Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas
.pdf154
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.
ADVANCED IMAGING OF THE ABDOMEN
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
155
JEJUNUM AND ILEUM
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-
156
ADVANCED IMAGING OF THE ABDOMEN
A B
Figure 4.17. Jejunal adenocarcinoma. A: A typical “napkin-ring” stricture is evident (arrow). B: CT in another patient reveals a heterogeneously enhancing tumor (cursors). Several adjacent jejunal loops are encased by this tumor. (Courtesy of Arunas Gasparaitis, M.D., University of Chicago.)
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
157
JEJUNUM AND ILEUM
Table 4.6. Tumor, node, metastasis (TNM) staging of small intestinal adenocarcinoma
Primary tumor: |
|
|
|
Tx |
Primary tumor cannot be assessed |
|
|
T0 |
No evidence of primary tumor |
|
|
Tis |
Carcinoma-in-situ |
|
|
Tl |
Tumor invades lamina propria or submucosa |
||
T2 |
Tumor invades muscularis propria |
|
|
T3 |
Tumor invades into subserosa; tumor invades |
||
|
through muscularis propria into |
|
|
|
surrounding tissue for less than 2 cm |
|
|
|
(between leaves of mesentery or into |
|
|
|
retroperitoneum if duodenal) |
|
|
T4 |
Tumor invades adjacent organs; tumor |
|
|
|
perforates serosa; tumor extends more |
|
|
|
than 2 cm into retroperitoneum or |
|
|
|
between leaves of mesentery |
|
|
Lymph nodes: |
|
|
|
Nx |
Regional lymph nodes cannot be assessed |
||
N0 |
No regional lymph node metastases |
|
|
Nl |
Regional lymph node metastasis |
|
|
Distant metastases: |
|
|
|
Mx |
Distant metastases cannot be assessed |
|
|
M0 |
No distant metastases |
|
|
Ml |
Distant metastasis |
|
|
Tumor stages: |
|
|
|
0 |
Tis |
N0 |
M0 |
I |
T1 |
N0 |
M0 |
|
T2 |
N0 |
M0 |
II |
T3 |
N0 |
M0 |
|
T4 |
N0 |
M0 |
III |
any T |
N1 |
M0 |
IV |
any T |
any N |
Ml |
|
|
|
|
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,
158
ADVANCED IMAGING OF THE ABDOMEN
A |
|
Figure 4.18. A: Ileal lymphoma (mucosa-associated lymphoid |
|
tissue, MALT) presenting as a dilated loop containing thickened |
|
valvulae conniventes (arrow). (Courtesy of Arunas Gasparaitis, M.D., |
|
University of Chicago.) B: Small bowel lymphoma diffusely thicken- |
|
ing bowel wall and obliterating valvulae conniventes. |
B |
a lobulated contour, heterogeneous contrast |
The 5-year survival rate for patients with a |
|
enhancement, mesenteric fat infiltration, ulcer- |
small bowel sarcoma is about 50%. |
|
ation, adenopathy, and an exophytic growth. |
|
|
The presence of calcifications does not aid |
Liposarcoma |
|
in differentiating benign from malignant. |
||
|
||
Similarly to adenocarcinomas, small bowel |
Many liposarcomas contain little or no fat. Thus |
|
leiomyosarcomas have been misdiagnosed as |
a CT finding of attenuation greater than fat in a |
|
ovarian tumors. |
tumor does not exclude a liposarcoma. |
A B
Figure 4.19. Non-Hodgkin’s lymphoma. A,B: Two CT images reveal a tumor (arrows) with a necrotic center (arrowheads). No bowel obstruction was present.
159
JEJUNUM AND ILEUM
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.
160
ADVANCED IMAGING OF THE ABDOMEN
B
A
|
Figure 4.21. A: Serosal metastasis resulting in an eccentric stric- |
|
ture. B: Ovarian carcinoma has led to a small bowel “napkin-ring” |
|
stricture (arrow). C: Metastatic breast carcinoma resulting in |
C |
extensive bowel wall and mesenteric invasion (arrows). |
Most nonplexiform neurofibromas occur on |
Magnetic resonance in a patient with neuro- |
|
the antimesenteric border. Imaging of the few |
fibromatosis revealed duodenal and jejunal |
|
published plexiform neurofibromas shows a |
polyps 1 to 2cm in diameter (79); the polyps |
|
mesenteric polypoid or bead-like mass infi- |
were isointense to bowel wall and enhanced |
|
ltrating a loop of bowel and its associated |
similarly. |
|
mesentery (88). The degree of small bowel and |
Carcinoid |
|
mesenteric involvement varies and is reflected |
||
on relative barium studies and CT of the |
Clinical |
|
involved bowel. Mesenteric angiography and CT |
||
|
||
angiography are helpful in localizing a bleeding |
Jejunoileal and colonic carcinoids tend to be |
|
tumor. |
rather aggressive tumors, with even small ones |
161
JEJUNUM AND ILEUM
showing local invasion and metastasis. Multiple |
Computed tomography was normal in 21% of |
|
carcinoids are not uncommon, and a case can |
patients with a known midgut carcinoid in spite |
|
be made for searching for additional tumors. |
of biochemical evidence for tumor (89); liver |
|
Abdominal carcinoids are not limited to bowel |
metastases were found in 68%, 20% had mesen- |
|
wall, and a mesenteric location is often favored. |
teric root involvement, and 24% had extraperi- |
|
Elevated serum serotonin and urine 5- |
toneal adenopathy, and the authors conclude |
|
hydroxyindoleacetic acid levels suggest liver |
that CT can miss a primary carcinoid but that it |
|
metastases, although high serotonin activity of |
is useful in gauging the extent of tumor spread |
|
some primary carcinoids results in a carcinoid |
and as an aid during follow-up. A common CT |
|
syndrome even without metastasis. |
appearance is a distinct mesenteric tumor, often |
|
|
containing characteristic linear radiating soft |
|
|
tissue spokes, which should be distinguished |
|
Imaging |
from mesenteric soft tissue stranding, which is |
|
found in a number of infiltrative conditions and |
||
|
||
Small bowel carcinoids range from intramural |
is a nonspecific finding. A minority of carci- |
|
polyps to an exuberant desmoplastic reaction to |
noids contain calcifications. |
|
the point of obstruction (Fig. 4.22). Intestinal |
Ultrasonography findings tend to be |
|
necrosis is a rare complication. |
nonspecific. |
A B
Figure 4.22. Spectrum of small bowel carcinoids. A: A tumor |
|
(arrow) is surrounded by fixed bowel loops. (Courtesy of Arunas |
|
Gasparaitis, M.D., University of Chicago.) B: Another carcinoid |
|
presents as a smooth, intramural tumor (arrows). C: Contrast- |
|
enhanced CT shows a large, necrotic, poorly enhancing tumor |
|
involving jejunal loops, an atypical appearance for a carcinoid.This |
|
complex tumor also contained nonneuroendocrine and signet |
|
ring cells. (Courtesy of Patrick Fultz, M.D., University of Rochester.) |
C |
162
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
metastases, |
and organ |
metastases achieved |
an overall |
sensitivity of |
65%; sensitivity of |
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
ADVANCED IMAGING OF THE ABDOMEN
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-
163
JEJUNUM AND ILEUM
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