ANATOMY OF ABDOMEN AND PELVIS1

The standard projections requested for abdominal radiographs are supine, erect and lateral decubitus. The outline of abdominal structures depends on the differences between their densities. These differences are less apparent on the abdominal radiograph as most soft tissue structures are of similar density. However the fat surrounding the organs delineate them.

Functional segmental anatomy of liver is based on distribution of three hepatic veins. Middle hepatic vein divides the liver into right and left lobes. Left hepatic vein divides the left lobe into medial and lateral parts. Right hepatic vein divides the right lobe into the anterior and posterior parts. An imaginary transverse line through the right and left portal vein divides these parts into superior and inferior segments which are numbered counterclockwise from the inferior vena cava.

It develops during the fourth week of gestation as the second endodermal diverticulum from foregut. The dorsal diverticulum forms the dorsal pancreas. Ventral diverticulum forms the ventral pancreas as well as the liver, gallbladder and bile ducts.

Spleen is formed during fifth week of gestational age from mesenchymal cells between layers of dorsal mesogastrium. Accessory spleen can be seen in 10–30% of patients. Spleen can even be attached to left testis or ovary as there is a close relationship between the left gonadal anlage and the splenic precursor mesenchymal cells (Spleno-Gonadal fusion). It has a weight up to 200 g and a length of 11 cm. The CT value of spleen is 5 HU less than the liver in plain scans (Figs 9 to 15).

The gastrointestinal system (Figs 9 to 22) originates from a pouch like extension of yolk sac starting from 6 weeks of gestational age. The foregut is supplied by celiac artery, midgut by superior mesenteric artery and the hindgut by inferior mesenteric artery.

Peritoneal spaces above transverse colon are:

Peritoneal spaces below transverse colon are:

Important Pelvic ligaments in relation to uterus are:

Normal liver parenchyma appears homogeneous and almost equal or mildly hyperintense in relation to spleen on T1W images (T1WI). Intravenous contrast agents like superparamagnetic iron oxide have been developed to improve the diagnostic capabilities of liver pathology. Fatty liver change can be made by comparing the homogeneous liver parenchymal signal intensity to the retroperitoneal fat on all magnetic resonance imaging (MRI) sequences. Simple hepatic cysts and abscesses are hypointense on T1W and hyperintense on T2W images. Complex hepatic cysts may show calcification in cyst walls appearing as signal void on MR pulse sequences. Hepatocellular carcinomas reveal variable appearance on T1W images and appear hyperintense on T2W images relative to the rest of hepatic parenchyma. Intrahepatic cholangiocarcinoma on MR appears as large central mass with irregular borders, hypointensity on T2WI relative to hepatic parenchyma. Most hepatic metastasis are hypointense relative to the normal liver, because these lesions are hypovascular and intravenous contrast can increase the difference in density between the metastatic lesion and normal hepatic parenchyma.

The gallbladder is located on the undersurface of the liver at the level of the tip of 9th costal cartilage on right side. It is a globular shaped fibromuscular sac measuring 8 cm in length and 4 cm in diameter when distended. The wall thickness of gallbladder on MRI is around 1–3 mm of thickness. Homogeneous enhancement of gallbladder wall is noted following intravenous contrast on MR. The capacity of gallbladder is about 50 mL of bile. On T1W images the gallbladder wall appears low signal intensity with hypointense signal in its lumen. On T2WI MR images the luminal contents appear hyperintense. Heterogeneous signal intensity is seen in cases of intraluminal tiny calculi and hemobilia. Large intraluminal calcium containing calculi may appear as signal void on MRI sequences. The gallbladder for descriptive purposes can be divided into fundus, body and neck regions. The neck region of gallbladder is narrow and gradually decreases in its caliber to connect with the biliary system. Mucosal folds of gallbladder neck (Hartmann's pouch) may be large enough for gall stones to be lodged. The duct connecting the gallbladder to the common hepatic duct is called the cystic duct it is around 3 cm in length with diameter 2–3 mm. The union of cystic duct with the common hepatic duct forms the common bile duct, this union normally occurs anterior to the right hepatic artery. The fundus and body of gallbladder are firmly attached to the liver by connective tissue. The peritoneum which covers the liver also covers the gallbladder, in certain cases the gallbladder may have its own narrow mesentery from the undersurface of liver and be mobile. In some cases the gallbladder may have internal septa. The cystic artery is a branch of the right hepatic artery which supplies the gallbladder and cystic duct. The cystic artery courses in the Calot's triangle, the margins of Calot's triangle are the liver, hepatic duct and the cystic duct. The lymphatics from gallbladder drain into the cystic node, eventually these lymph channels drain into the coeliac lymph nodes.

The common hepatic duct (CHD) is formed by the union of right and left hepatic ducts near the porta hepatis. The CHD is around 3–4 cm in length and around 1–3 mm in diameter, the CHD runs downwards towards the duodenum. On T1W images the CHD appears like a tubular hypointense structure at porta hepatis. The cystic duct joins the common hepatic duct to form the common bile duct (CBD). The CBD measures around 7–8 cm in length, its diameter in normal adults is 5–8 mm. In the elderly patients and postsurgical cases the CBD may have a larger diameter. The CBD has for descriptive purposes described in three segments—supraduodenal, retroduodenal and paraduodenal segments. In its supraduodenal segment the CBD lies anterior to the portal vein. In the retroduodenal segment the CBD lies posterior to the 1st part of duodenum. The paraduodenal segment of CBD lies posterior to the 2nd part of duodenum and the head of pancreas. The CBD appears as a low intensity structure in the posterior portion of pancreas on MR. The CBD and the main pancreatic duct enter the ampulla of Vater located on the posteromedial side of 2nd part of duodenum, about 10–12 cm from the pylorus. The sphincter of Oddi is made up of circular smooth muscle fibers that surround the ampulla of Vater. The cystic artery, right hepatic artery and superior pancreaticoduodenal arteries supply the CBD and common hepatic duct.

The portal vein is a large vein that is formed by the union of superior mesenteric vein and the splenic vein behind the neck of pancreas. The portal vein measures around 8–9 cm in length. Initially the proximal part of portal vein lies in front to the inferior vena cava and behind the first part of duodenum. The portal vein in its distal course runs in the lesser omentum behind the common bile duct and hepatic artery to reach porta hepatis. At porta hepatis the portal vein bifurcates into right and left portal veins to supply the right and left lobes of liver. The portal vein has no valves, it communicates with the systemic circulation through it small branches at the lower end of esophagus, bare area of liver, periumbilical region, upper anal canal region and some retroperitoneal regions. These communications of the portal vein with the systemic circulation serve as important anastomotic regions to maintain circulation and are enlarged when there is any block to the flow of blood in either the portal or systemic circulation.

The pancreas is a retroperitoneal structure located at the level of 1st lumbar vertebra; it measures around 13–15 cm in length. The pancreas crosses the midline transversely and has a feathery parenchymal appearance on MR imaging. The pancreas is subdivided into head, neck, body and tail regions. The splenic artery supplies the body and tail regions of pancreas while the head and neck regions are supplied by superior and inferior pancreatico duodenal arteries. On MRI the head of pancreas measures 10–22 mm in size, the body of pancreas measures 40–100 mm in length and the tail of pancreas measures 50–100 mm in size. The pancreatic shows intermediate signal intensity to that between the liver and spleen on T1W images with moderate enhancement following intravenous contrast.

The spleen develops around the fifth week of fetal life as a localized thickening of the mesoderm in the dorsal mesogastrium. Developmental rotations of the foregut occur during intrauterine growth leading to change in position of the stomach the spleen at birth. The part of the dorsal mesogastrium which intervened between the spleen and the greater curvature of the stomach 19 forms the gastrosplenic ligament. Accessory splenic tissue may occur due to nonfusion of some splenic tissue with the main organ.

The stomach is located in the upper abdomen and is the most distensible part of the gastrointestinal system. The stomach lies between the esophagus above and the duodenum below, it lies in front of the pancreas. The stomach has two margins called the greater and lesser curvature, a fold of peritoneum called as greater omentum hangs freely from the greater curvature. The stomach is divided into 4 parts—cardia, fundus, body, pylorus. The cardia is where the contents of esophagus enter the stomach. The fundus of stomach is the section formed by the upper margin of stomach under the left hemidiaphragm. The body of stomach is the central region of stomach. The pylorus of stomach is the distal section of stomach and can be subdivided into pyloric antrum and pyloric canal. The stomach ends at the gastroduodenal junction. The gastroesophageal sphincter is located at the junction of esophagus and stomach. The pyloric sphincter is located at the gastroduodenal junction. The upper limit of gastric wall thickness is 3.5 mm. On MR the muscular layer show low signal intensity on T1WI with moderate enhancement on contrast. On T2W images the inner mucosal layer shows intermediate signal intensity. The perigastric fat surrounding the stomach shows intermediate to high signal intensity on T1WI. The stomach receives blood supply from the left gastric artery, gastroduodenal artery, right gastric artery and short gastric arteries.

The small bowel is a hollow loop like structure that extends from the pylorus of stomach to the ileocecal junction. It is around six to seven meters in length. The luminal diameter of small bowel gradually decreases as it approaches the ileocecal junction. The small bowel wall measures 2–3 mm in thickness, at terminal ileum the upper limit of normal thickness is 5 mm. The muscular walls of small bowel show low to intermediate signal on T1W images. The inner mucosal layer of small bowel shows higher signal intensity on T2W images. The small bowel loops are located mainly in the central and lower part of the abdominal cavity. The small bowel loops are bounded on the sides and above by the large bowel. The omentum lies in front of the small bowel loops and a fold of peritoneum called the mesentery connects the small bowel to the lumbar spine. The main portions of the small bowel are the duodenum, jejunum and ileum. Widely spaced bowel loops on MR can be due to abscess, phlegmon, fibrofatty proliferation within mesentery, thickened bowel wall or mesenteric lymphadenopathy.

The large bowel is around 150 cm in length and extends from the ileocecal junction to the external anal opening. The large bowel has longitudinal muscle fibers in its walls which give the large bowel its haustral pattern. On T1W images the muscular layer of large bowel shows low signal intensity, on T2W images the inner mucosal layer shows high signal intensity. The fecal contents of large bowel show low to intermediate signal intensity. The large bowel for description purposes is divided into cecum, ascending colon, transverse colon, rectum and anal canal. The large bowel begins in the right iliac region and ascends in right lumbar to hypochondrium, then it lies under the inferior surface of liver (hepatic flexure), the large bowel then courses transversely to the left side of abdomen crossing the midline and then lies the inferior surface of spleen (splenic flexure), the large bowel courses downwards in the left hypochondrium to reach the left iliac region. In the pelvis the large bowel has a smooth bend (sigmoid colon) and beyond this bend the large bowel descends downwards to the anal canal. Some characteristics which help in recognizing 21 the large bowel from small bowel are the large caliber, is more fixed in its position and has sacculations in its walls. The intraluminal diameter diminishes in caliber gradually from cecum to rectum.

The kidneys are bean shaped organs located in the retroperitoneum, with its long axis parallel to the psoas muscle. The hilum of kidney is an opening on the medial side of kidney where the arteries and veins pass in and out of the kidney. Normal adult kidney measures around 12 cm in length, 6 cm in width and 3 cm in thickness and weighs around 135–145 g. The left kidney is slightly higher in location as compared to the right kidney. The upper pole of left kidney lies at the level of 11th rib while the upper pole of right kidney lies at the level of 12th rib. Posterior to the kidney is the diaphragm and quadratus lumborum muscles. Medially the kidney is related to the psoas muscle and laterally to transversus abdominis muscle. The suprarenal glands are located on the upper pole of each kidney. Anteriorly the upper pole of right kidney is related to the peritoneum and hepatorenal pouch. The left kidney upper pole is related anteriorly to peritoneum and lesser sac. The lower pole of right kidney is related anteriorly to the hepatic flexure. The lower pole of left kidney is related anteriorly to the splenic flexure. The perinephric fat envelops and supports the kidneys. The Gerota's fascia is condensed connective tissue which surrounds the perinephric fat.

Each kidney has its own ureter; it is a tube-like structure which allows the urine from the kidney to pass into the urinary bladder. On T2W images, the walls of the ureters show hypointense signal, the intraluminal contents show hyperintense signal. Although MR is not sensitive to calculi, the ureteral dilation in cases of obstruction and neoplastic etiology of ureter can be assessed. The extent of malignant ureteral pathology involving adjacent abdominal and pelvic tissues can be assessed. The ureter walls show moderate enhancement on intravenous gadolinium contrast. Each ureter is about 25–27 cm in length and has some narrowing at—(i) pelviureteric junction (PUJ) (ii) the point where the ureter crosses the pelvic brim and (iii) at the uretero-vesical (U-V-Junction) junction. The ureter begins at the pelviureteric junction at renal hilum. The ureter can be described into upper, mid and lower ureteric segments. The upper ureter is the segment that lies between the pelviureteric junction and the L3 transverse process level. The mid-ureteric segment extends from L3 transverse process level to the pelvic brim below. The lower ureteric segment is the part which lies in the pelvic cavity and ends at U-V junction. The right upper ureter lies behind the 3rd part of duodenum while the mid and lower right ureter lie behind the right colic vessels. The left lower ureter lies behind the left colic vessels and sigmoid mesocolon. Posteriorly the ureters lie on the psoas muscle fascia and at the point where the common iliac artery bifurcates the ureters cross over the sacro-iliac joint into the pelvic cavity. The lower ureter on either side courses downward into the pelvic cavity, along the anterior border of the greater sciatic notch and under the peritoneum. In the lower part of greater sciatic foramen the ureter turns medially to reach the lateral wall of bladder, finally it runs slightly oblique to enter the bladder wall. When the bladder is fully distended the distance between the openings of both the ureters is around 5–6 cm and when the bladder is empty this distance is reduced to less than 3 cm.

The urinary bladder is a musculomembranous sac which collects urine from both the kidneys. Its size, position change according to the amount of fluid it contains. On T1W images the walls of the bladder show slightly hypointense signals and its intraluminal contents appear as markedly hypointense consistent with fluid. On T2W images the intraluminal fluid 23 contents show high signal intensity. Vesical calculi are not well appreciated on MR, but abnormal pathology such as polyps appear isointense with the bladder wall on T1W and T2W images. Malignant lesions appear as hypointense signal with irregular margins on T1W and T2W images and show poor contrast enhancement.

The adrenal glands are located on the upper pole of each kidney; the left adrenal is located at a slightly higher level than the right adrenal. Each adrenal gland measures around 50 mm in length, 30 mm in width and around 10 mm in thickness. On MR scans both adrenals show similar signal intensity as the kidneys on T1W images. The right adrenal gland is more pyramidal in shape; it is related superiorly to the right hemidiaphragm above. Anteriorly the right adrenal is related to inferior vena cava and liver. The left adrenal appears crescentic or semilunar in shape; it is related superiorly to the left hemidiaphragm. Anteriorly the left adrenal is related to the lesser sac and tail of pancreas. Both adrenal glands have two small elongated process called as medial and lateral imbs. SStructurally the adrenal gland has an outer cortex and an inner medulla which are responsible for producing various hormones.

The thoracic aorta (Figs 40 and 41) enters the abdominal cavity by passing through the aortic hiatus in the diaphragm at T12 vertebral level and continues as the abdominal aorta. The abdominal aorta runs vertically slightly to the left of midline downwards till its bifurcation into common iliac arteries at L4 vertebral level.

The coeliac trunk is the artery of foregut, originates from the abdominal aorta at the level of T12 vertebra. At the upper border of pancreas the coeliac trunk branches into the left gastric artery, splenic artery and common hepatic arteries. The left gastric artery gives off smaller branches to the esophagus and stomach. The splenic artery branches are the left gastroepiploic artery, six short gastric arteries and terminal branch to the spleen. The common hepatic artery gives off—right gastric artery, gastroduodenal artery, right gastroepiploic artery, superior pancreaticoduodenal artery, small supraduodenal branches and terminal hepatic artery.

The superior mesenteric artery is the artery of midgut. It originates from the abdominal aorta at the level of L1 vertebral body and lower border of pancreas. Its main branches are inferior pancreaticoduodenal artery, small jejunal and ileal branches, right colic artery and middle colic artery.

The inferior mesenteric artery is the artery of hindgut. It originates from the abdominal aorta at the level of L3 vertebral body and lower border of 3rd part of duodenum. Its main branches are the left colic artery, small sigmoidal arteries and terminal superior rectal artery.

The renal arteries are direct branches of the abdominal aorta, behind the pancreas. Each renal artery courses almost at right angles to the abdominal aorta towards the renal hilum. At the renal hilum each renal artery divides into anterior and posterior division. Further branching in renal parenchyma gives the interlobar, arcuate and interlobular arteries.

The inferior vena cava (IVC) is a large vein that begins by the union of two common iliac veins at the level of L5 vertebra slightly to the right of midline. The IVC at its origin is located posterior to the right common iliac artery. The IVC runs upwards in the abdomen vertically, it lies parallel to the abdominal aorta higher in the abdomen. In the lower abdomen the IVC courses posterior to the peritoneal cavity, right gonadal artery and third part of duodenum. In the upper abdomen the IVC courses posterior to the portal vein, pancreatic head and bile duct. Near the right hemidiaphragm the IVC lies posterior to the bare area of liver. The IVC enters the thoracic cavity through the IVC hiatus in right crus of diaphragm at T8 vertebral level. After entering the thoracic cavity the IVC enters the right atrium. The main tributaries of the IVC are—the lumbar veins, gonadal veins, renal veins, left adrenal vein, inferior phrenic veins, iliolumbar veins, lateral sacral veins, medial sacral veins and hepatic veins.

The prostate appears like an inverted pyramid structure below the urinary bladder in males on coronal MRI images.

Normal prostate in adult measures around 40 mm × 30 mm × 20 mm. The base of prostate is in contact with the urinary bladder above; the apex of prostate is directed downwards. The prostate is related anteriorly to the retropubic space (Figs 29 and 30). The levator ani muscles are located lateral to the prostate. The lower part of rectum is located posterior to the prostate. The ejaculatory ducts enter the posterosuperior surface of prostate; they course in the prostatic parenchyma to empty into the prostatic urethra. The prostatic urethra measures around 30–40 mm in length travels through the prostate parenchyma downwards from the urinary bladder. The prostatic urethra receives the multiple small prostatic ducts along its course within prostate. The verumontanum is a smooth prominence seen on the posterior wall of prostatic urethra. True capsule of prostate is formed by the condensation of connective tissue at periphery of the prostate. False capsule of prostate is formed by condensation of pelvic fascia. The prostatic parenchyma has been divided into three zones for description purposes—the central zone, peripheral zone and transitional zone. The central zone of prostate shows lower signal intensity than the peripheral zone on MRI T1W images. The peripheral zone on MRI T2WI shows higher signal intensity.

The central zone of prostate appears wedge-shaped and it rarely gets involved in any major pathology. The peripheral zone of prostate surrounds the central zone, here the most common pathology to affect this zone is the carcinoma of prostate. The transitional zone of prostate lies around the distal portion of preprostatic urethra, the most common pathology to affect this zone is benign prostatic hypertrophy. The prostate is supplied by the inferior vesical artery, small branches from the middle rectal and internal pudendal arteries. The prostatic veins drain into vesicoprostatic venous plexus. Lymphatics drain into the internal and external iliac group of nodes.

The female pelvis delineates the uterus and adnexa on CT imaging (Figs 23 to 27). However, the genital organs are best seen on T2WI sagittal and axial planes on MRI imaging. Normal dimensions of the adult uterus are 80 mm × 50 mm × 3 0 mm (length × breadth × width). The uterus has a fundus, body and cervix. The lower part of cervix protrudes into the upper vagina. The fundus of uterus is the uterine part which lies above the level of the fallopian tube opening on either side. The cornu of uterus is the junction between the fundus and body of the uterus, the opening of the fallopian tube is located here. The body of uterus is related to the peritoneal folds and broad ligament on its lateral side. The uterine body and fundus is composed of a thick muscular layer the myometrium.

Male urethra is around 18 to 20 cm in length. For descriptive purposes the male urethra has been subdivided intoanterior urethra and posterior urethra. The posterior urethra has two segments—prostatic and membranous segments. The anterior urethra has two segments-bulbar and penile segments.