Thorax and Abdomen
2 Abdomen
Anatomy & Pathology
Key Anatomical Landmarks
- Diaphragm – Dome-shaped muscle separating thoracic and abdominal cavities; moves downward on inspiration, upward on expiration.
- Psoas muscles – Triangular soft-tissue shadows on either side of the lumbar spine; should be visible on well-exposed images.
- Lumbar spine (L1–L5) – Central midline reference; rotation errors show as asymmetric transverse processes.
- Kidneys – Retroperitoneal; right kidney slightly lower due to liver.
- Liver – Largest solid organ in upper right quadrant; inferior margin often visible on upright abdomen.
- Spleen – Upper left quadrant, posterolateral to stomach.
- Stomach – Left upper quadrant; gas pattern and air-fluid level important in obstruction studies.
- Small intestine – Occupies central abdomen; normal gas and fluid pattern should not show distention.
- Large intestine (colon) – Frames the abdomen; includes cecum, ascending, transverse, descending, sigmoidcolon, and rectum.
- Urinary bladder – Midline structure in lower pelvis; included on all KUB images.
- Iliac crests / ASIS / symphysis pubis – Key bony landmarks for centering and collimation.
Common Pathologies
- Bowel Obstruction (Small or Large) – Air-fluid levels and distended loops of bowel visible on upright or decubitus images.
- Ileus (Paralytic) – Lack of intestinal motility; air distributed uniformly throughout small and large bowel.
- Free Intraperitoneal Air (Pneumoperitoneum) – Air beneath diaphragm on upright or decubitus views, indicating perforation.
- Ascites – Fluid accumulation in peritoneal cavity; abdomen appears diffusely opaque and distended.
Exposure adjustment: Increase mAs or kVp moderately for fluid density. - Calcifications (Gallstones, Renal Stones, Aortic Calcifications) – Localized high-density opacities.
- Soft-Tissue Masses / Organomegaly – Enlarged organs (liver, spleen, kidney) or abnormal masses may displace bowel gas.
- Pneumoperitoneum (Post-operative) – Free air may appear normally after recent surgery but should resolve within days.
Routine Projections
AP Supine Abdomen (KUB)
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine, MSP centered to table
- Adjustments: Legs extended, pelvis not rotated, arms at sides
- CR: Perpendicular at level of iliac crests to include symphysis pubis
- Pt. Instructions: Suspend respiration at end expiration
- Exposure: On expiration
Evaluation Criteria
- Coverage: Diaphragm may not be included, but symphysis pubis must be visible
- Rotation checks: Iliac wings and obturator foramina symmetrical
- Motion checks: Sharp outlines of psoas muscles and spine
- Technique checks: Proper density to show kidneys and lumbar vertebrae
- Clinical aim: Baseline image for bowel gas pattern and urinary system survey
AP Upright Abdomen
CR Location & Positioning
- SID: 40 inches
- Patient position: Erect, back to IR, MSP centered
- Adjustments: Weight evenly distributed, arms at sides
- CR: Horizontal, 2 inches above iliac crests (to include diaphragms)
- Pt. Instructions: Suspend respiration at end expiration
- Exposure: On expiration
Evaluation Criteria
- Coverage: Diaphragms must be included
- Rotation checks: Iliac wings symmetrical
- Motion checks: Clear diaphragm and bowel gas outlines
- Technique checks: Appropriate contrast to show air-fluid levels
- Clinical aim: Detect intraperitoneal air and fluid levels
Lateral Decubitus Abdomen (Left Side Down)
CR Location & Positioning
- SID: 40 inches
- Patient position: Lateral recumbent, side down (left preferred to show free air against liver)
- Adjustments: Knees slightly flexed; arms above head; MSP parallel to IR
- CR: Horizontal, 2 inches above iliac crests
- Pt. Instructions: Suspend respiration at end expiration
- Exposure: On expiration
Evaluation Criteria
- Coverage: Diaphragms visible; lateral abdomen fully included
- Rotation checks: Iliac wings nearly symmetrical
- Motion checks: Sharp diaphragm borders, no motion
- Technique checks: Adequate contrast to show free air along right lateral abdomen
- Clinical aim: Detect free intraperitoneal air or fluid when upright not possible
Dorsal Decubitus Abdomen
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine; MCP centered; arms raised above head
- Adjustments: Legs extended; IR placed adjacent to right or left side
- CR: Horizontal, 2 inches above iliac crests
- Pt. Instructions: Suspend respiration at end expiration
- Exposure: On expiration
Evaluation Criteria
- Coverage: Diaphragms included; entire abdomen visualized
- Rotation checks: Posterior ribs and iliac wings superimposed
- Motion checks: Sharp organ outlines
- Technique checks: Proper exposure for visualization of gas patterns and soft tissue
- Clinical aim: Evaluate aneurysms, hernias, or air-fluid levels in immobile patients
Acute Abdominal Series (Three-View Abdomen)
Projections:
-
AP Supine Abdomen (KUB)
-
AP Upright Abdomen (or Left Lateral Decubitus if non-ambulatory)
-
PA Upright Chest
CR Location & Positioning (Summary):
Supine – Perpendicular at iliac crest (include symphysis pubis)
Upright – Horizontal 2″ above iliac crest (include diaphragms)
Chest – Perpendicular at T7 (include apices through costophrenic angles)
Evaluation Criteria:
All three images demonstrate proper inspiration/expiration, no rotation, and complete coverage of abdominal and thoracic structures relevant to the patient’s complaint.
Hows & Whys of Abdominal Radiography
Anatomy
- How many quadrants divide the abdomen, and why?
Four quadrants (RUQ, LUQ, RLQ, LLQ) — used for localizing pain and correlating organ position. - How are the psoas muscles useful on an abdominal radiograph?
Their visibility confirms correct exposure and proper soft-tissue contrast. - How can you tell the kidneys are correctly visualized?
Each kidney should appear faintly outlined lateral to the spine at the T12–L3 level.
Positioning
- Why are some abdominal images performed upright?
To demonstrate air-fluid levels and free intraperitoneal air. - Which side is down for a decubitus abdomen, and why?
Left side down — so free air rises over the liver margin, not the gastric bubble. - Why must the symphysis pubis be included on supine KUB?
To confirm bladder inclusion and complete urinary tract coverage. - Why is the CR placed 2 inches above the iliac crest for upright and decubitus views?
To ensure diaphragms are included for detection of free air. - Why perform all abdominal projections on expiration?
Expiration elevates the diaphragm, spreading abdominal organs for better visualization.
Technique & Image Evaluation
- How do you check for rotation on a KUB?
Symmetric iliac wings and obturator foramina indicate true AP alignment. - How do you confirm adequate exposure on an abdomen?
The outlines of the psoas muscles, kidneys, and lower liver margin should be visible without overexposure. - How do you evaluate motion?
Bowel gas margins and diaphragm edges should appear sharp. - How do you adjust technique for obese or fluid-filled abdomens?
Increase kVp or mAs moderately to penetrate denser tissue. - How do you adjust technique for bowel obstruction or ileus?
Decrease mAs or kVp slightly to improve contrast between air and tissue.
Clinical Applications
- Why is the upright abdomen essential in an acute abdominal series?
It shows free air and fluid levels not visible on supine images. - Why include a chest radiograph in the acute series?
To detect free air beneath the diaphragm and rule out thoracic causes of abdominal pain. - Why use the left lateral decubitus when upright imaging is not possible?
It keeps free air adjacent to the liver, away from the gastric bubble. - Why might a dorsal decubitus projection be requested?
To assess aneurysms, hernias, or air-fluid levels when the patient cannot be turned. - Why is precise centering critical in abdominal radiography?
Improper centering may exclude key anatomy such as the symphysis pubis or diaphragms, requiring repeats and added dose.
