RADIOLOGICAL

ANATOMY OF ABDOMEN

SYNOPSISLiver

Biliary tract

Spleen

Pancreas

Kidney

Adrenal gland

Gastrointestinal tract

Gross anatomyRadiological anatomy –USG / CT / MRIAnatomical variants / Congenital anamolies

LIVER

Liver - Anatomy

Largest of all abdominal organs, commands the right upper abdominal quadrant

Great transverse measurement -20 to 26 cm Vertically measurement- 15 to 21 cm Greatest anteroposterior diameter (determined at the level of the upper right kidney) - 7 to 12 cm.

The hepatic parenchyma is surrounded by a dense layer of connective tissue forming the liver capsule.

The liver’s

convex diaphragmatic surface

concave visceral surfaces.

Bare area , Fossa of the gallbladder, fossa of the inferior vena cava (IVC), and the suprarenal impression are not covered by peritoneum.

• Two sagitally oriented fissures linked centrally by transverse porta hepatis , form the letter H on visceral surface.

• Left fissure – Fissure for round ligament anteriorly and fissure for ligamemtumvenosum posteriorly.

• Right fissure – Fossa for GB anteriorly and groove for IVC posteriorly

Functional segmental anatomy Centrally located in each of the hepatic segments

segmental branch of the portal vein

hepatic artery

segmental bile duct.

The distal hepatic veins lie

between the individual

segments.

BISMUTH-COUINAUD SYSTEM Divides the liver into 8 functionally independent

segments.

Each segment has its own vascular inflow , outflow and biliary drainage.

Centre – Hepatic artery , portal vein and bile duct.

Periphery – Vascular outflow through hepatic veins.

Segment I -caudate lobe.

Receives branches from both the main portal vein and its right and left branches - portal trinity.

Does not drain into the hepatic veins but directly into the IVC.

All remaining liver segments (II to VIII) are defined by their positions relative to branches of the portal and hepatic veins.

Left portal vein

Gives off a caudate branch.Divides into its terminal branches - left lateral

and left medial portal venous branches.

The left lateral portal venous branch supplies superior segment II, located lateral to the left hepatic vein and above the portal venous plane.

The left medial portal venous branch supplies inferior segment III located laterally to the left hepatic vein and beneath the portal venous plane as well as segment IV.

• Segment IV is delineated

• Medially - middle hepatic vein and

• Laterally - left hepatic vein

• Subdivided into a superior segment IVa and an inferior segment IVb in regard to the portal venous plane.

Right portal vein

The right anterior portal venous branch

anterior-inferior segment V

anterior-superior segment VIII

The right posterior portal venous branch

posterior-inferior segment VI

posterior-superior segment VII

LIVER ANATOMY VARIANTS

• Horizontal elongation of the lateral segment (bismuth-couinaud segment II) of the left hepatic lobe, which can extend into the left upper abdominal quadrant and eventually abut or even wrap around the splenic contour.

• Riedel lobe - vertical elongation of the right lobe.

• Differentiated from extracapsular extension caused by a liver tumor ( hepatic adenoma or metastasis).

Precontrast T1-weighted hepatic MRI in a 77-year-old woman with Riedel’s lobe resulting from a prominent inferiorly positioned narrow right lobe of the liver that significantly extends the expected confines of the liver.

BILIARY TRACT AND GALL BLADDER

Biliarycanaliculi

Segmental bile duct

Common hepatic duct

Common bile duct

Ampulla of vater

Cystic duct

Pancreatic duct

IntraHepticDucts

On computed tomography , normal intra hepatic bile ducts appear as linear water-density structures accompanying the portal vein branches.

Normal IHDs measure less than 3 mm.

They appear to be randomly scattered throughout the liver but are confluent toward the hilum.

The IHDs from each lobe unite to form the right and left main hepatic ducts, which are located anterior to the portal veins .

Common hepatic duct

The right and left main hepatic ducts unite in the hilum to form the common hepatic duct (CHD).

The CHD usually courses along a 45-degree oblique plane with reference to the midline sagittal plane, which lies to the right and lateral to the proper hepatic artery.

On CT the CHD usually measures 3 to 6 mm in short-axis diameter

Common Hepatic Duct at hilum

Common bile duct The common bile duct (CBD) forms when the

cystic duct joins the CHD.

This union occurs at varying levels, from high in the porta hepatis to near the ampulla of Vater.

Because the union is usually not demonstrated on CT, the term common duct is used when the CHD and CBD cannot be differentiated.

Intrapancreatic Common BileDuct

THE UNION IS USUALLY NOT DEMONSTRATED ON CT, THE TERM COMMON DUCT IS USED

WHEN THE CHD AND CBD CANNOT BE DIFFERENTIATED.

Ampulla of Vater

The CBD enters the pancreas and typically lies along the posterior and lateral aspect of the pancreatic head.

The distal CBD and main pancreatic duct come into contact on the medial side of the descending part of the duodenum.

The two ducts pass separately through the wall of the duodenum and unite to form a short dilated tube—the ampulla of Vater

• The sphincter of Oddi is the circular muscle complex around the CBD, pancreatic duct, and ampulla of Vater; it consists of the

• sphincter choledochus

• sphincter pancreaticus and

• sphincter ampullae

On endoscopic retrograde cholangio pancreatography, the ampullary segment is usually not visualized

because of the contraction of the sphincter of Odd.

Gall bladder• The gallbladder is a blind

pouch lying along the undersurface of the liver.

• The normal gallbladder wall thickness ranges from 1 to 3.5 mm. On US, 3 mm might be a reasonable upper limit of normal.

USG anatomy

Normal GB wall appears s a pencil thin echogenic line at sonography

Minimum 6 hrs fasting

Subcoastal or intercostalapproach

Supine – LLD

GB wall - <3mm

Transverse diameter - <4cm

CT Anatomy

• On transverse CT images, the gallbladder is a rounded structure with a maximum diameter of less than 4 to 5 cm in the distended state.

• Visualization of the gallbladder wall depends on the degree of gallbladder distention and the presence of abnormality.

• Enhancement of the gallbladder wall on CT and MRI is normal after the intravenous administration of contrast medium.

• The density of the gallbladder lumen is generally that of water (0-20 Hounsfield units [HU]).

• After intravenous contrast administration, an increase in density is observed on CT.

Normal GB wall apperas as a thin rim of soft tissue density that

enhances on contrast administration.

Congenital anamolies

Phrygian Cap

The most common anomaly of the entire biliary tree

Septation in the distal fundus of the gallbladder, which results in the configuration called a phrygian cap.

In the retroserosal or concealed type, the mucosal fold projecting into the lumen may not be visible externally.

In the serosal or visible type, the peritoneum follows the bend in the fundusand then reflects on itself as the fundusoverlies the body.

Ectopic gall bladder

The gallbladder can be located in various positions

PANCREAS

PANCREAS

Retroperitoneal organ

Pancreas is an exocrine and endocrine organ

Approximately 15 cm long

Related to the stomach, duodenum, colon, and spleen.

USG anatomy

CT ANATOMY - PANCREAS

The density of the nonenhanced pancreas is normally the same as that of soft tissue, between 30 and 50 HU. It increases to 100 to 150 HU after intravenous administration of iodine-based contrast agents.

Homogeneous enhancement of the normal gland is a useful sign for excluding necrosis in pancreatitis.

PANCREAS LIES OBLIQUE , HENCE ALL PARTS ARE NOT AT THE SAME

TRANSVERSE LEVEL

MRI ANATOMY - PANCREAS

On T1-weighted images, the normal gland, owing to the aqueous protein content, reveals higher signal intensity than nonfatty tissue such as liver and muscle.

On fat-suppressed T1-weighted sequences, the relatively high signal intensity of the pancreas increases

On T2-weighted sequences, the normal pancreas is slightly hyperintense to muscle, whereas on fat-suppressed T2-weighted images, the contrast between the normal pancreas and surrounding suppressed fat is minimal

Being a very vascular organ, the pancreas shows intense contrast enhancement in the arterial phase, followed by a rapid washout (

Variant anatomy

PANCREAS DIVISUM • Failure of fusion of the dorsal and ventral ducts • Separate drainage into duodenum - Predominant

drainage occurs through dorsal duct system (duct of Santorini).

• Classified into three types– I (classic form): complete lack of fusion between

dorsal and ventral duct systems – II: absent duct of Wirsung– III: small communicating branch connects dorsal and

ventral duct systems

Annular Pancreas portion of pancreatic tissue in continuity with the

head that partially or completely circumscribes duodenum.

encircles second portion of duodenum

associated with duodenal anomalies such as atresia, atrophy, or stenosis .

discovered in childhood because of upper GI obstruction

Agenesis of pancreas

Agenesis of pancreas

Complete agenesis

Incompatible with life

Partial agenesis

Agenesis of ventral pancreas

Agenesis of dorsal pancreas

Complete agenesis

Partial

agenesis

Complete agenesis of the dorsal pancreas: body and tail of pancreas and whole dorsal duct system, including minor papilla and accessory duct, are absent.

Partial agenesis of the dorsal pancreas: distal part of pancreatic body or at least a remnant of accessory duct and minor papilla are found.

the rounded head of the pancreas (arrow) and the absence of the neck and body.

the abnormal position of the bowel loops (arrowheads) behind the stomach.

polysplenia

SPLEEN

SPLEEN

• Lies within the left upper quadrant.

• Weighs 100 to 200 g

• The max craniocaudallength is 12 cm.

• The normal spleen may have rib notching and clefts that should not be confused with lacerations in patients who have experienced trauma.

The spleen is a network of white and red pulp.

The white pulp consists of lymphocytes, plasma cells, and macrophages.

The red pulp contains splenic cords, splenicsinuses, terminal branches of the central arteries, and pulp veins.

The visceral surface of the spleen is adjacent to the stomach, left kidney, splenic flexure of the

colon, and tail of the pancreas.

Spleen – USG anatomy

Best assessed in left lateral position with left arm behind the head.

Visualised best obliquely in 9th or 10th intercostal space.

Higher echogenicity than liver.

CT anatomy

On unenhanced CT scans, the normal splenicparenchyma is homogeneous - it measures 40 to 60 Hounsfield units (HU), usually 5 to 10 HU less than the normal liver.

ROI measurement is placed within the liver and spleen. If spleen measures 10HU or more than the liver – fatty

infiltrate is indicated.

VZAfter intravenous (IV)

contrast enhancement, the spleen can have a heterogeneous appearance on early arterial-phase images on both CT and MRI.

This appearance is believed to be due to the differential enhancement of red and white pulp

Patterns of spleen enhancement in arterial phase

MRI anatomy

T1-weighted MRI the normal spleen has a signal intensity equal to or less than that of normal liver.

T2-weighted images, the spleen has uniformly high signal intensity.

Splenic Variants

Splenic cleft

Easily recognized because of their sharp, smooth borders.

Typically located superiorly and medially.

Splenic clefts are not associated with perisplenicedema ,seen with spleniclaceration.

• Accessory spleen represents normal splenic tissue in ectopic sites

• Arising from failure of fusion of some of the multiple buds of splenic tissue in the dorsal mesogastrium during embryologic life.

• They are typically located near the splenic hilum but can be found anywhere in the peritoneal cavity

Wandering spleen - or ectopic spleen is a rare entity whereby the spleen migrates from its normal site in the left upper quadrant.

Nonunion of the peritoneum of the lesser and greater sacs, creating a longer splenicmesentery and highly mobile spleen.

IMAGING TECHNIQUES Computed Tomography Single breath hold and IV administration of a contrast

agent. 90-ml bolus of nonionic contrast agent is administered

at a rate of 2 to 3 ml/second. Scanning is performed during the portal venous phase

at 60 seconds after the bolus is given. This approach typically provides uniform enhancement of the spleen and enhancement of the liver during the portal venous phase.

In any patient being evaluated for trauma, delayed scans taken 2.5 to 3 minutes after the bolus can often exclude lacerations of the spleen or other abdominal organs.

KIDNEY

Kidney - anatomy

Paired retroperitoneal organ.

Located on posterior abdominal wall.

Lies between T12 to L3.

Size

Adult male – 10 to 14cm

Adult female – 9-13cm

KIDNEYS

Anterior renal fascia(gerota’s fascia)

Posterior renal fascia(Zuckerkandl’s fascia)

The renal fascial layers divide the general retroperitoneal space into three compartments extending from the diaphragm to the pelvic brim—the anterior pararenal space, the perinephric space, and the posterior pararenal space

PERINEPHRIC SPACE

Contains the kidney, adrenal gland, renal pelvis, proximal ureter, renal blood vessels, renal capsular vessels, and perinephric fat.

It is bounded by the anterior and posterior renal fascial layers and is demarcated by their sites of fusion.

Above -- two fascial layers fuse and adhere firmly to the diaphragmatic fascia;

Laterally -- the layers fuse behind the ascending or descending colon to form the lateroconal fascia

Medially -- the anterior renal fascia blends into the connective tissue near the midline . posterior renal fascia fuses with the psoas or quadratuslumborum fascia.

Inferiorly -- fascial cone forming a caudal extension of the main perinephric space containing the proximal ureter and gonadal vessels

Lateroconal fascia

Anterior renal fascia(gerota’s fascia)

Posterior renal fascia(Zuckerkandl’s fascia)

Kidney – USG anatomy

Cortex is less echogenic than liver and more echogenic than medulla.

Renal sinus consisting of calyces , renal pelvis and fat appears echogenic than cortex.

CT ANATOMY• CT protocol for evaluation of the kidneys

consists of both non enhanced and contrast-enhanced CT scans obtained in suspended respiration to overcome the motion artifact.

• Non enhanced CT scans the normal renal parenchyma has an attenuation value of 30 to 50 hounsfield units (HU), depending on patient hydration, and the cortex and medulla show no visible density differences.

• Nonenhanced scans

permit contrast enhancement of a renal lesion to be measured

ensure that renal parenchymal calcifications, renal calculi, renal and perinephric hemorrhage and fat, and calcification in a renal mass will not be obscured by contrast medium.

• The accuracy of attenuation values should also be tested by measuring the attenuation value of the gallbladder contents before and after IV administration of contrast medium.

Corticomedullary phase

Corticomedullary phase occurs between 25 and 70 seconds after the start of contrast administration.

Renal cortex can be differentiated from renal medulla at this stage because (1) the vascularity of the cortex is greater than that of the medulla, and (2) contrast material has not yet reached the distal aspect of the renal tubules

Maximal opacification of the renal vein and arteries occurs during this phase, allowing confident diagnosis of tumor extension to the vein.

CORTICAL NEPHROGRAM

Nephrographic Phase.

The nephrographic phase starts about 80 seconds and lasts up to 180 seconds after the start of injection.

Offers the best opportunity for discrimination between the normal renal medulla and a renal mass

The nephrographic phase is the most valuable for detecting renal masses and characterizing indeterminate lesions

Excretory Phase

Approximately 180 seconds after the start of contrast injection, the excretory phase begins.

The contrast material is excreted into the collecting system, so the attenuation of the nephrogramprogressively .

Delineate the relationship of a centrally located mass with the collecting system. Evaluating urothelial masses.

Normal renal MRI.

A T1-weighted (gradient opposed-phase) image

T2-weighted axial image (B) demonstrate the normal appearance of the kidneys

T1-weighted axial Gd-enhanced image reveals normal enhancing renal

parenchyma

Congenital anamolies

Horse shoe kidney : MC renal fusion anamoly.

Renal ectopia - Abnormal anatomical location of one or both the kidney.

Adrenal gland

Retroperitoneal structures Normal adrenal glands have a characteristic

inverted Y, V, or T shape Right adrenal gland is typically superior to the

right kidney. Left adrenal gland is usually anterior to the

superior pole of the left kidney. The adrenals are typically described as having

a central body and two (medial and lateral) limbs. The body and limbs are typically smoothly shaped and measure less than 10 mm in thickness; the limbs can measure up to 4 cm in length

CT anatomy

Normal adrenal glands. Axial and coronal contrast-enhanced CT demonstrates typical location and appearance of the right

and left adrenal glands

MRI anatomy

Axial contrast-enhanced T1-weighted MRI shows normal thickness of the adrenal body and limbs (<1 cm) and normal

length (<4 cm).

GI Tract

Gastrointestinal TractStomach These areas of the stomach

cannot always be well demonstrated on axial CT or MRI,

so coronal or sagittal imaging planes are very helpful to

understand the exact anatomic location of the lesion .

Normal gastric wall is 2 to 5 mm thick, with 10 mm being the upper limit.

In an air-filled stomach - 3mm or 5mm

On CT or MRI, the normal gastric wall enhances homogeneously but shows a two- or three-layered structure .

Inner layer- mucosal layer, enhances markedly.

Intermediate layer - submucosallayer low attenuation

Outer layer – muscular serosallayer of slightly higher attenuation

Normal gastric wall of the stomach (S) with layered appearance.

In a suboptimally distended stomach, false-positive gastric wall thickening can frequently

occur.

When air techniques are used the right decubitus position -proximal stomach

and GEJ

the left decubitus position -distal stomach and duodenum

[reverse of the patient’s position when barium or water is used].

Left posterior oblique position - gastric antrum

Prone position - gastric fundus

SMALL INTESTINE

CT

Normal small bowel wall thickness - 2 to 3 mm

In the terminal ileum where 5 mm is considered the upper limit of normal.

The thickness of valvulae conniventesshould not exceed 3 mm.

Normal coronal CT enterographyimage after oral administration of

Gastrografin as a positive oral contrast.

Normal coronal CT enterographyimage after oral administration of water as a neutral oral contrast.

Colon and Rectum

Colon is visualized on CT by itsanatomic location

typical haustral morphology

Ringlike or tubular structure, depending on the orientation and position relative to the scanning plane.

The wall of the normal colon measures 3 mm or less in thickness when the colon is distended with oral contrast material.

• Caecum - Recognition of the cecum is facilitated by visualization of the terminal ileum, ileocecal valve, or appendix.

Appendix - The appendix appears as a small ringlike or tubular structure. The presence of air or contrast material in the appendix along with normal-appearing surrounding fat may be indicative of absence of appendicitis.

• The ascending and descending colons are within the anterior pararenal space and usually are surrounded by homogeneous fatty tissue.

• The rectum is about 12 to 15 cm in length.

• The peritoneum covers the anterior surface of the upper rectum

• Lower two thirds of the rectum are enveloped by extraperitoneal connective and adipose tissue.