THE ABDOMEN
Table 5 . 4
Portosystemic anastomoses
Site
Portal system
Systemic veins
Gastro-
Left gastric vein
Oesophageal veins
esophageal junction Rectum
Superior rectal veins
Inferior rectal veins
Retroperitoneum
Tributaries in mesentery
Retroperitoneal, renal, lumbar and phrenic veins
Umbilicus
Paraumbilical veins
Veins of the
with ligamentum teres
abdominal wall
Radiological features of the portal venous system Plain films of the abdomen The normal portal veins are not visible. If there is gas in the portal veins in the liver it is distinguished from gas in the bile ducts by its peripheral position. Ultrasound (see Fig. 5.34) The portal vein is visible on ultrasound as it passes towards the liver posterior to the common bile duct and hepatic artery (see Fig. 5.34). Its diameter is variable but always greater than that of the normal bile duct. Portal vein branches in the liver are seen as having more echogenic walls than the branches of the bile duct. The splenic vein is an important landmark in ultrasound as it passes posterior to the pancreas (see Fig. 5.42). It helps identify this gland. An abnormality in its course around the vertebrae and prevertebral structures can be an indication of a mass here. The direction and velocity of blood flow, which is important to evaluate in portal hypertension, can be assessed with pulsed Doppler interrogation of the portal vein. Colour flow Doppler helps in identification of the vessels. Computed tomography (see Figs 5.2, 5.3, 5.10 and 5.11) The portal vein can be seen in the porta hepatis and its branches are seen in the liver, generally posterior to the bile duct and hepatic artery branches. The portal vein is seen posteriorly in the free edge of the lesser omentum. Here it is separated from the IVC by the epiploic foramen. This space, called the portocaval space, is the site of normal lymph nodes in the lesser omentum and of abnormal masses. The splenic vein is seen posterior to the pancreas and unites with the inferior mesenteric vein laterally and the superior mesenteric vein behind the neck of the pancreas (see Fig. 5.42). It lies anterior to the left kidney and its hilum, the renal vein and IVC.
The superior mesenteric vein is seen to the right of the artery on lower slices, and together these pass anterior to the uncinate process of the pancreas. Magnetic resonance imaging MR angiography of the portal system is an excellent method of providing detailed information regarding portal vein anatomy and portosystemic collateral vessels. Information regarding portal blood flow direction and velocity can be obtained using 'time of flight' or phase contrast angiographic techniques without the need for intravenous contrast. With these techniques the background signal is suppressed and flowing blood is bright. The source data can be viewed as MR angiograms. Portography Direct portography This includes splenoportography, where contrast is introduced directly into the spleen and outlines the splenic vein and the portal vein. A transhepatic route can also be used to cannulate the portal vein and via this the splenic or other veins. This can be used to sclerose oesophageal varices or to do venous sampling to isolate a hormone-producing pancreatic tumour. Transumbilical portography can be performed in the neonate by catheterizing the umbilical vein, which drains to the left portal vein. Indirect portography Contrast is injected into the coeliac and superior mesenteric arteries (sequentially or together) and films are taken in the venous phase. To show the inferior mesenteric vein that artery must also be injected. Images can be acquired by film-screen techniques, or more easily using digital subtraction angiography. If the spleen is very large the splenic vein may be difficult to visualize because of pooling of contrast in the spleen. Flow of blood in the portal venous system is slow and there is poor mixing of blood from the splenic and superior mesenteric veins, with the former supplying principally the left lobe of the liver and the latter the right lobe. This should not be misinterpreted when only one artery is injected. Contrast in the liver preferentially fills the right lobe of the liver in the supine patient because of the more posterior position of this lobe. To overcome this effect of gravity it may be necessary to rotate the patient. THE KIDNEYS (Figs 5.46-5.49) The kidneys lie retroperitoneally in the paravertebral gutters of the posterior abdominal wall. They lie obliquely with their upper poles more medial and more posterior than their lower. The kidneys measure 10-15 cm in length, the left being commonly 1.5 cm longer than the right. Their size is approximately that of three-and-a-half lumbar vertebrae and their associated discs on a radiograph.
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