Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

polycystic disease. A majority of patients with sufficient polycystic renal involvement requiring transplantation also have hepatic cysts, and in an occasional postrenal transplant patient symptomatic liver cystic disease is the primarily cause of death.

Clinical findings consist of abdominal pain, ascites, or leg edema.

Marked hepatomegaly develops, with CT showing multiple homogeneous, hypodense, variable-size cysts scattered throughout the liver (Fig. 7.4). The remaining parenchyma is compressed by these cysts, which in some patients replace most of the liver parenchyma. No wall or cyst enhancement is evident postcontrast.

As expected, these cysts are hypointense on T1and homogeneously hyperintense on T2weighted MR images. They do not enhance postcontrast. Intracyst hemorrhage varies their appearance even to the point of suggesting a neoplasm. An occasional patient presents with an elevated a-fetoprotein level, suggesting a hepatocellular carcinoma.

These cysts can become infected. Indium 111 leukocyte scintigraphy aids in localizing such an infected cyst.

Some of these patients develop sufficient symptoms to require surgical intervention. Either cyst fenestration or partial hepatic resection is a viable option. Aspirated fluid cytology and cyst wall biopsy to exclude a neoplasm

ADVANCED IMAGING OF THE ABDOMEN

appear reasonable prior to fenestration and fluid spread into the peritoneal cavity. Ascites is a postoperative complication of cyst fenestration. Hepatomegaly occasionally recurs.

Congenital Hepatic Fibrosis (Caroli’s Disease)

The current definition of Caroli’s disease is somewhat muddled. Some authors limit this term to the rare communicating biliary saccular ectasia as originally describe by Caroli (12); whereas others apply it to a broader spectrum of congenital hepatic fibrosis. Some employ Caroli’s disease as a descriptive term and then associate it either with congenital hepatic fibrosis or infantile polycystic kidney disease. Still others use the term Caroli’s disease to describe the rarer isolated biliary ectasia and Caroli’s syndrome if both ectasia and hepatic fibrosis are evident.

Embryologically, intrahepatic bile ducts develop from liver progenitor cells adjacent to portal vein mesenchyme and form ductal plates, which eventually evolve into bile ducts. Lack of or disordered ductal plate remodeling leads to a number of congenital intrahepatic bile duct disorders, including congenital hepatic fibrosis. The latter represents a cholangiopathy with surrounding fibrosis. Kidney involvement is variable in these patients and ranges from renal tubular ectasia to various forms of polycystic disease, most often autosomal recessive and

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only rarely autosomal dominant. An interesting association is Caroli’s disease and occasional acute pancreatitis.

Histologically, bile ductules become ectatic but still maintain their communication with bile ducts. As an isolated finding, ectasia is found only in a minority; over time, progressive fibrosis ensues and accounts for the typical appearance of extensive periportal fibrosis and biliary dilation.

Two manifestations predominate: (1) Those patients developing extensive fibrosis early in life tend not to have prominent intrahepatic bile duct dilation. (2) Dilation is more common in those individuals who are asymptomatic until adulthood, when signs and symptoms of chronic bile stasis develop. In some of the latter patients dilated ducts predominate in one lobe. Also, in some patients Caroli’s disease is associated with extrahepatic bile duct dilation, and in an occasional patient an extrahepatic choledochal cyst is the initial presentation. Calculi tend to develop either in intrahepatic cysts, in bile ducts, or both.

With severe involvement death occurs during the neonatal period from renal causes, before liver damage is evident. Some develop hepatomegaly early in life, others remain asymptomatic, and still others progress to fibrosis, liver failure, and portal hypertension; varices and hematemesis is their initial presentation. Bile stasis and infection lead to acute cholangitis, at times at an early age. Intrahepatic calculi develop in some, but the involved ducts are not obstructed. Hepatic function tends to be preserved until relatively late. Generally most of the liver is involved, although occasionally cysts are limited to one lobe; some patients have a preponderance of left lobe disease.

These patients are at increased risk of a developing hepatocellular carcinomas or cholangiocarcinomas. A not uncommon progression consists of calculi formation, pyogenic cholangitis, intrahepatic abscesses, and eventual cholangiocarcinoma.

In adults, Caroli’s disease can be suspected with most imaging (Fig. 7.5). Findings include hepatomegaly, parenchymal fibrosis, numerous cysts scattered throughout the liver, or multiple cyst-like dilated bile ducts. It is necessary to show that the cysts communicate with bile ducts to distinguish this condition from autosomaldominant polycystic disease and multiple

intrahepatic abscesses. Although some intrahepatic abscesses do communicate with bile ducts, differentiation of abscesses from dilated bile ducts is generally straightforward with cholangiography.

Ultrasonography reveals extensive collaterals and a prominent periportal hyperechoic pattern, a finding also seen in other conditions. Liver parenchymal texture tends toward a heterogeneous appearance containing multiple high echoes.

With progressive fibrosis adults tend to develop portal hypertension, generally attributable to intrahepatic compression of portal vein branches by fibrosis. Doppler US findings vary depending on underlying hemodynamics. Intrinsically, the portal vein is patent, although some patients have portal vein thrombosis and cavernous transformation.

Magnetic resonance cholangiography also detects cystic intrahepatic biliary dilation.

Technetium-99m-IDA scintigraphy reveals bile stasis in cystic structures. Scintigraphy confirms that these cysts communicate with the bile ducts.

Regardless of how it is performed, cholangiography confirms the diagnosis by identifying numerous segmental dilated intrahepatic bile ducts, either saccular or fusiform in appearance.

Hereditary Hemorrhagic

Telangiectasia (Osler-Weber-Rendu

Disease)

Hereditary hemorrhagic telangiectasia, or

Osler-Weber-Rendu disease, is an autosomaldominant disorder manifesting primarily by epistaxis, yet numerous vascular malformations consisting of telangiectasias, arteriovenous fistulas, and aneurysms in the liver, spleen, and other organs often have more serious ramifications. An association of hereditary hemorrhagic telangiectasia and familial juvenile polyposis has been described in several families. Liver arteriovenous malformations are associated with fibrosis and progression to cirrhosis. High output cardiac failure, liver failure, and orthotopic liver transplantation are not uncommon. Why fibrosis and atrophy of some segments and enlargement of others develop is unknown although focal ischemia appears to play a role.

Telangiectasia defines a collection of dilated small vessels. Dilated, tangled vascular communications are common and range from small to large confluent vascular masses. Arteriovenous communications are either to a hepatic or portal vein. Extensive arteriohepatic venous shunting leads to hepatomegaly and congestive heart failure or abdominal angina. Arterioportal shunting, on the other hand, tends to progress to portal hypertension.

In a setting of extensive vascular malformation, the hepatic artery blood flow increases and the artery and its branches dilate and become

tortuous. Arterial phase and portal venous phase CT detects arteriovenous shunting, and reveals enlarged feeding vessels and tangled vessels within a malformation.

Doppler US can screen relatives of patients with hereditary hemorrhagic telangiectasia to detect intrahepatic arteriovenous shunts. Dilated vessels tend to mimic bile ducts with gray-scale US, but color Doppler US waveforms should confirm extensive shunting and suggest the diagnosis. Doppler US shows a marked increase in hepatic artery mean velocity. Enlarged hepatic veins are common. In most

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patients portal venous blood velocity is unchanged compared to normals, except with an arterioportal or even portovenous shunt.

Contrast-enhanced MRI also identified shunting and simultaneous enhancement of both hepatic arteries and veins.

Angiography readily identifies multiple intrahepatic vascular malformations. The hepatic arteries have been successfully embolized in stages in symptomatic patients with hereditary hemorrhagic telangiectasia; this is not an innocuous procedure, however, and deaths from hepatic infarction and necrosis have been reported (13).

Trauma

Management Issues

Previously in a number of institutions peritoneal lavage was an initial diagnostic study performed in patients with suspected blunt abdominal trauma, and, if positive, the patient underwent laparotomy. It was difficult to evaluate outcomes of various liver injuries because investigators used their own nomenclature for liver injury. To overcome some of these problems, a liver injury classification scale was devised by the American Association for the Surgery of Trauma (Table 7.2).

Currently peritoneal lavage has been abandoned in favor of CT and a more conservative approach than used previously. Emergency CT has evolved as the first diagnostic procedure performed in trauma patients. It has led to more nonsurgical management of liver trauma, especially in hemodynamically stable patients. Even those with a moderate-to-large hemoperitoneum have been managed nonoperatively as long as the patient is hemodynamically stable. Thus some patients with grade III liver injuries are managed nonoperatively. Most blunt hepatic trauma in children is also managed conservatively without surgery; in general, the severity of injury as detected by CT does not correlate with a need for surgery.

Severe liver trauma (such as type V on the liver injury scale in Table 7.2) has a very poor prognosis. A majority of patients with grade V injuries are unstable and require laparotomy. The initial surgical concern is control of hemorrhage. One approach is percutaneous intraaortic balloon occlusion using a femoral route, followed by surgical vascular exclusion of the liver.

One complication of liver trauma is a subsequent abscess. Risk of developing an abscess is higher in patients undergoing surgery than in those managed nonoperatively. Presumably infection develops from infected bile or is spread hematogenously, with a hematoma or

necrotic liver tissue acting as a nidus. Abscess imaging is discussed in a later section; suffice it here to state that trauma-associated abscesses are readily identified by CT. The presence of gas bubbles is almost pathognomonic, although occasionally noninfected necrotic liver parenchyma secondary to ischemia has a similar appearance.

Blunt trauma is a rare cause of a major liver infarct.

Imaging

Computed tomography is the first imaging study performed in a hemodynamically stable patient suspected of blunt liver injury. A CT blunt trauma liver injury grading scale has been proposed based on CT findings (Table 7.3). Its initial function was to assess the value of CT in predicting outcome following blunt liver injury. It should be kept in mind that this CT grading scale employs different parameters than the liver injury classification scale devised by trauma surgeons; the latter is meant to help standardize surgical findings and outcomes. Also, although CT aids in establishing the scope of an initial insult and detects complications, whether a patient is explored or managed conservatively often continues to be based on clinical findings.

The most sensitive US finding in patients with liver trauma is intraabdominal free fluid; actual liver injury is detected less often. Mild parenchyma injury appears as a focal hyperechoic region.

Table 7.3. Computed tomography (CT)-based liver injury grading system

Grade Criteria

1Capsular avulsion Superficial laceration <1 cm

Subcapsular hematoma <1 cm

2Lacerations 1–3 cm deep Central/subcapsular hematoma 1–3 cm deep

3Lacerations >3 cm deep Central/subcapsular hematoma >3 cm deep

4Massive hematoma >10 cm Lobar tissue destruction/ischemia

5Bilobar tissue destruction/ischemia

Source: Adapted from Mirvis et al. (15).

ADVANCED IMAGING OF THE ABDOMEN

Is CT helpful in monitoring healing? In most patients routine follow-up CT does not influence subsequent treatment and often is not necessary; a need for follow-up CT should be based on the patient’s clinical course.

Hemorrhage/Hematoma

A hematoma is the most common liver injury. These hematomas range from subcapsular to intraparenchymal in location. Not all are due to trauma. Common nontraumatic causes of liver hemorrhage include bleeding from a hepatic neoplasm; bleeding also occurs with hemangiomas, focal nodular hyperplasia, an occasional metastasis, the HELLP syndrome (discussed later; see Liver in Pregnancy), and such infiltrative conditions as amyloidosis.

A subcapsular hematoma tends to compress the underlying liver parenchyma, thus differentiating it from intraperitoneal fluid. An appropriately located hematoma will obstruct the intrahepatic portion of the inferior vena cava and hepatic veins and result in Budd-Chiari syndrome (discussed in Chapter 17). At times a slow bleed and resultant liver hematoma manifests by anemia.

Contrast extravasation is detected by CT if bleeding is sufficiently brisk. This extravasated contrast–blood mixture has a higher attenuation than an old hematoma, and the two are readily differentiated. If needed, transcatheter embolization is an option for hemorrhage control.

Unenhanced blood has an attenuation value of 30HU or greater. Initially clotted blood has a CT density higher than nonclotted blood, and an acute hematoma is thus hyperdense compared to normal liver parenchyma. Some liver lacerations, however, result in a hypodense hematoma, presumably due to simultaneous leakage of bile. Postcontrast, a hematoma has a lower density than normally enhancing liver parenchyma.

Occasionally CT identifies regions of low attenuation parallel to the portal vein and its branches. Such periportal tracking of blood was initially believed to be a marker of significant liver injury, although currently most radiologists tend to ascribe periportal tracking to lymphatic distention and edema and discount its importance as a marker for significant trauma.

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Its use as a guide for management decisions is controversial; whether periportal tracking is present or not has no effect on the eventual success rate of nonoperative management or on the complication rate. Such tracking usually does not signify a laceration.

The sonographic appearance of a hematoma depends on its age. Initially a hematoma is mostly hyperechoic, with some having a complex, heterogeneous appearance. With time, a hypoechoic pattern predominates as a clot lyses and eventually the hematoma becomes anechoic. A similar US pattern is found with hematomas in other solid organs.

Surgical retractors cause transient focal liver injury. Computed tomography in one patient revealed a focal, sharply marginated hypodense lesion, while MR identified a hypointense, poorly defined region on T1-weighted sequences that was heterogeneous and hyperintense on T2-weighted sequences (16).

Some traumatic liver hematomas are associated with pseudoaneurysms or arteriovenous fistulas; these should be detected by CTA and, if diagnosis is unclear, angiography.

Most subcapsular hematomas resolve within several months. Parenchymal hematomas, on the other hand, take considerably longer to resolve, probably because of their common admixture of bile.

Laceration/Rupture

Numerous nontraumatic disorders are associated with spontaneous liver rupture (Table 7.4).

Postcontrast CT reveals a liver laceration as an irregular linear or branching hypodense region surrounded by enhancing liver parenchyma (Fig. 7.6). Some of these linear lacerations mimic dilated bile ducts. Lacerations of the porta hepatis commonly involve bile ducts and major vessels. Extensive lacerations tend to radiate toward the periphery. A hemoperitoneum is common; the exception being with injury to the liver bare area (segment VII) when a hemoretroperitoneum can develop (17).

Biloma

Table 7.4. Conditions associated with spontaneous liver rupture

Infections

Typhoid

Malaria

Nonneoplastic tumors

Hemangioma

Peliosis hepatis

Focal nodular hyperplasia

Nodular regenerative hyperplasia

Angiomyolipoma

Neoplastic tumors

Hepatic adenoma

Hepatocellular carcinoma

Metastasis

Other

HELLP syndrome

Amyloid infiltration

Polyarteritis nodosa

Ehlers-Danlos syndrome

tion of contrast from the duct involved, thus establishing the diagnosis.

At times differentiation among a resolving hematoma, abscess, and biloma is difficult both by CT and US. Ultrasonography simply reveals an encapsulated fluid collection. In such a scenario either scintigraphy or percutaneous aspiration of fluid is necessary to establish the diagnosis.

Magnetic resonance imaging differentiates between a simple intrahepatic biloma and

Spontaneous perforation of an intrahepatic bile duct is uncommon.A cholangiogram,regardless of how it is performed, should reveal extravasa-

Figure 7.6. Liver laceration in a 25-year-old man (arrows). (Courtesy of Patrick Fultz, M.D., University of Rochester.)

hematoma. A biloma is hypointense on T1weighted images and hyperintense on T2weighted images; a hematoma is hyperintense on both T1and T2-weighted images; in practice, blood and proteins modify the MR appearance of a biloma. Nevertheless, a differentiation is more than theoretical because most bilomas should be drained, but hematomas tend to resolve spontaneously.

Porta Hepatis Injury

Although injury to the porta hepatis region is rare, it is often lethal; intraoperative exsanguination is not an uncommon cause of death in these patients, and thus hemorrhage control is the first priority. Other associated injuries are common. Suspected bile duct injuries are identified by intraoperative cholangiography and repaired either primarily or an enteric anastomosis.

In stable patients with blunt liver injury undergoing both CT and hepatic angiography, CT was only 65% sensitive and 85% specific in detecting arterial injury (18); if in doubt, hepatic angiography is the gold standard in detecting arterial injury.

Infection/Inflammation

Abscess

Pyogenic

Clinical

A pyogenic liver abscesses is often associated with bile duct stones and duct obstruction. At times simply removing retained stones, stenting, and medical management are therapeutic.

A not uncommon etiology for multiple liver abscesses is sigmoid diverticulitis and hematogenous spread of infection to the liver. Most abdominal radiologists have encountered patients with not only multiple liver abscesses but also portal venous gas; Bacteroides fragilis, producing gas by fermentation, is one such responsible organism. Chlamydia trachomatis is a common organism found in Fitz-Hugh–Curtis syndrome. A rare liver abscess evolves with this infection. The specific diagnosis is often established by percutaneous aspiration.

ADVANCED IMAGING OF THE ABDOMEN

An uncommon association for a liver abscess is with a silent colon adenocarcinoma.

The most common clinical presentation in patients with a pyogenic hepatic abscess is fever and leukocytosis, and less often abdominal pain, hepatomegaly, and abnormal liver tests. Jaundice and marked elevation of alkaline phosphatase are common with a biliary tract abscess origin.

The intervening liver capsule, subphrenic space, and diaphragm prevent spread of most intrahepatic abscesses into the thorax. An exception is an abscess involving the liver bare area; an occasional such abscess communicates with the mediastinum.

A liver abscess is a recognized complication of Crohn’s disease. Steroids and other therapy tend to mask the presence of these abscesses. Occasionally such an abscess is the initial presentation of Crohn’s disease.

Diabetes mellitus is a major predisposing factor for pyogenic liver abscesses even without a known infectious focus; gas within the abscess, detected with conventional radiography, CT, or US, is associated with a higher mortality than one without gas. Diabetes tends to predispose rupture of a pyogenic liver abscess. These patients have a high mortality even with prompt surgical cleansing of the abdominal cavity. Klebsiella pneumoniae is a common bacterium isolated.

Rarely, rupture of an infected gallbladder leads to a liver abscess. After percutaneous abscess drainage, contrast injected into the abscess cavity should reveal a fistulous communication with the gallbladder. A rare pyogenic liver abscess contains multiple stones and is caused by perforation of an adjacent necrotic gallbladder; these combined gallbladder and liver abscesses are notoriously difficult to evaluate adequately.

Imaging

Liver abscesses range from solitary to multiple; unilocular to multiloculated; large to small. In most instances imaging findings strongly suggest an abscess. It should be realized, however, that an occasional necrotic, or infected liver cancer presents as a hepatic abscess. These include primary liver cancer, gallbladder carcinoma, or even a metastasis. Imaging does not always differentiate a complex necrotic and

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cystic cancer from a liver abscess. Likewise, a pyogenic abscess appears similar to an amebic abscess.

Only an occasional hepatic abscess contains gas.

Computed tomography shows most intrahepatic abscesses as hypodense regions but with peripheral contrast enhancement. Nevertheless, the appearance of pyogenic abscesses varies from homogeneous and hypodense to a heterogeneous solid tumor. Some abscesses incite little surrounding inflammation; others induce considerable inflammation, and postcontrast they appear target-like, with the hypodense abscess surrounded by a hyperdense rim, which in turn is surrounded by a thin hypodense periphery thought to represent liver edema. Segmental hepatic enhancement during arte- rial-phase CT is common, presumably caused by decreased portal flow resulting from portal tract inflammation.

Ultrasonography shows approximately two thirds of pyogenic abscesses to be hypoechoic, with the rest inhomogeneous. Anechoic abscesses are uncommon. Approximately half have a smooth wall. With progression and necrosis, abscesses tend to become more hypoechoic. The abscess wall thickens with chronicity. Gas within an abscess is hyperechoic and reveals shadowing. Contrast aided phase inversion harmonic mode US reveals rim enhancement, arteries along abscess margins and internal septa, septal enhancement, absent circulation in fluid and arterial hypervascularity surrounding the abscess (19).

Magnetic resonance imaging appearance of a liver abscess is similar to other fluid-containing lesions. Most are hypointense on T1and hyperintense on T2-weighted images. Most abscesses tend to be better defined postcontrast and often show an immediate hyperintense enhancing rim, which persists in later phases, but the central portion, as expected, does not enhance. It should be kept in mind that many metastases also have rim enhancement. An absolute distinction between an abscess and a neoplasm is often not possible purely on MRI findings alone. An infected neoplasm complicates this issue further.

Indium 111 leukocyte scintigraphy readily detects most hepatic abscesses. An occasional abscess, however, results in a cold defect.

Normal hepatic uptake of gallium 67 makes interpretation of a Ga-67 abscess scan difficult;

a Tc-99m sulfur colloid scan performed in conjunction with Ga 67 scintigraphy is helpful for interpretation.

Therapy

Although some of the smaller abscesses can be treated medically, most require either percutaneous or surgical drainage, at which time diagnostic aspiration confirms the diagnosis. In many centers surgical drainage is reserved for abscesses that cannot be adequately drained percutaneously. Percutaneous drainage using a large-caliber drainage catheter is associated with few complications and has a high success rate. Both multiloculated and multiple abscesses are drained successfully. Previously considered a lethal disease, currently abscess cure rates up to 93% are typical with percutaneous drainage. The success rate is lower in patients with an underlying malignancy. Abscesses with a biliary fistula are also drainable percutaneously. Most of the underlying fistulas resolve unless distal bile duct obstruction is present.

The major complication of percutaneous drainage is bleeding, at times massive. An occasional patient develops an empyema due to pleural puncture. Mortality associated with percutaneous abscess drainage is less than 10%.

Common dogma holds that simple percutaneous needle aspiration is not a viable therapeutic option for a pyogenic liver abscess. Yet, percutaneous needle aspiration and systemic antibiotics can be successful in healing most abscesses, realizing that about half of these abscesses require two or more sessions (20). Large gauge trocar needles and US guidance, and complete pus removal are necessary.

Currently surgical drainage or liver resection are reserved for complex abscesses, those without a safe percutaneous access route, and failed percutaneous drainages.

Amebic

Clinical

Amebic liver abscesses have a wide geographic variation. Men predominate by a ratio of about 10 : 1. In nonendemic regions a high proportion of amebiasis is found in AIDS patients.

The diagnosis is usually suspected from serologic testing. Fever, abdominal pain, and hepatomegaly are common presentations.

Independent mortality risk factors include an elevated bilirubin level, the presence of encephalopathy, the size of the abscess cavity, hypoalbuminemia, and the number of abscesses present; mortality is independent of the duration of symptoms.

Imaging

In general, imaging cannot differentiate an amebic abscess from a pyogenic abscess, and the imaging findings described above for pyogenic abscesses also apply to amebic abscesses.

Some patients develop a right pleural effusion, right basal atelectasis, a perihepatic fluid collection, or simply elevation of right hemidiaphragm. These findings occur with intraabdominal abscesses and do not necessarily imply intrathoracic abscess spread. Nevertheless, a rare liver amebic abscess evolves into a hepatopulmonary fistula or a hepatocolic fistula, or even spreads into the mediastinum and pericardium.

In patients resistant to medical therapy, abscess communication with the bile ducts should be suspected. These patients tend to be jaundiced. A minority of amebic liver abscesses are associated with focal intrahepatic bile duct dilation, presumably secondary to obstruction.

Amebic liver abscesses range from solitary to multiple. Single abscesses tend to be more common in tropical amebiasis. A right lobe location predominates. On precontrast CT these abscesses range from isoto hyperdense. Mother cysts tends to be hyperdense compared to daughter cysts. Secondary bacterial infection further increases the mother cyst or daughter cyst CT density. Most abscess walls enhance postcontrast. Internal septations develop in some.

Ultrasonography reveals most to have a smooth wall and a homogeneous, hypoechoic US appearance. Because of retained necrotic debris and blood, some abscesses appear solid and contain hypoor hyperechoic internal echoes. They are surrounded by a hyperechoic wall, which in turn is surrounded by a thin peripheral halo. Some are multiseptate.

An occasional amebic liver abscess is imaged by triple-phase bone scintigraphy.

ADVANCED IMAGING OF THE ABDOMEN

Therapy

Medical therapy of an amebic abscess is highly successful; only those not responding to medical therapy or clinically appear to be at risk of impending rupture require drainage. Nevertheless, especially with a larger abscess, faster recovery is often achieved if drug therapy is combined with image-guided percutaneous aspiration.About a third or so of these abscesses yield no Entamoeba histolytica. If the material obtained initially is nondiagnostic, the final aspirate should be examined because it is most often diagnostic. The aspirate usually is rather viscous, and aspiration through a large-bore catheter is helpful. Abscesses tend to recur if no drain is left in place.

Presence of a biliary fistula does not influence the cure rate for percutaneous drainage. Most biliary fistulas close spontaneously. Likewise, even with a ruptured amebic abscess percutaneous catheter drainage of extrahepatic collections appears safe and effective.

Hydatid Cyst

Clinical

Infection with Echinococcus granulosus, or hydatid disease, is uncommon in Northwestern Europe and North America but is endemic in the Mediterranean basin and several regions of East Africa. Dogs, ruminants, and humans constitute the infestation cycle. Most common ruminants involved are sheep and goats, although apparently camels also have a role in parts of Africa; the highest prevalence in humans is in the Turkana district of Kenya. Hydatid disease exists in cattle in West and Southern Africa but human infestation here is rare.

The most common sites of involvement are the liver and lungs, the right lobe more often than the left, adjacent to Glisson’s capsule. Echinococcal cysts are slow growing and often take years to reach a large size.

Cysts contain two layers. The outer pericyst, composed of an avascular layer, is a host response to infestation. An inner endocyst, about 2mm in thickness,is produced by the parasite and normally is adjacent to the pericyst, except when ruptured.

Although some cysts rupture spontaneously, especially when large, unrelated trauma is a not uncommon cause of cyst rupture. In a contained rupture only the endocyst ruptures, with the

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pericyst remaining intact. Some cysts rupture into an adjacent bile duct or directly into the peritoneal cavity, subphrenic space, or, rarely, into the gastrointestinal tract. Communication with a gas-containing viscus leads to a gas-fluid level within the cyst. Occasional cysts have transdiaphragmatic spread to the thorax. Rupture into a blood vessel, such as a hepatic vein, is rare.

Jaundice is usually due to cyst rupture into bile ducts and intrabiliary spread of the hydatid content. The resultant obstruction is generally treated by endoscopic sphincterotomy and removal of the obstructing intrabiliary daughter cysts. Occasionally, a hydatid cyst obstructs the bile ducts close to the liver hilum simply by compression, and jaundice develops even without a biliary communication. Likewise, a hydatid cyst close to the liver hilum can lead to cavernous portal vein transformation.

In some patients hydatid cyst content in the bile ducts induces a hypersensitivity reaction, even to the point of anaphylaxis. A ruptured cyst is one cause of a relapsing generalized anaphylactic reaction, including life-threatening laryngospasm. An occasional one results in an eosinophilic cholecystitis.

The less common echinococcal infection with

Echinococcus multilocularis is encountered in colder climates. Liver screening with US is practiced in some endemic regions, and screening appears to contribute to early detection.

A local form of aggressive hydatid disease is found in Central and South American neotropical zones. The infectious agent is Echinococcus vogelii, paca, and other wild rodents are intermediate hosts, and the bush dog the final host. The liver is most often involved, with metacestodes spreading into the peritoneal cavity and eventually invading other abdominal and thoracic organs. Infected patients most often present with hard, round tumors in or adjacent to the liver, hepatomegaly, increased abdominal girth, pain, and cachexia; liver involvement progresses to portal hypertension. Imaging reveals a polycystic liver disease pattern. Extensive involvement leads to numerous cysts in the liver, pancreas, spleen, and peritoneal cavity. Cyst calcifications are common.

Imaging

Type 1: Simple unilocular cyst. Believed to be an early stage in hydatid cyst formation, these cysts are water dense on CT and anechoic on US. Cyst wall and septal enhancement is seen with CT and MR contrast, thus differentiating this entity from simple cysts.

Type 2: Cyst containing daughter cysts. Round or irregular daughter cysts are surrounded by a higher density fluid in the mother cyst.

Type 3: Dead cysts containing extensive calcifications.

Type 4: Complex cysts. These consist of superinfected cysts or ones that have ruptured. In a contained rupture imaging identifies endocyst separation from the surrounding pericyst. Bacterial superinfection implies the presence of cyst rupture.

Curvilinear cyst rim calcifications are common with Echinococcus granulosus infections, and a cystic pattern is detected in about half.

The CT appearance varies, although a hydatid cyst tends to have an oval and sharply defined outline. Precontrast, cysts are mostly inhomogeneous and of low density; postcontrast enhancement is inhomogeneous (Figs. 7.7 and 7.8). The sonographic appearance ranges from an anechoic cyst, mural nodules, and visualization of the endocyst, to a complex multicystic structure.

Debris either tends to be displaced toward the center of the primary cyst or is located in

An imaging classification of hydatid cysts consists of the following:

Figure 7.7. Hydatid cyst. CT also identifies a soft-tissue component within the cyst (arrow). (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)