Chapter 15: Digestive System

Detailed Chapter Overview

Chapter 15 provides a definitive and expansive exploration of the entire digestive system, covering a wide range of diagnostic procedures from the salivary glands down through the entire alimentary canal, and also including the accessory organs of the biliary system. This chapter is fundamental, as it integrates complex anatomy and physiology with a diverse array of fluoroscopic and radiographic studies. The chapter begins with the salivary glands and the specialized procedure of sialography. It then systematically follows the path of digestion, detailing the dynamic process of deglutition and the esophagogram used to evaluate it. The core of the chapter is a deep dive into the upper and lower GI series, emphasizing the critical role of contrast media (both barium and water-soluble agents) and the principles of single- and double-contrast techniques. It masterfully explains how body habitus and patient positioning are used to manipulate gravity, separating overlapping structures and coating mucosal surfaces to reveal pathology. A significant portion is dedicated to the accessory organs—the liver, gallbladder, and pancreas—and the complex network of the biliary ducts. The chapter details both historical procedures like the OCG and modern, advanced interventional procedures like ERCP, highlighting the radiographer's evolving role in these therapeutic examinations. For every procedure covered, from the simplest to the most complex, rigorous protocols, patient preparation guidelines, and exacting image evaluation criteria are meticulously established. This ensures the radiographer can produce diagnostically optimal images for this vast and varied system.

In-Depth Study Guide

Part I: Salivary Glands and Pharynx

Anatomy of the Salivary Glands

The process of chemical digestion begins in the mouth with the action of saliva, which is produced by three pairs of extrinsic salivary glands. Saliva moistens food, binds it into a bolus, and contains enzymes like amylase that begin carbohydrate digestion.

• Parotid Glands: The largest pair, located anterior and inferior to the EAM, extending down to the mandibular angle. The parotid duct (Stensen's duct) opens into the oral cavity opposite the second upper molar.
• Submandibular Glands: Located inferior to the body of the mandible. Their ducts (Wharton's ducts) open on either side of the lingual frenulum.
• Sublingual Glands: The smallest pair, situated on the floor of the mouth beneath the tongue. They have numerous small ducts that open into the floor of the mouth.

Sialography

• Purpose: This specialized procedure provides a detailed view of the salivary ducts to identify abnormalities. Its primary indications include the detection of calculi (stones) or strictures (narrowing) within the ducts, which can cause pain and swelling.
• Procedure: After preliminary radiographs, the patient is given a sialogogue (a substance like lemon juice) to stimulate salivation, making the ductal opening easier to find. A small cannula or catheter is then inserted into the duct orifice, and a small amount of water-soluble iodinated contrast medium is injected under fluoroscopic observation. Key projections include a tangential view to project the parotid gland away from the mandibular ramus, and a true lateral to visualize the extent of the ductal system.

Anatomy and Physiology of the Pharynx and Esophagus

• Pharynx: This muscular tube, about 12.5 cm long, serves a dual role in both respiration and digestion. It is divided into three regions: the nasopharynx (posterior to the nose, purely respiratory), the oropharynx (posterior to the mouth), and the laryngopharynx (extending from the epiglottis to the esophagus).
• Deglutition (The Act of Swallowing): This is a complex, rapid process. The first stage is voluntary, where the tongue pushes the bolus of food into the oropharynx. The second stage is involuntary and reflexive: the soft palate closes off the nasopharynx, and the epiglottis covers the laryngeal opening to prevent aspiration, while peristaltic waves propel the bolus down the esophagus.
• Esophagus: A collapsible muscular tube, approximately 25 cm long and 2 cm in diameter, that connects the pharynx to the stomach. It lies posterior to the trachea. The esophagus exhibits two natural indentations: one from the aortic arch and one from the left primary bronchus. It passes through the diaphragm at an opening called the esophageal hiatus.

Esophagogram (Barium Swallow)

• Purpose: A dynamic, real-time study of the form and function of the swallowing mechanism and the esophagus. It is used to diagnose a wide range of conditions including GERD (gastroesophageal reflux disease), hiatal hernias, tumors, strictures, foreign bodies, and esophageal varices (dilated veins, often due to liver disease).
• Contrast: A thick, high-density barium sulfate suspension is used to thoroughly coat the esophageal mucosa. For functional studies of swallowing, thinner barium may be used.
• RAO Position (35-40 degrees): This is the single most important projection for the esophagus. In the AP/PA and lateral positions, the esophagus is largely superimposed by the thoracic spine and heart. The RAO rotation shifts the esophagus off the spine and places it perfectly within the homogenous heart shadow, providing a clear, unobstructed profile view.
• Special Maneuvers: To detect reflux, the patient may be placed in a Trendelenburg position. The Valsalva maneuver (bearing down) increases intrathoracic pressure and can demonstrate esophageal varices or small hiatal hernias that may not otherwise be visible.

Part II: The Stomach and Small Intestine

Anatomy and Physiology of the Stomach

The stomach acts as a reservoir for food, where it is mixed with gastric secretions (hydrochloric acid and pepsin) to form a semifluid mass called chyme. Its position and shape vary significantly based on body habitus.

• Divisions: Cardia (surrounds the esophageal opening), Fundus (superior portion), Body (corpus, the main central region), and Pyloric portion (distal part, including the pyloric antrum and canal). The internal folds are called rugae.
• Air-Barium Distribution (A Critical Concept for Positioning): Understanding how air and barium shift with changes in body position is essential for all UGI studies. Barium is heavier than air and is affected by gravity. Air is lighter and rises.
  • Erect: Gravity pulls the heavy barium down into the pylorus and lower body. The lighter air rises to fill the superiorly located fundus. This is why a gas bubble is normally seen in the fundus on an erect chest or abdomen x-ray.
  • Supine (AP): The patient is lying on their back. The fundus is the most posterior part of the stomach, so gravity causes the barium to pool there. The body and pylorus are more anterior, so the air rises to fill these structures, providing a double-contrast view of them.
  • Prone (PA): The patient is lying on their stomach. Now, the body and pylorus are the most dependent (gravity-affected) parts, so they fill with barium. The air rises to the more posterior fundus, creating a double-contrast view of the fundus.

Upper GI Series (UGI)

• Purpose: A detailed study of the distal esophagus, stomach, and duodenum. It is a primary tool for diagnosing peptic ulcer disease, hiatal hernias, gastritis, tumors, and pyloric stenosis.
• Patient Prep: The patient must be NPO (nothing by mouth) for a minimum of 8 hours before the exam to ensure the stomach is empty and free of food particles that could mimic pathology. Patients should also avoid smoking and chewing gum, as these activities increase gastric secretions and can dilute the barium.
• Double-Contrast Technique: This is the preferred method for most UGI exams. The patient first ingests gas-producing crystals (e.g., calcium and magnesium citrate) which create carbon dioxide gas to distend the stomach. Then, a small amount of high-density barium is ingested. By rotating the patient through various positions (supine, prone, oblique), the gas forces the thin layer of barium against the stomach walls, providing an exquisite, detailed view of the mucosal lining (rugae).
• Routine Projections: A standard overhead series after fluoroscopy includes the RAO (opens the C-loop of the duodenum and shows the pyloric canal), PA (barium-filled body/pylorus), Right Lateral (shows the retrogastric space and duodenal loop), and LPO (shows the barium-filled fundus).

Small Intestine Radiography

Small Bowel Follow-Through (SBFT)

• Purpose: A functional study designed to evaluate the motility and mucosal pattern of the entire small intestine. It is most often performed to investigate conditions like Crohn's disease (an inflammatory bowel disease), malabsorption syndromes, or to locate the site of a small bowel obstruction.
• Procedure: The patient drinks two full cups (about 16 oz) of a thin barium suspension. The time of ingestion is noted. A series of overhead PA or AP radiographs are taken at specific time intervals (e.g., 15-30 minutes). The radiologist may use fluoroscopy to check the progress and use a compression paddle to separate overlapping loops of bowel. The exam is considered complete only when the head of the barium column has been visualized reaching the ileocecal valve and flowing into the cecum. The total exam time can vary from 1 to several hours depending on the patient's motility.

Part III: The Biliary System and Accessory Organs

Anatomy and Function

• Liver: This vital, large organ, located mainly in the RUQ, has hundreds of functions. In the context of digestion, its key role is the continuous **production of bile**.
• Gallbladder: A pear-shaped sac nestled on the inferior surface of the liver. It does not produce bile; its only functions are to **store, concentrate, and release bile**.
• Bile and its Function: Bile is not an enzyme; it is an emulsifier. It breaks down large fat globules into smaller particles, increasing the surface area for enzymes to act upon, which is essential for fat digestion and absorption.
• Biliary Ductal System: Bile travels from the liver through the right and left hepatic ducts, which unite to form the **common hepatic duct**. The **cystic duct** from the gallbladder joins the common hepatic duct to form the **common bile duct**. The common bile duct joins with the main pancreatic duct at the **hepatopancreatic ampulla (ampulla of Vater)** before emptying into the duodenum. The flow is controlled by the **sphincter of Oddi**.

Radiographic Procedures

Endoscopic Retrograde Cholangiopancreatography (ERCP)

• Purpose: An advanced, invasive procedure that is both diagnostic and therapeutic. It is used to visualize the biliary and pancreatic ducts to diagnose and treat conditions like choledocholithiasis (stones in the common bile duct), strictures, leaks, and tumors.
• Procedure: Performed under sedation by a gastroenterologist, ERCP involves passing a duodenoscope through the mouth to the duodenum. The physician cannulates the ampulla of Vater and injects a water-soluble iodinated contrast agent directly into the ducts under fluoroscopic guidance.
• Radiographer's Critical Role: The radiographer is an essential member of the ERCP team. Responsibilities include preparing and operating the C-arm fluoroscopy unit, adhering to sterile technique, assisting with contrast injection, acquiring and archiving the fluoroscopic images, and providing radiation protection for the patient, physician, and staff.
• Therapeutic Interventions: The great value of ERCP is its therapeutic capability. During the procedure, the physician can perform a sphincterotomy (cutting the sphincter to widen the opening), retrieve gallstones with a basket or balloon, dilate strictures, and place plastic or metal stents to maintain ductal patency.