Mesentery (Celiac, Upper and Lower Mesenteric Station)

Atypical patterns

Small confluent nodes, bulky formation

c Extended tumor formation encasing the superior mesenteric artery (AMS) in mantle cell lymphoma.

f Longitudinal epigastric view with extended, partly nodular, ill-defined tumor formation in CLL. AO = aorta; L = liver; MES = mesentery.

i Homogeneous hyperechoic tumor formation invading all of the mesenteric root in Hodgkin disease. A = ascites; AO = aorta; b = bowel; VMS = superior mesenteric vein; WS = vertebral column.

The differential diagnosis of an isolated mesenteric mass covers a whole gamut of possibilities, including:

●Intestinal loop, mesenteric cyst (Fig. 6.35), necrosis in pancreatitis, abscess

●Mesenteric varicosities in portal hypertension, thrombosis of the mesenteric vessels, mesenteric artery aneurysms, hematoma

●Inflammatory pseudotumor (Fig. 6.36), lipoma, primary mesenchymal tumor

Fig. 6.35 Transverse epigastric view with anechoic mesenteric mass corresponding with a cyst (C). AO = aorta; L = liver; MA = stomach; MES = mesentery.

In humans food is ingested, transported, and stored by the gastrointestinal (GI) tract, which may be subdivided into four major sections: esophagus, stomach, small intestine, and large intestine.

The esophagus propels food taken up in the mouth into the stomach, where it is mixed, ground down, digested, and stored. The ingesta are then passed into the small intestine, where they undergo selective absorption. The residual indigestible parts of our food become thick-

ened in the large intestine and are stored there until evacuation.

In addition, the GI tract plays an important role as defensive barrier, and its Peyer’s patches and lymphocytes make up the most extensive immune system in the human body.

Successful realization of these functions is based on a common wall structure of the GI tube. The innermost mucous membrane secretes and absorbs; the submucous layer in the middle with its blood vessels, lymphatics,

and nerves and most of the enteric lymphatic organ lets the outer tela muscularis slide with respect to the mucous membrane. This muscular coat is characterized by longitudinal and circular muscle fibers arranged in different fashions in the various parts of the GI tract; their peristaltic contraction mixes and propels the intestinal contents as well as controlling this transport by means of special sphincter organs.

Esophagus. The esophagus is a muscular tube about 40 cm long connecting the pharynx to the stomach and is located in the posterior mediastinum. Its distal segment is anterior and parallel to the aorta. It penetrates the diaphragm through the so-called esophageal hiatus, terminating at the esophagocardiac transition zone.

Stomach. The stomach is one of the intraperitoneal organs and is situated in the left upper quadrant. This hookor comma-shaped hollow organ runs from the cephalad left to the caudad right and has a length of approximately 20 cm. Under fasting conditions it contains about 50 mL of fluid, but will expand physiologically up to a maximum capacity of 2 liters.

The stomach is subdivided into cardia, fundus/fornix, body, and pyloric antrum. The cardiac opening of the stomach is situated anterior to the aorta at the level of the esophageal hiatus; its anterior aspect is covered by the inferior surface of the left hepatic lobe. The fundus/ fornix of the stomach with its round dome fills the space in the left upper quadrant, the superior border of which is formed by the diaphragm, while the left fundic aspect is in contact with the spleen. The distal part of the stomach is made up of the body and the pyloric antrum, which are defined on the anterior aspect by the left hepatic lobe. The muscular opening into the duodenum is the pyloric orifice, which is in direct contact with the head of the pancreas.

Small intestine. The small intestine is subdivided into the duodenum, which is about 30 cm long, and the jejunum plus ileum, which altogether measure approximately 1.5 m. The duodenum is a C-shaped arc (slightly more than a semicircle) that encloses the head of the pancreas, only partially covered by peritoneum. The superior duodenal segment crosses the structures of the porta hepatis, i. e., common bile duct, portal vein, and hepatic artery, while the gallbladder is anterolateral to it. The descending duodenal segment parallels the posterolateral vena cava and borders the right

Esophagus

●Wall thickness 3–4 mm

●Length 40 cm

●Transitory transport of the ingesta

Stomach

●Wall thickness 5–7 mm

●Length 20 cm

●Capacity: fasting 50 mL; postprandial up to 2000 mL

Duodenum

●Wall thickness 3–4 mm

●Length 30 cm

●Transitory transport of the chyme

Jejunum and Ileum

●Wall thickness 3–4 mm

●Total length 150 cm

●Transitory transport of the chyme

Colon

●Wall thickness 2–3 mm

●Length 120 cm

●Scybala/air

kidney. The inferior duodenal segment, situated about two fingerbreadths inferior to the mesenteric root and caudad of the pancreas, crosses anterior to the vena cava and the aorta. Cross-sectional views of the superior mesenteric artery and vein can be demonstrated anterior to the intestine. The retroperitoneal duodenum continues as an intraperitoneal organ at the level of the mesenteric root slightly left of the aorta.

The jejunum and ileum are attached to the posterior abdominal wall by the fan-shaped folds of the peritoneum known as the mesentery proper; most of the coils of the small intestine are found in the left upper (jejunum) and lower (ileum) quadrant. The jejunum and ileum account for most of the intraperitoneal

Fig 7.1 Topographic anatomy of the intestinal tract.

space. The terminal ileum ascends from the pelvis over the right psoas major muscle, the iliac vessels paralleling its medial margin, at the level of McBurney’s point, and ends in the right iliac fossa by opening into the medial side of the cecum at the ileocecal (Bauhin’s) valve.

Colon. The colon is about 120 cm long and runs along the margins of the intraperitoneal cavity. In a clockwise direction (from the subject’s point of view) it is subdivided into cecum, ascending, transverse, descending, and sigmoid colon, and rectum.

The cecum is situated anterior to the psoas major muscle in the right lower quadrant. The ascending colon parallels the right lateral abdominal wall and its superior segment is

anterior to the inferolateral part of the right kidney. The hepatic flexure comprises the terminal part of the ascending colon and the commencement of the transverse colon; usually, the convex part of this flexure hugs the gallbladder and the lower right margin of the liver, but as a variant it may dome up far superior and may even be situated between

Ultrasound Morphology

With enough training and experience the sonographer will be able to identify and demonstrate most regions of the GI tract ( 7.1).

Esophagus. The proximal part of the cervical esophagus posterior to the left lobe of the thyroid lends itself to sonographic study ( 7.1a); although for all practical purposes the thoracic segment is beyond the reach of transcutaneous ultrasound, ultrasonography can demonstrate the distal esophagus at the level of the gastric cardia. Endoscopic ultrasonography (EUS) is employed for the sonographic evaluation of the esophagus and the neighboring mediastinum. The esophagocardial junction can be better evaluated by transabdominal ultrasound.

the lower right thoracic wall and the right hepatic lobe (so-called Chilaiditi syndrome). The transverse colon touches the anterior abdominal wall and traverses the upper abdomen from right to left; elongated variants may have a drooping center section reaching all the way into the pelvis. The splenic flexure connects the transverse and descending colon, its convex

Stomach and duodenum. With appropriate techniques all of the stomach and duodenum are accessible to sonographic study.

Jejunum and ileum. In the fasting state the individual loops of the jejunum cannot be differentiated, while their mesenteric attachment and the circular (Kerckring’s) folds characterize the postprandial fluid-filled loops. Later, the postprandial ileum loops in the lower abdomen will also be fluid-filled and display vivid peristalsis.

Colon. Usually, the ileocecum and colon can be visualized reasonably well and, taking into account the anatomic relations, the topography of these parts of the GI tract can be identified

aspect being inferior to the spleen. The proximal part of the descending colon is anterior and lateral to the left kidney. The S-shaped sigmoid colon forms a loop that, depending on its length, frames the roof of the urinary bladder more or less markedly. The rectum descends into the pelvis posterior to the bladder and terminates at the anus.

with certainty. Sometimes an elongated sigmoid colon or drooping transverse colon may interfere and be the reason for misidentification. Almost always the colon will appear as a tube filled with a varying amount of feces and lacking peristalsis.

Gut signature. The morphological criterion to look for in ultrasound studies of the GI tract is the so-called gut signature. At any point in the gut this cross-sectional view will demonstrate the GI tube as an annular structure, while the longitudinal view depicts it as a tubular structure. Assessment of the intestinal wall and differentiation of its characteristic layered structure becomes possible only with high-fre- quency probes (5, 7, or 7.5 MHz) (Table 7.1).

Gut Signature

The normal “gut signature” is known as the “cockade,” a term whose origins date back to Old French and refer to a rosette-shaped badge. In ultrasonography, the gut signature and its pathological variant the “target sign” have come to denote the round crosssectional view of the intestinal wall with its different layers. Although it has also been used to describe the longitudinal view of the GI tube, this usage is reserved exclusively for the GI tract and should not be applied to other rosette-shaped structures, such as hepatic metastasis, which are given their own terms (e. g., “halo” or “bull’s-eye” sign).

Thus, in the cross-sectional view the gut signature/target sign refers to the annular image of the layered intestinal wall and its center, while in the longitudinal view it refers to the parallel arrangement of adjacent intestinal segments with the lumen in between.

Physiological gut signature. A so-called physiological gut signature refers to a lumen that is normal for this segment, a wall structure with characteristic layering, welldefined wall thickness, and normal peristalsis.

Pathological gut signature. A so-called pathological gut signature is characterized by changes in the normal appearance of lumen, wall thickness, and/or peristalsis/pliability (Table 7.2). Pathological wall changes may be circular or eccentric and focal, or they may be continuous or discontinuous along the longitudinal axis. Intussusception will be seen on ultrasound as the “target/doughnut sign” (cross-sectional view) or “pseudokidney sign” (longitudinal view). Adjacent target signs fixed in space are characteristic of a conglomerate.

Gut signature criteria. Detection of a gut signature calls for analysis of its ultrasound morphology, lumen, and peristalsis as well as of the surroundings and the intestinal segments upstream and downstream. Only all of these criteria together will permit typing and assessment with a clear-cut diagnosis in many cases. The criteria to be considered are as follows.

●Location/relation. Specifying the position of a gut signature offers the first chance of defining the anatomical relations. It is important to recognize whether the pathology exhibits a fixed location or is mobile.

●Echogenicity. Basically, the echogenicity of the gut signature may be normal, hypoechoic, or hyperechoic; since one is dealing with a layered anatomical structure, this criterion has to be applied to the predominant elements of the wall or should reflect the possible loss of layering.

●Wall thickness. Numerous standard dimensions have been listed for the wall thickness (see above). Measurement of the wall thickness has to account for the degree of filling and the functional state (contraction/dilatation) of the intestines, and also has to specify whether the wall is thickened or thinned out or whether its outline is interrupted or ill defined.

●Wall layering. The wall may display a normal layered structure or may be markedly layered, may demonstrate thickening of just one particular layer, may present with layer defects, or may have lost its layered structure altogether.

●Shape/texture of the gut signature. The cross-sectional view of the gut signature may be round or ovoid, or it may be characterized by asymmetrical changes; one characteristic pathological sign is the socalled target or bull’s-eye sign.

●Extent. In terms of length, short focal or extended lesions have to be differentiated from the diffuse changes in the intestinal wall.

●Internal delineation. Assessment of the luminal surface of the bowel wall will be able to differentiate between normal as well as pathological circular folds and haustra (preserved or lost) and also focal polypoid/tumorous changes, ulcers, and other surface irregularities.

●External delineation. The outside of the GI tube may be smooth or irregular; it may exhibit pod-like extensions or invade other structures.

●Changes in the surroundings. The surroundings of the gut signature may be absolutely normal or there may be free air or fluid; ultrasound may be able to visualize focal reactions in the surrounding tissue, e. g., hyperechoic pannus, abscess and fistula formation, as well as lymphadenopathy.

●Lumen. Assessment of the lumen has to account for the functional state/degree of contraction; it may be normal, distended, or reduced/narrowed.

●Stenosis. A stenosis is a circumscribed functionally narrowed lumen of the bowel with prestenotic distension and poststenotic narrowing of the intestinal tube; usually, a pathological gut signature or target sign will be seen at the stenosis itself.

●Contents. Within the lumen of the gut there may be anechoic fluid, echogenic chyme/ ingesta, hyperechoic feces/scybala, air, and foreign bodies.

●Peristalsis. The assessment of peristalsis differentiates between physiological contraction/distension on the one hand and normal, missing, increased, impaired, or

pathological peristalsis (pendulating peristalsis) on the other.

●Texture/pliability. Normal intestinal loops are compliant; lack of pliability suggests invasive inflammatory or malignant changes.

●Painfulness. A normal gut signature is not painful; if localized pain can be triggered at a focal change in the intestinal wall, this indicates either an inflammatory or a functional disorder (e. g., irritable bowel syndrome).

●Vascularity. The intestinal wall and its surrounding tissues may be hypervascular or avascular, or the vascularity may be normal.

●Changes in intestinal segments upstream. Upstream of a focal lesion one may find characteristic changes in the intestinal lumen (e. g., distension in the presence of ileus), peristalsis, and texture (e. g., thickening of the muscularis propria in case of hypertrophy).

●Changes in intestinal segments downstream. In case of mechanical ileus, a typical sign downstream is the so-called “starvation gut.”

●Changes observed over time. The quality and/or quantity of one or more of the observed criteria may become more or less pronounced.

●Changes in other organs. Additional findings in other organs may help immensely in the correct interpretation of intestinal sonographic observations (e. g., gallbladder, biliary tree, and liver in case of gallstone ileus; assessment of the flow signals in the superior and inferior mesenteric artery in ischemic and inflammatory bowel disease).

Peristalsis

Peristalsis consists of a wave of contraction, followed by immediate relaxation, passing along the longitudinal axis of the intestinal tube; under physiological conditions it may be visualized sonographically at the esophagus, stomach, and small intestine. Next to a segment actively contracting and displaying a thickened intestinal wall with narrowing of the lumen, the segments upstream and downstream will undergo concurrent relaxation of the intestinal wall musculature with thinning of the wall, dilatation, and widening of the lumen.

Esophagus. Rhythmic contractions are deliberately activated during deglutition and can be evaluated typically at the dorsal side of the left thyroid lobe.

Stomach. Gastric peristalsis is best seen around the antrum and is characterized by

rhythmic contractions with a frequency of 2–4 times per minute and a period of 2–20 s. The dilatation phases with discernible passage of chyme subsequent to swallowing are best observed at the esophagogastric junction.

Small intestine. Under fasting conditions, the empty small intestine (the Latin word “jejunus” means empty, dry, barren) is hard to visualize sonographically and thus its peristaltic movement remains hidden. Under physiological conditions the postprandial peristalsis usually appears as a steady and orderly alternation of muscular contraction and dilatation propelling chyme and fluid in a directed fashion.

In inflammatory disorders the lumen of the small intestine will still be filled with some fluid even when fasting; by itself this demonstration of intraluminal fluid (e. g., within

the duodenum in case of pancreatitis or cholecystitis) may be enough to suggest the underlying inflammation. Depending on the severity of the inflammation, the peristalsis may be vigorous, increased or even turbulent (e. g., the so-called “tumbler phenomenon” in celiac disease). Disturbed peristalsis with pathological contractions, mandatory pathological distension of the lumen, and conspicuously uncoordinated movement of the intestinal wall and intraluminal chyme/fluid are characteristic of ileus (see below).

Large intestine. Under physiological conditions, ultrasonography cannot visualize peristaltic activity in the large intestine. However, the characteristically impaired peristalsis in the presence of ileus does become evident on ultrasound.

a Demonstration of the cervical normal esophagus posterior to the left thyroid gland.

d Normal gastric corpus folds.

g Normal gut signature of the inferior part of the duodenum where it crosses the aorta (AO). VMS = superior mesenteric vein; WK = spine; Darm = intestine.

b Typical gut signature of the esophagocardiac junction anterior to the aorta at the esophageal hiatus.

e Physiological target sign of the antrum.

h Jejunum, slightly fluid-enhanced, after oral fluid intake; note the clearly delineated circular folds.

c The same segment, longitudinal scan: gut signature of the esophagocardiac junction dorsal to the left liver lobe and transition into the gastic fornix.

f Fluid-filled stomach caused by a pyloric stenosis.

Colon

o and p Haustra in colon.

r Hydrocolonic sonography demonstrating the ileocolic valve; the terminal ileum runs into the colon, normal finding with normal haustra.

q Hydrocolonic sonography demonstrating the intestinal wall of the descending colon.