Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

children. Curiously, about half of these tumors have an associated primary or metastatic central nervous system tumor.

This is a solid renal hilum tumor. A CT finding of a heterogeneous tumor, together with a peripheral eccentric crescent having an attenuation of fluid, representing subcapsular hemorrhage or peripheral tumor necrosis adjacent to tumor lobules, is common with these tumors. Some contain calcifications outlining tumor lobules. These findings are not pathognomonic, however, and are seen with other renal neoplasms in children. A Wilms’ tumor is generally in the differential diagnosis—the distinction is important because of the considerably better prognosis with a Wilms’ tumor.

Small Cell Carcinoma

The descriptive term small cell tumor is applied to a group of neoplasms having similar morphologic characteristics. These include lymphomas and various neuroendocrine carcinomas. Nonneuroendocrine small cell carcinomas, also called nonfunctioning neuroendocrine carcinomas or simply small cell carcinomas, are very rare in the kidneys and ureters. Some contain hyperchromatic small cells with neurosecretory granules. Others combine features with a transitional cell carcinoma, and still others exhibit adenomatous or squamous differentiation.

These are very malignant tumors with metastases common at the initial presentation. Often metastases are discovered first.

Neuroendocrine Tumors

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Neurilemoma (Schwannoma)

A neurilemoma originates from neural Schwann cells and is rare in the kidneys. It has no specific imaging findings. Some extraperitoneal ones simply surround and obstruct the ureter or ureteropelvic junction.

Wide excision is the treatment of choice, because up to one third are malignant. They can recur locally or metastasize.

Carcinoid Tumor

Primary renal carcinoid tumors are rare. These tend to be solid, with an occasional one containing calcifications. An occasional one originates from a cyst wall. For some reason renal carcinoids are more common in horseshoe kidneys (89), developing even in a horseshoe kidney isthmus.

Some larger carcinoids undergo hemorrhagic necrosis.

Dilated Urinary Tract

Renal collecting system dilation, or hydronephrosis, can be secondary to obstruction or functional in nature. Hydronephrosis varies from barely discernible to gigantic. At times huge hydronephrosis displaces the kidney away from its usual location. Complicating the issue, an occasional patient has intermittent hydronephrosis and, when asymptomatic, IV urography or US reveal only mild renal pelvic dilatation at best. These patients should thus be studied when symptomatic.

An occasional malignant primitive renal neuroectodermal tumor is encountered that cannot be readily classified.

A possible relationship between H2 receptor antagonist therapy and neuroendocrine carcinomas is suggested.

Neuroblastoma

Most neuroblastomas originate in the adrenal glands (discussed in Chapter 16). They occur primarily in the young and often are in the differential diagnosis of a renal or suprarenal tumor.

Urinary Obstruction

General

The etiologies for urinary tract obstruction range from intrinsic to the urinary tract to extrinsic, benign conditions to a malignancy. Some unilateral urinary obstructions are insidious, and renal function is lost before damage is clinically suspected. As an example, in a patient with terminal ileal Crohn’s disease, a surrounding inflammatory mass (phlegmon) can involve and obstruct the right ureter and lead to irreversible loss of renal function before damage is detected.

KIDNEYS AND URETERS

In general, with unilateral ureteral obstruction at an early age, ipsilateral kidney growth is impaired while the contralateral kidney undergoes compensatory growth. This association of ureteral obstruction and subsequent renal growth retardation is probably mediated by complex regulation of cell proliferation, destruction, and other factors.

Urinary perforation proximal to an obstruction is generally secondary to an acute obstruction, typically a stone. An occasional neoplasm, such as a transitional cell carcinoma, not only obstructs the urinary tract but also weakens the ureteral wall sufficiently to produce rupture, but the tumor then continues to grow in the perirenal urinoma. Resultant extensive deformity makes tumor identification difficult.

Imaging

Most imaging of suspected obstruction is straightforward. A contrast study of an acute obstruction shows a persistent unilateral nephrogram and delayed collecting system filling. It should be kept in mind that these findings are not pathognomonic for urinary tract obstruction, and similar changes may be seen with vascular obstruction, a vasculitis, infection, and a number of other causes of renal failure, including acute glomerulonephritis and acute tubular necrosis.

Normally a pyelogram persists for a short period of time after IV contrast. With obstruction, a pyelogram can persist for hours. A pyelogram occasionally persists for weeks if the ureter is encased by tumor, and the lymphatics are an obstruction.

Occasionally urinary tract obstruction cannot be readily distinguished from nonobstructive hydroureteronephrosis. In such a setting the Whitaker perfusion test still has a role. This is an invasive test requiring percutaneous renal pelvis puncture, bladder catheterization, and measurement of pressure differences between the two sites during contrast infusion.

A prominent extrarenal pelvis detected with CT is differentiated from hydronephrosis by evaluating the superior and inferior pole collecting systems; calyces are not dilated in the presence of a simple extrarenal pelvis. Computed tomography readily identifies a dilated ureter, keeping in mind that on occasion a

dilated gonadal vein mimics the appearance of a dilated ureter.

In a majority of patients with acute renal obstruction, unenhanced helical CT reveals that an obstructed kidney is less dense than the contralateral unobstructed kidney (90); this difference is visually detected and constitutes a secondary sign for acute obstruction.

Gray-scale US readily detects moderate hydronephrosis as an anechoic region conforming to the renal collecting system. Mild hydronephrosis can be confused with prominent renal veins; Doppler US is helpful in this setting. Determining whether hydronephrosis is indeed secondary to urinary tract obstruction is more problematic. Also, the etiology of an obstruction may not be evident with US.

Gray-scale US is reasonable in monitoring residual postoperative urinary tract dilation in children. Normally infundibular and ureteral dilation decrease gradually after surgery. In children, US findings pointing to obstruction include a hyperechoic renal appearance, a parenchymal rim of 5 mm or less, contralateral hypertrophy, a resistive index (RI) of 1.10 or greater, an RI difference with diuresis of 70% or greater, a ureter diameter 10 mm or greater, and an aperistaltic ureter (91); how practical these findings are in differentiating obstructive from nonobstructive hydronephrosis is not clear.

Resistance to renal arterial blood flow (RI), as measured with renal Doppler US, is increased in most acute obstructive uropathies. With unilateral obstruction, RI and the pulsatility index (PI) are greater in the obstructed kidney than a contralateral kidney, keeping in mind that RI increases with age and some nephropathies. The published RI sensitivities in detecting an acute urinary obstruction depend on accepted RI cutoff values, but even then they vary considerably, and some authors have concluded that RI values are not useful enough to detect acute urinary tract obstruction.

Occasionally endorectal or endovaginal 3D US is helpful with suspected distal ureteral or ureterovesical junction obstruction. Normal ureters have forceful ureteral jets. Initially after stenting, ureters have passive flow, with peristaltic activity only gradually becoming evident; Doppler US is not reliable in diagnosing an obstruction in a stented ureter.

A respiratory-triggered 3D fast spin-echo MR technique can detect urinary tract dilation

without the use of IV contrast. Comparing breath-hold heavily T2-weighted sequences (thin-slice HASTE) with gadolinium-enhanced 3D FLASH MR sequences in patients with acute flank pain, both MR urography techniques were comparable in detecting obstruction (92); however, 3D FLASH was superior for stone detection, reaching a sensitivity of 96% to 100% and specificity of 100% compared with a sensitivity of 54% to 58% and specificity of 100% for HASTE T2-weighted sequences. For the best results, both sequences are recommended in patients with acute flank pain.

Perirenal fluid is common in acutely obstructed kidneys but generally not present with a chronic obstruction. The HASTE MR urography, which identifies not only ureteral obstruction but also presence of any perirenal fluid, detects a high perirenal signal in most patients with acute ureteric obstruction and is thus useful in differentiating acute from chronic obstruction.

Radionuclide renography also identifies most ureteral obstruction if renal function is adequate, but does not provide anatomic detail. Either Tc-99m-DTPA or Tc-99m-MAG3 is commonly used. Radiotracer stasis is evident in a dilated system, be it obstructed or not. In the absence of obstruction, vigorous hydration or the use of a diuretic result in a washout, but an obstruction leads to prolonged retention. Comparing furosemide injected 20 minutes after renography versus furosemide injected 15 minutes before renography in children, the earlier furosemide injection revealed seven times more obstructions than the later injection (93).

Obstruction by Stones (Renal Colic)

Clinical

Acute flank pain and hematuria are typical presentations in patients with acute obstruction. About 85% of these patients have hematuria. An obstruction induces prostaglandin secretion, which increases renal blood flow and glomerular filtration rate and results in renal colic. Nevertheless, a clinical diagnosis of obstruction is not straightforward and is accurate in only about half of the patients.

Although a stone can obstruct anywhere in the ureter, the most common site is at the

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ureterovesical junction, followed by a site just distal to the ureteropelvic junction and midureter at the iliac vessel level.

One complication among patients with a chronically impacted ureteral stone is a stricture; a previous ureteral perforation at the site of a stone appears to be a risk factor for stricture.

Not all flank pain is due to renal colic. Among less common causes mimicking the flank pain of renal colic are ovarian vein thrombophlebitis. Acute renal artery obstruction can mimic renal colic pain. A unique cause of renal colic was a bird shot calculus after a gunshot wound to the abdomen (94).

Imaging

Considerable literature analyzed the role of conventional radiography and US in patients with flank pain believed to be renal colic in origin, with conventional teaching being that about 80% of ureteral stones contain sufficient calcium to be radiopaque and thus could be identified with conventional radiography. Some studies reported a sensitivity and specificity of over 90% in detecting a calculus, especially if conventional radiography and US are combined. More critical studies, however, do not achieve such optimistic results. In some studies conventional radiographs could detect ureteral calculi with a sensitivity of <50%. A prospective study of patients with renal colic achieved US sensitivity of only 61% in detecting ureteral calculi, while CT sensitivity was 96% (95). Incidentally, conventional abdominal radiography and CT scout radiography have different resolution; thus abdominal radiography revealed 60% and CT scout radiography only 47% of ureteral calculi (96). Digital abdominal radiography identifies most urinary tract calculi >5mm and those calculi having a CT attenuation of >300HU (97). Stone visualization can be enhanced on a CT scout view by a low kilovoltage setting.

Traditionally, IV urography was performed for suspected renal colic, but it has been supplanted to a large extent by noncontrast CT. That noncontrast CT is as effective as or better than IV urography in identifying ureteral calculi and equally effective in determining the presence or absence of ureteral obstruction has been confirmed by numerous studies. Helical CT also

KIDNEYS AND URETERS

provides such accessory signs as periureteral and perirenal fat infiltration and ureteral edema, and, in fact, achieves greater detection of renal and ureteral calculi in patients with acute flank pain than IV urography. Most helical CT studies report sensitivities and specificities in the high 90% for detecting obstructing ureteral calculi. Also, in the absence of urolithiasis an alternative diagnoses can often be established. As a result, in a number of countries, including the United States, noncontrast helical CT is preferred; in other countries,such as France,US has a more prominent role.

One side effect of clinicians becoming more comfortable with unenhanced helical CT in a setting of suspected acute urinary tract obstruction is that the indications for this technique are being arbitrarily expanded. As a result, the number of stones being detected is decreasing and the number of other diagnoses is increasing, a trend bound to continue. Given such expanded indications, an argument can be made for adding a contrast-enhanced CT study if the initial unenhanced CT is negative for obstruction by stones, yet little published data supports such an approach.

Phleboliths are in the differential diagnosis of pelvic calcifications. Most phleboliths are round and on conventional abdominal radiographs often have a radiolucent center, a finding identified with CT in about 10%. Their size and appearance varies and no pathognomonic sign differentiates ureteral stones from phleboliths aside from establishing ureteral continuity for a stone. Especially difficult with noncontrast CT is distinguishing pelvic phleboliths from minimally obstructing distal ureteral calculi, at times leading to unnecessary retrograde urography. Many ureteral stones are surrounded by a soft tissue rim (soft tissue rim sign), representing ureteral wall and possible edema, although an occasional phlebolith also has a soft tissue rim. Surrounding edema is less evident in patients with chronic renal failure.

An eccentric, tapering soft tissue density pelvic vein adjacent to a calcification identifies a phlebolith (comet sign or tail sign); this appearance also is not pathognomonic and a curvilinear nondilated ureter can mimic this appearance. Prevalence of the comet sign varies—some studies find it only in a minority of phlebolith while others believe it is present in a majority.

A passed stone is occasionally detected in the bladder. Secondary signs of prior obstruction, namely, a surrounding soft tissue rim without evidence of a ureteral stone, are often still evident.

A caveat is in order for the patient with clinically suspected obstruction due to a renal stone but a normal unenhanced CT study. Renal ischemia secondary to renal artery obstruction is in the differential diagnosis and will not be detected with precontrast CT. Also, a rare renal carcinoma, not evident on precontrast CT, first presents with flank pain (98), and thus postcontrast CT is necessary in these patients. Alternatively, Doppler US should detect major renal ischemia.

Once a site of obstruction is identified, narrow collimation scanning of the region in question often identifies an etiology. Calculi as small as 1mm in diameter are detected. Secondary signs of obstruction consist of hydroureter, perinephric and periureteral stranding, ureterovesical junction edema, hydronephrosis, and renal enlargement; keep in mind that calyces are not unduly dilated and little perinephric stranding is evident early in an obstruction or with an incomplete obstruction. In general, the extent of perinephric edema correlates with the degree of ureteral obstruction. Although an indirect sign, attenuation difference between kidneys greater than or equal to 5.0 on unenhanced helical CT achieved a 60% sensitivity and 100% specificity in detecting ureteral obstruction (99).

Whether a stone is impacted at the ureterovesical junction or is already in the bladder can be difficult to determine for stones located close to the ureterovesical junction. If the diagnosis is uncertain with the patient imaged in the supine position, a prone position image will usually distinguish between these entities.

Color Doppler US in patients with complete ureteral obstruction by a stone revealed the absence of a normally present ureterovesical jet; with incomplete obstruction the jet tended to be continuous but decreased in volume and intensity. Also, a distal ureteral stone diverts the jet orientation from its usual anteromedial direction. These findings are indirect evidence for a ureteral stone.

Contrast enhanced MRI can also suggest an obstructed ureter. Defining renal transit time as

the time interval between contrast appearance in the kidney and in the ureter at or below the lower pole level, a time >490sec suggests an obstructed ureter (100); a normal transit time is <245sec.

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while some evolve into renal damage. If stable, US is useful in following this condition every 6 months or so.

Imaging

Therapy

The degree of obstruction, the stone location, and the underlying renal function are not major issues during initial therapy of patients with acute ureteral stone obstruction. More relevant is whether CT can predict which stones will pass spontaneously. In patients with a single stone, the mean stone diameter in those with spontaneous stone passage was 2.9mm and in those with failed conservative therapy 7.8mm (101); also, perinephric fat stranding and perinephric fluid collections were significantly higher in patients with spontaneous stone passage.

The usual options for obstruction and infection due to ureteral calculi are percutaneous nephrostomy or retrograde ureteral catheterization with drainage. Both procedures relieved an obstruction and infection; the choice depends on procedure availability and cost.

Ureteropelvic Junction Obstruction

Clinical

Ureteropelvic junction obstruction is more common on the left side. Depending on chronicity, the resultant distention ranges from an almost-normal mild renal pelvis prominence to severe hydronephrosis. Likewise, patients can be asymptomatic or have a palpable mass, with the later more common in infants.

Both intrinsic and extrinsic obstructions develop at the ureteropelvic junction. Intrinsic obstructions include congenital strictures, ischemic strictures, fibrosis, redundant mucosa, ureteral kinks, or even a peristaltic abnormality. With some apparent strictures no underlying abnormality is found. Extrinsic obstructions include fibrosis and compression by adjacent blood vessels; the latter are usually incidental findings, although an occasional crossing renal vessel leads to intermittent ureteral obstruction.

Most mild ureteropelvic junction obstructions remain stable for prolonged periods of time. An occasional one resolves spontaneously

Urography readily detects a ureteropelvic junction obstruction. Some have suggested that US overdiagnoses this condition, although others disagree and believe that US allows the detection of an obstruction at an earlier age.

Computed tomography angiography of patients with symptomatic ureteropelvic junction obstruction should achieve near 100% seusitivity and specificity in detecting crossing arteries. These vessels are either posterior or anterior to the ureter. Axial CT generally is sufficient, although 3D reconstruction provides better spatial orientation of the surrounding structures.

Nonenhanced color Doppler US detected only 65% of crossing vessels in patients with ureteropelvic junction obstruction, while con- trast-enhanced Doppler US detected 96% (102). Doppler US proponents thus maintain that their detection results are equivalent to those achieved by CT.

Occasionally an infant with a ureteropelvic junction obstruction has such large renal pelves that they extend almost to the bladder. In such a setting US may suggest an incorrect diagnosis of ureterovesical junction obstruction. Likewise, occasionally vesicoureteral reflux mimics a ureteropelvic junction obstruction.

Patients with an unilateral ureteropelvic junction obstruction have increased differential function as shown with DTPA scintigraphy, possibly due to compensatory hyperfunction.

Therapy and Outcome

Over half of neonates with unilateral hydronephrosis detected by preand postnatal US and followed with sequential nuclear renograms eventually revert to normal renal function and washout; those with poor or worsening kidney function and drainage require pyeloplasty.

Adjacent arteries are a relative contraindication to endopyelotomy, and recognition of crossing vessels is of prognostic value for successful endopyelotomy. Nevertheless, the need for preoperative imaging has been questioned