First principleschapter 1

For convenience the abdominal cavity is divided into nine segments (Figure 1.1). These regions can be demarcated on an abdominal radiograph by drawing a horizontal line through the 9th ribs and the pelvic brim, and two vertical lines from the centre of the costal cartilage of the 9th rib to the middle of the inguinal ligament. The organs (Figure 1.2) contained in these segments are as follows:

The liver is the largest organ in the abdomen and consists of a right and a left lobe, divided by the longitudinal fissure, which is seen as a notch in the liver contour (Figure 1.3). On radiographs, the liver is seen as a triangular structure on the right; its undersurface may be outlined by fat and is visible across the right hypochondrium or lumbar region. On cross-sectional imaging the liver is seen on the right. If intravenous contrast is used, the blood vessels appear of higher signal density than the liver parenchyma.3

Lobes of the liver The right lobe is much larger than the left, though this morphology may be reversed in certain pathologies such as cirrhosis, where the right lobe atrophies and appears to be of similar size or smaller than the left lobe. Between 4% and 14% of the population have a prominent inferior extension of the right lower lobe, known as Riedel's lobe. This lobe usually extends caudally below the iliac crest.

Blood supply The liver derives its blood supply from two sources: the hepatic artery and the portal vein.

The pancreas consists of a head, body and tail and is situated transversely across the posterior wall of the abdomen, with its body at the level of T12 (Figure 1.6). The head of the pancreas is curved on itself and located along the concavity of the second and third parts of the duodenum. The body of the pancreas is covered anteriorly by layers of the transverse mesocolon and posterior surface of the stomach. Therefore inflammation of the pancreas can involve the colon via the mesocolon and the stomach bed. The pancreas is supplied by the pancreaticoduodenal branch of the hepatic artery and branches from the splenic arteries. Its venous drainage is into the splenic and superior mesenteric veins.6

The gallbladder is a pear-shaped structure located on the undersurface of the liver, consisting of a fundus, body and neck. It is a storage organ for bile produced by the liver, and drains via the cystic duct into the common hepatic duct to form the common bile duct (CBD). The common hepatic duct is formed by the union of the right and left hepatic ducts (Figure 1.7). The CBD joins the pancreatic ducts and opens at the duodenal ampulla. It lies to the right of the hepatic artery and in front of the portal vein as it descends to open out at the ampulla.

The spleen is an oblong organ located in the left hypochondrium beneath the left 10th rib and hemidiaphragm posterolateral to the gastric fundus (Figure 1.8). It is attached to the stomach by the gastrosplenic ligaments, which contain the vascular supply consisting of the splenic artery and veins. The spleen is usually <12 cm in length in an adult, and the splenic vein measures up to 1 cm in diameter. The most inferior surface of the spleen abuts the phrenicocolic ligament, a peritoneal fold that marks the anatomical splenic flexure of the colon.7

The kidneys are located at the posterior wall of the abdomen in the retroperitoneal region. They extend from approximately the 11th rib to the iliac crests. The right kidney is located slightly lower than the left due to the large size of the liver. The external or lateral border is convex, whereas the internal border is concave, and contains a deep notch, which is known at the hilum of the kidney (Figure 1.9). The renal vessels, nerves and uterus enter the kidney at the hilum.

The stomach is situated mainly in the left hypochondrium and epigastrium (Figure 1.10). It consists of the fundus, body and antrum. The distal-most part of the stomach is the pyloric canal, which communicates with the first part of the duodenum. The lesser curvature of the stomach extends from the oesophageal to the pyloric orifice, along the upper border of the organ, and is attached to the undersurface of the liver via the lesser omentum; the greater curvature is along the outer border and gives attachment to the deep greater omentum. The stomach is supplied by the right gastroepiploic branches of the hepatic and the left gastroepiploic branches of the splenic arteries.

At between 3 and 7 metres, the adult small intestine consists of:

The colon starts at the caecum, where it communicates with the terminal ileum via the ileocaecal valve, and extends clockwise 1.5 m around the abdomen to the rectum. The colon is larger in diameter and more fixed in position than the small intestine, and has characteristic small pouches, called ‘haustra’, caused by sacculations (folding) of the colonic wall. The caecum is located in the right iliac fossa (Figure 1.12). The ileocaecal valve is usually on the posteromedial wall of the caecum, and the appendix approximately 2 cm below the valve.

The ascending and descending portions of the colon are retroperitoneal and therefore fixed in position. Conversely, the transverse colon, caecum and sigmoid colon are attached to their respective mesocolon (a double layer of peritoneum) and hang freely within the abdominal cavity. Therefore, these segments may be tortuous or mobile. At the left iliac fossa the colon becomes more tortuous as it forms the sigmoid colon. It enters the pelvis and descends along the posterior wall and presacral space, to form the rectum and anal canal.

The abdominal aorta starts at the diaphragmatic opening at the level of the T12 vertebral. It usually descends slightly to the left of the vertebral column and terminates at the level of L4 where it divides into the two common iliac arteries (Figure 1.13). The major visceral branches of the aorta are the coeliac axis, superior and inferior mesenteric arteries, suprarenal arteries, renal arteries and spermatic arteries. Parietal branches (parietal = relating to the walls of a part or cavity) are the phrenic, lumbar and sacral arteries.

Radiographs are formed by X-rays passing through the body and forming a latent image on the film or sensor placed behind. Different tissues of the body absorb different amounts of X-ray photons, producing different densities on the image – a ‘shadow’ of tissues. Bones absorb most of the X-ray photons because they have a higher electron density than soft tissues. When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film.

Several other techniques are used to enhance the normal X-ray examination. For example, injection of iodine-containing intravenous contrast can delineate the vasculature (iodine, being dense, outlines the blood vessels against the soft tissues). This technique is known as angiography (Figure 1.14).14

Fluoroscopy is another technique in which X-rays are used to image the body in real time. In this type of examination, continuous X-ray exposure of the body (e.g. 2 frames/s or more) produces a cine image.15

For example, fluoroscopy may be used in assessing peristalsis of the bowel on a barium follow-through examination or the swallowing mechanism in a barium swallow examination. Fluoroscopy is also particularly useful in guiding interventional procedures such as biopsies and drainages.

Intravenous and oral contrast are usually administered during abdominal CT examinations to delineate the GI tract and vasculature. Intravenous contrast is particularly useful in assessing the vascularity or enhancement patterns of tumours and abnormal tissue. Intravenous contrast contains iodine, which appears hyperdense on CT images so blood vessels appear dense. Recent advances in CT technology allow scans to be acquired within seconds and powerful software allows detailed three-dimensional images of the body.17

Ultrasound (US) uses high-frequency sound waves (>2 mHz) to visualise body tissues with real-time sonographic images. It was originally developed from military SONAR technology used in the navy, and is now one of the most widely used diagnostic tools.

A handheld transducer (probe) emits high frequency sound waves (above 20 kHz) that are reflected to varying degrees by the body tissues. These reflected echoes are sensed by the same probe and used to create the image. Dense tissues and material reflect more soundwaves (hyperechoic) and hence appear light (Figure 1.18). Conversely, fluid and air reflect less (hypoechoic) and transmit more of the soundwaves and appear dark. The advantages of US include its lack of radiation and the ability to visualise tissue in real time.

US examinations can be supplemented with Doppler scans that assess blood flow in body tissues (Figure 1.19). Doppler imaging uses the physical principle of the Doppler effect to assess whether blood is moving towards or away from the probe, and its relative velocity.

By convention, blood flowing towards the probe is labelled red whereas blood flowing away from it is labelled blue on US images.

First used in 1977, magnetic resonance imaging (MRI) uses the spin properties of hydrogen nuclei to generate images. It uses a powerful magnetic field to align all the hydrogen atoms in the body. Once aligned the atoms have a net longitudinal magnetic moment. A switching radiofrequency pulse is then used to disrupt this alignment. The radiofrequency pulse imparts extra energy to the atoms and they spin out of the longitudinal arrangement into a transverse orientation. Once the pulse is switched off, the hydrogen atoms again realign longitudinally to the magnetic field. In returning from a transverse to a longitudinal orientation, a rotating, diminishing magnetic field 20is created. The return of the excited nuclei from the high-energy to the low-energy state is associated with the loss of energy to the surrounding nuclei and MR images are based on the observation of this relaxation that takes place after the radiofrequency pulse has stopped.

MR images take longer to acquire because a number of different sequences are required to define anatomical detail, organ-specific sequences and sequences for highlighting specific disease processes. Each individual sequence may take about 5 min and a typical examination may use several sequences. The dynamic enhancement patterns of abdominal organs can also be assessed by using ultrafast sequences. MRI has the advantage over CT and radiographs that ionising radiation is not involved.

Intravenous contrast agents used in MRI contain gadolinium or manganese, which have paramagnetic properties. Unlike CT or US, which use only X-rays or sound waves to generate images, MRI exploits a long list of tissue properties to generate images and thus can be more tissue specific. For example, blood flow within the arteries can be used to generate angiographic images without having to use a contrast agent.

Nuclear imaging relies on specific radionuclides that are injected into the body for production of images. A radionuclide is an atom with an unstable nucleus, which undergoes radioactive decay and in doing so produces ionising radiation such as gamma rays and subatomic particles. These emissions are detected by cameras to produce images. Radionuclides may occur naturally, but can also be artificially produced. Some commonly used radionuclides are isotopes of technetium (^99mTc) and iodine (¹²³I and ¹³¹I), thallium-201, gallium-67 and indium-111.

Abdominal interventions such as biopsies or drainages may be carried out by radiologists under imaging guidance using fluoroscopy, US, CT or MRI. These imaging modalities are typically used to guide needles or catheters into correct anatomical locations. For example US or fluoroscopy may be used to guide puncture of the bile duct in a percutaneous transhepatic cholangiogram examination (Figures 1.22 and 1.23).

Imaging is also required for more complex interventions such as placing of stents or angioplasties.