Abdomen and Pelvis Anatomy - Structures and Variants

Overview

• Recognition of normal anatomical variants is essential for the interpreting radiologist.
• Failure to identify a variant may lead to misdiagnosis of pathology, incorrect surgical planning, or inadvertent intraoperative injury.
• Variants are particularly consequential in hepatic resection, liver transplantation, biliary surgery, and vascular intervention.

Major Abdominal Aorta and Branches

Standard Vertebral Origins

• The proximal aorta measures ~2.3 cm in men and ~1.9 cm in women, tapering caudally.
• Spectral Doppler demonstrates a triphasic waveform.

Coeliac Trunk Variants

The coeliac trunk classically trifurcates into the left gastric, splenic, and common hepatic arteries; this is present in only ~55-65% of individuals. Recognised variants include:

• Hepatosplenic trunk with separate left gastric artery
• Hepatogastric trunk with separate splenic artery
• Coeliacomesenteric trunk: coeliac axis and SMA share a common origin, occlusion jeopardises both foregut and midgut simultaneously
• All three branches arising independently from the aorta (absent coeliac trunk)
• Arc of Buehler: persistent ventral anastomosis between coeliac and SMA, rare but haemodynamically significant

Inferior Vena Cava Variants

• The left renal vein is approximately three times the length of the right.
• In its standard course it passes anterior to the aorta and posterior to the SMA.
• When the IVC is absent above the renal veins, the azygos vein replaces it, passing posterior to the diaphragmatic crura into the chest, while hepatic veins drain directly to a cardiac atrium.

Hepatic Vasculature Variants

Hepatic Arterial Variants

• The common hepatic artery is one of three major branches of the coeliac axis.
• After giving off the gastroduodenal artery, the main hepatic artery continues and divides into right and left hepatic arteries.
• Hepatic arterial supply has the highest variant frequency of any abdominal organ system.

The Michels classification (10 types) is most widely used; clinically relevant variants:

• A replaced RHA from the SMA typically courses in the hepatoduodenal ligament posterior to the bile duct, critical during cholecystectomy and Whipple procedure.
• A replaced LHA from the LGA travels in the gastrohepatic ligament.

Portal Vein Variants

The portal vein divides into right and left branches at the hepatic hilum; variations are infrequent but surgically important:

• Trifurcation of the main portal vein into right anterior, right posterior, and left branches (~10-15%)
• Early origin of the right posterior branch from the main trunk before the standard bifurcation
• Early branches arising from the main trunk may complicate hepatic resection

Hepatic Vein Variants

Three major hepatic veins (right, middle, left) drain to the IVC in ~70% of cases. In the remaining ~30%:

• Accessory inferior right hepatic veins draining directly to the IVC (not via the main right hepatic vein) are seen in up to 30% of individuals and must be identified before right hepatectomy.
• The corrected sinusoidal pressure (wedged hepatic vein pressure minus IVC pressure) is normally 4-8 mmHg.

Biliary Tree Variants

Normal biliary anatomy is present in only ~58% of individuals.

Intrahepatic Duct Variants

• The caudate lobe (segment 1) duct drains into either the right or left hepatic duct and may have dual drainage.
• The right hepatic duct drains segments 5-8 (right anterior duct: segments 5 and 8; right posterior duct: segments 6 and 7); the left hepatic duct drains segments 2, 3, and 4.
• Normal intrahepatic bile ducts do not exceed 2 mm in the central liver or 40% of the diameter of the adjacent portal vein.
• The normal extrahepatic CBD does not exceed 6-7 mm internal diameter on ultrasound.
• Ducts appear larger on contrast cholangiography due to injection distension and radiographic magnification.

Cystic Duct Variants

These are particularly important during cholecystectomy and biliary intervention:

• Low insertion: cystic duct joins CBD near the ampulla rather than mid-hepatoduodenal ligament
• Long parallel course alongside the common hepatic duct before joining
• Medial insertion into the CBD rather than the standard lateral insertion
• Spiral course (anterior or posterior) around the CHD
• Short cystic duct or, rarely, direct gallbladder insertion into the common hepatic duct

The common bile duct joins the pancreatic duct at an angle of ~60° at the hepatopancreatic ampulla (of Vater), opening at the major duodenal papilla on the posteromedial wall of the second part of the duodenum, ~10 cm from the pylorus.

Splanchnic Arteries and Veins

Splenic Artery

• The splenic artery supplies the neck, body, and tail of the pancreas; a prominent branch is the arteria pancreatica magna.
• It characteristically courses tortuously along the superior border of the pancreas.

Notable variants:

• Early branching into superior and inferior terminal divisions (up to 3-4 cm from the hilum), relevant for splenectomy and distal pancreatectomy
• Superior polar artery to the upper pole of the spleen
• Short gastric arteries (usually 3-6) represent terminal splenic branches

Inferior Mesenteric Vein

The IMV ascends, receives the left colic and sigmoid tributaries, and normally drains into the splenic vein posterior to the pancreatic body. Variants:

• Terminates at the portosplenic confluence (junction of splenic vein and SMV)
• Drains directly into the SMV

This variation is relevant during pancreatic surgery and left colectomy.

Superior Mesenteric Artery Variants

• Origin may be shared with the coeliac trunk (coeliacomesenteric trunk)
• Middle colic artery may arise as two separate vessels or be absent with collateral supply from the marginal artery

Pancreatic Variants

Pancreatic Divisum

The most common congenital pancreatic variant (~5-10%). Failure of fusion of the dorsal and ventral pancreatic ducts results in:

• Dominant dorsal duct (duct of Santorini) draining most of the pancreas through the minor papilla (~2 cm proximal to the major papilla)
• Smaller ventral duct (duct of Wirsung) draining only the head/uncinate through the major papilla
• On MRCP, the dorsal duct courses to the minor papilla without communication with the ventral duct.
• May predispose to recurrent acute pancreatitis due to the relatively narrow minor papilla.
• Secretin-enhanced MRCP improves dorsal duct visualisation.

Annular Pancreas

Failure of rotation/regression of the ventral pancreatic bud produces a ring of pancreatic tissue encircling the second part of the duodenum, potentially causing obstruction.

Ansa Pancreatica

An inferior loop of the main pancreatic duct (MPD) that may mimic a dilated side branch on MRCP.

Normal Pancreatic Duct Calibre

Pancreatic Blood Supply

• The head is supplied by the superior and inferior pancreaticoduodenal arteries.
• The neck, body, and tail are supplied principally by the splenic artery.
• Venous drainage is via the splenic vein (body/tail), superior pancreaticoduodenal vein into the portal vein (head, upper), and inferior pancreaticoduodenal vein into the SMV (head, lower/uncinate).

Renal Arteries

Normal Renal Artery Anatomy

• Each kidney receives a single renal artery, a lateral branch of the abdominal aorta, arising just inferior to the SMA origin at approximately L1-L2.
• The left renal artery usually arises slightly higher than the right; the right renal artery is longer and passes posterior to the IVC.

Each artery divides near the hilum into anterior and posterior divisions:

• Posterior division → posterior segment
• Anterior division → apical, upper, middle, and lower segments
• There are always five renal arterial segments with no collateral circulation between them.
• The pattern of branching may vary but the segments are constant.
• The usual order of structures at the renal hilum (anterior to posterior) is: vein, artery, ureter.

Accessory Renal Arteries

Accessory (supernumerary) renal arteries occur in approximately 20-30% of individuals. Key features:

• Arise from the lateral abdominal aorta, either above or below the primary renal artery
• May enter the hilum alongside the main artery or pass directly into the renal parenchyma at the poles (polar arteries, particularly lower polar)
• Represent persistence of fetal segmental vessels; each is a true end-artery with no collateral circulation
• Accessory lower pole arteries crossing anterior to the ureter may cause ureteropelvic junction (UPJ) obstruction
• Not usually accompanied by veins
• Critical to identify before nephrectomy, partial nephrectomy, and renal transplantation

Renal Veins

• Multiple renal veins contribute to the formation of the left and right renal veins; both are anterior to the renal arteries.
• The left renal vein is ~3× the length of the right and crosses the midline anterior to the abdominal aorta.

Renal venous variants:

Unlike the renal arteries, veins from the renal segments communicate with one another and form five or six vessels that unite at the hilum into a single renal vein.

Urogenital Tract, Male

Renal Collecting System

• Duplex collecting system (~1% of population): the most common congenital urinary tract anomaly; may be partial (bifid renal pelvis fusing before the UPJ) or complete (two separate ureters with separate vesical orifices). The Weigert-Meyer rule applies: the upper moiety ureter inserts ectopically (caudal and medial), while the lower moiety ureter inserts in the normal orthotopic position.
• Bifid renal pelvis: partial duplication with two pelvises fusing into a single ureter at variable levels, a normal variant of no clinical significance in the majority.

Ureter

• Retroaortic (circumcaval) ureter: the right ureter passes posterior to the IVC due to abnormal persistence of the right subcardinal vein. Produces a "reversed J" or "fish-hook" deformity on IVP/CT urography; may cause right-sided hydronephrosis.
• Y-shaped ureter and ureteral bifurcation variants also described.

Male Gonadal Vasculature

• Testicular arteries arise from the anterolateral aorta below the renal arteries (approximately L2-L3).
• Variant origins: may arise from the renal artery, an accessory renal artery, or (rarely) the IMA.
• The pampiniform plexus drains:
  • Right: directly into the IVC at an oblique angle
  • Left: into the left renal vein at a right angle, this angulation predisposes to higher venous pressure and left-sided varicocele

Urogenital Tract, Female

Uterine Artery

• Arises from the anterior division of the internal iliac artery, courses medially and anteriorly in the base of the broad ligament to reach the cervix, then ascends along the lateral margin of the uterus to anastomose with the ovarian artery at the uterine tube.
• Key relationship: the uterine artery crosses superior to the ureter at the level of the internal cervical os ("water under the bridge"), critical during hysterectomy.
• Variant origins include: common trunk with the superior vesical artery, or origin from the internal pudendal artery.
• The uterine artery is the major blood supply to the uterus and enlarges significantly during pregnancy.

Ovarian Arteries

• Normally arise from the anterolateral abdominal aorta below the renal arteries, descend to cross the pelvic inlet, and anastomose with terminal branches of the uterine artery.
• Variant origins: from the renal artery, from an accessory renal artery, or asymmetric origin (one from the aorta, one from the renal artery).
• Ovarian veins drain: right to IVC, left to left renal vein (parallel to male gonadal venous drainage).
• Ovarian vein incompetence and reflux is associated with pelvic congestion syndrome.
• The ovarian arteries enlarge significantly during pregnancy to augment uterine blood supply.

Female Collecting System

• Duplex collecting system occurs with similar frequency to males.
• In females, an ectopic upper moiety ureter may drain below the external sphincter (urethral or vaginal), causing continuous dribbling incontinence (in contrast to males, where the ectopic ureter always inserts above the external sphincter, so continence is maintained).

Imaging Recognition of Key Variants

Clinical and Radiological Pitfalls

• A replaced RHA posterior to the common bile duct may be injured during laparoscopic cholecystectomy or Whipple procedure if not identified pre-operatively.
• A retroaortic left renal vein may be mistaken for an enlarged lymph node and may complicate aortic surgery, left nephrectomy, and living donor nephrectomy.
• Accessory lower pole renal arteries crossing the UPJ are a surgically correctable cause of hydronephrosis and must be reported on CT urography or CTA.
• Cystic duct variants (low insertion, parallel course, medial insertion, spiral course) dramatically increase the risk of biliary duct injury during cholecystectomy.
• IVC duplication may be misidentified as bilateral lymphadenopathy on non-contrast CT.
• Pancreatic divisum may be missed on standard MRCP; secretin-enhanced MRCP is required for optimal dorsal duct visualisation.
• A coeliacomesenteric trunk means a single occlusion (e.g. median arcuate ligament syndrome) may simultaneously jeopardise both foregut and midgut.
• Anomalous biliary anatomy (right posterior duct draining to LHD, triple confluence) must be identified before hepatic resection to avoid inadvertent biliary transection.
• The nutcracker phenomenon occurs when the left renal vein is compressed between the aorta and SMA (in retroaortic variants or when the aorto-mesenteric angle is reduced); presents with haematuria and left flank pain.
• In females, an ectopic upper moiety ureter inserting below the external sphincter causes continuous incontinence despite normal voiding, distinguishing it from stress incontinence.
• Hepatic vein accessory drainage (accessory inferior right hepatic veins to IVC) must be identified pre-operatively before right hepatectomy to avoid uncontrolled haemorrhage.