Abdomen and Gastrointestinal System

I. Anatomy & Physiology – Essentials

Abdomen

• Houses major organs: liver, gallbladder, pancreas, spleen, stomach, and intestines.

• Divided into quadrants (RUQ, LUQ, RLQ, LLQ) and regions (9-region method).

• Imaging relevance: quadrant pain localization → helps in differential diagnosis.

GI System (Basic Function)

• Ingestion → digestion → absorption → elimination.

• Normal motility is essential; any disruption leads to obstruction or ileus.

II. Imaging Considerations (What Each Modality Does Best)

Radiography (X-ray)

• Best initial tool for: bowel obstruction, perforation (free air), calcifications.

• Upright abdomen: look for air-fluid levels.

• Supine abdomen: look for gas patterns.

Computed Tomography (CT)

• Most important modality for abdominal pathology

• Excellent for: tumors, trauma, infection, vascular issues, and obstruction.

Magnetic Resonance Imaging (MRI)

• Soft-tissue contrast is superior.

• Best for: liver lesions, biliary tree (MRCP), and inflammatory bowel diseases.

Sonography

• 1st choice for: gallbladder disease, pediatric abdomen, obstetric pelvis.

• No radiation.

Nuclear Medicine

• Functional studies: GI bleeding scan, gastric emptying, biliary obstruction.

Endoscopy

• Direct visualization is best for biopsy.

• Used for: ulcers, esophagitis, polyps, and cancers.

Abdominal Tubes & Catheters

• NG tube (decompression)

• G-tube and J-tube (feeding)

• Colostomy/ileostomy (diversion)

• Drains (abscess, biliary)

III. Congenital & Hereditary Anomalies

Esophageal Atresia

• Failure of the esophagus to form a continuous tube.

• Often with tracheoesophageal fistula (TEF).

• Imaging: NG tube coils in upper pouch.

Bowel Atresia

• The ileum is the most common.

• Obstructs newborns.

• Imaging: multiple dilated loops; “double bubble” sign for duodenal obstruction.

Hypertrophic Pyloric Stenosis (HPS)

• Thickened pyloric muscle → severe vomiting in infants.

• Sonography: elongated pylorus, increased muscle thickness.

Malrotation

• Abnormal rotation of the intestines in the fetus.

• Risk of midgut volvulus (deadly).

• UGI: abnormal position of the ligament of Treitz.

Hirschsprung Disease

• Absence of ganglion cells → no peristalsis.

• The newborn fails to pass meconium.

• Imaging: narrow rectum, dilated colon above.

Meckel Diverticulum

• Remnant of the vitelline duct.

• May bleed; diagnosed on nuclear medicine Meckel scan.

Gluten-Sensitive Enteropathy (Celiac Disease)

• Immune reaction → damaged villi → malabsorption.

• Imaging: fold thickening or scalloping.

Carbohydrate Intolerance

• Usually, lactase deficiency → bloating, diarrhea.

• Not a radiographic diagnosis (clinical).

IV. Inflammatory Diseases

Esophageal Strictures

• Narrowing due to chronic irritation.

• Barium swallow: smooth or irregular narrowing.

GERD

• Chronic reflux → esophagitis.

• It can progress to Barrett's esophagus, a precancerous condition.

Peptic Ulcer Disease

• Gastric or duodenal mucosal erosion.

• Imaging: niche or crater on UGI.

Gastroenteritis

• Inflammation of the stomach + intestines.

• CT: bowel wall thickening; usually, the diagnosis is clinical.

Crohn Disease (Regional Enteritis)

• Skip lesions, transmural inflammation.

• Imaging: string sign, fistulas, cobblestone appearance.

Appendicitis

• RLQ pain is the most common emergency surgery.

• CT: dilated appendix >6mm, fat stranding.

Ulcerative Colitis

• Continuous colon inflammation starts in the rectum.

• Lead pipe colon on X-ray.

V. Esophageal Varices

• Dilated veins due to portal hypertension.

• High risk of rupture → GI bleeding.

VI. Degenerative Diseases

Herniation

• Protrusion of an organ through a weak wall.

• Hiatal hernia → stomach into chest.

Hiatal Hernia

• Sliding: GE junction displaced upward.

• Paraesophageal: stomach herniates beside the esophagus.

VII. Bowel Obstruction Disorders

Mechanical Bowel Obstruction

• Physical blockage (adhesions, hernia, tumor).

• X-ray: air-fluid levels, dilated proximal loops.

Paralytic Ileus

• No peristalsis; caused by infection, surgery, or medications.

• Uniformly dilated bowel without obstruction.

VIII. Neurogenic Diseases

Achalasia

• LES fails to relax → food stuck.

• Imaging: “bird-beak sign.”

IX. Diverticular Diseases

Esophageal Diverticula

• Outpouchings may cause dysphagia.

• Zenker’s diverticulum: posterior neck pouch.

Colonic Diverticula

• Common in the elderly.

• Diverticulitis → LLQ pain, wall thickening on CT.

X. Neoplastic Diseases

Esophageal Tumors

• Squamous cell, adenocarcinoma

• Barium: irregular narrowing or “apple core.”

Stomach Tumors

• Adenocarcinoma most common.

• CT: wall thickening, mass.

Small Bowel Neoplasms

• Rare.

• CT enterography is useful.

Colonic Polyps

• Precursors to cancer.

• Detected by colonoscopy or CT colonography.

Colon Cancer

• “Apple core” lesion on barium.

• CT staging needed.

KEY TERMS 

Esophageal & Upper GI Conditions

• Achalasia – Failure of the lower esophageal sphincter to relax → difficulty swallowing (dysphagia).

• Adenocarcinomas – Cancer arising from glandular tissue; in the GI tract, often the esophagus or stomach.

• Dysphagia – Difficulty swallowing.

• Esophageal varices – Dilated veins in the esophagus, usually from portal hypertension, risk of bleeding.

• Gastroesophageal reflux disease (GERD) – Stomach acid flows back into the esophagus → heartburn, inflammation.

• Reflux esophagitis – Inflammation of the esophagus due to acid reflux.

• Peptic ulcer – Open sore in the stomach or duodenum, often from H. pylori infection or NSAIDs.

• Hiatal hernia – Stomach pushes through the diaphragm into the chest, often worsening reflux.

• Endoscopy – Procedure to visually examine the GI tract with a flexible scope.

Small & Large Bowel Conditions

• Adynamic (Paralytic) Ileus – Non-mechanical bowel obstruction due to reduced intestinal motility.

• Mechanical bowel obstruction – Physical blockage in intestines → pain, vomiting, distention.

• Gallstone ileus – Intestinal blockage caused by a gallstone entering the bowel.

• Volvulus – Twisting of the intestine → obstruction and potential ischemia.

• Intussusception – One part of the intestine slides into another → obstruction, common in children.

• Malrotation – Abnormal rotation of the intestine during fetal development → risk of obstruction.

• Atresia – Congenital absence or closure of a normal body opening (e.g., bowel, esophagus).

Inflammatory & Infectious Disorders

• Gastroenteritis – Inflammation of the stomach and intestines → diarrhea, vomiting, abdominal pain.

• Diverticulum – Outpouching of the intestinal wall.

• Diverticulitis – Infection/inflammation of a diverticulum.

• Crohn disease / Regional enteritis / Granulomatous colitis – Chronic inflammation of the GI tract, often patchy, can affect any segment.

• Ulcerative colitis – Chronic inflammation limited to the colon, continuous lesions, ulceration.

Congenital & Developmental Conditions

• Anal agenesis / Imperforate anus – Congenital absence of a normal anal opening.

• Hirschsprung disease – Missing nerve cells in part of colon → severe constipation, bowel obstruction.

• Hypertrophic pyloric stenosis – Thickened pylorus in infants → projectile vomiting.

• Situs inversus – Organs are mirrored from the normal position.

Surgical & Supportive Procedures

• Colostomy / Ileostomy – Surgical opening of the colon or ileum to the abdominal wall for stool diversion.

Metabolic & Functional Conditions

• Carbohydrate intolerance – Difficulty digesting sugars → diarrhea, bloating.

Other GI Conditions & Tumors

• Hernia – Protrusion of an organ through its wall (e.g., inguinal, umbilical).

• Leiomyomas – Benign smooth muscle tumors; can occur in the GI tract.