Chapter 4: Abdomen

Detailed Chapter Overview

Abdominal radiography provides a critical, non-invasive window into the complex workings of the digestive, urinary, and hepatobiliary systems. This chapter delves into the anatomical and procedural knowledge required to produce diagnostic images of this challenging region. Unlike the high-contrast environment of the chest, the abdomen is composed of soft tissues with similar densities, making proper technique and patient preparation paramount. A core focus of the chapter is the detailed mapping of the abdominopelvic cavity, using both the quadrant and nine-region methods to precisely localize organs and potential pathologies. Understanding the relationship between intraperitoneal and retroperitoneal structures is essential for interpreting images correctly. The chapter thoroughly covers patient preparation protocols, emphasizing the need to reduce bowel gas and fecal material that can obscure vital anatomy. It meticulously explains the rationale behind specific technical factors, such as the moderate kVp range required for soft tissue visualization and the necessity of short exposure times to combat involuntary peristaltic motion. A significant portion is dedicated to the "acute abdomen series"—a set of projections including supine, erect, and decubitus views—designed to diagnose serious conditions like bowel obstructions and pneumoperitoneum. For each projection, the text provides detailed positioning instructions and stringent evaluation criteria, training the radiographer to identify key anatomical landmarks, such as the psoas muscles and kidney outlines, which are indicators of a high-quality, diagnostic image.

In-Depth Study Guide

Abdominopelvic Cavity: Anatomy and Topography

The abdominopelvic cavity is a large, continuous space extending from the diaphragm down to the bony pelvis. For localization, it is divided using two common methods.

1. Four Quadrants:

Two imaginary perpendicular planes intersect at the umbilicus, creating four quadrants:

• Right Upper Quadrant (RUQ): Contains the liver, gallbladder, right kidney, hepatic flexure of the colon, and head of the pancreas.
• Left Upper Quadrant (LUQ): Contains the spleen, stomach, left kidney, splenic flexure of the colon, and tail of the pancreas.
• Right Lower Quadrant (RLQ): Contains the cecum, appendix, and ascending colon.
• Left Lower Quadrant (LLQ): Contains the descending and sigmoid colon.

2. Nine Regions:

A more detailed system using two horizontal and two vertical planes:

• Top Row: Right Hypochondriac, Epigastric, Left Hypochondriac.
• Middle Row: Right Lateral (Lumbar), Umbilical, Left Lateral (Lumbar).
• Bottom Row: Right Inguinal (Iliac), Pubic (Hypogastric), Left Inguinal (Iliac).

Digestive and Urinary System Anatomy

• Stomach: An expandable organ for food digestion, located in the LUQ.
• Small Intestine: About 15-20 feet long, divided into three parts: the **duodenum** (shortest, widest, C-shaped), the **jejunum** (feathery appearance), and the **ileum** (longest section).
• Large Intestine: Begins in the RLQ with the cecum and appendix, and continues as the ascending, transverse, descending, and sigmoid colon, rectum, and anus.
• Accessory Organs: The **liver** (largest solid organ, produces bile), **gallbladder** (stores bile), and **pancreas** (produces digestive enzymes and insulin).
• Urinary System: Includes two **kidneys**, two **ureters**, one **urinary bladder**, and one **urethra**.

The Peritoneum

The peritoneum is a large serous membrane that lines the abdominal cavity.

• Parietal Peritoneum: The outer layer that adheres to the abdominal wall.
• Visceral Peritoneum: The inner layer that covers the organs.
• Peritoneal Cavity: The potential space between the two layers, containing serous fluid. An abnormal accumulation of this fluid is called **ascites**.
• Mesentery and Omentum: Folds of the peritoneum that support and suspend organs. The **mesentery** is a double fold that holds the small intestine in place. The **greater omentum**, often called the "fatty apron," drapes over the transverse colon and small intestine.
• Intraperitoneal vs. Retroperitoneal:
  • Intraperitoneal Organs: Organs within the peritoneal cavity, covered by visceral peritoneum. Includes the liver, gallbladder, spleen, stomach, jejunum, ileum, and transverse colon.
  • Retroperitoneal Organs: Organs located behind the peritoneum. Includes the kidneys, ureters, adrenal glands, pancreas, duodenum, and ascending/descending colon.

Positioning Considerations and Technical Factors

Patient Preparation and Breathing:

• Preparation: For certain procedures, meticulous bowel preparation (low-residue diet, laxatives) is required to eliminate gas and fecal matter that can obscure anatomy.
• Breathing: Exposures for abdominal radiographs are always taken on **full expiration**. This raises the diaphragm to its highest position, reducing compression of the abdominal organs and providing better visualization.

Topographic Landmarks for Positioning:

• Xiphoid Process (T9-T10): Superior margin of the abdomen.
• Inferior Costal (Rib) Margin (L2-L3): Used to locate upper abdominal organs like the gallbladder and stomach.
• Iliac Crest (L4-L5): The most commonly used landmark for abdominal radiography. Corresponds to the mid-abdominopelvic region. Centering at the crest for a KUB includes most essential anatomy.
• Anterior Superior Iliac Spine (ASIS): A prominent landmark often used for positioning the pelvis and hip.

Technical Factors:

• Kilovoltage (kVp): A medium kVp range (70-80 kV) is used to provide sufficient contrast to differentiate between the various soft tissue densities (fat, muscle, organs).
• Exposure Time: Should be as short as possible to minimize motion blur caused by involuntary peristalsis.
• Grids: Essential for all abdominal radiography to reduce scatter and improve image contrast.

Routine and Special Projections: The Acute Abdomen Series

An acute abdomen series is performed to evaluate patients with sudden, severe abdominal pain. The goal is to identify bowel obstruction, perforations (free air), or other emergent conditions. A typical series includes two or three projections.

1. AP Supine (KUB - Kidneys, Ureters, Bladder)

• Purpose: The foundational survey image. Used to assess organ size and shape, identify calcifications (e.g., kidney stones), and view the overall gas pattern.
• Positioning: Patient is supine with the midsagittal plane centered to the table. The CR is perpendicular, centered to the level of the **iliac crest**. Ensure the pubic symphysis is included at the bottom of the image.
• Evaluation Criteria: Symmetric iliac wings, visible psoas muscle outlines (indicating good soft tissue contrast), and inclusion of the diaphragm superiorly and pubic symphysis inferiorly.

2. AP Erect

• Purpose: Primarily to demonstrate **air-fluid levels** within the bowel (a sign of obstruction) and **pneumoperitoneum** (free air under the diaphragm, indicating a perforated organ). The patient should be upright for a minimum of 5 minutes before exposure.
• Positioning: Patient is standing or seated fully erect. CR is horizontal, centered about 2 inches above the iliac crest to ensure the diaphragm is included.
• Evaluation Criteria: The diaphragm must be visible and clearly defined. Air-fluid levels appear as straight horizontal lines within the bowel loops.

3. Left Lateral Decubitus (AP Projection)

• Purpose: An alternative to the erect view for patients who cannot stand. It also demonstrates air-fluid levels and pneumoperitoneum.
• Positioning: Patient lies on their left side for at least 5 minutes. A pad should be placed under them to ensure the entire abdomen is visible. CR is horizontal, centered to the level of the iliac crest.
• Why Left?: Placing the patient on their left side allows any free intraperitoneal air to rise and be visualized along the lower edge of the liver in the RUQ. This prevents the gas bubble normally present in the stomach from being mistaken for free air.

Other Projections:

• Dorsal Decubitus (Lateral Projection): Patient is supine, CR is horizontal, entering the side of the patient. Useful for detecting aortic aneurysms and their anterior calcification.