Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

Table 14.1. Etiology of rectus sheath hematomas

Spontaneous Anticoagulant therapy Steroid therapy Thrombocytopenia Hemodialysis Collagen disease

Idiopathic (especially in elderly patients)

Traumatic

Direct trauma, including surgical

Vigorous muscle exertion

Physical exertion

Coughing

Seizures

Pregnancy and labor

Tetanus

ADVANCED IMAGING OF THE ABDOMEN

Cysts

Lymphangiomas, lymphoceles, urinomas, and cystic mesotheliomas are discussed separately in this chapter. Cystic tumors also include rare primary cysts, cystic neoplasms, hematomas, aneurysms, and related entities. Anecdotal reports also describe extraperitoneal bronchogenic cysts (40). Pancreatic pseudocysts (which are not necessarily in the pancreas) and enteric duplications are also in the differential diagnosis if a cyst origin cannot be precisely identified. Incidentally, not all pseudocysts are pancreatic in origin; a pseudocyst developing around the tip of a ventriculoperitoneal shunt contains cerebrospinal fluid (41).

transversalis fascia, and rupture of the inferior epigastric artery branches in the preperitoneal fat results in a hematoma throughout this region.

A typical clinical presentation is a rapidly growing painful abdominal wall swelling. Some patients develop anemia. Therapy in most patients is conservative once imaging confirms the diagnosis.

Computed tomography reveals an acute hematoma to be hyperdense compared to adjacent muscles, primarily because of its hemoglobin content. Most so-called rectus sheath hematomas are located posterior to the rectus abdominis muscle but do involve the muscle and result in muscle enlargement. Some hematomas also contain a fluid–fluid level due to a hematocrit effect. In time, a hematoma loses protein and becomes isodense or hypodense relative to adjacent muscle. Some eventually become surrounded by a capsule.

Ultrasonography results vary depending on hematoma age. An inhomogeneous appearance due to blood and blood clots is found in some. Similar to CT, a fluid–fluid level is evident in others, especially larger ones. Ultrasonography, however, can confuse a hematoma with an abdominal wall neoplasm.

Magnetic resonance imaging reveals a heterogeneous hyperintense signals both on T1and T2-weighted images. Fluid–fluid levels are also detected on MR images.

Mesenteric Cysts

The term mesenteric cyst is a descriptive one for cystic structures located in the mesentery. Most occur close to the mesenteric root. They have several etiologies, although some cysts are difficult to classify even after histopathologic study. Cyst content varies considerably. Some cysts lack a mucosal lining, thus distinguishing them from a bowel duplication, while others are lined by ciliated, cuboidal or columnar cells. The cyst wall and any septa can contain smooth muscle, fat or lymphovascular tissue. In adults, many are inclusion cysts (discussed later), while in children lymphangiomas are high in the differential.

A number of these cysts are discovered incidentally. Some patients have chronic pain. Cyst rupture or torsion may result in an acute abdomen. A mesenteric cyst tends to change its position as the mesenteric structures shift.

These cysts vary in size. The cyst wall is thin to the point of being barely perceptible, thus differentiating it from an enteric duplication, which has a thick wall. Most of these cysts contain clear fluid, although more proximal jejunal cysts tend to contain chyle. Some of these chylous cysts have a characteristic appearance; they tend to be unilocular and contain a fluid–fluid level visible by imaging. The two fluids are of fat and water densities, with the uppermost containing fat. The two fluids mix with a change in patient position.

Their MR appearance varies depending on cyst content. The cyst wall enhances postcontrast.

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Mesenteric cysts should probably be completely excised whenever possible; incompletely excised cysts tend to recur and an occasional one even undergoes malignant transformation.

Omental Cysts

In general, the types of cyst found in the mesentery also develop in the omentum. The most common omental cyst is a lymphangioma.

Inclusion (Mesothelial) Cysts

Peritoneal inclusion cysts, also known as mesothelial cysts and benign cystic mesotheliomas, are found in premenopausal women and typically are located adjacent to an ovary or surround the ovary. Histologically, the cyst wall is of mesothelial origin, although occasionally it undergoes squamous metaplasia. Their relationship to benign mesotheliomas (discussed in a later section) is unclear. They are not neoplastic or premalignant.

Some peritoneal fluid originates as an exudate from ovaries. This fluid is normally absorbed by the peritoneum, except in a setting of an injured peritoneum and resultant fibrosis when fluid tends to accumulate in discrete cavities surrounded by mesothelial proliferation.

These women present with pelvic pain. Most have had previous surgery, pelvic inflammatory disease, or endometriosis.

Imaging shows a single cyst, at times containing septations, or a multilocular structure. Computed tomography and US identify multiseptated, thinor thick-walled cysts. In some women a spider-in-a-web appearance is found, with the spider representing an entrapped ovary (42). In general, in premenopausal women with prior pelvic surgery, an US finding of an ovary inside a complex cyst is typical for a peritoneal inclusion cyst. At times the appearance mimics a hydrosalpinx or an ovarian malignant cystic neoplasm if a separate ovary cannot be identified. A paraovarian cyst is also in the differential diagnosis, although in this entity a distinct and separate ovary is identified.

Endovaginal Doppler US reveals low resistive flow within the septations.

Some of these cysts are adequately treated by simple drainage, while others recur. Recurrent cysts can be treated by transvaginal US-guided drainage and ethanol instillation into the cavity;

any subsequent recurrence is also similarly treated.

Presacral Cysts

A list of presacral cystic tumors is rather extensive (Table 14.2). Discussed here are developmental cysts, which include epidermoid cysts, dermoid cysts, enteric cysts, and neurenteric cysts. A rare mesenteric cyst is presacral in location.

As the name implies, these cysts are located anterior to the sacrum and posterior to the rectum, are lined by epithelium, and are believed to originate from residual embryonic tissue. Some manifest in children; in adults these cysts are more common in women. The rare Currarino syndrome consists of an anorectal malformation, a sacral bone defect, and a presacral tumor such as a teratoma, meningocele, or a developmental cyst.

Epidermoid cysts are lined with squamous epithelium and contain a clear fluid. Dermoid cysts are also lined with squamous epithelium but contain additional dermoid components— hair follicles, teeth structures, and so on. They also contain lipid material. Rarely, multiple dermoid cysts develop. The enteric origin tailgut cysts are lined by a variety of epithelium, at times containing columnar, squamous, and transitional epithelium. To be considered a rectal duplication cyst it should be part of the

Table 14.2. Cystic presacral tumors

Development cysts

Epidermoid cysts

Dermoid cysts

Neurenteric cysts

Cystic hamartomas

Rectal duplications

Neoplasms

Sacral origin neoplasms

Chordoma

Teratoma

Cystic meningocele

Necrotic presacral neoplasms

Necrotic rectal neoplasms

Other

Cystic lymphangioma

Abscess

Hemangioma

Figure 14.12. Remnants of a presacral cyst (arrow) after partial resection.The rectum is displaced to the left. (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

rectal wall, contain rectal mucosa, and be surrounded by smooth muscle. The contained fluid has a mucous consistency.

Many presacral cysts are discovered incidentally. Some are detected on rectal examination as a posterior rectal tumor or a chronic fistula posterior to the anus. If sufficiently large, bowel obstructive symptoms or dysuria develop.

Imaging reveals a thin-walled, unilocular cyst, thus distinguishing these cysts from necrotic neoplasms and abscesses. A barium enema shows a soft tissue tumor posterior to the rectum; occasionally contrast outlines a fistulous tract. Calcifications are rare. A typical MR appearance is that of a hypointense lesion on T1and hyperintense on T2-weighted images. Hyperintense regions on T1-weighted images should suggest fat in a dermoid cyst; occasionally old blood or mucus will have a similar appearance.

Complications include infection, fistula, and malignant degeneration (Fig. 14.12). Especially if infected, a cyst will appear thick-walled. Associated soft tissue nodules or thick septa, especially if irregular in appearance, are seen with a malignancy.

Hemangioma

Hemangiomas represent a developmental anomaly rather than a neoplasm and consist of endothelial-lined blood-filled channels. Most common are cavernous hemangiomas, then

capillary ones, and least common and most difficult to diagnose and resect are extraperitoneal venous hemangiomas, which when large are called monstrous hemangiomas. Histology of the latter reveals blood vessel walls. A transition from hemangioma to angiomyoma exists with these tumors. At times their large size and slow blood flow preclude even an angiographic diagnosis (43).

In distinction to the liver, most mesenteric hemangiomas occur in the pediatric population; only an occasional one is detected in an adult.

Lymphangioma

Cystic lymphangiomas are either congenital or acquired, with many found in children. Often located at the root of the mesentery, these chylecontaining fluid collections represent lymphatic structures analogous to hemangiomas. They are lined by normal endothelial cells. Lymphoid tissue is often found at the periphery. An occasional one is extraperitoneal in location. They range from unilocular to multilocular, and from single to diffuse lymphangiomatosis.

Some lymphangiomas are detected incidentally. An occasional one results in intractable chyluria, bowel obstruction or an acute abdomen. It is mostly the mesenteric lymphangiomas which result in an acute abdomen due to bowel obstruction and related complications. Some lymphangiomas are associated with recurrent gastric or small bowel volvulus. An occasional one enlarges rapidly secondary to intracystic hemorrhage. Some of these lymphangioma are associated with recurrent gastric or small bowel volvulus.

Some lymphangiomas can be suspected with imaging, with the diagnosis confirmed by aspiration cytology. Especially in children, conventional radiography detects a large, noncalcified tumor. Ultrasonography reveals a cystic tumor containing thin septations. Some cystic lymphangiomas in adults contain calcifications. The solid tumor portion and capsule enhance postcontrast. The cyst content is either of fluid attenuation or higher, presumably because of bleeding or infection. A lipid-containing lymphangioma suggests a lipomatous tumor.

Ultrasonography shows cystic lymphangiomas as cystic tumors containing septations

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

and occasionally nodules. These lesions are predominantly anechoic. Any hyperechoic debris is presumably secondary to either hemorrhage or infection. Cyst septations are better identified with US than with CT. Magnetic resonance findings vary depending on the amount of protein and blood present. The multilocular appearance and enhancing stroma and capsule as shown by CT should suggest the diagnosis.

An occasional multicystic mesothelioma or cystadenocarcinoma has a similar imaging appearance, and tissue and fluid are necessary to confirm the diagnosis. Aspiration cytology reveals macrophages and abundant lymphocytes.

Percutaneous lymphangioma sclerotherapy with an alcoholic solution results in inconsistent results.

Pseudomyxoma Peritonei

Clinical

Pseudomyxoma peritonei is a poorly understood condition and its definition is controversial. It is characterized by mucinous ascites and peritoneal tumor implants. Rarely, it is associated with a paraneoplastic syndrome. Some authors use the term pseudomyxoma peritonei only for a malignancy, while others also include benign tumors. The most common primary site is the ovary; less common is an appendiceal origin, and perhaps least common is an urachal origin. A rare intraductal papillary mucinous neoplasm of the pancreas is associated with pseudomyxoma peritonei (44). At times synchronous mucinous tumors are detected both in the appendix and ovary, or colon and ovary, together with pseudomyxoma peritonei; the former tumors probably are not independent, but originate one from the other and rupture of one of these tumors probably results in pseudomyxoma peritonei, produced by welldifferentiated columnar epithelium.

Only rarely does pseudomyxoma involve extraperitoneal tissues, appropriately called pseudomyxoma retroperitonei. Most of these are focal rather than diffuse. Thus an occasional mucinous tumor in a retrocecal appendix bursts and results in a retroperitoneal cystic tumor, detectable by imaging.

Some authors subdivide this condition into two distinct categories (45): a disseminated

peritoneal adenomucinosis and a peritoneal mucinous carcinomatosis. The former is composed of extracellular mucin and a mucinous epithelium with little cytologic atypia or mitotic activity, at times with an associated appendiceal mucinous adenoma. The latter is composed of a more abundant mucinous epithelium having features of a carcinoma, at times with an associated mucinous adenocarcinoma. In some patients this condition is intermediate between the adenomucinosis and carcinomatosis varieties. The primary reason in subdividing this entity is a difference in prognosis; the 5-year survival rates for patients with adenomucinosis are 84%, with the intermediate form 38%, and 7% for patients with the carcinomatosis variety (45).

Only rarely can the entire tumor be resected, with debulking being the most common operation performed. Because of extensive peritoneal involvement, these patients tend to develop recurrent bowel obstructions. The results of chemotherapy vary.

Imaging

With the peritoneal cavity filled with sebaceous material, abdominal radiography reveals a wide fat stripe between the parietal peritoneum and lateral wall of the ascending colon, mimicking increased intraperitoneal fat. Occasionally curvilinear calcifications develop here. Right lower quadrant calcifications can also be seen in myxoglobulosis.

Computed tomography reveals pseudomyxoma peritonei as numerous, septated, heterogeneous intraabdominal cysts, with cyst content having a density higher than water. The liver and spleen outlines are often scalloped due to adjacent implants. Ultrasonography shows that the viscous fluid is not mobile, with numerous echoes corresponding to the gelatinous masses. Occasionally the viscous fluid and cyst walls have a highly hyperechoic appearance. Ultrasonography is useful in identifying less viscid material for biopsy.

Pseudomyxoma nodules are hypointense on T1-weighted MR images (Fig. 14.13). Any adenocarcinoma component enhances after iv gadolinium.

At times differentiation of pseudomyxoma peritonei from ascites can be difficult. With ascites, small bowel loops tend to float on top;

with pseudomyxoma peritonei bowel loops are more centrally displaced and compressed.

Solid Tumors

Discussed here are intraperitoneal and primary extraperitoneal tumors originating outside discrete organs. They are not common. A majority are malignant. Clinical presentation with these tumors varies depending on the structure affected. An example of an unusual presentation is a primitive extraperitoneal tumor manifesting as a varicocele (46).

the other hand, no significant differences were evident between the two imaging modalities.

In general, CT criteria do not distinguish between benign and malignant tumors, although a malignancy is suggested if the patient is symptomatic and imaging detects a large tumor with irregular margins. Calcifications suggest a benign tumor, but exceptions do occur. Invasion of surrounding structures is an obvious feature of malignancy tumors. Contrast enhancement of a solid component is common with a malignancy but is also seen with benign tumors.

Imaging

Intraand extraperitoneal tumors show considerable variability in their imaging findings, and the individual tumor types are thus discussed separately. Nevertheless, some general conclusions are apparent. For solid tumor detection MRI appears superior to CT. In patients with known or suspected extrahepatic malignancies, abdominal contrast-enhanced CT detected 70% of proven tumor sites compared with 91% for MRI (47); MRI detected significantly more tumors involving peritoneum, bowel, vascular structures, and bones. For benign disease, on

Adenopathy

Benign lymph node hyperplasia occurs in a number of chronic disorders, with Laënnec’s cirrhosis and Crohn’s disease being common examples. The prevalence of node hyperplasia increases with the severity of disease, being especially common in end-stage cirrhosis.

Tumor-Associated Adenopathy

Heavily calcified lymph nodes generally imply prior infection, but calcifications, often visible by CT, do occur with malignant infiltration.

PERITONEUM, MESENTERY, AND EXTRAPERITONEAL SOFT TISSUES

Para-aortic adenopathy is more common with non-Hodgkin’s lymphoma than with Hodgkin’s lymphoma. Imaging shows either discretely enlarged nodes or confluent tumors. These nodes are hypoechoic or even anechoic with US. Nonlymphomatous tumor involvement generally results in a more hyperechoic and heterogeneous lymph node appearance.

With most tumors, including lymphoma, CT and MR simply detect whether lymph nodes are enlarged or not. Most authors simply assume that nodes above a certain size are malignant, with 10mm often being a cutoff size used; this limit achieves a low sensitivity but specificity is >90% in detecting lymph node metastasis. CT and MRI appear comparable in detecting most lymph node enlargement. Also, one should keep in mind that asymmetry of small pelvic nodes is not uncommon, even with normal nodes. Magnetic resonance readily differentiates lymph nodes from vascular structures, a task occasionally difficult with CT. At times even contrast-enhanced CT does not differentiate between some pelvic lymph nodes and venous structures because some nodes also enhance.

Based on tissue characterization, neither CT nor unenhanced MR differentiates between neoplastic nodal infiltration and inflammatory conditions. T1 and T2 relaxation times of normal, inflammatory, and neoplastic nodes overlap and are not reliable in differentiating these conditions. Even gadolinium-enhanced MRI does not differentiate between normal nodes and metastasis-containing nodes. Exceptions to the above are nodes with central necro- sis—regardless of size, these generally represent metastases, although exceptions also occur here because similar changes are found with neurofibromatosis, abdominal tuberculosis, and Whipple’s disease.

Potentially, color Doppler US can differentiate between reactive and malignant adenopathy by detecting intranodal focal perfusion defects, aberrant central vessels, displacement of intranodal vessels, and the presence of subcapsular vessels, findings pointing toward a malignancy. One color Doppler US study of superficial lymph nodes achieved a 96% sensitivity and 77% specificity in differentiating reactive and malignant nodes (48).

Laparoscopic US shows most benign lymph nodes to have a hyperechoic center and an oval

shape, while neoplastic nodes tend to be more round and inhomogeneous. There is, however, considerable overlap in the appearance of benign and malignant nodes. Also, fatty infiltration makes some nodes blend into the surrounding soft tissues.

Ultrasmall superparamagnetic iron oxide particles (ferumoxtran), however, are taken up by normal lymph tissue but not metastatic foci, and the use of this contrast agent thus selectively decreases T2-weighted signal intensity of even normal-sized nodes but not those containing metastases. Ferumoxtran-enhanced MRI is unique among present-day imaging modalities in being able to detect metastases even in normal-sized lymph nodes. Potentially, FDGPET has similar capabilities in visualizing lymph node metastases,although it accumulates in the bladder and obscures visualization of pelvic nodes.

Once enlarged lymph nodes are detected, fine-needle aspiration using appropriate image guidance is accurate in differentiating between neoplastic and inflammatory nodes. Some lymph nodes are accessible for endoscopic USguided aspiration.

Mesenteric Adenitis

Mesenteric adenitis is a description of pathologic findings rather than a specific disease. Clinically, it often mimics appendicitis. A not uncommon presentation is a patient with an acute onset of right lower quadrant pain, imaging reveals enlarged mesenteric lymph nodes, and a normal appendix is found at surgery. Associated ileal or ileocecal wall thickening is identified in some patients. Yersinia enterocolitica infection is a common cause, although in only about half of these patients is an eventual etiology identified.

Castleman’s Disease

Angiofollicular lymph node hyperplasia, or Castleman’s disease, is not common. Enlarged nodes range from unicentric (focal) to multicentric (generalized). An isolated extraperitoneal conglomerate tumor is one form of presentation. Less common is extranodal involvement, including intrahepatic disease. Two histologic types of this disease exist—a more