Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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Technique

Conventional Cystography

Cystography is a well-established technique in evaluating the bladder lumen and providing access for biopsies. It is often used as a gold standard when evaluating other imaging procedures. Cystography as used here implies conventional cystography, which consists of lumen distention with a contrast agent introduced via a catheter and appropriate filming. Cystography is also feasible using computed tomography (CT), ultrasonography (US), magnetic resonance (MR), and radionuclide techniques.

Voiding cystourethrography continues to be the gold standard for imaging evaluation children with urinary tract infection, male urethra, functional bladder conditions and in detecting vesicoureteral reflux. Although radionuclide and contrast enhanced US cystography have a role in screening and follow-up of established conditions, most initial evaluation of function and anatomy appears best served by voiding cystourethrography.

Computed Tomography

A typical CT cystography technique consists of contiguous 5-mm axial scans after bladder distention with a 4% iodinated solution as a positive contrast agent. Air or carbon dioxide is

occasionally used for bladder distention, but a potential complication of air is air embolism. Imaging in both the supine and prone positions is necessary when using gas to detect tumors otherwise covered with fluid.

Similar to CT colonoscopy, CT cystoscopy (also called virtual cystoscopy) is feasible with a contrast filled bladder. Computed tomography cystoscopy is an alternate procedure in patients with urethral strictures who cannot undergo conventional cystoscopy or cystography but indications are expanding and some enthusiasts believe it will replace a large part of conventional cystoscopy. Three-dimensional (3D) CT cystography using perspective volume rendering and a shaded-surface 3D outline evaluates tumor size and shape and provides information about its relationship to adjoining mucosa.With current techniques, however, endoscopic cystoscopy has better spatial resolution in detecting small bladder tumors; thus CT cystoscopy detected 90% of bladder tumors, with all undetected tumors being <5mm in diameter (1); transverse and virtual images are complemen- tary—some small tumors are identified only on virtual images but bladder wall thickening is more apparent on transverse images.

Flow of opacified urine into the bladder during contrast-enhanced CT results in intraluminal artifacts due to incomplete mixing. If necessary, delayed bladder images should be obtained to eliminate these artifacts.

Ultrasonography

Bladder US is performed using a transabdominal, endorectal, endovaginal, or endourethral approach. Color Doppler US detects ureteral jets and with unilateral ureteral obstruction, Doppler US reveals asymmetry to these ureteral jets. The presence of normal ureteral jets excludes significant obstruction.

Intravesical instillation of an US contrast agent aids detection of certain abnormalities, such as vesicoureteral reflux. A galactose-based microbubble contrast agent, Levovist, is typically employed. Contrast duration for this agent varies when mixed with normal saline. A high concentration of dissolved oxygen in normal saline appears to prevent diffusion of gas from the microbubbles into the solution and as a result contrast duration is longer than with use of a low oxygen concentration solution, such as is found in vacuum sealed containers (2).

Magnetic Resonance Imaging

On T1-weighted images urine is homogeneously hypointense and blends into the bladder wall. On T2-weighted images urine is hyperintense while bladder wall muscle layers remain hypointense and the two can be differentiated from each other. Immediate postcontrast images reveal little or no urine enhancement, but with renal excretion urine becomes hyperintense on delayed images.

Bladder instillation of a superparamagnetic iron oxide contrast agent aids in evaluating intraluminal extension of bladder tumors. High-resolution T2-weighted turbo spin echo (TSE) sequences detect tumors as small as 4mm and estimate the depth of infiltration by identifying inner and outer bladder wall margins (3); this technique, however, does not readily identify different bladder wall layers.

Magnetic resonance (MR) cystoscopy is based on principles similar to CT cystoscopy. After filling the bladder with a negative contrast agent and using a heavily T2-weighted sequence such as half-Fourier acquisition single-shot turbo spin echo (HASTE), axial and virtual cystoscopic images achieved a sensitivity of about 90% in detecting tumors <1cm and 100% for tumors 1cm or larger (4), results not differing statistically from CT cystoscopy or conventional cystoscopy.

ADVANCED IMAGING OF THE ABDOMEN

Virtual MR cystoscopy without bladder filling with contrast or use of IV contrast is also feasible; tumors >1cm are readily detected (5). The role of such a contrast-less MR study in screening and diagnosis remains to be established.

Scintigraphy

Radionuclide cystography is less often performed than conventional cystography because of its poorer spatial resolution. Technetium99m (Tc-99m)–sulfur colloid is not absorbed from the bladder and is thus employed. Indirect radionuclide cystography using Tc-99m- mercaptoacetylglycilglycilglycine (MAG3) may be sufficient for some indications.

Congenital Abnormalities

Agenesis

Bladder agenesis is very rare and is usually associated with other major anomalies. Most affected neonates are stillborn.

Duplication

Bladder duplication is rare. With complete duplication each kidney and ureter drains into its separate bladder, which then drains by separate urethras. Some duplications are incomplete and the two bladders communicate through an isthmus. Some bladders only appear duplicated because of an internal septum. An “hourglass” bladder or a large diverticulum arising from the dome of the bladder also mimics an incomplete duplication.

Cystography readily detects a bladder duplication.

A rare bladder duplication is associated with exstrophy, epispadias, and other abnormalities.

Cloacal Exstrophy

Various gradations in failure of fusion of the inferior abdominal wall range from an omphalocele, to cloacal (bladder) exstrophy, to epispadias. This condition is more common in males. In some, the anterior abdominal muscles and anterior bladder wall are absent.

Cloacal exstrophy is associated with an omphalocele, imperforate anus, exstrophy of

BLADDER

two hemibladders, and an everted cecum between which lie the hemibladders. Epispadias is part of an exstrophy abnormality. Cryptorchidism and an umbilical hernia are common. Extensive bowel anomalies include a blind-ending hindgut and an omphalocele. The terminal ileum may prolapse out of the cecum. Spinal anomalies are common and the spinal cord is tethered. An association exists between bladder exstrophy and subsequent development of a bladder malignancy.

Conventional radiographs reveal symphysis pubis widening. Such widening is not pathognomonic for cloacal exstrophy but is also found in cloacal malformation, epispadias, and other hindgut malformations. In exstrophy the iliac bones are rotated laterally.

Surgical treatment of these patients is complex and includes closure of an omphalocele, separation of the gastrointestinal tract from hemibladders, and creation of a single bladder. A bladder continence mechanism needs to be created. Lastly, if possible, a colon pull-through is performed.

Previous therapy for cloacal exstrophy was cystectomy and ureterosigmoidostomy. A number of complications develop with this procedure, including urinary infections and renal damage, with the major long-term complication being development of rectosigmoid adenocarcinoma. The risk of carcinoma is much less after creation of a urinary colonic conduit than with a simple ureteral sigmoidostomy.

Serial renal US is worthwhile in these patients to assess renal status and check for development of hydronephrosis. Renal ectopia is common. These patients are prone to reflux.

A cloacal malformation is a different entity, occurs only in girls, and is not related to cloacal exstrophy; it is discussed in Chapter 12.

Ureterocele

Clinical

A ureterocele is a congenital anomaly consisting of a dilated intramural ureteral segment. It is associated with urinary retention and infection. Ureteroceles are mostly a pediatric diagnosis but they occasionally first manifest in adulthood. They are classified into simple and ectopic, depending on the distal orifice. About 10% are bilateral. Some authors suggest that sib-

lings of children with ureteroceles be screened for urogenital abnormalities.

Ectopic ureteroceles are often part of a duplicated renal collecting systems, with most terminating a ureter draining the upper renal pole. They are more common in girls and are a common cause of bladder obstruction in girls. An extreme example consists of bilateral ureteroceles and bilateral duplex systems. An ectopic ureterocele does occur in patients with a single kidney or a horseshoe kidney.

An ectopic ureterocele can distort an adjacent normally located ureterovesical junction to the point of inducing reflux. A rare ureterocele prolapses and even strangulates. Or occasionally a simple ureterocele even prolapses into the urethra and leads to bladder outlet obstruction. Bladder outlet obstruction can also be due to a simple ureterocele if it is large enough.

If necessary, a ureterocele is treated by endoscopic ureterocele incision; the success rate is lower with ectopic ureteroceles. Vesicoureteral reflux develops after such endoscopic incision in about half the patients, with reflux being more common with an ectopic ureterocele.

Imaging

Intravenous urography of a simple ureterocele is usually diagnostic. A ureterocele is seen as a cyst-like dilation of the distal ureter protruding into the bladder lumen; the ureterocele wall appears as a thin radiolucent line, although infection or stasis results in thickening. The typical “cobra-head” appearance is familiar to most radiologists. At times urography simply reveals a large radiolucent tumor in the bladder in association with nonvisualization of the ipsilateral kidney upper pole.

A cystogram detects a ureterocele, provided instilled contrast does not efface or obscure it. Serial visualization during bladder filling and voiding is helpful.

Computed tomography of a ureterocele reveals a well-marginated soft tissue tumor at the ureteral insertion (Fig. 11.1). A simple ureterocele is seen on US as a thin-walled, fluid-filled structure at the trigone.

Occasionally a diminutive renal upper pole is drained by a small duplicated ureter that terminates in a relatively large ureterocele. This condition, called ureterocele disproportion, is

Figure 11.1. Ureterocele. Excretory phase three-dimensional (3D) computed tomography (CT) reconstruction identifies hydronephrosis, hydroureter, and a ureterocele (arrow). (Source: Joffe SA, Servaes S, Okon S, Horowitz M. Multi-detector row CT urography in the evaluation of hematuria. RadioGraphics 2003;23(6):1441–1456, with permission from the Radiological Society of North America.)

ADVANCED IMAGING OF THE ABDOMEN

midline extending from the anterior superior bladder wall to the umbilicus. This embryonic communication normally closes before birth, but patency after birth results in luminal communication between the bladder and the umbilicus. Partial obliteration leads to a blind ending sinus at the umbilical end, or, with failure of the bladder end to close, a sinus communicates with the bladder (vesicourachal diverticulum). A urachal cyst (omphalovesical cyst) represents a cavity in the urachus with both the bladder and umbilical ends closed. In children with urachal abnormalities, urachal sinuses and cysts predominate, with a patent urachus and a urachal diverticulum being less common.

Ultrasonography identifies a midline mass between the rectus abdominis muscle and the superior-anterior bladder wall, representing a urachal remnant, in about one third of young adults. A majority of these urachal remnants are nodular, with a minority being tubular in shape. Their prevalence decreases with age, presumably due to involution. In most people a urachal remnant is a normal variant. Occasionally one manifests by hematuria and is resected.

difficult to diagnose with most imaging modalities.

Some ureteral duplications and ectopic ureteroceles are associated with ureteral obstruction, marked hydronephrosis, and dilation of the involved ureter to the point that adjacent nonobstructed collecting systems are displaced inferiorly. The reverse can also occur—occa- sionally an obstructed normal ureter mimics the appearance of a ureterocele.

Stones develop in some ureteroceles.

The term ureterocele eversion describes a ureterocele that everts or intussuscepts retrograde into the ureter during voiding cystourethrography. Radiographically, the outpouching can appear similar to a paraureteral bladder diverticulum. At times fluoroscopy aids in detecting these eversions.

Urachal Abnormalities

The urachus, or median umbilical ligament, is a residual fibromuscular cord located in the

Fistula and Sinus Tract

Urachal anomalies are more common in males. The prevalence of vesicourachal fistulas is increased in the prune belly syndrome. A patent urachus is also associated with urethral obstruction, such as posterior urethral valves or urethral atresia.

The presence of a patent urachus is generally suspected in the neonate and is best established by direct contrast injection into the umbilical opening. A voiding cystourethrogram also detects patency, provided the images are obtained with the infant in a lateral projection. At times an almost patent urachus is initially asymptomatic but manifests later in life when a bladder outlet obstruction develops.

A vesicourachal diverticulum is a midline outpouching extending from the superioranterior bladder wall. Most are discovered incidentally. Occasionally stasis results in recurrent urinary tract infection and stone formation. Cancer originating in a vesicourachal diverticulum is an uncommon occurrence developing in adults.