On completion of this chapter, you should be able to:
Describe the anatomy and relational landmarks of the gastrointestinal system
Discuss the size of wall thickness and diameters of the gastrointestinal tract
Describe the sonographic technique used to image the gastrointestinal tract and appendix
Differentiate the sonographic appearances of the pathologies covered in this chapter
Sonography is not the primary imaging tool to investigate the gastrointestinal system due to the limited visualization of many structures. However, there are patients who have nonspecific complaints related to the gastrointestinal tract that sonography may be able to help direct the further workup of the patient. The gastrointestinal tract may be difficult to image with ultrasound in most patients unless they ingest fluids or some other acoustic transmittable contrast agent. Many laboratories have begun to investigate various contrast agents in pursuit of the ideal medium for imaging the stomach, duodenum, small bowel, and colon. The retrograde infusion of water can also be used to distend the colon to evaluate for abnormalities.
Anatomy of the gastrointestinal tract
The digestive tract, also known as the alimentary tract, is a tube about 8 m long extending from the mouth to the anus ( Figure 13-1 ). The gastrointestinal tract is that part of the digestive system below the diaphragm. Together, the digestive tract and gastrointestinal tract comprise the digestive system. The sequential parts of the digestive system include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Three types of accessory digestive glands—the salivary glands, liver, and pancreas—secrete digestive juices into the digestive system.
The esophagus extends from the pharynx through the thoracic cavity, then passes through the diaphragm and empties into the stomach (see Figure 13-1 ). The lower end of the esophagus is a circular muscle that acts as a sphincter, constricting the tube so that the entrance to the stomach, at the cardiac orifice, is generally closed. This helps to prevent gastric acid from moving up into the esophagus.
The stomach is a large, smooth, muscular organ that has two surfaces: the lesser curvature and the greater curvature ( Figure 13-2 ). The stomach is divided into three parts: The fundus is found in the superior aspect, the body makes up the major central axis, and the pylorus is the lower aspect. The pylorus is further subdivided into the antrum, the pyloric canal, and the pyloric sphincter. The pyloric canal is a muscle that connects the stomach to the proximal duodenum.
Supporting ligaments of the greater curvature of the stomach include the greater omentum, the gastrophrenic ligament, the gastrosplenic ligament, and the lienorenal ligament. Ligaments that support the lesser curvature of the stomach include the gastrohepatic ligament of the lesser omentum. Folds of the mucosa and submucosa are called rugae.
The small intestine is a long, coiled tube about 5 m long by 4 cm in diameter (see Figure 13-1 ). The first 22 cm is the duodenum, which is curved like the letter C. The duodenum is subdivided into four segments: (1) superior, (2) descending, (3) transverse, and (4) ascending ( Figure 13-3 ). The first part of the duodenum is not attached to the mesentery; the remainder of the small intestine, including the rest of the duodenum, is attached to the mesentery. The mesentery projects from the parietal peritoneum and attaches to the small intestine to anchor it to the posterior abdominal wall.
The first part of the duodenum, the duodenal bulb, begins at the pylorus and terminates at the neck of the gallbladder, posterior to the left lobe of the liver and medial to the gallbladder. The duodenal bulb is peritoneal, supported by the hepatoduodenal ligament, and passes anterior to the common bile duct, gastroduodenal artery, common hepatic artery, hepatic portal vein, and head of the pancreas. This is an important point for sonographers to recognize: if there is air in the duodenal bulb, it will be more difficult to image the common bile duct and smaller vessels that help to define the head of the pancreas. This is avoided by changing patient position (decubitus or upright) or giving the patient water to fill the duodenal loop and serve as an acoustic window to image these structures.
The second part (descending) of the duodenum is retroperitoneal and runs parallel, posterior, and to the right of the spine. The transverse colon crosses anterior to the middle third of the descending duodenum. The pancreatic head is medial to the duodenum at this point. The common bile duct joins the pancreatic duct to enter the ampulla of Vater.
The third part (transverse) of the duodenum begins at the right of the fourth lumbar vertebra and passes anterior to the aorta, inferior vena cava, and crura of the diaphragm. The superior mesenteric vessels course anterior to the duodenum.
The fourth part (ascending) of the duodenum ascends superiorly to the left of the spine and aorta to the second lumbar vertebra, where it joins the proximal jejunum (duodenojejunal flexure). This portion lies on the left crus of the diaphragm. It is held in place by the ligament of Treitz (which courses from the left toward the right crus of the diaphragm).
As the duodenum turns downward, it is called the jejunum ( Figure 13-4 ). The jejunum extends for about 2 m before becoming the ileum. The inner wall of the small intestine is marked by circular folds of the mucous membrane, villi. The valvulae conniventes are large folds of mucous membrane that project into the lumen of the bowel and help retard the passage of food to provide greater absorption. The lower part of the small intestine is the ileum. The ileocecal orifice marks the entry into the large intestine and prevents food from reentering the small intestine.
This is the last part of the digestive system. The large intestine is larger in diameter and shorter in length than the small intestine. The cecum and vermiform appendix, ascending colon, transverse colon, and descending colon, sigmoid colon, and rectum all make up the large intestine ( Figure 13-5 ). The haustra (singular haustrum ) of the colon are the small pouches caused by sacculation, which give the colon its segmented appearance ( Figure 13-6 ). The ascending colon extends from the cecum vertically to the lower part of the liver. It turns horizontally at the hepatic flexure and moves to become the transverse colon. On the left side of the abdomen, at the splenic flexure, it then descends vertically to become the descending colon and eventually the sigmoid colon, which empties into the rectum. The rectum is 12 cm long, terminating at the anus. The mucosa of the large intestine lacks villi and produces no digestive enzymes. The surface epithelium consists of cells specialized for absorption and goblet cells that secrete mucus.
The arteries that supply the esophagus rise from the high, middle, and lower sections of this muscular tube. The inferior thyroid branch of the subclavian artery supplies the upper esophagus, the descending thoracic aorta supplies the middle of the esophagus, and the gastric branch of the celiac axis and the left inferior phrenic artery of the abdominal aorta supply the lower end of the esophagus. Varices may be seen to rise from the gastroesophageal arteries ( Figure 13-7 ).
The vascular supply to the stomach is provided by the right gastric arterial branch, pyloric and right gastroepiploic branches of the hepatic artery, left gastroepiploic branch and vasa brevia of the splenic artery, and left gastric artery (see Figure 13-6 ). The venous system of the stomach is parallel to the arterial vessels, which drain into the portal venous system.
The mesentery outlines the small intestine and contains superior mesenteric vessels, nerves, lymphatic glands, and fat between its two layers. The celiac axis supplies the duodenum through its right gastric, gastroduodenal, and superior pancreaticoduodenal branches ( Figure 13-8 ). The superior mesenteric artery has multiple branches to the small bowel, which include the inferior pancreaticoduodenal, jejunal, and ileal arteries. The venous system parallels the arterial system and empties into the portal venous system.
The celiac, superior mesenteric, and inferior mesenteric arteries supply both the small and the large intestine. The superior mesenteric arterial branches include the ileocolic, right colic, and middle colic arteries ( Figure 13-9 ). The inferior mesenteric artery supplies the intestine from the left border of the transverse colon to the rectum, rising from the anterior surface of the abdominal aorta at the level of the third lumbar vertebra and descending retroperitoneally. Branches of the inferior mesenteric artery include the left colic, sigmoid, and superior rectal arteries. The venous system parallels the arterial system and empties into the portal venous system.
Physiology and laboratory data of the gastrointestinal tract
Digestion and absorption are the primary functions of the gastrointestinal tract. Food is ingested through the mouth, chewed, and swallowed. The molecules of food must be further digested, or mechanically broken down, and chemically split into small molecules. The chemical digestion of food breaks down long-chain organic molecules (i.e., polysaccharides or proteins). Each reaction is carried on with the help of a specific enzyme produced by cells of the digestive tract or its accessory glands. When these particles are small enough, nutrient molecules pass through the wall of the intestine into the blood or lymph system by absorption.
Nutrients are transported to the liver after they are absorbed by the blood; the liver processes and stores nutrients. Remaining nutrients in the blood are transported to cells throughout the body. Undigested and unabsorbed food is eliminated from the digestive tract by the process of defecation.
When food enters the stomach, the rugae gradually smooth out, causing the stomach to stretch and increase its capacity for food intake. Contractions of the stomach help to mix the food. The three layers of smooth muscle in the wall enable the stomach to mash and churn food and move it along through peristalsis. Large amounts of mucus are secreted in the stomach. Gastric glands secrete gastric juice containing hydrochloric acid and enzymes. Over a 3- to 4-hour period, food is converted into chyme. This soupy mixture is moved toward the pylorus and into the small intestine. Small quantities of water, salts, and lipid-soluble substances, such as alcohol, are absorbed through the stomach mucosa. The pyloric sphincter is a strong band of muscle that relaxes at the time necessary to release the food.
Villi within the small intestine increase its surface area for digestion and absorption of nutrients. If the villi were not present, food would move quickly through the intestine without time for absorption. The intestinal glands are found between the villi and secrete large amounts of fluid that serve as a medium for digestion and absorption of nutrients. The hormone gastrin, which is released by the stomach mucosa, stimulates the gastric glands to secrete. Most digestion occurs within the duodenum. Bile and enzymes from the liver and pancreas are secreted into the duodenum to act on the chyme and break down the food particles for absorption. The intestinal glands are stimulated to release their fluid mainly by local reflexes initiated when the small intestine is distended by chyme.
Other gastrointestinal hormones include cholecystokinin and secretin. Cholecystokinin is released by the presence of fat in the intestine and regulates gallbladder contraction and gastric emptying. Secretin is released from the small bowel to stimulate the secretion of bicarbonate to decrease the acid content of the intestine.
A period of 1 to 3 days or longer may be required for the journey through the large intestine. Within the large intestine, undigestible chime becomes stool as most of the remaining sodium and water are reabsorbed. Some of the undigestible remains of the chime are devoured by gut bacteria. Most of the absorption process of sodium and water occurs in the cecum.
The most common laboratory data the sonographer may come across in a patient with gastrointestinal disease relate to the presence of blood in the stool. If chronic, this blood loss can lead to anemia. Blood in the stool indicates the presence of a bleed somewhere in the gastrointestinal system. Infection would show elevation of the white blood count. An increase in the carcinoembryonic antigen is found in patients with inflammatory bowel disease.
Clinical signs and symptoms of nausea, vomiting, and diarrhea are common with gastrointestinal problems. Abdominal pain and fever may also be present with gastrointestinal conditions, such as colitis, bowel abscess, acute diverticulitis, and appendicitis.
Sonographic evaluation of the gastrointestinal tract
Visualization of the gastrointestinal tract with ultrasound may be difficult because intraluminal air produces an echogenic shadow, which prevents the sound beam from penetrating structures posteriorly. The scattering and reflection effect of gas in the gastrointestinal tract often produces an incomplete or mottled distal acoustic shadow. The rim of lucency represents the wall (i.e., intima, media, and serosa), and its periserosal fat produces the outer echogenic border of the tract wall.
The bowel wall consists of five layers ( Box 13-1 ). The odd-numbered walls (first, third, and fifth) are echogenic, and the even-numbered walls (second and fourth) are hypoechoic, with an average total thickness of 3 mm if distended and 5 mm if undistended.
Mucosa: directly contacts the intraluminal contents; lined with epithelial folds; echogenic
Submucosa: contains blood vessels and lymph channels
Muscularis: contains circular and longitudinal bands of fiber
Serosa: thin, loose layer of connective tissue
Mesothelium: covers intraperitoneal bowel loops
The technique used to observe the upper gastrointestinal tract is for the patient to drink 10 to 40 oz of water through a straw after a baseline ultrasound study of the upper abdomen is completed. The straw helps prevent ingestion of excess air when the water is consumed. The patient should be in an upright position for the examination; this causes air in the stomach to rise to the fundus of the stomach and not interfere with the ultrasound beam ( Figure 13-10 ). The lower gastrointestinal tract requires no preparation. When imaging the lower colon, it may be useful to give the patient a water enema to better delineate the colon.
The gastroesophageal junction is seen on the sagittal scan to the left of the midline as a bull’s-eye or target-shaped structure anterior to the aorta, posterior to the left lobe of the liver, and inferior to the hemidiaphragm ( Figure 13-11 ). The left lobe of the patient’s liver must be large enough to allow imaging of the gastroesophageal junction. The gastric antrum can be seen as a target shape in the midline ( Figure 13-12 ). The remainder of the stomach usually is not visualized well unless dilated with fluid ( Figure 13-13 ).
When pathology is present, the serosal layer of the normal gastric wall is seen running toward the serous side of a tumor, which allows differentiation of intramural from extraserosal tumors. If a serosal bridging layer (three layers are seen on the mucosal side of the tumor, and at least two of them are continuous with the first and second layers of normal gastric wall) is present, the tumor lies within the gastric wall. If mucosal bridging is continuous with the mucosal layers of the normal gastric wall, is intramucosal, or is deeply infiltrated, carcinoma can be excluded. The sonographer should orient the transducer vertical to the area of transition between the lesion and the stomach wall to show their relationship.
Cystic mass in the left upper quadrant.
If a patient has a cystic mass in the left upper quadrant, several measurements can be taken to determine whether the mass is the fluid-filled stomach or another mass arising from adjacent organs. The sonographer may give the patient a carbonated drink to see bubbles in the stomach, ask the clinician to place a nasogastric tube for drainage, watch for a change in the shape or size of the “stomach” mass with ingestion of fluids, alter the patient’s position by scanning in an upright or left or right lateral decubitus position, watch for peristalsis, or ask the patient to drink water to see the swirling effect.
Usually, only the gas-filled duodenal cap is seen to the right of the pancreas. As discussed previously, the duodenum is divided into the following four segments:
A superior portion that courses anteroposteriorly from the pylorus to the level of the neck of the gallbladder
A sharp bend in the duodenum into the descending portion that runs along the inferior vena cava at the level of L4
A transverse portion that passes right to left with a slight inclination upward in front of the great vessels and crura
An ascending portion that rises to the right of the aorta and reaches the upper border at L2, where at the duodenojejunal flexure, it turns forward to become the jejunum (usually not seen with ultrasound)
The duodenum can be outlined easily with water ingestion or a change in position ( Figure 13-14 ). Generally, the right lateral decubitus position allows the fluid to drain from the antrum of the stomach into the duodenum. Observation of peristalsis is useful to delineate the duodenum.