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ANATOMY FOR DIAGNOSTIC IMAGING
the duodenum can always be identified passing between the superior mesenteric vessels and the aorta. The fourth part of the duodenum or the duodenojejunal flexure is visible at the L2 level. The calibre of the duodenum varies according to its position and contents: for example, the junction of second and third part may be quite redundant and distended with fluid, whereas the third part may be collapsed and attenuated as it crosses between aorta and superior mesenteric vessels. Angiography For full angiographic assessment of the duodenum, both the coeliac trunk and the superior mesenteric arteries must be visualized (Fig. 5.12; see also Fig. 5.21).
two-fifths of the small intestine is called the jejunum and the distal three-fifths the ileum, although the boundary between these is not well defined. The circular mucosal folds - known as valvulae conniventes or plicae semilunaris - are seen in the duodenum and continued in the small intestine, although they are less prominent distally and may be absent in distended views on barium studies. Lymphoid follicles found in the mucous membrane throughout the intestinal tract become increasingly more numerous along the length of the small intestine. In the distal ileum they become aggregated together into patches called Peyer's patches. They are oval in shape and found on the antimesenteric border of the ileum. Differences between jejunum and ileum are outlined in Table 5.1.
Ultrasound Gas in the duodenum may hinder visualization of other organs, particularly the common bile duct, which runs behind the first part. The duodenum may be identified adjacent to the head of the pancreas by ultrasound. THE SMALL INTESTINE The small intestine begins where the intestine assumes a mesentery at the duodenojejunal flexure and ends at the ileocaecal junction. It varies in length from 3-10 m, with an average length of 6 m. The root of its mesentery extends from the left of L2 to the right sacroiliac joint and is only 15 cm long (Fig. 5.17). The small intestine is very mobile and lies in mobile coils in the central abdomen. The proximal
Table 5.1
C o m p a r i s o n b e t w e e n j e j u n u m and i l e u m Jejunum
Ileum
Diameter
Wider (3.0-3.5 cm)
Narrower (2.5 cm)
Wall thickness
Thicker
Thinner
Position
Left upper abdomen
Right lower abdomen
Valvulae
Thicker and more
Thinner and less
conniventes
prominent
prominent
Peyer's patches
Fewer and bigger
More numerous
One or two w i t h
Four to five with
fewer long branches
many short branches
Arterial arcades
Fig. 5 . 1 7 Posterior abdominal wall showing peritoneal a t t a c h m e n t of mesentery.
