Axial Anatomy

D  Karthikeyan; Deepa  Chegu

Axial AnatomyCHAPTER 1

NORMAL AXIAL CT ANATOMY

FIGURE 1.1: Axial CT sections showing abdominal wall muscles.

Liver Segments

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SURFACE REFORMATION

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FIGURE 1.8: Pancreas.

FIGURE 1.9: Axial section at the celiac axis level.

FIGURE 1.10: Axial sections at the level of SMA.

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FIGURE 1.12: Axial section at the level of umbilicus.

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FIGURE 1.14: Axial section at the level of caecum.

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FIGURE 1.17: Axial section at the level of urinary bladder.

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Uterus and Ovaries

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Muscles and Pelvic Structures

FIGURE 1.23: Axial section at the level of acetabular roof. RA—Rectus abdominus, PS—Psoas, GMIN—Gluteus minimus GMED—Gluteus medius, GM—Gluteus maximus, PI—Piriformis.

FIGURE 1.24: Axial CT at the level of pubic symphysis. OI—Obturator internus, OE—Obturator externus, SA—Sartorius, PEC—Pectineus, PS—Psoas, RF—Rectus femoris.

Individual Bowel Patterns

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FIGURE 1.31:

CT Peritoneogram—Showing Peritoneal Spaces

Peritoneal Spaces

The peritoneal cavity is divided into two main compartments, the supramesocolic and the inframesocolic, by the transverse colon and its mesentery connecting it to the posterior abdominal wall. The root of the transverse mesocolon subextends across the infra-ampullary segment of the descending duodenum, the head of the pancreas, and continues along the lower edge of the body and tail of the pancreas.

The Supramesocolic Compartment

The supramesocolic compartment can be arbitrarily divided into right and left supramesocolic peritoneal spaces. These regions can also be divided into a number of subspaces, which are normally in communication, but often become separated by inflammatory membranes in disease.

Right Supramesocolic Space

The right supramesocolic space has three subextends spaces: (a) the right subphrenic space; (b) the right subhepatic space, which can be further divided arbitrarily into anterior and posterior areas; and (c) the lesser sac. The right 8subphrenic space extends over the diaphragmatic surface of the right lobe of the liver to the right coronary ligament posteroinferiorly and the falciform ligament medially, which separates it from the left subphrenic space. In the presence of infected fluid, pyogenic membranes may divide the right subphrenic space into anterior and posterior compartments.

The right subhepatic space can be arbitrarily divided into anterior and posterior spaces. The anterior right subhepatic space is limited inferiorly by the transverse colon and its mesentery. The posterior right subhepatic space, also known as the hepatorenal fossa or Morison's pouch, extends posteriorly to the parietal peritoneum overlying the right kidney. Superiorly the right subhepatic space is bounded by the inferior surface of the right lobe of the liver.

It communicates freely with the right subphrenic space and the right paracolic gutter. In the supine patient, the posterior right subhepatic space (the hepatorenal fossa or Morison's pouch) is more dependent than the right paracolic gutter, and thus under the force of gravity, fluid collections are common in this location.

The lesser sac extends to the left, behind the stomach and anterior to the pancreas. It is considered to be part of the right supramesocolic space, as embryologically the growth of the liver into the right peritoneal space stretches the dorsal mesentery and forms the future lesser sac posterior to the stomach. It communicates with the rest of the peritoneal cavity through a narrow inlet, the epiploic foramen (foramen of Winslow), between the inferior vena cava and the free margin of the hepatoduodenal ligament. The lesser sac lies posterior to the lesser omentum, stomach, duodenal bulb and gastrocolic ligament.

FIGURE 1.32: Coronal CT reformation after CT peritoneogram revealing the peritoneal spaces. RSP—right subphrenic space, SHS—subhepatic space, RPCS—right paracolic space, LPCS—left paracolic space.

A prominent oblique fold of peritoneum is raised on the posterior wall of the lesser sac by the left gastric artery, dividing it into two major recesses. The smaller superior recess completely encloses the caudate lobe of the liver. At the porta hepatis this recess lies posterior to the portal vein. Superiorly it extends deep into the fissure for the ligamentum venosum and posteriorly lies adjacent to the right diaphragmatic crus. The larger inferior recess lies between the stomach and the pancreas. It is bounded inferiorly by the transverse colon and its mesentery, but can extend for a variable distance between the leaves of the greater omentum. To the left it is bounded by the gastrosplenic and splenorenal ligaments which meet at the splenic hilum.

Left Supramesocolic Space

The left supramesocolic space has four arbitrary subspaces, which are in communication in normal anatomy: (a) the anterior left perihepatic space; (b) the posterior left perihepatic space, surrounding the lateral segment of the left hepatic lobe; (c) the anterior left subphrenic spaces;and (d) the posterior left subphrenic (perisplenic) space, superior to gastric fundus and spleen.

The left anterior perihepatic space is bounded medially by the falciform ligament, posteriorly by the liver surface and left coronary ligament, and anteriorly by the diaphragm. It communicates superiorly and to the left with the left anterior perihepatic space is bounded medially by the falciform ligament, posteriorly by the liver surface and left coronary ligament, and anteriorly by the diaphragm. It communicates superiorly and to the left with the left anterior subphrenic space, and inferiorly with the greater peritoneal cavity over the surface of the transverse mesocolon.

FIGURE 1.33: CT peritoneogram showing right subphrenic space (RSP), left subphrenic space (LSP), phrenicolic ligament (PCL).

FIGURE 1.34: CT peritoneogram showing the gastro-hepatic ligament.

FIGURE 1.35: CT peritoneogram showing the recess of lesser sac.

The Inframesocolic Compartment

The inframesocolic compartment is divided into two unequal spaces posteriorly by the root of the small bowel mesentery, as this runs from the duodenojejunal flexure in the left upper quadrant to the ileocaecal valve in the right lower quadrant. It also contains the right and left paracolic gutter lateral to the ascending and descending colon.

The Right Inframesocolic Space

This triangular space is smaller than its counterpart on the left. It is bounded by the transverse colon superiorly and to the right, and by the root of the small bowel mesentery, as this runs from the duodenojejunal flexure to the ileocaecal junction inferiorly and to the left.

The Left Inframesocolic Space

This space is larger than its counterpart on the right and is in free communication with the pelvis on the right of the midline. The sigmoid colon and its associated mesentery form a partial barrier on the left of the midline.10

The Paracolic Gutters

These are the peritoneal recesses on the posterior gastroabdominal wall lateral to the ascending and descending colon. The right paracolic gutter is continuous superiorly with the right subhepatic and subphrenic spaces. It is larger than the left paracolic gutter, which is partially separated from the left subphrenic spaces by the phrenicocolic ligament. Both paracolic spaces are in continuity with the pelvic peritoneal spaces.

The Pelvic Peritoneal Spaces

Inferiorly the peritoneum is reflected over the fundus of the bladder, the anterior and posterior surface of the uterus and upper posterior vagina in females, and on to the front of the rectum at the junction of its middle and lower thirds. The urinary bladder subdivides the pelvis into right and left paravesical spaces. In men parathere is only one potential space for fluid collection posterior to the bladder, the rectovesical pouch.

In women there are two potential spaces posterior to the bladder, the uterovesical pouch, and posterior to the uterus the deeper rectouterine pouch (pouch of Douglas). The layers of peritoneum on the anterior and posterior surcolon faces of the uterus are reflected laterally to the pelvic side walls as the broad ligaments, containing the uterine (Fallopian) tubes.

Omentum

This is a double-layered sheet or fold of peritoneum.
The lesser and greater omentum attach the stomach to the body wall or to other abdominal organs.

The Lesser Omentum

This fold of peritoneum connects the lesser curvature of the stomach and the proximal part of the duodenum to the liver.
Individually, these connections are referred to as the gastrohepatic ligament and the hepatoduodenal ligament.
The lesser omentum lies posterior to the left lobe of the liver and is attached to the liver in the fissure for the ligamentum venosum.
It is also attached to the porta hepatis, the transverse fissure or gate (L. porta) on the inferior surface of the liver through which the bile duct, vessels, and nerves enter or leave the liver.

The Greater Omentum

This is a fat-laden fold of peritoneum that hangs down from the greater curvature of the stomach and connects the stomach with the diaphragm, spleen, and transverse colon.
This double-layered peritoneal fold normally fuses during the fetal period, thereby obliterating the inferior recess of the omental bursa.
As a result, the apron-like greater omentum is composed of four layers of peritoneum.
After passing inferiorly as far as the pelvis, the greater omentum loops back on itself, overlying and attaching to the transverse colon.

Peritoneal Ligaments

A peritoneal ligament is a double layer of peritoneum that connects an organ with another organ or with the abdominal wall.
Ligaments may contain blood vessels or remnants of vessels (e.g. the falciform ligament contains the ligamentum teres, a remnant of the fetal umbilical vein).11
The greater omentum is divided into 3 parts:
1. The apron-like part, called the gastrocolic ligament, is attached to the transverse colon.
2. The left part, called the gastrosplenic ligament (gastrolienal ligament), connects the hilum of the spleen to the greater curvature and fundus of the stomach.
3. The superior part called the gastrophrenic ligament is attached to the diaphragm and the posterior aspect of the fundus and the esophagus.
The falciform ligament extends from the liver to the anterior abdominal wall and the diaphragm.
The ligamentum teres is the obliterated remnant of the left umbilical vein, lying in the free edge of the falciform ligament and extending from the groove for the ligamentum teres to the umbilicus.
The superior (anterior) and inferior (posterior) layers of the coronary ligament are reflections of the peritoneum, which surround the bare area of the liver.
The left and right triangular ligaments are where the layers of the coronary ligament meet to the left and right respectively.
The falciform, coronary and triangular ligaments are derived from that part of the ventral mesogastrium connecting the liver to the body wall.
The gastrohepatic and hepatoduodenal ligaments are derived from that part of the ventral mesogastrium connecting the stomach and the liver.
The gastrosplenic and gastrophrenic, as well as the lienorenal and phrenicolienal ligaments are derived from the dorsal mesogastrium.

FIGURE 1.36: CT peritoneogram showing folds of mesentery.

The Peritoneal Folds

A peritoneal fold (L. plica) is a reflection of peritoneum with more or less sharp borders. Often it is formed by peritoneum that covers blood vessels, ducts, and obliterated fetal vessels.
Several folds are visible on the parietal peritoneum on the interior of the anterior abdominal wall.
The median umbilical fold contains the urachus, which extends from the urinary bladder to the umbilicus.
The medial umbilical folds are raised by the obliterated umbilical arteries, extending from the internal iliac arteries to the umbilicus.
The lateral umbilical folds are raised by the inferior epigastric arteries, extending from the deep inguinal rings on each side to the arcuate lines.12

Peritoneal Pouches

The rectouterine pouch (in females) separating the rectum from the bladder.
The rectovesical pouch (in males) separating the rectum from the bladder.
The vesicouterine pouch (in females) separating the bladder from the uterus.

FIGURE 1.37: CT peritoneogram showing pelvic spaces.

FIGURE 1.38:
Falciform ligament (FL)
Lienorenal ligament (LR)
Gastrolienal ligament (GL)
Lesser omentum (LO)

RETROPERITONEUM

Extra peritoneal space is divided into the anterior and posterior pararenal space and the perinephric space by the anterior and posterior layers of the renal fascia. Both these fascia fuse to form the lateral conal fascia behind the descending colon.

ANTERIOR PARARENAL COMPARTMENT

The anterior pararenal compartment lies between the anterior renal fascia and the posterior parietal peritoneum. The lateral border is defined by the lateroconal fascia and the compartment is potentially contiguous across the midline. Contents include pancreas, the descending, horizontal and terminal portions of duodenum, the ascending and descending colon.

Perinephric Compartment

Formed by fusion of the anterior (gerotas) and posterior (zuckercandles) fascia superiorly it fuses with the diaphragmatic fascia and laterally with the lateroconal fascia.

The inferior portion of the space is open towards the iliac fossa. Medially the posterior renal fascia fuses with the quadratus and psoas fascia.

Contents include adrenals, kidneys, renal vasculature, proximal part of renal collecting system.13

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Posterior Pararenal Compartment

It lies between the posterior renal fascia and the transversalis fascia, it contains fat tissue and continues laterally as the peritoneal fat. The space is open inferiorly at the iliac crest. Medially the space is contained by the fusion of the transversalis and psoas fascias.

Chapter Notes

Axial AnatomyCHAPTER 1