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

Lymphangioma

Lymphangiomas, or malformations of lymphatic tissue, are more common in the mesentery than in small bowel wall (they are discussed in more detail in Chapter 14).

A not uncommon presentation of diffuse lymphangiomatosis is intestinal bleeding.

Occasionally small bowel lymphangiomatosis appears as multiple polyps, and at times both large and small ones are present. If large enough, a small bowel or adjacent mesenteric lymphangioma obstructs the bowel lumen.

Endometriosis

Colonic endometriosis is more common than small bowel involvement. Distal ileal involvement predominates in small bowel endometriosis and results in fibrosis and strictures mimicking ileal Crohn’s disease (Fig. 4.15). Complicating the issue is the occasional woman with antecedent Crohn’s colitis who develops ileal endometriosis.

Clinical presentation of small bowel endometriosis varies. Symptoms do not necessarily occur cyclically, and these women develop pain, hematochezia, or small bowel obstruction.

Figure 4.15. Terminal ileal endometriosis (arrows). Superficially it mimics Crohn’s disease.

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Barium studies in five women revealed endometrial implants within 10cm of the ileocecal valve in four and more proximal in one (78); findings ranged from an annular infiltration, extrinsic tumor, and spiculations, to a plaque-like tumor. Of interest is that rectosigmoid endometriosis was also found in four who underwent a barium enema.

Benign Neoplasms

Adenoma

Adenomas are most common in the duodenum. Similar to other intraluminal polyps, an adenoma can bleed, intussuscept, or obstruct. When resected, some adenomas already contain a focus of adenocarcinoma.

Adenomas appear as intraluminal soft tissue tumors. Differentiation from carcinoma is based mostly on size and whether bowel wall invasion is present, keeping in mind that for most smaller ones imaging differentiation is not possible.

Leiomyoma

The most common small bowel neoplasm is a leiomyoma. It is rare in neonates but large, congenital ones are in the differential diagnosis of a palpable abdominal mass. An increased leiomyoma prevalence is reported in patients with neurofibromatosis.

Most are small and intramural and are discovered either at surgery or autopsy. Leiomyomas tend to be hypervascular to the point that some mimic an arteriovenous malformation. A mostly intraluminal leiomyoma suggests a muscularis mucosa origin. Yet even one originating from muscularis propria can ulcerate and bleed, at times massively, necessitating arterial embolization even to the point of inducing ischemia in order to stop bleeding.

Occasionally a leiomyoma presents as a giant abdominal or pelvic tumor, often partly necrotic.

Lipomatous Tumors

Small bowel lipomas are common, at times being multiple. In spite of their intramural origin they have a tendency to become pedunculated and then are indistinguishable from

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other pedunculated polyps. In fact, a lipoma should be high in the differential diagnosis for a small, smooth pedunculated small bowel polyp. In adults, lipomas are a relatively common lead point for an intussusception. Occasionally multiple lipomas result in multiple intussusceptions and small bowel obstruction.

Computed tomography shows a lipoma as a well-marginated, homogeneous tumor having fat attenuation.

Polyposis Syndromes

The polyposis syndromes are discussed in more detail in Chapter 5. Covered here are only those aspects pertinent to the small bowel.

Although not common, small bowel adenocarcinomas do develop in familial polyposis, even after a colectomy. Whether these carcinomas are more common after a colectomy due to ileal mucosa becoming dysplastic, as some investigators have suggested, is unknown.

Enteroenteric intussusceptions are common in Peutz-Jeghers syndrome (Fig. 4.16). Most are transient, but an occasional one evolves into an obstruction or ischemia. Magnetic resonance in

apatient with Peutz-Jeghers syndrome detected

asolitary 1.5-cm polyp associated with an enteroenteric intussusception (79); the polyp was isointense to bowel on precontrast images and enhanced similar to bowel both on early and late postcontrast images. A rare finding in

Figure 4.16. Small bowel intussusception in an 8–year-old boy with Peutz-Jeghers syndrome. The greatly dilated small bowel loop contains several nondilated loops in this CT image, reflecting the chronicity of this condition. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

Peutz-Jeghers syndrome is small bowel cystica profunda.

An occasional polyp in a patient with Peutz-Jeghers syndrome undergoes osseous metaplasia.

Malignant Neoplasms

Primary small bowel cancers are not common. The Utah Cancer Registry from 1966 through 1990 found the most common cancer to be a carcinoid (41%), followed by adenocarcinoma (24%), lymphoma (22%), and sarcoma (11%), with a small percentage unclassified (80); an age-adjusted incidence of small bowel cancers was 1.4 per 100,000; for comparison the colon cancer rate was 36 per 100,000 and breast cancer 93 per 100,000.

Carcinoids, lymphomas, and sarcomas decrease in frequency from the ileum to the duodenum (80); the reverse is true for adenocarcinomas. Of interest is that in industrialized countries adenocarcinomas tend to predominate but lymphomas are more common in developing countries (81).

Adenocarcinoma

Patients who have one small bowel adenocarcinoma are at increased risk of developing both other duodenal and jejunal tumors and other carcinomas. The risk of small bowel adenocarcinomas increases with age and is higher among men. Risk factors appear similar to those seen for colon cancer. An increased prevalence is found in patients with Crohn’s disease, celiac disease, cystic fibrosis, familial adenomatous polyposis, and Peutz-Jeghers syndrome.

The two current imaging modalities used for diagnosing small bowel cancers are barium studies and CT. The role of CT enteroclysis is promising but not yet established. The diagnosis is not always straightforward. An occasional small bowel adenocarcinoma metastatic to the ovary mimics a primary ovarian carcinoma.

A typical small bowel adenocarcinoma appears as a poorly marginated soft tissue tumor or localized infiltration often obstructing the bowel lumen (Fig. 4.17). A minority necrose extensively, do not obstruct, and mimic a mes-

enchymal tumor. A multicentric adenocarcinoma is rare.

The role of MRI is not established. Both precontrast T1-weighted SGE images and postgadolinium fat-suppressed images appear useful in visualizing these tumors. Malignant tumors tend toward heterogeneous enhancement on postgadolinium images.

Few studies have evaluated preoperative staging of small bowel adenocarcinomas (Table 4.6). A retrospective preoperative nonhelical CT study achieved an overall CT staging accuracy of only 47% (82); sensitivity and specificity for detecting adenopathy were 75% and 20% and for detecting distant metastases 58% and 63%, respectively. Staging errors were compounded by the presence of Crohn’s disease, other polyps, and small bowel obstruction.

The 5-year survival rate for patients with small bowel adenocarcinoma is about 20%; less than half of patients undergo a “curative” resection.

Lymphoma

Clinical

Patients with Crohn’s disease or celiac disease, immunocompromised patients, and those with dermatitis herpetiformis are at increased risk

for intestinal lymphomas. Most primary intestinal lymphomas are non-Hodgkin’s lymphomas. Lymphomas develop throughout the small bowel, the exception being in the setting of celiac disease, when proximal lymphomas are more common.

Mediterranean lymphoma probably is a variant of primary lymphoma.Weight loss, diarrhea, and general debility predominate in these patients. Affected are mostly young adults. In distinction to the stomach, only a minority of small bowel lymphomas are of the MALT type. Immunoproliferative disease is a rare lymphoproliferative disorder involving the small intestine. It appears to be a manifestation of MALT lymphoma.

Clinical findings range from abdominal pain, malabsorption, and hypoalbuminemia, to perforation, the latter finding seen more often with primary intestinal T lymphomas rather than B type. These T lymphomas tend to be rather aggressive.

Imaging

Primary intestinal lymphoma develops mostly as either a polypoid or an exophytic ulcerated lesion, with only an occasional one infiltrating diffusely (Fig. 4.18). Focal circumferential involvement is common. In some patients the

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Table 4.6. Tumor, node, metastasis (TNM) staging of small intestinal adenocarcinoma

Source: From the AJCC Cancer Staging Manual, 6th edition (2002), published by Springer-Verlag, New York, NY, used with permission of the American Joint Committee on Cancer (AJCC), Chicago, IL.

bowel lumen is aneurysmally dilated, in others it is narrowed. Bowel obstruction is not a common feature even with extensive lymphomatous involvement and, if present, should suggest another diagnosis. At times ulcers are quite large and out of proportion to the tumor mass, an appearance mimicking a necrotic mesenchymal tumor (Fig. 4.19).

In the terminal ileum primary lymphomas tend to have a tumor-like appearance; more proximal ones more often are ulcerated. Extensive fistulas are rare with both primary and secondary small bowel lymphoma. Non-Hodgkin’s lymphomas also present as multiple polyps

scattered throughout the gastrointestinal tract. These polyps range from relatively flat ones, with an overall appearance similar to that of lymphonodular hyperplasia, to mostly intraluminal ones mimicking familial polyposis. A polypoid appearance is more common with Mediterranean lymphomas and in immunocompromised patients.

Therapy

Surgery is performed to obtain a biopsy, establish the extent of involvement, correct any obstruction or perforation, and, at times, debulk.

After cytoreductive chemotherapy some of these patients develop intramural fat deposition, detected by serial CT as fat-attenuating bowel wall thickening. It should be kept in mind that submucosal fat deposition also occurs in inflammatory bowel disease.

Small bowel fibrosis and resultant obstruction are a complication of chemotherapy for intestinal lymphoma.

Sarcoma

Leiomyosarcoma

The pathologic borderland between leiomyoma and leiomyosarcoma is rather murky, and the same applies to a larger extent in radiology. Presence of metastases obviously signifies malignancy. A small bowel leiomyosarcoma often presents as a large tumor with a prominent, irregular central cavity that fills with barium. The cavity tends to overwhelm any surrounding soft tissue component. In spite of the large size, lumen obstruction is not common.

Computed tomography reveals a small bowel mesenchymal neoplasm, regardless of whether it is benign or malignant, as a large, bulky, eccentric mass, often with a low attenuation center or cavity (Fig. 4.20). Some investigators have used specific CT criteria to suggest malignancy but with limited success. The CT findings of 21 leiomyomas and 24 leiomyosarcomas were subdivided into three groups—I, probably benign; II, probably malignant; and III, malignant—and the results compared with histopathologic diagnosis (83). In addition to direct invasion and metastasis, CT findings favoring malignancy included tumor size >5cm,

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Figure 4.20. Small bowel leiomyosarcoma, probably a gastrointestinal stromal tumor (GIST), in a 47–year-old woman with fever, pain, and a palpable right lower quadrant tumor. Oral and IV contrast-enhanced CT reveals a complex, heterogeneously enhancing tumor (arrows). Other images identified surrounding mesenteric infiltration. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

Other Primary Malignancies

A primary jejunal choriocarcinoma in a man led to melena and a markedly elevated serum b- human chorionic gonadotrophin level (84); the authors suggest metaplasia to primitive trophoblasts as an etiology for this tumor.

Small bowel sarcomatoid carcinomas are rare. They tend to be necrotic, ulcerated, and hemorrhagic. The prognosis was poor in the few reported patients.

Metastasis or Direct Invasion

Tissue from intestinal metastases can be successfully obtained using US-guided 22-gauge fine-needle aspiration or 18-gauge core biopsy (85).

Melanoma

A small bowel melanoma, even if a primary is not known, is most likely metastatic. The time interval between initial skin lesion and detection of intestinal metastases is typically several years. Bleeding and obstruction are common

presentations. An Armed Forces Institute of Pathology review of 103 patients with small bowel malignant melanoma suggests two subsets: one occurs in younger patients and is more aggressive with rapid metastasis and early death, and the other is a more indolent form occurring in older patients that tends to metastasize less often (86). A rare melanoma intussuscepts (87).

Most metastatic melanomas to the small bowel present as multiple small, solid nodules scattered haphazardly. An occasional metastasis is solitary, at times quite large, and even cystic in appearance.

Other

Breast, ovarian, lung, and other similar metastases to bowel tend to obstruct, often at multiple sites, or ulcerate and bleed. Their imaging appearance ranges from minor infiltration to large, necrotic tumors (Fig. 4.21). Enteroclysis is necessary to detect subtle ones, keeping in mind that most metastases are closer to the serosa than mucosa (mucosal metastases are rare). The larger ulcerated tumors are detected by most imaging modalities.

Neuroendocrine Tumors

A number of neuroendocrine tumors are of mesenteric origin and rightfully should be discussed in Chapter 14. In clinical and radiologic practice, however, these tumors often are in the differential diagnosis of small bowel and colonic tumors.

Schwannoma/Neurilemmoma

Small bowel schwannomas are uncommon. An occasional one bleeds. These are mostly solid soft tissue tumors.

Neurofibroma

Neurofibromatosis is discussed in more detail in Chapter 14.

Many patients are asymptomatic but an occasional tumor erodes through the mucosa and bleeds or acts as a lead point for an intussusception. When large, tumor bulk may obstruct either the small bowel or adjacent urinary tract.

Magnetic resonance imaging in patients with carcinoids identified a primary tumor in 67% (90); these tumors were best seen on postcontrast T1-weighted fat-suppressed images, and their appearance ranged from a nodular tumor originating from bowel wall to focal bowel wall thickening having homogeneously moderate contrast enhancement. Mesenteric nodular metastases were associated with mesenteric stranding in most. Most liver metastases were hypervascular with moderate contrast enhancement, although 6% were hypovascular; of note is that some metastases were visible only on immediate postcontrast images. Carcinoids are hypointense both on T1and T2-weighted MR images. Infiltration from a central tumor into surrounding hyperintense mesenteric fat results in a spoke-wheel appearance.

Carcinoids contain increased somatostatin receptors, and thus scintigraphy with radiolabeled somatostatin analogues is potentially useful. Initial studies with octreotide labeled with radioactive iodine were discouraging but In-111–pentetreotide appears promising; SPECT is more sensitive than planar scintigraphy in detecting both primary carcinoids and metastases. Indium 111–pentetreotide is more sensitive than CT.

Whole-body 18F-dihydroxyphenylalanine (DOPA) positron emission tomography (PET) of a mix of primary tumors, lymph node

18-fluorodeoxyglucose (FDG)-PET was 29%, somatostatin receptor scintigraphy 57%, and morphologic procedures 73% (91). The FDGPET imaging was falsely negative in a patient with a known metastatic carcinoid and a positive octreotide scan (92).

Therapy

Therapy for carcinoids varies depending on tumor size and presence of metastases. Curative radical resection should be considered for small intestine carcinoids. With some metastatic carcinoids a palliative resection decreases symptoms. Medical therapy with somatostatin analogues and a-interferon can prolong survival. The 5-year survival rate for patients with small bowel carcinoids is <50%.

In appropriate patients metaiodobenzylguanidine (MIBG) scintigraphy appears to

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have a role in planning iodine 131 MIBG therapy.

Dilation

In ancient Greece the term ileus described intestinal obstruction. The Romans narrowed this definition to include primarily intestinal volvulus and intussusception, and in this sense ileus was used up to the 18th century. The Oxford English Dictionary defines ileus as a painful affection due to intestinal obstruction. Ileus as a synonym for intestinal obstruction, regardless of cause (mechanical ileus, adynamic or paralytic ileus, ischemic ileus, and so on), is used in Europe and Japan, but in the United States ileus has been subverted to be synonymous with adynamic ileus only, leading to confusing terminology. Even some of the more respected American medical and surgical textbooks have adopted this narrower definition of ileus.

Mechanical Obstruction

Table 4.7 lists some of the conditions associated with mechanical small bowel obstruction. Both internal and external hernias are common causes of small bowel obstruction; these are discussed in Chapter 14. Ileocolic intussusception is covered in Chapter 5.

Radiologists typically subdivide small bowel obstruction into complete and incomplete. Incomplete obstruction is then subdivided further into low grade, high grade, and similar variants. Restraint should be used in drawing clinical prognostic conclusions from such radiographic classification. Some have equated incomplete obstruction to mean insignificant obstruction, although these two terms have different meanings. Whether to suggest surgical correction of an incomplete obstruction should be based more on clinical findings (presence of nausea, vomiting, pain, inability to maintain weight, etc.) rather than on a specific radiographic appearance, with some exceptions; thus the presence of bowel wall edema or pneumatosis suggest underlying ischemia.

Detection

Many patients with suspected small bowel obstruction undergo passage of a decompres-

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Table 4.7. Conditions associated with mechanical small bowel obstruction

Intraluminal

Bezoar (foreign body)

Gallstone

Intussusception

Parasites

Polyp (neoplastic or nonneoplastic)

Meconium

Intramural

Infiltrating neoplasm

Hematoma and hemorrhage

Inflammation

Crohn’s disease

Infection

Ischemia

Radiation enteritis

Congenital stricture

Extrinsic Adhesions

Adjacent inflammation/abscess involving bowel Hernia (internal or external)

Volvulus

Extrinsic neoplasm, cyst, etc. compressing bowel Congenital bands

Endometriosis

sion tube early in their course, even before a definitive diagnosis is established, followed by watchful observation, a poorly defined term having a different meanings for different physicians. Gastrointestinal radiologists prefer that these patients undergo imaging studies without undue delay. What should be the sequence of imaging studies?

Because of ready availability, conventional radiographs should be obtained before bowel decompression. Likewise, CT is best performed before bowel decompression. If conventional radiographs do not reveal small bowel dilation, the decompression catheter, preferably a sumptype catheter rather than a simple nasogastric tube, is advanced distal to the pylorus. Barium instilled through the tube provides a fluoroscopic small bowel study, and is used for enteroclysis or as an aid to identify the small bowel during a subsequent CT study.

Occasional but recurrent publications advocate the use of a water-soluble contrast agent to

study small bowel obstruction. Although in infants use of a nonionic agent is advantageous, in the study of adult small bowel obstruction the water-soluble contrast agents have little to recommend. Erratic amounts of additional fluid are drawn intraluminally, the already distended bowel becomes even more distended,the patient becomes more uncomfortable, and often the only conclusion possible from such a study is identifying whether the bowel is grossly dilated or not. In fact, some of the studies claiming a superiority for CT over a small bowel contrast study used water-soluble contrast agents rather than barium for the comparison study, leading to preordained results. In most patients, if bowel wall integrity is not compromised, a small bowel contrast study should be performed with barium.

Some investigators use solid, 4-mm radiopaque markers to identify a partial small bowel obstruction (93); the markers coalesced close to the obstruction. This technique is currently little used and appears inferior to more conventional imaging tests.

Small bowel dilation per se, without colonic dilation, is not pathognomonic of obstruction; for example, patients with celiac sprue tend to have dilated, fluid-filled loops of small bowel, with an overall appearance similar to distal small bowel obstruction. On the other hand, regardless of the imaging study performed, small bowel obstruction is implied if proximal small bowel is dilated while more distal loops are collapsed. These dilated proximal loops tend to be filled mostly with liquid intestinal content. Numerous bowel caliber dimensions have been published in an attempt to differentiate between normal and dilated caliber bowel, but these measurements have little use in clinical practice; the borderland between normal caliber and dilated bowel is sufficiently vague and varies considerably depending on site, degree, and age of an obstruction, resultant peristaltic activity, and presence of underlying disease.

The presence of massively dilated small bowel loops suggests a chronic, long-standing obstruction.

Conventional Radiography

A number of investigators advocate CT rather than conventional radiographs when bowel obstruction is suspected clinically. Generally