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

[Japanese] Nippon Shokakibyo Gakkai Zasshi 1998;95: 895–899.

30.Novacek G,Walgram M, Bauer P, Schofl R, Gangl A, Potzi R. The relationship between juxtapapillary duodenal diverticula and biliary stone disease. Eur J Gastroenterol Hepatol 1997;9:375–379.

31.L’Helgouarc’h JL, Peschaud F, Benoit L, Goudet P, Cougard P. [Treatment of perforated duodenal ulcer by

laparoscopy. 35 cases.] [French] Presse Med 2000;29: 1504–1506.

32.Dhadphale S, Sawant P, Rathi P, et al. Bleeding duodenal varix in splenic vein thrombosis and chronic pancreatitis. Indian J Gastroenterol 1998;17:29–30.

Technique

Contrast Studies

Barium Sulfate

Accepted clinical indications for a barium small bowel study include (1) unexplained gastrointestinal bleeding, (2) suspected small bowel tumor, (3) suspected small bowel obstruction,

(4) Crohn’s disease, and (5) malabsorption. Computed tomography (CT) has made inroads in some of these indications, especially with suspected obstruction, and whether a barium study or CT is performed varies considerably between institutions. The role of a swallowed capsule is still being established. Nevertheless, a majority of American radiologists both in academia and private practice continue to perform small bowel follow through examinations (1).

This is not the place to discuss relative merits of a conventional small bowel study versus enteroclysis and other more specialized examinations.

The limitations of a conventional small bowel study are well known. Enteroclysis has achieved a sensitivity and specificity of over 90% in detecting abnormalities responsible for a patient’s symptoms, and some experts are adamant that the conventional small bowel follow-through examination should be abandoned; still, this examination continues to be performed throughout most of the world.

The small bowel is studied using a barium sulfate suspension in water (including various

additives). One novel approach is to substitute a methylcellulose solution instead of water as the suspending agent. Use of a 40% barium suspension in methylcellulose improved small bowel image quality (compared to a water suspension)

(2). Whether such a modification is overall advantageous remains to be established.

The peroral pneumocolon and gas-enhanced double-contrast study have carved a very limited niche in the study of suspected small bowel disease. Retrograde ileography using an endoscopically introduced occluding balloon is an option (3); its complexity argues against wide use.

Enteroscopy visualizes the proximal small bowel. Some authors believe that a combination of enteroscopy and enteroclysis via a catheter inserted on enteroscope withdrawal offers the advantages of both studies.

Water-Soluble Agents

Ionic water soluble contrast agents beloved by some surgeons have a very limited role in small bowel studies. They are hyperosmolar and draw fluid into the small bowel lumen, resulting in distention and dilution. Especially in neonates they tend to damage mucosa and induce hypovolemia. Considerably more harm is produced if these agents are aspirated into the lungs than with barium.

Most nonionic agents are still hyperosmolar, but are associated with fewer side effects and complications than ionic agents. They lead to

less dilution and are better visualized in the small bowel than are ionic agents. Nevertheless, for optimal visualization with conventional radiography barium sulfate is preferred in most patients.

Normally ionic water-soluble contrast agents are minimally absorbed after oral use, although idiosyncratic absorption exists. An occasional patient without underlying small bowel disease has sufficient absorption to opacify renal collecting systems, but, in general, such absorption should be viewed with suspicion. Common causes of visualizing the renal collecting systems after oral administration are bowel perforation and impaired bowel mucosal integrity.

Prevalence of contrast sensitivity secondary to oral contrast ingestion is much less than with intravenous (IV) injection, although contrast reactions occur even with dilute solutions, such as used to opacify the gut for CT studies.

ADVANCED IMAGING OF THE ABDOMEN

zation and gross artifacts. Commercial CT barium manufacturers overcome this settling tendency, in part, by using rather small barium sulfate crystal particles and various high viscosity antisettling additives.

A concentration of 1.5% to 2.0% small particle barium sulfate preparations is suitable for oral small bowel opacification during abdominal CT; as a further refinement, a 2.0% concentration provided better jejunal contrast,whereas a slightly lower concentration is better suitable for pelvic structures.

Other CT oral contrast agents are feasible and at times preferred. Among whole milk, 2% milk, water, barium suspension, and no oral contrast, whole milk was superior to the others (4).

The term double-contrast abdominal CT is used in the trauma literature to specify use of both intravenous and per oral contrast; this is a misuse of the traditional connotation of “double contrast” in radiology and is best avoided to prevent confusion.

JEJUNUM AND ILEUM

Magnetic Resonance

Technique

Currently magnetic resonance (MR) has a limited role in small bowel disease, although potential applications exist. After CT enteroclysis, it was only a question of time before MR enteroclysis was also performed. Single breath-hold magnetic resonance imaging (MRI) after enteroclysis with oral iron particles in Crohn’s patients detected most stenoses, fistulas, and marked bowel wall thickening with prominent contrast enhancement (5). Magnetic resonance enteroclysis using methylcellulose in patients with suspected inflammatory bowel disease or small bowel obstruction achieves similar results to those obtained with conventional enteroclysis or surgery (6). Another MR enteroclysis study using oral and IV gadolinium–diethylene triamine pentaacetic acid (DTPA) reached similar conclusions, noting that a prerequisite for an excellent study is good bowel distention and a homogeneous appearance (7). Although one study of CT enteroclysis immediately followed by MR found CT sensitivity higher than MR in detecting small bowel wall thickening, wall enhancement and detection of adenopathy (8), the relative merits of CT enteroclysis versus MR enteroclysis remain to be explored.

Although techniques vary, precontrast T1weighted spoiled gradient echo (SGE) images, T2-weighted images, and early and late postcontrast SGE images are commonly obtained. Single breath-hold MR sequences minimize peristaltic artifacts and aid in evaluating dilated bowel.

in detecting a sinus tract; on the other hand, they mask intraluminal contents and make bowel wall visualization difficult.

A distinction between positive and negative contrast agents is not absolute, and MR properties of some contrast agents change both with dilution and MR sequence used. For example, in vivo ferric ammonium citrate is hyperintense on both T1and T2-weighted turbo spin echo (TSE) and fat-suppression images 20 minutes after contrast administration at concentrations <45mg/mL (9); on the other hand, at higher concentrations and at 10 to 20mg/mL, bowel loops are hypointense on T2-weighted TSE and short-time inversion recovery (STIR) images both at 20 minutes and 2 hours. A more relevant issue is whether this contrast improves sensitivity and specificity for detecting abnormalities; the current results are not clear (10). Gadolinium is a positive contrast agent and shortens T1 in the small bowel, but when concentrated in the colon acts as a negative contrast agent.

A dilute barium sulfate suspension is a useful negative agent. Air and water are also MR contrast agents. The perfluorocarbons lack hydrogen protons and do not produce a MR signal on either T1or T2-weighted images. Their role in the gastrointestinal tract is not established.

Positive contrast agents accentuate motion artifacts, which are reduced by choosing short scanning times. On the other hand, contrast artifacts are more common with negative agents, yet bowel wall detail is accentuated with negative agents. Use of antiperistaltic pharmacologic agents is not common.

Contrast Agents

The primary objective of oral MR bowel contrast agents is to identify bowel lumen and differentiate normal bowel wall from an abnormal process. For most MR enteroclysis, contrast is injected via a nasojejunal catheter.

Oral MR contrast agents are subdivided into positive contrast agents, which predominantly shorten T1 and increase MR signal intensity on T1-weighted images, and negative contrast agents, which either shorten T2 and decrease signal intensity or simply lack hydrogen protons. Positive contrast agents include various iron, manganese, and gadolinium paramagnetic compounds. Their hyperintense signal is useful

Scintigraphy

Technetium-99m (Tc-99m)–hexamethylpropy- leneamine oxime (HMPAO)–labeled leukocyte scintigraphy is useful in detecting inflammation. Its major application is with a suspected abscess, in Crohn’s disease, and other inflammatory processes.

Capsule Endoscopy/Biopsy

Capsule endoscopy is an endoscopic procedure and is not covered in this work, but mentioned must be its relevance to small bowel imaging

studies. The most common abnormal capsule finding is angioectasia and similar submucosal malformations, lesions not detected by imaging studies (11); also, more ulcers are detected with a capsule than with barium or CT.

Percutaneous 18-gauge core biopsies or 21gauge aspiration of bowel wall tumors using CT and US guidance are performed similarly to other abdominal site biopsies.

Percutaneous Jejunostomy

A percutaneous jejunostomy is believed to be difficult to perform due to inconstant small bowel position and mobility. Yet one study achieved a 95% success rate for new feeding jejunostomies and an 81% success rate for replacement jejunostomies (12); these jejunostomies aid drainage, dilation, stone extraction, and bile duct or intestinal recanalization. Leakage is a possible complication.

During esophagectomy, a loop of jejunum can be surgically fixed to the anterior abdominal wall and marked with metal clips; later these clips can be used as guides for percutaneous access for a feeding jejunostomy in those requiring additional nutritional support.

Congenital Abnormalities

Rotation Anomalies

Midgut malrotation implies an arrest in the usual rotation at any one position, with portions of bowel being not in their usual place, that is, they are malpositioned. Such malposition is usually accompanied by lack of fixation and it is usually this fixation anomaly that allows the bowel to twist and form a volvulus.

Patients with congenital diaphragmatic hernias have a high prevalence of midgut rotational abnormalities, and those with rightsided hernias have a more obvious anomaly than those with left-sided ones. Yet mid-gut volvulus is uncommon among patients operated upon for a congenital diaphragmatic hernia, probably related to postoperative adhesions limiting development of a volvulus in these patients with rotational abnormalities.

Malrotation does occur in each of a pair of identical twins. Midgut malrotation is common

ADVANCED IMAGING OF THE ABDOMEN

in patients with asplenia and polysplenia, and a barium study to detect malrotation is suggested in these patients (situs inversus and heterotaxy syndrome are discussed in Chapter 14).

Presenting symptoms in a setting of malrotation reflect the degree of obstruction and are age dependent. In neonates, bilious vomiting predominates. In older children, pain, bilious vomiting, and failure to thrive are common. Teenagers and young adults tend to have chronic nonspecific pain. Obstruction due to malrotation is accentuated in pregnancy.

With incomplete rotation the cecum is located more medial than usual. Fibrous bands extending diagonally from a malpositioned cecum to the right upper quadrant (Ladd bands) tend to compress adjacent small bowel and result in duodenal or jejunal obstruction of varying severity.

In many of these individuals a diagnosis of malrotation is made either by an upper gastrointestinal examination or a barium enema; the former is more sensitive and is preferred (Fig. 4.1). Subtle rotational anomalies are not uncommon, and the diagnosis is not as straightforward as generally taught. Some children have an unusual duodenal redundancy or a duodenojejunal junction located somewhat more medial than usual. In fact, in some of these children an upper gastrointestinal examination is believed to be grossly “normal.” The status of duodenal redundancy with a normal duodenojejunal junction as a marker for subtle intestinal malrotation is not known. The reverse is also true—false-positive diagnoses result from failure to recognize normal variants in jejunum position. Manual epigastric compression during an upper gastrointestinal examination is useful to detect some neonate intestinal malrotations. For instance, manual compression of a nearnormal duodenojejunal junction can detect abnormal mobility, suggesting malrotation; in some infants with malrotation and volvulus, manual compression induces contrast to pass beyond the point of obstruction and identifies a twist.

A reversed relationship between the superior mesenteric artery and vein, a sign of midgut volvulus, is not always present with malrotation, and this sign cannot be relied on to detect malrotation. A deep ileocolic intussusception also distorts normal superior mesenteric vessel anatomy.

Occasionally a malrotation is suspected with radionuclide gastric emptying studies performed for other reasons.

detects some. An occasional one contains ectopic pancreatic tissue.

Duplication

A small bowel duplication is lined with intestinal mucosa and is usually intramural in location. It may or may not communicate with the true lumen. Ileal duplications are more common than jejunal. Multiple duplications are rare.

Some patients with a duplication are asymptomatic, whereas others present with a palpable mass or even obstruction. In general, most larger duplications manifest early in life. A communicating ileal duplication is an occasional cause of massive hemorrhage.

Imaging typically shows a duplication as an oval or elongated cystic structure adjacent to bowel lumen. It generally has a thick wall, thus distinguishing it from a mesenteric cyst, which is thin-walled. Ultrasonography reveals the cystic nature of a duplication; in addition, US shows a characteristic inner hyperechoic mucosa and an outer hypoechoic muscle layer.

Many duplications contain ectopic gastric mucosa, and thus scintigraphy (Meckel’s scan)

Obstruction

Atresia

Small bowel atresia, believed to be secondary to intrauterine ischemia, ranges from complete to partial and involves the jejunum, the ileum, or both. It is associated with midgut malrotation and is more prevalent in a setting of cystic fibrosis. Autosomal dominance is evident in some families with bowel atresia. Not all atresias are detected in early life; a rare infant develops an ileocolonic fistula to bypass an atretic segment.

Some authors subdivide jejunoileal atresia into four types:

Type I: a short, web-like narrowing

Type II: blind-ending proximal and distal tracts with a fibrous connection

Type III: a short, atretic segment in the proximal jejunum

Type IV: multiple atretic segments

In the rare apple-peel type of atresia the distal duodenum or proximal jejunum ends blindly, the distal superior mesenteric artery is absent, and the distal small bowel is foreshortened and spirals around its rudimentary blood supply, appearing similar to an apple peel. This condition presumably is due to a distal superior mesenteric artery occlusion or failure to develop.

Meconium Ileus

Meconium ileus is in the differential diagnosis with a suspected low intestinal obstruction. Almost all neonates developing meconium ileus have cystic fibrosis. Some have an underlying atresia, stenosis, or volvulus. Untreated, meconium ileus may progress to bowel necrosis and perforation (meconium peritonitis is discussed in Chapter 14).

Conventional radiographs reveal a low obstruction. Other findings, such as a bubbly appearance in the right lower quadrant, lack sufficient sensitivity and specificity to be diagnostic.

A water-soluble contrast enema is not only diagnostic but also often therapeutic. It identifies a small-caliber colon (microcolon), but keep in mind that both a meconium ileus and distal small bowel atresia result in a small caliber colon. With a distal ileal obstruction, however, the entire colon has a small caliber; otherwise a colonic abnormality should be sought Meconium ileus results in intraluminal content at the site of obstruction.

Ultrasonography in six neonates with a meconium ileus revealed multiple loops of bowel filled with hyperechoic material (13); this is in distinction to neonates with ileal atresia who have dilated loops of bowel filled with fluid and gas but not hyperechoic content.

A distinction between a meconium ileus and ileal atresia is more than academic, because the former is usually relieved by a contrast enema but the latter requires surgical intervention.

Cystic Fibrosis

Cystic fibrosis, with a recessive-autosomal inheritance pattern, involves multiple organs,

ADVANCED IMAGING OF THE ABDOMEN

with the lungs and pancreas most often affected. A number of affected patients eventually develop chronic liver disease. The sweat test should initially be performed when cystic diagnosis is suspected.

In patients eventually shown to have cystic fibrosis, newborn meconium ileus occurs as an initial presentation only in a minority. On the other hand, those with a meconium ileus almost all have cystic fibrosis.

The bowel wall becomes thickened in these children; histology reveals extensive intramural fibrosis and fatty infiltration, but unlike Crohn’s disease, acute inflammation is not a prominent feature.

Distal ileal obstruction due to inspissated intestinal content developing after the neonatal period is called meconium ileus equivalent. The prevalence of such obstruction increases with age. Especially during the first such obstructive episode, acute appendicitis is often in the differential diagnosis. A contrast enema is generally diagnostic. Often contrast does not reflux into the terminal ileum. Computed tomography is occasionally used to monitor these patients. The diagnosis can be suspected with US.

Other causes of bowel obstruction in these patients include intussusception and colonic strictures. One child with cystic fibrosis developed partial ascending colon obstruction secondary to diverticulitis (14), a very rare complication indeed.

A number of cystic fibrosis patients now reach adulthood and they are developing new complications, such as cancer, which is not common in the pediatric age group.

A barium enema reveals an irregular and spiculated colon outline, nodules, and loss of haustra. Irreversible and progressive colonic strictures tend to develop, at times to the point of almost complete obstruction.

Thickened small bowel folds and nodularity are familiar to most radiologists. Conventional abdominal radiographs reveal calcifications only in a minority ranging from small specks of calcium to extensive curvilinear calcifications; most calcifications are intramural in location, with a minority being intraluminal or serosal. An occasional newborn with cystic fibrosis has multiple atresias and calcified intraluminal meconium.

JEJUNUM AND ILEUM

Celiac Disease (Sprue)

Clinical

Celiac disease, or gluten-sensitive enteropathy, is a familial genetically determined disease associated with human leukocyte antigen (HLA)-B8-DR3 and manifesting by a digestive tract cytotoxic T-lymphocyte reaction. Affected individuals have a lifelong intolerance to dietary gluten. Certain foods, such as wheat, barley, rye, and others, exacerbate the symptoms of this protein-losing enteropathy. A strict gluten-free diet protects against known complications.

Considerable variability exists in presentation. A latent form of celiac disease appears to exist; affected individuals have subtle small intestine abnormalities consisting of increased intraepithelial lymphocyte levels, abnormal mucosal permeability, and elevated levels of secretory immunoglobulin A (IgA) and IgM antibody to gliadin. Occasionally celiac disease is first diagnosed even in elderly patients, presumably as an expression of silent disease that has been present since childhood. At times unrelated abdominal surgery unmasks latent celiac disease. It does present in the puerperium. It is a complex disease; variants such as celiac disease with immunologic activation of normal small bowel mucosa have been suggested. Some patients have liver disease, detected as a hypertransaminasemia and often labeled a nonspecific reactive hepatitis, which reverts to normal on a gluten-free diet. In an occasional patient an immunologic basis is suggested by finding associated primary biliary cirrhosis, primary sclerosing cholangitis, or autoimmune hepatitis.

Worldwide prevalence of celiac disease varies considerably. An interesting postulate is that the introduction of a high gluten content diet at an early age increases the risk and affects the symptomatology of celiac disease in a specific population. A study involving roughly half of the French pediatric population found an annual incidence of one per 2419 (15); the diagnosis was made before age 2 years in 77% of affected individuals, with the symptoms being failure to thrive, diarrhea, anorexia, abdominal distention, weight under two standard deviations, and short stature.

Orocecal transit time is delayed, and small bowel villous atrophy develops in untreated patients.

Highly sensitive and specific noninvasive blood tests for celiac disease are available, and imaging is relegated to a study of complications. A small bowel biopsy appears superfluous to confirming the diagnosis in symptomatic adults with antibodies to IgA antiendomysial.

Imaging

Patients with celiac disease have fasting or postprandial bowel motor abnormalities, which in most affected individuals are similar to those found in neuropathic disorders (Fig. 4.2). Dilated distal small bowel containing excessive fluid is a late but characteristic finding, at times mimicking partial distal small bowel obstruction. These abnormalities decrease on a glutenfree diet.

Enteroclysis achieves an almost 100% specificity but only about 80% sensitivity in detecting celiac disease in adults (16).

Ultrasonography of infants with known celiac disease detects abdominal fluid and hyperperistalsis in most. The US findings in adults include fluid-filled, dilated small bowel. Disordered motility is common. Bowel wall thickening is mild; marked thickening suggests hypoproteinemia, and asymmetric thickening raises suspicion for a neoplasm. Abdominal fluid is found in a minority. None of these findings are pathognomonic, but detection of several findings should raise suspicion for this disease.

Mild-to-moderate mesenteric adenopathy is common. If an untreated patient is placed on a gluten-free diet, serial CT should show a reduction in node size. Mesenteric lymph node necrosis develops in a rare patient.

Superior mesenteric artery Doppler US in untreated patients reveals significantly higher fasting peak systolic velocity, end diastolic velocity, mean velocity, and flow volume than in controls, and a resistive index that is significantly lower (17). Of interest is that the superior mesenteric artery and portal vein tend to dilate. The effect of therapy on the superior mesenteric artery resistive index is illustrated in Table 4.1.

Protein-losing enteropathy can be studied with several scintigraphic agents, including Tc99m human serum albumin (HSA), Tc-99m human immunoglobulin, indium 111 transferrin, and Tc-99m-dextran.

Comparing pretreatment and follow-up enteroclysis in adults with celiac disease, clinical response to a gluten-free diet correlated better with enteroclysis findings than with repeat biopsy (19).

JEJUNUM AND ILEUM

Associated Conditions/Complications

Lymphocytic gastritis develops in a minority of patients with celiac disease. Of interest is that gastric mucosa-associated lymphoid tissue (MALT) regresses in Helicobacter pylori– negative celiac patients when treated with a gluten-free diet.

Patients with long-standing celiac disease are at increased risk for malignancies, with small bowel high-grade T-cell lymphoma being most common and tending to involve the proximal small bowel. This is in distinction to lymphoma in normal small bowel, which is usually B cell in origin. Celiac disease and lymphoma have been diagnosed during the same presentation. A common appearance is that of small, discrete nodules varying in size or localized fold thickening. Nodules are not found in uncomplicated celiac disease, and such a finding should suggest superimposed lymphoma. A CT finding of marked adenopathy also suggests lymphoma.At times a barium study outlines a shaggy, ulcerated bowel segment, mimicking inflammatory bowel disease. Complicating the issue is that these lymphomas are often associated with considerable inflammation, and a biopsy may suggest inflammatory bowel disease. Enteropathyassociated lymphoma has a poor prognosis, with malignant ulcers not uncommonly resulting in bowel perforation.

Small bowel adenocarcinomas also occur in these patients. In fact, an occasional patient presents with small bowel adenocarcinoma, and celiac disease is diagnosed only after cancer resection. The prevalence of pharyngeal and esophageal carcinomas is also increased in these patients.

An occasional patient develops intestinal ulcerations, loss of valvulae conniventes, and a thickened, tube-like bowel wall; the overall imaging appearance mimics chronic ischemia.

Type 1 diabetes mellitus is found in an occasional patient with celiac disease diagnosed in adulthood. An anecdotal association exists with primary sclerosing cholangitis, lupus, and antiphospholipid syndrome. Some evidence suggests an increased prevalence of celiac disease in patients with primary biliary cirrhosis. An autoimmune linkage is suggested with idiopathic thrombocytopenic purpura and hepatic granulomatous disease.

Splenic atrophy is common in these individuals. Whether splenic infarction and splenic

venous thrombosis in one patient was fortuitous is conjecture (20).

Bone scintigraphy is positive for sacroiliitis in some adults with celiac disease.

Some patients with unresponsive celiac disease are treated with immunosuppressive therapy. A lack of response to therapy should raise the possibility of lymphoma, a setting contraindicating immunosuppressive therapy.

Agammaglobulinemia

An arrest in B-lymphocyte development leads to agammaglobulinemia. The primary defect involves mutations consisting of missense, nonsense, and splice mutations as well as deletion and insertion mutations in the gene encoding Btk (Bruton tyrosine kinase) (21).

Because maternal IgG passes through the placenta, affected newborns initially have normal serum IgG levels, but then these levels decrease and hypogammaglobulinemia ensues. Affected patients are prone to mostly bacterial infections. They have normal resistance to viral infections, except for enteroviral infections, leading to vaccine-related paralytic poliomyelitis and a dermatomyositismeningoencephalitis syndrome (21). Intestinal giardiasis is common in these patients.

A barium small bowel study often is diagnostic in affected patients; marked lymphonodular hyperplasia is evident throughout the small bowel, especially the jejunum, a site where even in pediatric patients lymphonodular hyperplasia is uncommon (Fig. 4.3).

Figure 4.3. Agammaglobulinemia. Small nodules are scattered in the jejunum (arrows).