Abdomen, Liver & Pancreas MRI

Abdomen, Liver & Pancreas MRI

Abdominal Visceral Anatomy & Functional Topography

Functional Hepatic Segmentation (Couinaud Classification)

Rather than relying on superficial anatomical fissures, clinical MRI evaluates the liver according to the Couinaud segmentation system, which partitions the liver into eight functionally independent segments (I through VIII). Each segment possesses its own dual vascular inflow (portal vein branch and hepatic artery branch), biliary drainage duct, and dedicated hepatic venous outflow:

• Segment I (Caudate Lobe): Positioned posteriorly between the inferior vena cava (IVC) and the fissure for the ligamentum venosum. Unlike other segments, Segment I receives vascular inflow from both the left and right portal triads and drains blood directly into the IVC via autonomous emissary veins, often sparing it from cirrhosis-induced atrophy.
• Left Hepatic Lobe: Separated from the right lobe by the middle hepatic vein (Cantlie's line). Subdivided into:
  • Segment II: Superior left lateral segment.
  • Segment III: Inferior left lateral segment.
  • Segment IV: Left medial segment (historical quadrate lobe), divided into superior Segment IVa and inferior Segment IVb.
• Right Hepatic Lobe: Divided by the right hepatic vein into anterior and posterior sectors:
  • Segment V: Inferior right anterior segment.
  • Segment VI: Inferior right posterior segment.
  • Segment VII: Superior right posterior segment.
  • Segment VIII: Superior right anterior segment.

Couinaud Functional Liver Anatomy:
Superior Plane: Segment II ➔ Segment IVa ➔ Segment VIII ➔ Segment VII
Inferior Plane: Segment III ➔ Segment IVb ➔ Segment V ➔ Segment VI
Posterior / Independent: Segment I (Caudate Lobe, direct IVC drainage)

The Biliary Tree and Pancreatic Architecture

• Biliary System: Intrahepatic bile ducts parallel the portal venules, uniting into the right and left hepatic ducts, which converge at the porta hepatis to form the common hepatic duct (CHD). The cystic duct (draining the gallbladder) joins the CHD to form the common bile duct (CBD), measuring up to 6 mm in healthy young adults.
• Pancreas: An elongated retroperitoneal organ oriented obliquely across the posterior abdominal wall from the duodenal C-loop to the splenic hilum. It is divided anatomically into the head, uncinate process (a hook-like projection curving posterior to the superior mesenteric vein and artery [SMV/SMA]), neck, body, and tail.
• Pancreatic Ductal System: The main pancreatic duct (duct of Wirsung) traverses the length of the gland, joining the common bile duct at the hepatopancreatic ampulla (ampulla of Vater) to empty into the second portion of the duodenum via the major duodenal papilla (controlled by the sphincter of Oddi). The accessory pancreatic duct (duct of Santorini) drains the upper pancreatic head into the minor papilla.

Adrenal Glands and Kidneys

• Adrenal (Suprarenal) Glands: Paired retroperitoneal endocrine organs situated superior and anteromedial to the upper renal poles within Gerota's fascia, surrounded by perinephric fat. The right adrenal resembles an inverted "V" or linear ridge abutting the IVC; the left adrenal resembles an inverted "Y" or crescent lateral to the abdominal aorta. The adrenal cortex produces corticosteroids, aldosterone, and androgens; the adrenal medulla synthesizes epinephrine and norepinephrine.
• Kidneys: Retroperitoneal organs located between T12 and L3. Composed of an outer renal cortex, inner renal medulla (pyramids), renal calyces, and the renal pelvis tapering into the ureter.

Patient Positioning, Coil Setup & Landmarking

Patient Positioning & Comfort Stabilization

1. Patient Orientation: The patient is positioned supine, head-first on the scanner table (feet-first is acceptable on modern wide-bore scanners if coil cable lengths permit, but head-first remains the standard to keep abdominal cables routed neatly toward gantry receptacles).
2. Coil Selection and Placement: A multi-channel phased-array torso/body coil is positioned over the abdomen, extending from the lower costal margin up to the mid-sternum to ensure complete coverage of the liver dome and adrenal glands.
3. Snug Coil Fastening: The torso coil must be secured snugly against the patient's anterior abdominal wall using wide elastic Velcro compression straps. A loose coil vibrates and bounces with diaphragmatic excursion, magnifying respiratory phase ghosting across the abdominal viscera.
4. Knee Bolster: A triangular or contoured foam wedge is placed under the patient's knees. Flexing the knees reduces lumbar lordosis, relaxes the rectus abdominis muscles, stabilizes abdominal wall movement, and alleviates lower back discomfort during long breath-hold sequences.

Laser Landmarking

• Centering Coordinate: The transverse laser centering crosshair is positioned at the xiphoid process (corresponding to the T9–T10 vertebral level) or the lower costal margin (L2–L3 level). Centering at the xiphoid process places the liver dome, caudate lobe, gallbladder, and pancreatic parenchyma precisely at magnet isocenter.

Respiratory Motion Suppression: Breath-Holding vs. Navigator Gating

Respiratory motion is the primary cause of image degradation in abdominal MRI. Two distinct motion-control methodologies are utilized clinically:

The End-Expiration Breath-Holding Standard

Dynamic Contrast-Enhanced (DCE) 3D T1-weighted gradient-echo sequences (e.g., LAVA, VIBE, THRIVE) require breath-holding periods lasting 15 to 22 seconds per phase.

Prospective Respiratory Triggering & Navigator Gating

High-resolution T2-weighted fast spin-echo (FSE/TSE) sequences and diffusion-weighted imaging (DWI) require minutes of acquisition time and cannot be acquired within a single breath-hold. These sequences utilize:

• Respiratory Bellows: Fastened over the area of maximum abdominal excursion (epigastrium) to monitor the respiratory wave, acquiring data only during the quiescent expiratory resting phase.
• Diaphragmatic Navigator Echoes: A 2D pencil-beam navigator pulse is placed over the dome of the right hemi-diaphragm (the lung-liver interface). The scanner continuously samples the position of the diaphragm and triggers RF acquisitions only when the liver falls within a narrow 3 to 5 mm tracking window during quiet end-expiration.

Kidneys, MR Urography and Abdominal Coverage

Kidneys lie retroperitoneally on either side of the spine, with ureters descending to the bladder. MR urography evaluates the collecting systems and ureters with techniques selected for the clinical question. Some techniques emphasize fluid without IV contrast and some use excreted contrast; do not assume all urography follows a liver protocol. Confirm requested superior and inferior coverage, coil reach, renal/contrast screening, and any prescribed hydration, medication or voiding instructions. The assistant does not choose a diuretic or force extra fluid in a patient with restrictions.

Breath-hold style and duration depend on the protocol and patient's capability. Practice the technologist's instructions before scanning and report dyspnea or inability to comply. Do not insist on complete exhalation or strap a coil so tightly that breathing is restricted. Consistent comfortable breaths often matter more than a forced maximum breath. Preserve line slack and required RF spacing while supporting the abdomen and back.