THE ABDOMEN 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 with 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 with 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 fill the acini. This is undesirable, as it is associated with a higher incidence of post-ERCP complications.
Fig. 5.43
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 with 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, with 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
Spleen: (a) visceral surface; (b) diaphragmatic surface and peritoneal attachments.
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