and aorta but posterior to the inferior mesenteric artery. As a result, renal assent is arrested at this level. The isthmus is composed of either functioning renal parenchyma or fibrous tissue ( Fig. 27.14 ). Horseshoe kidney is associated with a 30% incidence of ureteropelvic junction obstruction, stone formation in 30% and 10% chance of ureteric duplication. There is an increased incidence of genital, cardiovascular and anorectal abnormalities. Patients with horseshoe kidneys also have increased risk of renal tumours, including Wilms tumour, traumatic injury or infection.

Occasionally, one kidney crosses the midline and fuses with the contralateral kidney, a configuration known as crossed fused ectopia ( Fig. 27.15 ). Unlike horseshoe kidneys, both kidneys lie on one side of the spine. Generally, the ureters drain in an orthopic manner into the bladder when crossed fused ectopia exists. This fusion pattern is more common in men and the left kidney is more commonly ectopically located.

Overall, abnormalities of the renal vasculature occur in almost 25% of the population. Ectopic kidneys have a higher incidence of anomalous blood supply, and renal artery duplication is the most common vascular abnormality (see Fig. 27.4 ). The accessory artery normally supplies the lower pole renal territory. Accessory veins are much less common than accessory arteries. Vessels crossing the ureteropelvic junction can cause complete segmental renal obstruction, which can simulate duplication (pseudodupliction). Anomalous vessels generally cause few symptoms; however, inadvertent injury during open aortic and renal surgery, or ureteroscopic procedures, can occur. The tissues anterior to the ure­teropelvic junction, in particular, should be examined carefully on pre-procedure imaging, because anomalous vessels traverse anterior more frequently than posterior to the ureteropelvic junction.

Ureter and Pelvis

Congenital ureteric anomalies can be generally subdivided into those of ureteric number, direction, diameter and filling defects. The most common ureteric anomaly is that of ureteric number, which was observed in 0.7% of autopsies in one series. The renal parenchyma may be divided into upper and lower components by a septum of Bertin. This anomaly is associated with a bifid collecting system consisting of two pelvices joined proximal to the ureteropelvic junction ( Fig. 27.16 ). The upper calyces drain into the upper pelvis, while the mid and lower calyces drain into the lower pelvis. The ureters may also join distal to the ureteropelvic junction ( Fig. 27.17 ) with a Y-configuration. This is normally asymptomatic unless urine refluxes from one ureter up the other causing ureteroureteric reflux, also termed yo-yo reflux , which should be considered if one ureteric segment is disproportionately enlarged. This anomaly can precipitate recurrent infections and obstruction of a ureteric segment.

When there is complete duplication of the renal collecting system, the ureter draining the upper moiety tends to insert ectopically below and medial to the normal ureteric orifice or sometimes outside the bladder (extravesical). The ureter draining the upper moiety tends to drain ectopically due to delayed separation of the ureteric bud from its parent Wolffian duct, which migrates in a caudal direction during development. For this reason, it is suggested that if the upper urinary tract is normal, the ureter is likely to have a normal insertion; however, a dysplastic or malfunctioning upper tract is likely associated with an ectopic insertion. The ectopic ureter often has a strictured distal element, which causes proximal dilatation and ureterocele formation in 75% of affected patients and is eight times more common in women than men ( Figs 27.18 and 27.19 ). A ureterocele contains layers of ureteric and bladder epithelium, which are optimally appreciated on early imaging on a voiding cystourethrogram (VCUG) but can be obscured by contrast medium, effaced or everted similar to a diverticulum when the bladder is full. A cobraneck appearance is observed at IVU consisting of a dilated contrast-filled ureter surrounded by thin rim of ureteric wall and bladder mucosa measuring less than 2 mm in diameter ( Fig. 27.20 ). Ectopic ureteroceles often project into the bladder and are associated with recurrent infection, stone formation, complete ipsilateral or bilateral ureteric obstruction. An ectopic ureter with extravesical insertion below the bladder neck, into the urethra or vagina, can cause urinary incontinence, infection and ureteric obstruction from fibrotic stricture formation. The ectopic ureter generally inserts above the external sphincter in men, thereby inhibiting incontinence, but in approximately 50% of women this does not occur and patients present with dribbling incontinence even though they can void normally ( Fig. 27.21 ).

The ureter draining the lower moiety of a duplicated collecting system usually inserts in the normal anatomical location but tends to reflux; in fact, reflux is the most common association of ureteric duplication. Lower pole hydronephrosis in the setting of duplication is mainly due to reflux rather than an obstructing ureterocele or stricture ( Fig. 27.22 ). In duplicated systems, the dilated upper pole ureter can impinge and displace the lower pole ureter, which can be appreciated on VCUG. Conformity of a duplicated collecting system with this pattern of ureteric insertion is known as the Weigert–Meyer rule. Approximately one-third of patients with complete ureteric duplication have an additional congenital anomaly such as horseshoe kidney or multicystic dysplastic kidney.

Abnormalities of the course of a ureter generally occur due to extrinsic processes, which push or pull complete or segmental portions of the ureter in medial or lateral directions. Potential causes of medial ureteric deviation include retrocaval ureter ( Fig. 27.23 ), laterally based renal, retroperitoneal, lymph node, central pelvic or psoas masses, pelvic lipomatosis, vascular aneurysms or haematomata, retroperitoneal fibrosis ( Fig. 27.24 ) and surgical resection of pelvic organs such as the entire rectum. Lateral ureteric deviation is caused by retroperitoneal and central pelvic masses such as lymphadenopathy (testicular metastases), aortic aneurysm, psoas enlargement (young men) and abnormal renal position (malrotated or horseshoe kidney). Cross-sectional imaging is usually required to confirm the aetiology. Acute medial deviation of the right ureter medial to corresponding vertebral pedicles on an abdominal radiograph is virtually diagnostic of a retrocaval ureter. Pelvic lipomatosis pushes the distal ureters medially. This condition is more common in African-American men and can induce bilateral hydronephrosis. A characteristic pear-shaped bladder may be observed and cystitis glandularis of the bladder is an associated premalignant condition. Retroperitoneal fibrosis (periaortitis) pulls one or both ureters medially in approximately half of cases . Psoas muscle hypertrophy can be recognised as the cause of lateral ureteric deviation if the lateral psoas border exceeds 8 cm from the lateral edge of the corresponding vertebral body at the level of the iliac crest .

Other congenital ureteric anomalies of note include ureteric stricture, megaureter and vesicoureteric reflux. Ureteric strictures are the most common of these anomalies. The ureteropelvic junction, in particular, and the ureterovesical junction to a lesser extent, are predisposed to stricture formation. Ischaemia during development is thought to incite fibrosis and stricture formation ( Fig. 27.25 ). There is consequent dilatation of the proximal ureter and possibly the renal pelvis, which may be detected by screening or as an incidental finding in asymptomatic patients, or associated with flank pain, recurrent infection, haematuria or stone formation, anywhere from in utero to adulthood.

Ureteric diameter exceeding 7 mm is termed megaureter. Megaureter may be a congenital phenomenon or secondary to a process in the bladder or urethra ( Fig. 27.26 ). Primary megaureter is due to deficient ureteric musculature, which limits peristalsis and leads to upstream dilatation. Three forms are described: obstructing primary megaureter; refluxing primary megaureter; and non-obstructing non-refluxing primary megaureter. Primary refluxing megaureter is due to inadequate one-way valve mechanism at the vesicoureteric junction. In non-obstructing non-refluxing primary megaureter, the calyces should maintain a crisp outline and not be blunted as with primary obstructing megaureter, and there should be normal excretion. Three-quarters of cases are unilateral; there is left-sided male predominance.

Vesicoureteric reflux is caused by maldevelopment of the distal ureter characterised by a shortened intramucosal bladder tract with a deficient valve-like mechanism. Reflux increases the risks of renal infection, scarring and reflux nephropathy. Vesicoureteric reflux represents the most common cause of antenatal hydronephrosis, responsible for 40% of cases. Grading is between 1 and 5 according to the International Reflux Study Committee, as determined by the most severe degree of reflux at voiding cystourethrography: reflux into the ureter only (I); to the renal pelvis without calyceal dilatation (II); with mild or moderate ureteric and calyceal dilatation but preserved fornices (III); severe ureteric and calyceal dilatation with obliteration of forniceal angles but maintained greater calyceal papillary impressions (IV); and, finally, gross dilatation with effacement of the papillary impressions (V) ( Fig. 27.27 ).