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AN51.1-2 | Sectional Anatomy — Part 1
CLINICAL SCENARIO
A 45-year-old man from Pondicherry presents to the emergency department with acute severe epigastric pain radiating to the back. He is a chronic alcohol user. His serum amylase is 1,800 U/L. A contrast-enhanced CT of the abdomen is ordered.
The radiologist's report describes: "peripancreatic fat stranding at the level of L1, with the pancreatic body seen posterior to the stomach; the splenic vein and superior mesenteric vein visible at this level."
What structures are seen at the L1 (transpyloric) level on CT? Why is the pancreas vulnerable at this retroperitoneal level? How do you orient a CT cross-section?
This module teaches you to read CT and MRI cross-sections of the abdomen and pelvis — an essential skill for clinical radiology in India.
WHY THIS MATTERS
Sectional anatomy is directly applied every day in Indian hospitals:
- CT Abdomen — the most commonly ordered abdominal imaging in tertiary care; every radiologist and clinician must correlate axial slices with anatomy
- MRI Pelvis — used for staging cervical cancer, rectal cancer, and prostate cancer; pelvic midsagittal anatomy is essential
- Ultrasound guidance — sectional anatomy guides needle placement for biopsies, drains, and nerve blocks
- NMC 2024 curriculum (AN51.1–51.2) — specifically mandates identification of cross-sections at T8, T10, and L1 and midsagittal sections of the pelvis
- Laparoscopic surgery — the surgeon views anatomy as a cross-section on the screen; orientation requires solid understanding of sectional anatomy
RECALL
Before we begin, recall:
- Vertebral level landmarks: T8 = inferior vena cava pierces diaphragm; T10 = oesophagus pierces diaphragm; L1 (transpyloric plane) = midpoint between suprasternal notch and pubic symphysis; level of the pylorus, fundus of gallbladder, duodenojejunal flexure, root of mesentery, hilum of kidneys, origin of superior mesenteric artery
- Retroperitoneal structures (SAD PUCKER): Suprarenal glands, Aorta and IVC, Duodenum (2nd–4th parts), Pancreas, Ureters, Colon (ascending and descending), Kidneys, Oesophagus, Rectum
- Orientation of CT axial slices: View from below (as if looking up from the patient's feet); left side of patient = right side of image; structures are seen as they appear at that vertebral level
Part 1: Cross-Section at T8 Level (AN51.1)
T8 Level — Key Structures
At the T8 vertebral level (approximately at the level of the inferior vena cava foramen in the diaphragm):
Midline structures:
• Thoracic vertebra T8 — vertebral body (anterior) + spinal cord (in vertebral canal)
• Descending thoracic aorta — posterior to oesophagus, slightly left of midline
• Oesophagus — posterior mediastinum, between aorta and vertebral body
Right side:
• Inferior vena cava — pierces the central tendon of the diaphragm at T8; lies in the right caval foramen
• Right lobe of liver — the dome of the right lobe is at T8; the diaphragm curves upward to T8 on the right
• Right lung base — at this level, the right lung base is seen just posterior to the liver
Left side:
• Heart — the lower cardiac chambers (left and right ventricles) are still present at T8 level
• Left lobe of liver — extends to the left of midline at this level
• Spleen — the fundus of the spleen at T8-T9
• Stomach fundus — the gastric fundus is at T8-T9 in the left upper quadrant
Clinical correlation:
• IVC laceration at T8 level → massive haemorrhage (IVC carries high-flow venous blood)
• Hepatic veins drain into the IVC just below the diaphragm at T8 — important in Budd-Chiari syndrome (hepatic vein thrombosis causing liver congestion)
• Diaphragmatic rupture (road traffic accident in India) → herniation of abdominal contents into T8 level chest
Part 2: Cross-Section at T10 Level (AN51.1)
T10 Level — Key Structures
At the T10 vertebral level (the level of the oesophageal hiatus in the diaphragm):
Midline and posterior:
• T10 vertebral body + spinal cord
• Descending thoracic aorta — will pierce the diaphragm at T12
• Thoracic duct — runs between aorta and azygos vein posterior to oesophagus
Anterior structures:
• Oesophagus with the left and right vagus nerves — the oesophagus pierces the diaphragm at T10; left vagus becomes anterior; right vagus becomes posterior at this level
• Liver — still present at T10 level; right lobe is large
• Stomach (body) — the body of the stomach lies to the left; the lesser curvature faces right and upward
• Spleen — upper pole of spleen visible at T10
Clinical correlation:
• Oesophageal varices — occur at the T10 oesophageal hiatus level due to portosystemic anastomosis between left gastric vein (portal) and oesophageal veins (systemic); rupture causes haematemesis — a major cause of upper GI bleeding in Indian cirrhotic patients
• Sliding hiatal hernia — stomach herniates through T10 oesophageal hiatus into the chest; common in obese patients in India; causes GERD
• Penetrating injury to T10 level: Stab wounds at the lower thorax in India can damage the oesophagus, thoracic duct, and aorta
Part 3: Cross-Section at L1 — Transpyloric Plane (AN51.1)
L1 (Transpyloric Plane) — Key Structures
The transpyloric plane passes through L1 vertebra. It is the most important abdominal cross-sectional level because of the density of structures:
Named structures AT the transpyloric plane (L1):
• Pylorus of the stomach (hence "transpyloric")
• Fundus of the gallbladder (tip of gallbladder, visible between right lobe of liver and transverse colon)
• Duodenojejunal flexure (DJ flexure, ligament of Treitz) — left of midline, retroperitoneal
• Hilum of both kidneys — the renal pelvis, vessels, and ureter emerge at L1
• Origin of the Superior Mesenteric Artery (SMA) — arises from the aorta at L1
• Root of the mesentery — attaches to the posterior abdominal wall at L1, running from DJ flexure to right iliac fossa
• Neck and body of pancreas — the neck of the pancreas overlies the SMA and SMV at L1; the body extends to the left
• Head of pancreas — in the C-curve of the duodenum, slightly to the right of L1
Relationships at the pancreatic neck (critical for surgery):
| Structure | Position |
|---|---|
| Stomach | Anterior |
| Pancreatic neck | Between stomach (anterior) and portal vein (posterior) |
| Superior mesenteric vein (SMV) | Posterior to pancreatic neck |
| Superior mesenteric artery (SMA) | Behind SMV, anterior to aorta |
| Aorta | Posterior |
| Inferior vena cava | Right of aorta |
Clinical significance:
• Acute pancreatitis at L1: CT shows peripancreatic fat stranding at L1; the pancreatic body and SMA/SMV are seen at this level
• Pyloric stenosis (adults: peptic stricture; infants: hypertrophic pyloric stenosis) — the pylorus is palpable at the transpyloric plane
• Renal transplant: Donor kidney hilum (L1) orientation must be maintained during transplantation
• Portosystemic shunting (spleno-renal shunt in portal hypertension) — operates at the L1 level where the renal vein and splenic vein can be anastomosed