Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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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.