The kidneys

Table 5.4 Portosystemic anastomoses

Site Portal system Systemic veins

Gastroesophageal junction Left gastric vein Oesophageal veins

Rectum Superior rectal veins Inferior rectal veins

Retroperitoneum Tributaries in mesentery Retroperitoneal, renal, lumbar and phrenic veins

Umbilicus Paraumbilical veins with ligamentum teres Veins of the 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 w i th 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 w i th 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. 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 d i f f icult 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, w i th 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 w i th 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.

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.

Fig. 5.46 Structure of kidney.

On coronal cross-section (see Fig. 5.46) each kidney is seen to have an outer cortex and an inner medulla. Extensions of the cortex centrally as columns (of Bertin) separate the medulla into pyramids whose apices, jutting into the calyces, are called the papillae. Here are usually seven pairs of minor calyces, each pair having an anterior and a posterior calyx, although there is wide variation. Minor calyx pairs combine to form two or three major calyces, which in turn drain via their infundibula to the pelvis. This arrangement is quite variable, but when there are two infundibula these usually drain four calyces from the upper pole and three from the lower. When there are three infundibula there are usually three upper pole calyces, and two sets of two calyces draining the midpolar region and lower pole. The pelvis may be intrarenal or partially or entirely extrarenal. The gap between the renal substance and the pelvis is called the renal sinus and is filled w i th fat.

A simple calyx has one papilla indenting it; a compound calyx has more than one. Compound calyces are said to be less efficient at preventing intrarenal reflux of urine from the calyx and are more common in the upper pole.

The h i l um of the kidney lies medially, that of the left at L1 vertebral level and that of the right slightly lower at L1/L2 level, owing to the bulk of the liver above. At the hilum, the pelvis lies posteriorly and the renal vein anteriorly w i th the artery in between. The artery may branch early and a posterior arterial branch may enter the hilum posterior to the pelvis. Lymph vessels and nerves also enter at the hilum.

The functional subunit of the kidney is called the nephron and consists of a glomerulus in the cortex and a tubule in the medulla. This drains to a collecting duct, which empties into the calyx at the tip of the medulla. The kidney has approximately 1 million nephrons.

The relations of the kidneys

These are as follows:

• Posteriorly: — Upper third, diaphragm and twelfth rib and the costodiaphragmatic recess of the pleura — Lower third, medial to lateral: psoas, quadratus lumborum and transversus abdominis muscles. • Superiorly: the adrenal gland - more medial on the right kidney; and • Anteriorly: — Right kidney: liver second part of the duodenum ascending colon small intestinal loops — Left kidney: stomach pancreas and its vessels spleen splenic flexure of the colon jejunal loops.

Blood supply of the kidneys

The renal arteries normally arise from the aorta at L1/L2 level. The right renal artery is longer and lower than the left

Fig. 5.47 Coronal MRI through kidneys. Note - the axis of the kidneys as they lie on the psoas muscles and how the kidneys indent liver and spleen. The spinal canal is seen because of normal lordosis.

1. Right kidney 2. Right renal sinus 3. Posterior right lobe of liver 4. Spleen 5. Descending colon 6. L. vertebral body

7. L1/L2 disc space 8. Spinal canal containing cauda equina 9. Pulmonary vessels 10. Descending aorta 11. Psoas muscle

and passes posterior to the IVC. Both renal arteries usually have two divisions: one passes posterior to the renal pelvis and supplies the posterior upper part of the kidney; another anterior branch supplies the upper anterior kidney; a branch of the anterior division passes inferiorly and supplies the entire lower part of the kidney.

Within the hilum, the renal arteries divide inconsistently into five segmental branches which cross the renal sinus anterior and posterior to the pelvis and pierce the medulla in between the pyramids. These are termed interlobar arteries, as they pass between the lobes or pyramids. At the corticomedullary junction and the base of the pyramids the interlobar arteries become the arcuate arteries. They do not anastomose, but form arcades around the bases of the pyramids. Branches of the arcuate arteries give off the interlobular arteries, which run to the capsule. Afferent arterioles arise from the interlobular arteries to the glomeruli. Distal to the glomeruli, the efferent arterioles supply the convoluted tubules of the nephron unit.

Venous drainage

There is extensive anastomosis between the veins of the kidney. Five or six interlobular veins unite at the hilum to form the renal vein. The renal vein lies anterior to the pelvis at the hilum. The renal veins drain directly to the IVC. The left renal vein is much longer than the right and passes anterior to the aorta to reach the IVC. It also receives the inferior phrenic, adrenal and gonadal veins of that side. The right renal vein receives no extrarenal tributaries.

Lymphatic drainage

Lymph drainage follows the arteries to para-aortic nodes.

Fascial spaces around the kidneys (see Fig. 5.48) A true fibrous capsule surrounds the kidney. This, in turn, is surrounded by perirenal fat, which separates the kidney from the surrounding organs including the adrenal gland. A condensation of fibroareolar tissue around this fat forms the renal fascia. Thus the retroperitoneum is divided into three compartments: the perirenal space within the renal fascia, and the anterior and posterior pararenal spaces anterior and posterior to the renal fascia.

The renal fascia has an anterior leaf (Gerota's fascia) and a posterior leaf (Zuckerkandl's fascia). These fascial layers are fused laterally as the lateral conal fascia, which is continuous w i th the fascia on the deep surface of the trans¬ versalis abdominis muscle. Above, the layers blend w i th the diaphragmatic fascia. Medially, the anterior fascia fuses w i th the sheaths of the aorta and the IVC. The posterior fascia fuses w i th the psoas muscle. The perirenal space contains the kidneys and renal vasculature. There is no agreement in the literature regarding the upper limit of the perirenal space. The adrenal gland lies above the kidney, separated from it by fat. Most authors describe the adrenals as lying within the renal fascia.

Below, the perirenal space is relatively open. The anterior layer merges w i th the areolar tissue that binds the peritoneum w i th the posterior abdominal wall, preventing communication of the perirenal spaces across the midline. The posterior fascial layer becomes continuous w i th the fascia over the iliacus muscle. The fat within the perirenal space has septa that can lead to loculation of any urine, blood or pus that escapes into this space.

The space between the anterior renal fascia and the posterior peritoneum is called the anterior pararenal space. It is continuous across the midline and contains the pancreas, duodenum and ascending and descending colon. Some writers describe this as a multilaminar rather than a single space.

Posterior to the posterior renal fascia and anterior to the muscles of the posterior abdominal wall is the posterior pararenal space. This is limited medially by the attachment of the renal fascia to the psoas muscle, but is continuous laterally w i th the extraperitoneal fatty tissue deep to the transversalis muscle. It extends inferiorly to the fat anterior to the iliacus muscle. It contains only fat.

The development of the kidney

The glomerulus and proximal ductal system of the definitive kidney are derived from the metanephros. The collecting

Fig. 5.48 Fascial spaces of the retroperitoneum.

duct, calyces, pelvis and ureter are derived from the meta¬ nephric duct.

The kidney is formed in the pelvis from approximately 12 distinct lobules and assumes its adult position by the differential growth of the ureters relative to the trunk. It is supplied first by branches of the iliac artery and subsequently by series of vessels from the abdominal artery, each of which disappears as the kidney develops a new supply.

Developmental abnormalities and variants • The commonest anomaly of development is duplication of the collecting system, which occurs in 4% of individuals (see section on development of the ureter, below). • The adult kidney may retain some degree of fetal lobulation. This may involve the entire kidney or just the middle and lower thirds. It is frequently bilateral.

Fetal lobulation is distinguished from pathological scarring by the position of the surface notches: in fetal lobulation surface notches are between the calyces, whereas scarring occurs directly over the calyces. • It is common for the kidney to ascend to a normal position but retain a supply from one or more accessory arteries, which usually enter the lower pole below the hilum. • The kidneys may fuse during development and lead to a horseshoe kidney (1 in 700 births). In this condition, the kidney is fused across the midline in its lower pole.

The isthmus, which joins the kidneys, may be fibrous or composed of functioning renal tissue. In this condition

the kidney must fail to ascend, as branches of the aorta would impede its upward movement. Such kidneys are more prone to trauma as they lie across the vertebral column. • One or both kidneys may fail to migrate cranially, resulting in a persistent pelvic kidney supplied by a branch of the internal iliac artery (1 in 1500 births). • Rarely (1 in 2400 births) one kidney is absent. • The kidneys may be fused but lie on one side or the other in crossed fused ectopia. The lower pole of the normally situated kidney is fused w i th the upper pole of the ectopic kidney. • Very rarely both kidneys are completely fused in the pelvis in a condition referred to as pancake kidney. • A very rare anomaly called thoracic kidney (although the diaphragm is usually intact) occurs when the kidney is found much higher than its normal position; this may cause an opacity on a chest radiograph.

The adrenal glands are situated in their normal location in cases of abnormal renal migration, but their shape may be more discoid because of the absence of moulding by the kidneys during development.

Radiological features of the kidney

Plain films of the abdomen Perirenal fat often makes part or all of the renal outlines visible. Renal size is variable, w i th a normal range of 10-15 cm on a radiograph or approximately three-and-a-half vertebral

bodies in height. The left kidney is usually larger, but a difference in size of more than 2 cm is abnormal. The kidneys are relatively larger in the child (approximately four vertebral bodies in height), and normal limits of size have been plotted against height or age.

The kidneys are seen to move w i th changes from supine to erect positions and, because of their relationship w i th the diaphragm, to move w i th respiration. These moves may be more than 5 cm and are generally greater in women than men. As the kidney moves inferiorly its degree of tilt increases, w i th its lower pole becoming more anterior. It may, as a result, appear foreshortened on a radiograph.

Because the colon is anterior to both kidneys and the stomach is anterior to the left kidney and the duodenum to the right, gas in these organs may overlie the renal outlines in a radiograph. Confusing gas and faecal shadows in the colon can be minimized by bowel preparation prior to radiography. Stomach rugal-fold patterns can also cause misleading shadows. The anterior relationship of the stomach is used to advantage in filling the stomach w i th gas, for example by a carbonated drink in renal radiography, especially in children.

Intravenous urography (Fig. 5.49) The renal outline can be seen in the nephrographic phase of intravenous urography (IVU) in most cases. Tomography can help eliminate overlying bowel gas and other shadows. Tomographic cuts for the upper pole are more posterior to those for the lower pole because of the slope of the kidney. Features of size and of renal movement are as for plain films. Fetal lobulation and its relationship to the calyces can be appreciated. Prominence of the midportion of the lateral border of the left kidney is a normal variant called a dromedary hump or splenic hump (because of the proximity of the spleen).

In the urographic phase the calyceal system can be seen. Minor and major calyces are seen. These are connected to the pelvis of the kidney by infundibulae, which may be long or short. Occasionally a calyx is connected directly to the pelvis of the kidney. The papillae are conical and indent the calyces w i th surrounding sharp fornices. Several papillae may indent a single calyx - known as a complex calyx - an arrangement that is more common in the upper pole. Calyces viewed end-on w i ll appear foreshortened. Their infundibulae w i ll be better seen on oblique views.

Contrast in the collecting tubules in the papilla is responsible for the papillary blush that is sometimes seen.

Renal vessels passing close to the renal pelvis and calyces can cause filling defects. These vascular impressions are less obvious in well filled collecting systems.

The renal pelvis is bifid in 10% of cases. This may be associated w i th complete or partial renal duplication. Partial renal duplication may lead to hypertrophy of septal cortex in the midportion of the kidney - known as hypertrophied column of Bertin - causing a pseudomass appearance.

If renal sinus fat is prominent this may cause thinning and elongation of the infundibulae on IVU ('spidery' calyces).

Ultrasound examination of the kidneys The renal size is not magnified on ultrasound and so is smaller than on radiographs - normally 9-12 cm. The renal outline is usually smooth. The cortical thickness is uniform, but is slightly more prominent at the upper and lower poles. Persistent lobulation may produce subtle indentations but does not narrow the cortical thickness. The cortex may appear thicker on the lateral aspect of the left kidney when there is a splenic hump. An echogenic line of fat running from the echogenic sinus fat to the cortex may be seen; this is termed a junctional cortical defect.

The cortex is distinguishable from the relatively hypoechoic renal pyramids - a difference that is more marked in the young infant. The cortex is usually less echogenic than that of liver or spleen, but in the young infant is isoor hyperechoic w i th respect to these organs.

The renal sinus contains fat, calyces, infundibulae and vessels, and is highly echogenic because of the multiple tissue interfaces. The renal pelvis may be intrarenal or extrarenal. When extrarenal, it may appear dilated. Visualization of the collecting system is variable. It is best seen when the subject is well hydrated and/or in a state of diuresis.

The aorta, IVC and renal artery and vein are visible on ultrasound. The structures at the hilum (i.e. the pelvis, artery and vein from anterior to posterior) can also be identified. Blood flow can be assessed w i th colour flow Doppler and pulsed wave Doppler.

In addition, the relationship of the right kidney to the liver and the left to the spleen and pancreas can be appreciated. Gas in the stomach, duodenum, small intestine and colon, which are anterior to the kidneys, may obscure their view from an anterior scanning approach on ultrasound, but visualization is almost always possible from a posterolateral, lateral or posterior approach.

CT and MRI (see Figs 5.3, 5.4 and 5.47) The kidneys are seen on slices from T12 to L3 vertebral levels.

Posterior relations (i.e. diaphragm, pleura and ribs, psoas, quadratus lumborum and transversus abdominis muscles) and anterior relations (i.e. liver, pancreas, spleen and gastrointestinal tract) can be seen on axial CT images, but are very well appreciated on sagittal and coronal MR images.

The kidney is seen to be surrounded by perinephric fat. This is most abundant medial to the lower pole, and this is a favoured site of accumulation of blood or urine in the ruptured kidney and of pus in a perirenal abscess. The renal fascia is less than 1 mm thick in the normal subject and can