11.2 Abdominal Radiography: Acute Abdomen Series & Erect/Supine Protocols

11.2 Abdominal Radiography: Acute Abdomen Series & Erect/Supine Protocols

Abdominal radiography plays a vital role in evaluating non-traumatic and traumatic acute abdominal pain, bowel obstructions, calcifications, and gastrointestinal perforations. The standard routine abdominal projection—the AP supine view (KUB)—and the multi-view Acute Abdomen Series require meticulous technical control of patient position, central ray alignment, beam filtration, and respiration.

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Routine AP Supine Abdominal Radiography (KUB)

An AP supine abdominal radiograph is universally referred to as a KUB because it is designed to visualize the Kidneys, Ureters, and Bladder.

Technical & Positioning Parameters

1. Patient Position: Patient lies supine on the radiographic table with the midsagittal plane centered to the midline of the table/IR. Arms are placed at the sides away from the body or across the upper chest.
2. Central Ray (CR) Centering: CR is directed perpendicular to the IR at the level of the iliac crests (which corresponds to the L4-L5 intervertebral disc space).
3. Respiration Instructions: Exposure is taken at the end of full expiration. Taking the radiograph on expiration elevates the diaphragm superiorly into the thoracic cavity, compressing the abdominal viscera slightly and preventing diaphragmatic movement during exposure.
4. Image Evaluation Criteria:
   • Pubic Symphysis Inclusion: The inferior margin of the radiograph MUST include the pubic symphysis to verify complete coverage of the urinary bladder and lower pelvic cavity.
   • Soft Tissue Visualization: High-quality abdominal images display sharp outline boundaries of the bilateral psoas major muscle shadows, renal outlines, lumbar transverse processes, and the inferior margin of the liver.
   • Absence of Rotation: Evaluated by symmetrical appearance of the iliac wings, symmetric outer rib margins, and spinous processes aligned along the midline of the lumbar vertebrae.

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The Acute Abdomen Series (3-View & 2-View Protocols)

The Acute Abdomen Series (also termed the 2-view or 3-view abdominal series) is an emergency protocol indicated for severe acute abdominal pain, suspected intestinal perforation, bowel ischemia, or bowel obstruction.

Diagnostic Indications & Pathology Detection

• Free Intraperitoneal Air (Pneumoperitoneum): Air escaping from a perforated hollow viscus (e.g., peptic ulcer, perforated diverticulum) that collects underneath the diaphragm in an erect position.
• Bowel Obstruction: Differentiating mechanical obstruction (e.g., volvulus, intussusception, adhesions showing dilated bowel loops with air-fluid levels) from paralytic ileus (generalized gas distension throughout small and large intestines).
• Abdominal Air-Fluid Levels: Dynamic fluid and gas separation inside bowel loops demonstrated under a horizontal central ray.

The Standard 3-View Acute Abdomen Series Components

1. AP Supine Abdomen (KUB): Centered at the iliac crests; evaluates bowel gas distribution, soft tissue structures, organomegaly, and abdominal calcifications.
2. AP Erect Abdomen: Patient sits or stands upright for at least 5 minutes prior to exposure. CR is centered 2 inches (5 cm) above the iliac crests (or at the level of the axilla) to ensure the diaphragm and upper abdomen are included. Demonstrates air-fluid levels and subdiaphragmatic free air.
3. PA Chest (Erect): Included as a standard component of the 3-view series. CR is centered to T7. The upright PA chest is the most sensitive projection for detecting tiny amounts of pneumoperitoneum because the horizontal beam is tangential to the thin diaphragmatic domes.

Note on the 2-View Series: Consists of the AP Supine Abdomen and AP Erect Abdomen (or Left Lateral Decubitus Abdomen if the patient cannot stand).

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Left Lateral Decubitus Abdomen Protocol

When a patient is critically ill, hemodynamically unstable, or unable to stand for an erect abdominal radiograph, the Left Lateral Decubitus Abdomen projection replaces the erect view.

Technical Protocol & CR Centering

• Patient Placement: Patient lies on their LEFT side on a radiolucent pad with knees slightly flexed for stability. A horizontal central ray (parallel to the table surface) is used.
• Pre-exposure Resting Rule: Patient must remain in the left lateral decubitus position for at least 5 minutes (preferably 10 to 20 minutes) prior to exposure to allow free gas to migrate upward.
• CR Centering: CR is directed horizontal and perpendicular to the IR, centered 2 inches (5 cm) above the iliac crests to ensure inclusion of the upper abdominal borders and diaphragm.

Anatomical Rationale for LEFT Side Down

• Positioning the patient left side down causes free intraperitoneal air to rise toward the patient's right side, where it collects over the homogeneous, uniform density of the liver shadow.
• If the patient were placed right side down, free air would rise to the left side and coalesce with the normal gastric bubble in the stomach fundus and splenic flexure gas, making pneumoperitoneum virtually impossible to identify.

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Abdomen Positioning Comparison