Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

and a diameter greater than 6cm also imply a poor prognosis.

Epithelioid GISTs (Leiomyoblastomas)

Most leiomyoblastomas (also called epithelioid leiomyosarcomas or epithelioid stromal tumors) tend to be slow growing. About 25% are malignant. Although most are intramural in location, an occasional one grows primarily intraluminal, and even prolapsing into the duodenal bulb. Computed tomography revealed one connected to the serosal surface by a narrow stalk and changing its location in the peritoneal cavity during studies (59).

Their imaging appearance is similar to leiomyosarcomas (Fig. 2.19). Magnetic resonance imaging of a recurrent gastric leiomyoblastoma 14 years after initial presentation revealed a heterogeneous and moderately hypointense signal on T1-weighted images and a heterogeneous and moderately hyperintense signal on T2-weighted images (60); the tumor enhanced somewhat postgadolinium, with enhancement increasing on delayed images. The heterogeneous appearance suggested regions of necrosis.

Liposarcoma

Primary gastric liposarcomas are rare. An occasional one contains sufficient fat to be identified

Figure 2.19. Leiomyoblastoma. CT identifies a soft-tissue density tumor (arrows) containing a lucent center and focal gas. It was believed initially to represent an abscess.

Figure 2.20. Kaposi sarcoma seen as a submucosal nodule (arrow) in an AIDS patient.

by CT, but in general their imaging appearance is similar to that of other stromal tumors. One presented as a large ulcerated submucosal mass at the incisura (61).

Histiocytoma

A primary gastric malignant fibrous histiocytoma is very rare. These solid, intramural tumors are similar in appearance to other gastric sarcomas.

Angiosarcoma

A patient on hemodialysis for 21 years was found to have multicentric gastrointestinal angiosarcomas (62); autopsy revealed lung, bone, liver, gallbladder, and lymph node metastases.

Kaposi’s Sarcoma

Kaposi’s sarcomas are subdivided into the Mediterranean (non-AIDS) type and those associated with AIDS. Patients with Mediterranean Kaposi’s sarcoma also develop gastrointestinal tumors.A typical imaging appearance is that of a sessile polyp (Fig. 2.20).

Carcinosarcoma

Occasionally detected is a tumor containing an adenocarcinoma and mesenchymal elements such as a rhabdomyosarcoma or osteosarcoma and that is labeled carcinosarcoma by patholo-

STOMACH

Figure 2.21. Pancreatic body adenocarcinoma invading stomach (arrow).

gists (or sarcomatoid carcinoma, although these two terms are differentiated by pathology purists). In some cases, an immunocytochemical study suggests that the carcinomatous cells have transformed into sarcomatous cells or even vice versa. A number of these patients have had a previous partial gastrectomy performed years earlier.

Metastasis or Direct Invasion

With the exception of breast cancer and malignant melanoma, metastases to the stomach are not common; more often encountered is direct invasion from adjacent structures. Typical is pancreatic carcinoma, which readily invades the stomach, duodenum, and other adjacent struc-

Figure 2.23. Gastric melanoma presenting as multiple polyps (arrows). (Courtesy of Arunas Gasparaitis, M.D., University of Chicago.)

tures (Fig. 2.21). Peritoneal carcinomatosis also involves the stomach. A rare metastasis is from a renal cell carcinoma, at times one resected years previously. Establishing a primary site can be difficult with simultaneous gastric and ovarian carcinomas (Fig. 2.22). These metastatic carcinomas ulcerate, bleed, obstruct, or even perforate.

A vast majority of gastric melanomas are metastatic (Fig. 2.23). Only a rare one is believed to be a primary gastric melanoma, a diagnosis to be made with caution.

Not all tumor diagnosis is straightforward; at times imaging studies and endoscopy with biopsy suggest an inflammatory condition, such

as gastric Crohn’s disease, when in fact metastatic breast carcinoma or other metastasis to the stomach is responsible for an inflammatory reaction around sparse tumor cells, but this is a limitation of biopsy, and especially cytology, rather than an indictment of imaging or endoscopy.

Neuroendocrine Tumors

Autonomic Nerve Tumor

Gastrointestinal autonomic nerve (GAN) tumors morphologically resemble an enteric plexus and were originally called plexomas or plexosarcomas. They consist of spindle-shaped cells mimicking a smooth muscle tumor, such as a GIST, or schwannoma. They also contain synapse-like structures and contain neurosecretory granules, suggesting a myenteric plexus origin.

Schwannoma

A schwannoma, also known as neurilemmoma, neurinoma, and perineural fibroblastoma, originates from peripheral nerve sheaths (Schwann cells) and is found at peripheral neural tissue sites. Most are benign, with about half of the malignant ones occurring in a setting of neurofibromatosis type 1 (von Recklinghausen’s disease). Radiation therapy appears to predispose to schwannoma formation. These are rare tumors in children and are rare in the stomach. Clinically, schwannomas present with bleeding from erosions, obstruction, or, if large enough, a palpable mass.

The imaging appearance is similar to that of other stromal (mesenchymal) tumors. Heterogeneous tumor CT contrast enhancement is evident in some. An occasional schwannoma is partly cystic and shows peripheral contrast enhancement with CT and MRI.

Endoscopic biopsy is not always straightforward with these tumors, at times suggesting a GIST or leiomyoma, with only a resected specimen confirming a schwannoma.

Zollinger-Ellison Syndrome

Zollinger-Ellison syndrome develops as a result of a gastrin-secreting tumor, gastric acid hypersecretion, and resultant severe peptic ulcer

ADVANCED IMAGING OF THE ABDOMEN

disease (gastrinomas are discussed in more detail in Chapter 9). Primary gastric gastrinomas are uncommon; most are in the duodenopancreatic region. Clinically, these patients range from those with atypical peptic ulcer disease to those with malabsorption. Of note is that not all patients with an elevated gastrin level manifest Zollinger-Ellison syndrome.

H. pylori infection is not a risk factor for ulcers in these patients; in fact, prevalence of H. pylori infection in patients with ZollingerEllison syndrome is lower than in the general population and much lower than in patients with classic peptic ulcer disease.

Carcinoid

Contrary to earlier belief, gastric carcinoids are common and in fact may be the most common gastrointestinal location for these tumors. Gastric carcinoids can be subdivided into several categories:

1.In association with chronic atrophic gastritis, gastric enterochromaffin-like cell hyperplasia and pernicious anemia

2.In association with Zollinger-Ellison syndrome

3.In association with multiple endocrine neoplasia type 1

4.Sporadic carcinoids

Pathogenesis of all except the last type is influenced by promoting agents, such as hypergastrinemia and transforming agents, and carcinoids develop through a hyperplasia– dysplasia–neoplasia sequence. In general, these are associated with a good prognosis. Pathogenesis of the sporadic carcinoids is unknown; these patients have a poor prognosis.

A majority of gastric carcinoids are associated with chronic atrophic gastritis, and about half of these patients have multiple tumors. Gastric enterochromaffin-like cell hyperplasia is common in autoimmune gastritis, and presumably carcinoids found in some of these patients are related to hyperplasia. Evidence suggests that gastric enterochromaffin-like cell carcinoids constitute an independent tumor in multiple endocrine neoplasia type I (MEN-I) syndrome and develop with enteropancreatic and parathyroid MEN-I tumors via inactivation of the MEN-I gene.

STOMACH

The prevalence of gastric carcinoids is increased in a setting of hypergastrinemia. The drug omeprazole leads to an increase in serum gastrin levels and in laboratory animals induces gastric carcinoids. An achlorhydria–hypergas- trinemia–carcinoid sequence appears to exist. Hypergastrinemia-associated carcinoids tend to have a benign course and some are managed by endoscopic excision. Sporadic carcinoids occur throughout the stomach and are not related to gastrin levels. They are more likely to be malignant than the other types.

Contrast-enhanced imaging studies are necessary to detect these hypervascular tumors. Carcinoids tend to enhance more than normal gastric wall on contrast-enhanced MR.

Glomus Tumor

Computed tomography of two gastric glomus tumors revealed peripheral nodular or homogeneous arterial phase contrast enhancement, followed by prolonged late phase enhancement (63).

Gastric Dilation

Gastric Emptying Studies

Gastric emptying is influenced by the presence of duodenal acid or fat. Acid in the duodenum induces tonic duodenal constrictions and increases gastric emptying, but bile and fat delay gastric emptying by decreasing duodenal contractility. Animal and vegetable fats have similar effects on gastric emptying (64). Accelerated gastric emptying is found in several conditions, namely non–insulin-dependent diabetes, increased body mass index, hypertension, hyperthyroidism, and after some medications, such as erythromycin. Gastric motility in diabetes mellitus, however, is a complex issue (see Gastroparesis, below). Gastric liquid emptying is accelerated after truncal vagotomy; emptying of solids is also usually increased, but results are inconstant.

Gastric emptying and gastric motility studies are feasible with MRI. Adding gadolinium contrast to a liquid meal results in a bright gastric lumen. Serial 3-D T1-weighted gradient-echo MRI every 5 minutes or so assesses gastric

volumes and detects delayed gastric emptying (65). A T1/2 value can be calculated from a plot of gastric emptying. Whether such MR techniques have a role in clinical practice remains to be established.

Similar to MR, gastric emptying scintigraphy using various Tc-99m–labeled solids and liquids generate gastric radioisotope time-activity curves from which a gastric emptying rate and half-times are calculated. A two-point method (at 0 and 120 minutes) correlates well with a multiple point conventional method and saves substantial technologist and camera times (66). An emptying half-time implies exponential emptying, which is not necessarily true and thus introduces a variable; nevertheless, an emptying half-time is often used as a standard. To be reproducible, several corrections are incorporated, including radioisotope decay during test time and variation in tissue attenuation between patients. Men have a faster gastric emptying rate, shorter half-emptying time, and lower residual radioactivity than women. Patient positioning and meal composition and size need be standardized by each laboratory performing these studies and normal ranges established. Also, mechanical gastric outlet obstruction should be excluded prior to the test.

A solid test meal is more reliable and is employed more often than a liquid meal. The radioisotope should not disassociate from the test meal used, otherwise results will approach those of a liquid meal. Initially Tc-99m–sulfur colloid–labeled chicken liver was the test meal used, having been supplanted in most institutions by an egg meal. The half-times obtained with a solid liver meal differ from those of an egg meal; the latter is more of a semisolid consistency.

Other methods of measuring gastric emptying include oral ingestion of acetaminophen in a test meal followed by serial blood sampling for plasma acetaminophen. Breath sampling for 13CO2 after a liquid test meal containing 13C- acetate provides a gross estimate of gastric emptying time. Also, in experienced hands a barium upper gastrointestinal study is often adequate to evaluate gastric emptying.

In children, a Tc-99m–sulfur colloid milk scan is useful in evaluating both gastroesophageal reflux and gastric emptying. In-111- DTPA in water is a liquid marker. One

refinement is a dual-phase solid-liquid combination, although the added complexity is not necessary in many patients. Serial imaging allows calculation of activity as a percent of original; the gastric emptying half-time is then calculated.

The sulfamethizole absorption test evaluates gastric motility and gastric emptying. This test relies on a rapid absorption of sulfamethizole from proximal small bowel.

Gastric Outlet Obstruction

Infant Hypertrophic Pyloric Stenosis

Clinical

Etiology of infantile hypertrophic pyloric stenosis (IHPS) is not known. Hypergastrinemia appears to have a pathogenetic role. Genetic factors also are involved; IHPS is encountered more often if a parent or sibling has had the condition. It is more common in boys. A possible connection exists between IHPS and allergic gastroenteropathy; both entities narrow the pyloric region. In some infants IHPS is associated with eosinophilic gastroenteritis; histologically, these infants have pyloric circular muscle hypertrophy and hyperplasia. Jaundice in several infants with IHPS was believed to represent an early manifestation of Gilbert’s syndrome (67).

A surprising number of other bowel obstructions have been reported in a setting of IHPS, including a duodenal web, coexisting hypertrophic antral polyp, pyloric mucosal hypertrophy, an adjacent cyst or duplication, and a solid tumor. An occasional infant develops IHPS during the postoperative period after repair of some other congenital condition.

Some infants with cyanotic congenital heart disease undergo prostaglandin infusion therapy and some then develop gastric outlet obstruction. Persistent but asymptomatic gastric distention develops in some infants within 48 hours of initial prostaglandin therapy and in a minority evolves into feeding intolerance with continued prostaglandin therapy; gastric distention resolves in most when prostaglandin therapy is stopped. Although superficially similar to IHPS, these infants have mostly distal antral mucosal involvement. Barium studies reveal antral narrowing; US detects increased gastric mucosal thickening and distal antral and

ADVANCED IMAGING OF THE ABDOMEN

pyloric elongation but no muscular wall thickening. The lobular mucosal thickening consists of alternating hyperand hypoechoic structures and in these infants lumen narrowing appears to be secondary to mucosal rather than muscle thickening.

Imaging

Whether a surgeon is able to palpate a pyloric “olive” depends on pyloric muscle volume, which can be obtained from US. Whether such pyloric volume calculations are of clinical value is debatable.

If a typical pyloric “olive” is palpable, a strong argument can be made for no preoperative imaging studies. On the other hand, US in experienced hands achieves a high sensitivity and specificity in detecting pyloric stenosis. A major limitation of US is its inability to detect alternative diagnoses in an infant with nonbilious vomiting due to some other cause. Thus in an infant with projectile vomiting, if US excludes pyloric stenosis, a second imaging study, such as a barium examination, is often necessary to identify an esophageal, gastric, or duodenal abnormality to account for the infant’s symptomatology. In clinical practice a reasonable compromise seems to be to perform US in those infants with strong suspicion for pyloric stenosis whereas others undergo a barium study.

A number of authors have proposed normal and abnormal ranges of pyloric length, muscle thickness, and muscle volume (Fig. 2.24). Ultrasonography measurement of pyloric muscle thickness and pyloric length is easier if the muscle is enlarged. In healthy infants, pyloric muscle thickness measures about 2mm and pyloric length about 10mm; in infants with surgically confirmed IHPS, muscle thickness is about double and pyloric length increased by about 40%. Unfortunately, in actual practice results are not so clear; infants with an initial suspicion of IHPS but a final diagnosis of no IHPS have intermediate measurements. Also, overlap exists between normal and IHPS, and a diagnosis based solely on US measurement of pyloric muscle thickness is not straightforward. In particular, a contracted pylorus imaged tangentially will appear thickened. Some investigators prefer to rely primarily on pyloric length, yet critical analysis reveals diagnostic

problems similar to those when relying on pyloric width.

The commonly used US term pyloric muscle thickness appears to be anatomically incorrect. Closer study of the pyloric wall reveals that hypertrophied mucosa makes up about one third of the pyloric cross-sectional diameter and tends to fill the pyloric lumen (68). To add confusion, the term pylorospasm is applied to an apparently thickened pyloric muscle without evidence of IHPS. A time-varying pyloric muscle thickness and an incomplete obstruction to flow should differentiate pylorospasm from IHPS, which is fixed in outline, yet here also a borderline zone exists.

Infants with suspected IHPS and borderline US values for pyloric volume have been treated with an antispasmodic drug (metoclopramide) with inconstant results; obstruction has cleared in some, while others have progressed to IHPS.

IHPS is a rare cause of transient portal venous gas.

Delayed gastric emptying is encountered in some infants with few if any symptoms. Such “pylorospasm” is controversial and of dubious significance.

Therapy

Laparoscopic pyloromyotomy is being performed for IHPS and may result in a shorter hospital stay.

Both US and a barium study remain abnormal for some time after a pyloromyotomy for IHPS. However, no significant delay in gastric emptying should be evident.

In infants with IHPS treated medically, US identifies an initial pyloric channel shortening followed by muscular thinning as symptoms improved (69).

Other Causes in Infants

A congenital antral membrane is a rare cause of gastric outlet obstruction. Partial obstruction is more common than complete and some patients present later in life (Fig. 2.25). A barium study differentiates an antral web from IHPS; with a web a more distally located pylorus can be identified. Difficulty of differentiation exists, however, with complete obstruction.

A rare ectopic pancreas in the distal antrum causes gastric outlet obstruction. Ultrasonogra-

Figure 2.25. Antral web (arrow) in an adult. A peristaltic wave is thicker and changes with time.

phy in a 1-month-old infant with familial hyperlipidemia suspected of IHPS revealed an intense hyperechoic thickened pyloric muscle (70); surgery found a necrotic and inflamed pyloric muscle infiltrated by fat.

Gastric volvulus in infants tends to be part of a complex congenital visceral attachment anomaly, often involving the left hemidiaphragm.

Causes in Adults

In adults, gastric outlet obstruction was more common in the past. Prior to H2 blockers, peptic ulcer disease was the most common cause of gastric outlet obstruction; currently a malignancy is more common. A pedunculated antral polyp prolapsing intermittently through the pylorus into the duodenum is a cause of gastric outlet obstruction. Skill in differentiating between a prolapsing polyp and prolapsed gastric mucosa (which is a normal variant) should be acquired in a radiology residency.

An uncommon cause of gastric outlet obstruction is a cholecystogastric or a cholecystoduodenal fistula and resultant gallstone obstruction (Bouveret’s syndrome). This condition is discussed in Chapter 3.

Gastric outlet obstruction secondary to an enlarged gallbladder is rare; based on the few reported patients, a gallbladdder neoplasm or other infiltrative disease should be suspected.

ADVANCED IMAGING OF THE ABDOMEN

Antral diaphragms occur in both children and adults. Some are congenital, but an antral web in an adult is usually an acquired condition, presumably secondary to prior inflammation. Histologically, these webs consist of normal mucosa without significant inflammation. If sufficiently prominent, these webs and diaphragms result in obstruction.

Hypertrophic pyloric stenosis in adults is rare. In one elderly woman a barium study simply revealed markedly reduced gastric emptying (71). From a clinical and imaging viewpoint, adult hypertrophic pyloric stenosis should be considered only after more common etiologies of gastric outlet obstruction are excluded.

Balloon dilation of peptic ulcer–induced gastric outlet strictures is performed using endoscopic, fluoroscopic, or both modalities for guidance. Older children with delayed gastric emptying refractory to medical therapy have been treated with balloon pyloroplasty using fluoroscopic guidance (72); the full role of such therapy both in children and adults is yet to be established.

Volvulus

In the gastrointestinal tract torsion signifies a twist of the organ in question, whereas volvulus implies superimposed lumen obstruction. This is not a universally accepted definition; for instance, urologists use the term torsion for conditions most gastroenterologists would label as volvulus.

Gastric torsion occurs with hiatal hernias, especially larger ones, even to the point of volvulus. Similarly, volvulus occurs if the stomach herniates through some other point of weakness, such as foramen of Morgagni.

Gastric volvulus is encountered both in the very young and very old. Volvulus occurs either along the gastric longitudinal axis (organoaxial) or transverse axis (mesenteroaxial). The latter is more common; it is occasionally associated with recent trauma. An organoaxial volvulus obstructs either at the proximal point of twist, generally close to the esophagogastric junction, at the distal point, generally in the distal antrum, or at both. The site of greatest obstruction determines which organ is most dilated, and thus the resultant imaging appearance varies (Fig. 2.26).