Spleen

MRI of the pancreas (see Fig. 5.36) The pancreas has the shortest T1 of the abdominal organs and therefore a higher signal intensity on T1-weighted imaging, equivalent to or slightly higher then that of normal liver. Intrinsic contrast is good on T1 imaging, especially when the surrounding fat is suppressed. The pancreas is also well seen on T2-weighted imaging, and faster sequences, including breath-hold sequences, reduce artefact from breathing. The pancreas is very vascular and enhances intensely during the arterial phase of a gadolinium bolus.

MR pancreatography depicts the normal ductal system as well as congenital variations. The normal pancreatic duct is 2 mm and its numerous side branches can be seen draining from the lobules into the duct in a perpendicular fashion.

Endoscopic retrograde cholangiopancreatography (see Fig. 5.35) ERCP visualizes the pancreatic duct by injection of contrast after cannulation via duodenal endoscopy. The main duct is cannulated in common w i th the bile duct when the anatomy of the bile duct is normal. The main duct is seen to begin by the union of small ducts in the tail. It passes obliquely downwards and to the right across the L1 vertebra and ends in a dilated ampulla before entering the duodenum. The duct is 16 cm long and measures up to 4 mm in diameter in the head. The accessory duct may be filled via its communication w i th the main duct and is seen to pass anteriorly and superiorly to the main duct.

Branches join the main duct at right-angles. A pancreatogram is obtained when enough contrast is injected to f i ll the acini. This is undesirable, as it is associated w i th a higher incidence of post-ERCP complications.

The pancreatic duct is foreshortened in PA views on ERCP because of the posterior course of part of the gland between the head, which lies on the aorta and the IVC, and the tail, which lies in the paravertebral gutter. Right posterior oblique views may be helpful.

Pancreatic duct measurements are higher on ERCP than on MRCP and ultrasound because they distend as they are filled w i th contrast on ERCP.

Angiography of the pancreas This technique shows the vessels as described. Coeliac and superior mesenteric arteries must be opacified as the superior and inferior pancreaticoduodenal branches arise from them. These vessels may also be demonstrated by CT or MR angiography.

Venography of the pancreas This technique is sometimes undertaken for venous sampling to identify the site of a hormone-producing tumour. A transhepatic approach to the portal, splenic and superior mesenteric veins is used, w i th samples taken from various sites along these vessels in an attempt to identify a high hormone level at the site of the occult tumour.

THE SPLEEN (Fig. 5.43) The spleen is found in the left upper quadrant of the abdomen. It arises from a mass of mesenchymal cells located between the layers of the dorsal mesentery, between

the aorta and the stomach. As the stomach rotates to the right along its longitudinal axis during development, the spleen and dorsal mesentery are carried to the left along w i th the greater curvature of the stomach (see section and figures on development of mesentery later in chapter). The base of the dorsal mesentery then fuses w i th the peritoneum covering the left kidney, giving rise to the splenorenal ligament. A portion of the spleen may be adherent to the anterior surface of left kidney, giving rise to a 'bare area of the spleen', analogous to the bare area of the liver.

The spleen is the size of a fist, measuring up to 12 cm long, 7 cm wide and 3-4 cm thick. Its long axis is in the line of the tenth rib and its lower pole does not usually extend beyond the midaxillary line.

The spleen has a smooth diaphragmatic surface related through the diaphragm to the costodiaphragmatic recess of the pleura and the ninth, tenth and eleventh ribs. The visceral surface of the spleen faces anteroinferiorly and to the right. Its contours correspond to its relationship to the stomach anteriorly, the splenic flexure of the colon inferiorly and the left kidney posteriorly. The splenic artery enters and the splenic vein leaves the spleen as four or five branches each at the hilum between the gastric and renal impressions. The tail of the pancreas also lies at the splenic hilum.

The spleen is covered w i th peritoneum on its visceral and diaphragmatic surfaces. At the hilum it is attached by peritoneum to the posterior abdominal wall - the phrenico¬ splenic and splenorenal ligaments - and to the greater curve of the stomach - the gastrosplenic ligament. The gastrosplenic ligament is composed of two layers of dorsal mesentery, which separate the lesser sac posteriorly from the abdominal cavity (greater sac) anteriorly. The splenic vessels and the pancreatic tail pass towards the spleen in the former and the short gastric and left gastroepiploic vessels pass in the latter.

Blood supply of the spleen

The splenic artery arises from the coeliac trunk. The splenic vein receives the inferior mesenteric vein and joins w i th the superior mesenteric vein to form the portal vein.

Variants

The spleen develops in many parts in the dorsal mesentery of the stomach at 5-7 weeks of intrauterine life. These parts are represented in the adult by the segments supplied by each branch of the artery. The adult may, however, have multiple spleens or may retain some fetal lobulation. Up to 10% of normal adults have, in addition to a normal spleen, accessory spleens called splenules or splenunculi. These occur most commonly in the splenic hilum or near the pancreatic tail, but may also be found in the small intestinal mesentery or in the greater omentum.

Anatomical asplenia (no spleen) and polysplenia (several small rudimentary splenic bodies) are associated w i th congenital heart disease and isomerism (bilateral right- or left-sided anatomy of the heart, bronchi and abdominal structures). This reflects the development of the spleen and the heart at the same period of intrauterine life. Functional asplenia occurs where, because of an interruption to the blood supply or diffuse disease, no spleen is seen in imaging such as scintigraphy, which depends on functioning splenic tissue.

A 'wandering spleen' is a condition related to age, where lax suspensory ligaments allow great splenic mobility and the ectopic spleen may simulate a mass elsewhere.

Radiological features of the spleen

Plain films of the abdomen The spleen is often not visible but its lower pole may be outlined by fat. Its size and position may be deduced from the distance between air in the lung and its impression on the gastric or colonic gas shadows. Assessing its size by measurement of its w i d th 2 cm above the splenic tip on plain films is less accurate than measurement of its length from the dome of the diaphragm to the tip. This is usually less than 14 cm.

The relationship of the spleen to the ninth, tenth and eleventh ribs is important in plain films of trauma cases, where fracture of these ribs is associated w i th splenic rupture.

Ultrasound of the spleen The spleen is of similar echogenicity to the liver or even higher, although the liver may appear more echogenic because of the reflectivity of its many vessels. It usually measures less than 12 cm in its long axis. The splenic vessels are best seen when enlarged.

Anterior to the colon the spleen enlarges and becomes easier to see on ultrasound. Retroperitoneal masses, by comparison, may be masked by colonic gas.

Accessory spleens can be identified by ultrasound by demonstrating supply by the splenic artery and drainage by the splenic vein.

Computed tomography (see Figs 5.2, 5.3, 5.10 and 5.11) On CT the spleen is seen as homogeneously enhancing. In the arterial phase a pallisading pattern of enhancement may be seen. Its relationship to the diaphragm, pleura and ribs and to the pancreas, stomach, colon and left kidney can be appreciated. The spleen is enlarged if anterior to the aorta or extending below the ribs.

Magnetic resonance imaging (see Fig. 5.47) The spleen is readily visible on MRI and its relationships to the diaphragm, left kidney, and adrenal gland can be