ANATOMY ON X-RAY
Abdominal Radiograph
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.
Routine supine abdominal radiograph shows the following structures (Fig. 1):
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Dark margins outlining the spleen, liver, kidneys, bladder and psoas muscles due to presence of surrounding fat
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Gas in—body of stomach, descending colon, small intestines
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Fecal matter in cecum gives it a mottled appearance, seen as a mixture of gray densities representing a gas-liquid-solid mixture
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The heart shadow is seen be on the left side above the diaphragm
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Pelvic phleboliths—these are small round/oval calcific densities in pelvic cavity
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A dark skin fold across the upper abdomen is normal finding
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The bony pelvis, spine and visualized ribs
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Check whether the right ‘R’ marker is placed on the right side of the abdominal radiograph
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Make sure that the abdominal radiograph covers both the hemidiaphragms to the inguinal canal regions
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Check the lung bases.
On an erect abdominal radiograph the following changes occur:
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The air rises
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Fluid goes down due to gravity
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The transverse colon, small bowel loops and kidneys are seen relatively at inferior position
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A slight increase in radiographic density in lower abdomen
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The lung bases appear clearer as the diaphragms move down a little
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The liver and spleen become more visible.
The abdominal radiograph is most helpful in cases of acute abdomen. A normal initial abdominal radiograph does not exclude intra-abdominal trauma, follow-up radiographs, ultrasound, and CT scanning may be necessary. Abnormal air – fluid levels become easier to visualize on erect abdominal radiographs. Gas under diaphragm is seen in cases of perforated viscus. Also remember not to waste any time if the patient's condition is critical, stabilize the patient and shift the patient to operating theater if needed.
Radiation exposure in early pregnancy can be disastrous. It is always safer in female patients of reproductive age group to check the date of their last menstrual period.
Additional Points to Note While Examining Abdominal Radiographs
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Maximum diameter of small bowel should not exceed 3 cm and that of large bowel by more than 5 cm
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Cecum is said to be dilated if it measures more than 8 cm
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The haustra of the large bowel extends only a third of the way across the bowel from each side, whereas the valvulae conniventes of the small bowel transverse from wall-to-wall
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Presence of small amounts of intraluminal gas throughout the gut is normal, but if found in excess may be abnormal. Also absence of bowel gas in one area may indicate bowel pathology
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Presence of extraluminal gas is abnormal (look for it under the diaphragm, in the bowel wall, in biliary system)
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Look for nasogastric tube placements, catheters, etc. to mention them in your report
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Look for normal calcified structures which can cause diagnostic difficulty—excessive costal cartilage calcification, calcified aortic/splenic arteries, pelvic phleboliths, calcified mesenteric lymph nodes, etc.
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Normal liver has a fairly pointed tip, if this tip appears more rounded with displacement of adjacent intra-abdominal structures it is suggestive of hepatomegaly
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The spleen is not normally seen on abdominal radiographs, when spleen is enlarged more than 15 cm, it displaces the adjacent intra-abdominal organs and becomes more obvious on abdominal radiographs
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Normal kidneys extend from the lower margin of 12th dorsal vertebra to the upper margin of 3rd lumbar vertebra, the left kidney is usually slightly larger in size and slightly higher placed as compared to the right kidney. The outline of kidney visible on abdominal radiograph is due to perinephric fat
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An abdominal mass can arise anywhere in abdomen and would produce a dense area with displacement of bowel loops around it, calcification may also occur within it, CT scan may be required to investigate such masses
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A full bladder appears in the pelvic cavity as a smooth rounded mass of uniform density, the outline is due to perivesical fat tissue
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Retroperitoneal masses usually obscure or displace the psoas muscle outlines on abdominal radiographs
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An erect chest radiograph and not abdominal radiograph is the best projection to diagnose a pneumoperitoneum (gas in the peritoneal cavity).
ANATOMY ON CT SCAN
Abdomen and Pelvis
Liver
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.
The Couinaud classification of liver anatomy divides the liver into eight functionally independent segments. Each segment has its own vascular inflow, outflow and biliary drainage. In the center of each segment there is a branch of the portal vein, hepatic artery and bile duct. The numbering of the segments is in a clockwise manner (Figs 2 to 8).
Segment 1 (caudate lobe) is located posteriorly and extends between fissure of the ligamentum venosum anteriorly and the inferior vena cava posteriorly.
The longitudinal plane of the right hepatic vein divides segment 8 from segment 7 in the superior portion of the liver and in the inferior portion of the liver segment 5 from segment 6.
The longitudinal plane of the middle hepatic vein through the gallbladder fossa separates segment 4a from segment 8 in the superior liver and segment 4b from segment 5 in the inferior liver.
The longitudinal plane of the left hepatic vein and fissure of the ligamentum teres separates segment 4a from segment 2 in the superior liver and segment 4b from segment 3 in the inferior liver.
The axial plane of the left portal vein separates segment 4a superiorly from segment 4b inferiorly and segment 2 superiorly from segment 3 inferiorly in the left lobe.
The axial plane of the right portal vein separates segment 8 and segment 7 superiorly from segment 5 and segment 6 inferiorly in the right lobe.
Normal liver has a precontrast attenuation value of 45–65 H.U. and maximum enhancement occurs at 50–60 seconds after administration of contrast. Normal liver has a size up to 13 cm. Normal portal vein is 10–13 mm in diameter (Figs 9 to 17).
Normal gallbladder is up to 10 cm long and 4 cm wide (Figs 15 to 17). It has a normal wall thickness up to 3 mm. Gallbladder may have septae. Bifid appearance is due to longitudinal septum. Phrygian cap gallbladder is due to kink or septum at the neck. Ectopic gallbladder can be seen beneath the left lobe of liver or even in retrohepatic location. Floating gallbladder can result from loose peritoneal attachments.
Normal cystic duct is 2 cm long and 2 mm wide. Maximum diameter of normal common bile duct in an adult is 2–5 mm. Postcholecystectomy it can be up to 7 mm. Normal width increases by 1 mm per decade in elderly over 60 years of age.
Pancreas
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.
The main pancreatic duct is known as the duct of Wirsung. The angle between the pancreatic duct and common bile duct at their joining point is between 5°–30°. These ducts open into second part of duodenum through the ampulla of Vater which has a sphincter called the sphincter of Oddi.
The entire length of pancreas is 10–15 cm. Pancreatic tail is up to 1.6 cm thick; body is up to 1.1 cm thick and the head ranges from 1 cm to 2 cm in thickness (Figs 14 to 17).
Annular pancreas is a congenital anomaly in which the duodenum is enclosed on all sides by pancreas as a result of abnormal migration of ventral pancreas.
Spleen
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
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.
Upper gastrointestinal system starts from mouth and continues into oropharynx which continues into esophagus. Esophagus is a 25 cm long tubular structure which opens into the stomach via gastroesophageal junction. Parts of stomach are the fundus, body, greater and lesser curvatures, antrum and pylorus. Walls are 3–5 mm thick except in pylorus where it can be up to 7 mm thick.
4Small intestine can be up to 6 m long and extends from pyloric orifice of stomach up to ileocecal valve. Duodenum is 1 feet long, jejunum is around 10 feet and ileum is up to 8 feet. Fifteen centimeters long mesentery is located between ileocecal junction and ligament of Treitz. Circular folds of small bowel are called as valvulae conniventes.
Rule of three for normal small bowel states that its walls are 3 mm thick, valvulae conniventes are 3 mm thick, there are less than 3 air fluid levels and the diameter is up to 3 cm.
Large intestine is 1.5 m long and extends from ileum to anus. Its parts are cecum, ascending colon, hepatic flexure of colon, transverse colon, splenic flexure, descending colon, sigmoid colon, rectum and anal canal.
Peritoneal spaces above transverse colon are:
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Spaces on the right
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Right subphrenic space
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Anterior and posterior right subhepatic space
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Bare area of liver
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Lesser sac
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Spaces on the left
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Left subphrenic space
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Left subhepatic space
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Perisplenic space.
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Peritoneal spaces below transverse colon are:
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Superior and inferior ileocecal recesses
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Retrocecal space
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Right and left paracolic gutters
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Intersigmoid recess
Two folds of peritoneum supporting a structure within the peritoneal cavity together form a structure known as ligament.
When two folds of peritoneum connect a portion of bowel to the retroperitoneum it is known as mesentery. Ventral mesentery gives rise to falciform ligament, gastrohepatic ligament and hepatoduodenal ligament. Dorsal mesentery gives rise to gastrophrenic ligament, gastropancreatic ligament, phrenicocolic ligament, gastrosplenic ligament, splenorenal ligament and gastrocolic ligament. Dorsal mesentery also forms the small bowel mesentery and transverse as well as sigmoid mesocolon.
Omentum is a structure connecting stomach to an additional structure. Lesser omentum is formed by combination of hepatoduodenal and gastrohepatic ligament. Greater omentum is an inferior continuation of gastrocolic ligament and is composed of four layers of peritoneum resulting from double reflection of dorsal mesogastrium.
Anterior right subhepatic space located posterior to porta hepatic communicates with lesser sac through epiploic foramen also known as foramen of Winslow.
Urogenital System
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Kidneys arise from metanephros (of mesodermal origin) at fourth week of intrauterine life. Bladder, urethra and prostate are formed from urogenital sinus.
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Renal arteries can be multiple, aberrant, accessory and even supplementary. Single or multiple renal veins can exist.
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Retroperitoneum is the space between parietal peritoneum extending from diaphragm to pelvic brim and fascia transversalis.
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Adrenal glands (suprarenal glands) are situated on the top of kidneys and are 3 cm long and 1 cm thick.
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Average size of testis is 2.5 × 3.0 × 3.5 cm. Epididymis has a head, body and a tail.
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Spermatic cord consists of testicular artery, cremasteric artery, pampiniform plexus of veins, vas deferens, nerves and lymphatics.
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Gonadal artery arises from ventral surface of aorta slightly below the origin of renal arteries. Occasionally it can arise from renal artery.
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Gonadal veins drain in the IVC or renal vein on right and in the renal vein on left.
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Prostate has a normal size of 2.5 × 2.8 × 3.0 cm. It is composed of an outer part having a central and peripheral zone and an inner part made of periurethral and transitional zone.
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Male urethra is 15–20 cm long and has a posterior part composed of prostatic urethra and membranous urethra. The anterior part of urethra is composed of bulbar and penile urethra.
Female urethra is 2.5–5 cm long.
Adult uterus is 6–9 cm long, 2.5–4 cm anteroposterior and 3–4.5 cm transverse in dimension. Endometrium is the innermost zone of uterus. Serosa is the outermost zone. Myometrium is the middle layer. CT scan usually does not show them separately.
Fallopian tubes arise from upper and the outer aspect of uterus, and extend between the folds of broad ligament towards the pelvic side walls to open just above and anterior to ovaries located in ovarian fossa on each side.
Pelvic spaces formed due to relationship between urinary bladder, uterus and rectum are:
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Rectouterine pouch of Douglas
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Uterovesicle pouch
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Rectovesicle recess
Important Pelvic ligaments in relation to uterus are:
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Broad ligament—between uterus and pelvic sidewalls
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Round ligament—between uterus and labia majora
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Cardinal ligament/Mackenrodt ligament—between cervix and fascia of obturator internus
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Uterosacral ligament—between uterus and sacrum
Adult ovaries measure 0.5–1.5 cm × 1.5–3.0 cm × 2–3 cm.
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ANATOMY ON MAGNETIC RESONANCE IMAGING
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 liver receives blood from two sources—the hepatic artery and the portal vein. In each lobe of liver the arteries are end arteries, because of this the liver infarction in certain diseases can occur. The venous drainage of liver is through the three main hepatic veins which drain into the inferior vena cava.
The lymphatics of liver drain to the nodes at porta hepatis and from there drain to the coeliac group of lymph nodes. The liver is innervated by the sympathetic supply from coeliac plexus and parasympathetic supply from vagus nerve branches.
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.
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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 head of pancreas is tilted backwards into the right paravertebral gutter; it is broad and its lower end resembles a hook in appearance (uncinate process) and is closely related to the medial wall of 2nd part of duodenum. The inferior vena cava lies behind the head of pancreas. The neck of pancreas is a narrow region of pancreas which lies in front of the union between the superior mesenteric vein and splenic vein. The body of pancreas crosses the midline; its distal portion is tilted backwards into the left paravertebral gutter. The upper margin of body of pancreas crosses the coeliac trunk origin from the abdominal aorta. The inferior margin of body of pancreas is related posteriorly to the origin of superior mesenteric artery from abdominal aorta. The inferior mesenteric vein unites with the splenic vein behind the body of pancreas. The body of pancreas anteriorly is related to the lesser sac and the posterior gastric wall. The tail of pancreas is the distal portion of pancreas that lies within the splenorenal ligament. The splenic artery and splenic vein are also located within the splenorenal ligament along with the tail of pancreas. The tail of pancreas is located anteriorly to the left kidney and the tip of tail of pancreas lies in close relation to the splenic hilum.
In cases of acute pancreatitis the pancreatic margins appear to be blurred on all MRI sequences. The pancreas appears bulky and areas of necrosis within it may show high signal on T2WI. If a pseudopancreatic cyst forms then a thin low signal intensity rim may be seen around the central hyperintense collection on T2WI. Mesenteric fat stranding is seen on T1W sequences with intermediate to high signal intensity.
In chronic pancreatitis with fibrosis there is absence of signal on both T1WI and T2WI. Calcification is seen in chronic pancreatitis containing mature calcium deposits is seen as an area of signal void on all MR sequences.
Pancreatic endocrinal tumors show higher signal intensity on T2W images. Air within the pancreatic duct is seen as signal void on MR sequences.
The pancreatic duct is a long tube like structure that is present in pancreatic parenchyma from the tail to the head of pancreas. On MRI sequences the walls of the pancreatic duct appears to be hypointense as compared to its lumen. The intraluminal contents of pancreatic duct appear hyperintense on T2W images. The diameter of the duct gradually increases from the tail to head of pancreas. Sometimes calculi may be seen on MRI within the pancreatic duct. The pancreatic duct opens into the ampulla of Vater in the posteromedial aspect of 2nd part of duodenum. At the ampulla of Vater the common bile duct also enters making an acute angle with the pancreatic duct. The lower part of head of pancreas and uncinate process drain into the 2nd part of duodenum via the accessory pancreatic duct, this opening in 2nd part of duodenum is located around 1 to 2 cm above the ampulla of Vater. The pancreatic duct diameter is larger in neoplastic etiology of pancreas as compared to acute pancreatitis where the pancreatic duct size is about 6 mm.
During embryologic growth period the pancreas is formed by the union of dorsal and ventral pancreatic buds. In some cases the fusion is not normal and can encircle the 2nd part of duodenum like a ring resulting in annular pancreas.
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 spleen is located in the left hypochondrium, under the left hemidiaphragm. The size and weight of the spleen is variable at different periods of life, in different individuals, and in the same individual under different conditions. In the adult it is usually about 12 cm in length, 7 cm in breadth, and 4 cm in thickness and weighs about 200 gm. The maximum craniocaudal length on MR is 12–15 cm. The spleen is surrounded by peritoneum which is fixed to the splenic capsule. It is held in position by two folds of this membrane—the phrenicolineal and the gastrolineal ligaments. The lower end of the spleen is supported by the phrenicocolic ligament. The spleen has a smooth superior surface called the diaphragmatic surface of spleen. The anterior surface of spleen is related to the stomach and is called the gastric surface of spleen. The posterior surface of spleen is related to left kidney and is called the renal surface of spleen. The splenic pulp is a soft mass of dark brown mesh of fibers and trabeculae. The mesh of splenic pulp is filled with blood, which contain white blood corpuscles in large numbers.
On MR the splenic parenchyma has homogeneous appearance with MR signal intensity almost equal or mildly hypointense to liver on T1W images. On T2W images the spleen shows higher signal intensity as compared to liver. Following intravenous contrast, the spleen shows heterogeneous enhancement in early arterial phase on T1WI. Sometimes the heterogeneous enhancement might mimic a mass in splenic parenchyma in early arterial phase and a false diagnosis of splenic mass is made, this mistake can be avoided by taking delayed phase images of spleen. In lymphoma of spleen superparamagnetic iron oxide can improve the detection of lesions less than 1 cm in diameter as T1W and T2W images show the spleen and lymphomatous tissue to have the same relaxation times. Splenic cysts can be true cysts or false cysts, and both contain fluid to give high signal intensity on T2W images, following intravenous contrast with gadolinium chelates the splenic cyst shows no rim enhancement or enhancement of the cyst contents. Splenic hemangiomas have low signal intensity on T1W images and have higher signal intensity on T2WI images. Hemorrhagic infarcts of spleen can be single or multiple and appear wedge shaped; high signal intensity is noted on both T1W and T2W images.
The splenic artery arises from the coeliac trunk supplies the spleen and enters the hilum of spleen and small branches course through the parenchyma of spleen. The splenic veins are numerous small venules that accompany the splenic artery branches, eventually at the hilum they unite and form the splenic vein. The splenic vein joins the superior mesenteric vein at the neck of pancreas to form the portal vein.
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 duodenum begins at the gastroduodenal junction and ends at the ligament of Treitz. The duodenum is a hollow tube like structure about 25–30 cm long connecting the stomach to the jejunum. 20 On T1W images the muscular layer of duodenum appears as low signal intensity, on T2W images the inner mucosal layer shows higher signal intensity. The duodenum is divided into four sections for the purposes of description. The first three parts of duodenum form a “C”-loop concavity in which the head of the pancreas lies. The first 2 cm of duodenum is mobile; as it is covered by peritoneum. The rest of duodenum is retroperitoneal and fixed.
The first part of duodenum begins as a continuation of the duodenal end of the pylorus. It runs to the right side of abdomen for a short distance of 5 cm before making a sharp curve inferiorly into the superior duodenal flexure.
The second part of the duodenum begins at the superior duodenal flexure. It runs vertically downwards up to the lower border of vertebral body L3, before making a sharp turn medially into the inferior duodenal flexure. The pancreatic duct and the common bile duct enter the second part of duodenum through the ampulla of Vater. The accessory pancreatic duct also enters the second part of duodenum.
The third part, or inferior part or part horizontal of the duodenum begins at the inferior duodenal flexure and passes transversely to the left, passing in front of the inferior vena cava, abdominal aorta and the vertebral column. The superior mesenteric artery and vein are anterior to the third part of duodenum. This part may be compressed between aorta and SMA causing superior mesenteric artery syndrome.
The fourth part of duodenum passes upwards until it reaches the inferior border of the body of the pancreas and terminates at the duodenojejunal flexure. The duodenojejunal flexure is surrounded by a peritoneal fold containing muscle fibers called as ligament of Treitz.
The duodenum receives arterial blood from two sources—the gastroduodenal artery and the superior mesenteric artery branches. The venous drainage of the duodenum follows the arteries. The veins drain into the portal venous system eventually. The lymphatics drain to the celiac group of lymph nodes.
The proximal two-fifth of small bowel is called the jejunum and begins at the duodenojejunal flexure. The jejunum has characteristic feathery type of appearance due to its thick mucosal folds called valvulae conniventes. The intraluminal diameter (4 cm) of jejunum is wider than ileum. The jejunum also has thicker walls than the ileum. On T1W images the muscular layer shows low signal intensity, on T2W images the mucosal layer of jejunum shows intermediate to high signal intensity which may be patchy in distribution due to Peyer's patches. The Peyer's patches are lymph nodules in the mucosa and walls of small bowel, these are minimal or absent in upper jejunum and present to some extent in lower jejunum. These Peyer's patches are more common in the ileum.
The distal three-fifth of small bowel is called the ileum; it occupies the umbilical, hypogastric, right iliac, and pelvic regions. The ileum is recognized by its featureless pattern of small bowel on MR images. The intraluminal diameter of ileum is narrow (3.75 cm) as compared to jejunum; the Peyer's patches are abundant in lower ileum. On T1W images the muscular wall of ileum shows low signal intensity, T2W images show high signal intensity. The ileum along with jejunum is attached to the posterior abdominal wall by a fold of peritoneum called the mesentery, which gives mobility to small bowel loops, so that each loop of small bowel can change in form and position inside the abdominal cavity.
The mesentery is fan-shaped fold of peritoneum attached to the posterior abdominal wall from the left side of the body of the second lumbar vertebra to the right sacroiliac. The mesentery measures around 15 cm in length with width is around 20 cm. The mesentery contains arteries, veins, and lymph node. On MRI the mesenteric fat gives intermediate to high signal intensity on T1WI. Mesenteric vessels normally measures less than 3 mm in diameter on T1W images. The jejunum and ileum are supplied by the superior mesenteric artery intestinal branches, anastomosis with the inferior mesenteric branches also occur. The veins accompany their arteries. The small bowel is innervated by the sympathetic nerve plexuses around the superior mesenteric artery, and run in the small bowel wall forming the myenteric plexus situated between the circular and longitudinal muscular fibers. Smaller nerve fiber from myenteric nerve plexuses run deeper and terminate into the submucosal layer forming the Meissner's plexuses.
Mesenteric lymph nodes measures less than 3 mm in size normally and seen on T1WI and show mild enhancement.
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 cecum is located in the right iliac fossa and it begins at the ileocecal junction. The cecum is a distensible pouch whose lower end is blind and the superior end has a wider mouth which is continuous with the ascending colon. Its size is approximately 6.25 cm length and 7.5 cm in breadth. The cecum lies in front of the right psoas muscle; the anterior wall of cecum is related to the anterior abdominal wall when the cecum is distended. The cecum has its own mesentery and so can be mobile in the abdominal cavity. The posterior surface of cecum has loose connective tissue which connected to the iliac fascia.
The appendix is a long, narrow, blind-ended tube at the inferior aspect of the cecum (Fig. 28), a small fold of mucous membrane at that junction may act as a valve. The appendix measures 20 mm to 80 mm in length and around 5–8 mm intraluminal diameter. It has its mesentery so it is mobile and the appendicular artery courses through the mesentery.
The ascending colon is that portion of large bowel which continues above the cecum, it is around 15 cm in length and is smaller in caliber than the cecum. The ascending colon is mostly retroperitoneal in location, sometimes in 10–15% of cases it may have its own mesentery and be mobile in abdominal cavity. The ascending colon lies on the iliac fascia and the anterior layer of lumbar fascia and passes upward, to the under surface of the right lobe of the liver. The ascending colon then bends to the left forming the hepatic flexure, beyond this point the large bowel continues as the transverse colon.
The transverse colon courses transversely from the hepatic flexure in right hypochondrium to the splenic flexure in left hypochondrium, while doing so it crosses the midline. The transverse colon is mobile in abdomen due to its mesentery. The transverse colon is the longest part of the colon and measures 45–60 cm in length. The splenic flexure of transverse colon is located in left hypochondrium and is related to lower end of the spleen and the tail of the pancreas. The splenic flexure lies at a higher level than the hepatic flexure; it is an acute bend which results in a small portion of transverse colon to lie in front of the descending colon. Beyond the splenic flexure of transverse colon lies the descending colon.
The descending colon extends from the splenic flexure to the pelvic brim; it is around 30–40 cm in length. The luminal caliber of descending colon is smaller as compared to the ascending colon. The descending colon lies on the lumbar and iliac fascia, it passes almost vertically downward through the left hypochondrium and lumbar regions along the lateral border of the left kidney. The peritoneum covers the anterior surface and sides of descending colon to fix it to the posterior abdominal wall. The small bowel loops are located in front and medial to the descending colon.
The sigmoid colon is the continuation of descending colon into the pelvic cavity. It extends from the level of the pelvic brim to the third sacral vertebra, beyond this point lies the rectum. The sigmoid colon is around 40–50 cm in length, has its own mesentery called the sigmoid mesocolon. The sigmoid colon has S-shaped course in pelvic cavity and lies in front of the rectum. Behind the sigmoid colon are the left external iliac vessels, the left pyriformis muscle, and left sacral plexus of nerves. Anterior to the sigmoid colon is the bladder in males and the uterus in females, these structures are separated from the sigmoid colon by some loops of small bowel.
The ascending colon and proximal 2/3rd of transverse colon is supplied by the superior mesenteric artery branches (right colic, ileocolic and middle colic arteries). The inferior mesenteric artery branches (left colic, sigmoid arteries) supply the descending colon and distal 1/3rd of transverse colon. The marginal artery of Drummond is a crucial arterial anastomosis that runs along the inner margin of the large bowel. The veins of large bowel accompany their arteries and eventually reach the portal vein through the superior mesenteric vein and inferior mesenteric vein. The lymphatics drain into the superior and inferior mesenteric lymph nodes. The sympathetics from the T12 to L2 supply the large bowel; the parasympathetic supply is from the vagal fibers.
The rectum is continuous above with the sigmoid colon, while below it ends in the anal canal. The rectum begins at the level of 3rd sacral vertebra, is about 12–15 cm in length, has no mesentery but some loose connective tissue (mesorectum) which shows as intermediate signal on T2W images. the upper 1/3rd of rectum is covered with peritoneum on the front and sides. This peritoneal covering in front of rectum also covers the superior surface of bladder in males (rectovesical pouch) and the superior surface of uterus in females (rectouterine pouch). Some small bowel loops are normally found in these rectovesical and rectouterine pouches. The rectum runs downwards along the sacrococcygeal curve and forwards to the level of tip of coccyx. The rectum during its decent has two lateral curves, one lateral is to the right side at the level of S3–S4 vertebrae and the other is to the left side at the level of sacrococcygeal junction. The rectum has an intraluminal caliber similar as that of sigmoid colon, however just proximal to the anorectal junction the rectum appears dilated; this portion of rectum is referred to as the rectal ampulla. As compared to the rest of large bowel the rectum has no external sacculations.
The anal canal is the terminal portion of the large bowel, it is around 4–6 cm in length in males and 3–4 cm in females. It is a hollow muscular tube that continues below the rectum. The proximal portion of anal canal forms an angle with the distal rectum, called 22 the anorectal sling supported by the puborectalis muscle. Behind the anal canal is a fibromuscular tissue called the anococcygeal body that lies between the coccyx and anal canal. In females anterior to the anal canal is the perineal body which is a mass of fibrous tissue that separates the anal canal from the lower end of vagina. In males the anal canal is related anteriorly to the membranous portion and bulb of urethra. On the lateral aspect of the anal canal is dense connective fatty tissue located between the ischium and anal canal, this region is called the ischioanal fossa.
The anal canal is supplied by the middle hemorrhoidal artery from the hypogastric artery, and the inferior hemorrhoidal from the internal pudendal artery. The superior hemorrhoidal artery from the inferior mesenteric artery divides into six small branches at the lower rectum and seconds to the anal canal to form anastomoses with other hemorrhoidal vessels around the anal canal. Similarly the inferior, middle and superior hemorrhoidal veins form an plexus of venous network, this creates an anastomoses between portal and systemic veins. The sympathetic plexus innervates the anal canal and the vagal fibers provide the parasympathetic fibers.
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.
The renal parenchyma has a cortex and medulla. The cortex is the thick outer layer of kidney and contains the glomerular apparatus, proximal and distal tubules, afferent and efferent arterioles. The medulla of kidney comprises of loop of Henle, medullary pyramids, collecting tubules, vasa recta, major and minor calyces. In cases of chronic renal diseases the renal parenchyma may be thinned out and cortical scarring seen. The renal pelvis is the portion of kidney that is hollow and gradually tapers towards the ureter. Urine from the collecting ducts drain into the renal pelvis. In cases of obstructive uropathy the renal pelvis may be dilated resulting in hydronephrosis.
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.
Urinary bladder
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 trigone of bladder is a triangular area, the two orifices of ureters and the internal urethral orifice forms the imaginary boundaries for this area. The trigone of the bladder is the posteroinferior region of bladder; it is the least mobile part of bladder because it is fixed to the superior surface of prostate by connective tissue. The superior surface of urinary bladder is directed upwards; the peritoneal covering separates the bladder from sigmoid colon and the loops of small bowel. The inferior surface of urinary bladder is directed downward and is not covered by peritoneum. The inferior surface may be divided into a prostatic surface and two lateral surfaces. The fatty tissue located behind the symphysis pubis interposes between the two lateral surfaces of the inferior surface of bladder. When the bladder is empty it is located completely within the pelvis. As the bladder distends with urine, its superior surface gradually rises into the abdominal cavity. In males, the rectovesical pouch lies behind the urinary bladder, it is the peritoneal fold which continues downwards and backwards to cover the posterior surface of urinary bladder. This rectovesical pouch then continues up to the rectum, when the urinary bladder is distended the distance between the bladder and rectum is around 8–10 cm. The ductus deferens is located in the posterior aspect of bladder in males. The peritoneum on the superior aspect of bladder runs backwards to cover the uterus in females. The superior and inferior vesical arteries provide blood supply to the urinary bladder, the small vesical veins drain into the vesicoprostatic plexus in males and vesicouterine plexus in females. The lymph drains into the external and internal iliac group of nodes. The superior and inferior hypogastric plexus provide the innervation of urinary bladder.
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 adrenal glands receive blood supply from small branches of abdominal aorta, renal artery and inferior phrenic arteries. The right adrenal vein drains into the inferior vena cava, while the left adrenal vein drains into the left renal vein. The lymph drains into the para-aortic lymph node group. The sympathetic nerve supply is from the splanchnic nerves.
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 branches of abdominal aorta (from above downwards) are divided into following groups:
Anterior Branches
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Coeliac trunk (T12 vertebral level)
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Superior mesenteric trunk (L1 vertebral level)
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Inferior mesenteric (L3 vertebral level)
Lateral Branches
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Inferior phrenic
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Suprarenal
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Renal
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Gonadal
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Spinal/lumbar
Terminal Branches
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Right and left common iliacs
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Median sacral
Coeliac trunk
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.
Inferior Mesenteric 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 subcostal arteries originate at the origin of the abdominal aorta in the abdominal cavity. The lumbar arteries are four in number and related to the upper four lumbar vertebrae. The iliolumbar artery is related to the last lumbar vertebra.
Renal Artery (Fig. 84)
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 abdominal aorta bifurcates into two common iliac arteries at the level of L4 vertebra. The common iliac arteries run downwards obliquely and in front of the sacroiliac joint bifurcate into external and internal iliac arteries.
The external iliac artery exits the pelvis to continue as the common femoral artery below the inguinal canal. The internal iliac artery runs downwards in the pelvis to divide into anterior and posterior division. The branches of anterior division of internal iliac artery are—superior vesical artery, inferior vesical artery, middle rectal artery, uterine artery, vaginal artery, obturator artery, inferior gluteal artery and internal pudendal artery. The branches of posterior division of internal iliac artery are—Iliolumbar artery, lateral sacral artery and superior gluteal artery.
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 Male Genital System
The prostate appears like an inverted pyramid structure below the urinary bladder in males on coronal MRI images.
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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.
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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.
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The ductus deferens (vas deferens) originates from the epididymis and travels through the inguinal canal and reaches the pelvic floor. The ducts deferens ascends to the posterior wall of bladder and turns downwards, a small dilated segment called the ampulla is noted parallel and medial to the seminal vesicles. The ductus deferens finally ends by emptying into the ejaculatory duct. The ductus deferens on MRI gives intermediate to high signal intensity on T2W images. The ductus deferens is supplied by the superior vesical artery.
The seminal vesicles are two lobulated sacs located on the posteroinferior surface of the bladder. On MRI images the seminal vesicle glandular tissue shows high signal intensity on T2WI and the walls of seminal vesicle show low signal intensity. The seminal vesicle produces the seminal fluid which empties into the ejaculatory duct. The seminal vesicles are supplied by inferior vesical artery and middle rectal artery.
The scrotum is a sac like structure located outside the pelvic cavity. The scrotal sac appears low signal intensity structure on T1WI and T2WI MRI images, the contents of scrotum show higher signal on MRI T2WI. The outer layer of scrotum is form by loose skin which allows the internal temperature of scrotum below the body temperature so that its contents—the testes and spermatic cord and epididymis maintain normal function and capability. The subcutaneous layer of scrotum contains dartos muscle; no fatty tissue is noted in subcutaneous layer. A thin midline septum in scrotum divides it into right and left scrotal sacs. The blood supply to the scrotum is from the superficial and deep external pudendal arteries. The venous drainage is to the external pudendal veins and great saphenous veins. The lymph drains to the superficial inguinal nodes. The genitofemoral nerve supplies the scrotal sac.
The testis shows high signal intensity on T2WI MRI images. Sometimes low signal intensity areas may be seen due to the radiating septae from mediastinum of testis. The right testis is located in the scrotal sac and the left testis is located in left scrotal sac. The right and left testis in the scrotal sac are separated by the midline septum. Each adult testis measures 50 mm × 25 mm × 30 mm (length × breadth × AP). The appendix of testis a small cyst like structure located at upper pole of each testis. The testicular artery from the abdominal aorta 45 supplies each testis, in the scrotal sac it divides into medial and lateral branches. The venules of testis pass through the mediastinum of testis and course in a mesh like fashion forming the pampiniform plexus, which surrounds the testicular artery. At the deep inguinal ring the pampiniform plexus unite to form the testicular vein. The left testicular vein joins with the left renal vein and the right testicular vein drains directly into the IVC. The testicular veins have valves and varicoceles (varicosities of pampiniform plexus) are more common on left side. Lymphatic drainage is to the para-aortic group of nodes. The testis is innervated by sympathetic T10 segment.
The epididymis is located on the posterior aspect of each testis. It appears like a coiled tubes compressed by fibrous tissue. Each epididymis has a head, body and tail. The head is connected to the testis, the tail of epididymis continues as the ductus deferens. The epididymis is supplied by the branches of testicular artery. The epididymis on MRI reveals inhomogeneous intermediate signal intensity on T2W images.
The Female Genital Organs
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.
The endometrium is a triple layered mucous membrane structure, it is a potential cavity where blood or fluid can collect and implantation in pregnancy takes place in endometrium. The endometrium is located in the middle of uterine body and fundus; it runs vertically downwards to the internal Os of cervix. Under normal circumstances in adult female the endometrium does not contain any fluid and shows high signal intensity on T2WI on MRI. The surrounding myometrium layer shows low signal intensity on T2WI on MRI. In postmenopausal women the myometrium shows very low signal intensity, probably related to undergoing atrophic changes. The subserosal layer of myometrium shows a patchy layer of intermediate signal intensity on T2WI. During menstruation or any pathology when fluid or blood accumulates within the endometrial cavity variable signal intensity on T2WI can be found.
The cervix is the lowermost part of uterus, and is more cylindrical in shape. On MRI T2WI the cervix shows low signal intensity similar to the myometrium. The junction of the cervix and body of uterus is the internal Os which opens into the cervical canal. The cervical canal is a narrow slit like canal and allows the passage of fluid from endometrial cavity into the vagina below. The opening of the cervical canal into the vagina is called the external Os.
The round ligament in the parametrium shows low signal intensity on T2WI. Each fallopian tube is around 100 mm in length; its medial end is located within the muscular walls of uterus. Normal fallopian tubes are not normally identified on MRI but when they contain blood or fluid they are readily seen as tortuous high signal intensity structures lying laterally in the parametrium, in such cases the muscle layer of the fallopian tube shows low signal intensity.
The ovary is an elliptical to ovoid structure located in the parametrium on either side. Each ovary measures 30 mm × 20 mm × 10 mm in size. The ovaries appear as intermediate intensity structures on T2WI on MRI. The follicular cyst wall appears as low signal intensity while its inner fluid contents appear as high signal intensity on T2WI.
The uterus receives its blood supply from the uterine artery, which is a branch of internal pudendal artery. The veins from the uterus form a plexus at the pelvic floor and eventually drain into the internal iliac veins. The lymph drains into the external and internal iliac nodes. The inferior hypogastric plexus from T10 to L1 segments innervates the uterus and cervix. The ovaries receive the ovarian arteries directly from the abdominal aorta. The lymph for the ovaries drains into the para-aortic nodes. The sympathetic fibers from T10 and T11 segments supply the ovaries.
The vagina is located below the cervix, is a hollow fibromuscular tube. The vagina is around 80 mm to 110 mm in length. On MRI T2WI the vaginal wall shows intermediate to low signal intensity. At the upper end of vagina the cervix projects opens into it creating anterior, posterior and lateral fornices in the vagina. The lower end of vagina opens into the vaginal orifice located between the labia minora. The urethral opening is located just anterior to the vaginal orifice. The internal iliac artery through its branches supplies the vagina. The venous drainage from vagina eventually drains into the internal iliac vein. The lymph from vagina drains into the external and internal iliac nodes. The lymph from the area around the vaginal orifice drains into the superficial inguinal nodes. The innervation of vagina is from small branches from the internal pudendal nerve.
The labia majora is formed by the lateral extension of subcutaneous fat (mons pubis) and skin anterior to the pubic symphysis. The subcutaneous fat is thrown into folds which cover the vaginal orifice, external urethral orifice and labia minora on either side, leaving 46 a small midline slit as an opening. The labia majora are seen as intermediate to high signal on T1WI MRI and low signal on STIR images.
The labia minora are cutaneous folds of tissue without any fatty tissue in it. The labia minora are located deeper to the labia majora; it covers the vaginal orifice and external urethral orifice. The labia minora gives an intermediate to low signal on T1W1 MRI images.
Male and Female urethra
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.
The prostatic urethra extends from the internal urethral orifice at bladder neck coursing in the parenchyma of prostate and exiting to the lower end of prostate. It has openings for the ejaculatory duct and prostatic ducts in its walls.
The membranous urethra extends from the lower end of prostate to the perineal membrane. It measures 1.5 cm in length, the shortest segment of male urethra. The membranous segment of posterior urethra is the least dilatable segment of male urethra.
The sphincter urethrae is also called as external urethral sphincter. The muscle fibers of sphincter urethrae appear like a pyramidal shape. The uppermost muscle fibers of sphincter urethrae forming its apex surround the lower prostatic urethra, and it extends downwards forming a base proximal to the perineal membrane.
The bulbar urethra extends from the perineal membrane to the base of penis. The bulbar urethra is a slightly dilated segment of male urethra; it courses almost at 90° from the posterior urethra to enter the penis.
The penile urethra runs in the ventral aspect of penis until the external urethral orifice. There is a small dilation proximal to the external urethral orifice; this dilatation is like a small cavity called the navicular fossa. The navicular fossa has a roof higher than the penile urethra and instrumentation into this area may create a false passage.
The female urethra is about 40–50 mm in length; due to the smaller length of urethra in females it is more prone for urinary tract infections. It extends from the neck of urinary bladder to the external urethral meatus. The external urethral meatus in females lies anterior to the vaginal orifice. Most of the course of female urethra is straight, so catheterization is much easier in females. Normal adult female urethra gives low signal intensity on T1WI and T2WI MRI images.