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ANATOMY FOR DIAGNOSTIC IMAGING
Radiological features of the ileocaecal valve Plain films of the abdomen Gaseous distension of the colon is seen proximal to a site of colonic obstruction. In some of these cases the ileocaecal valve remains competent, so that marked distension of the caecum can occur with or without distension of the small intestine. In other patients the valve is incompetent and there is distension of both large and small intestine without excessive distension of the caecum. Barium-enema examinations The ileocaecal valve may present a filling defect in the posteromedial wall of the caecum. This may be polypoid or bilabial, depending on the state of contraction of the valve. In the contracted valve, barium may fill a narrow slit between the folds like a linear ulcer. The valve is at the site of the first completely transverse haustral fold. The thickened posterior ends of this haustra are the frenula of the valve and should not be greater than 3 mm in diameter. Computed tomography Fat accumulation around the ileocaecal valve makes it easily visible in many abdominal CT scans. This can be very marked in some individuals, particularly elderly women. THE APPENDIX (Figs 5.18-5.20) The appendix arises at the convergence of the taenia coli on the posteromedial wall of the caecum about 2 cm below the ileocaecal valve. It is a thin structure containing lymphoid tissue. Its length is very variable - between 12 and 24 cm long. It has its own mesentery - a triangular fold from the lower border of the ileum - and as a result is mobile. Its position is variable with the incidence of the commonest positions, as follows: • Retrocaecal - 64% • Inferomedial - 36%
When the appendix lies behind the caecum it is quite free if the caecum is completely invested in peritoneum and is itself mobile. Occasionally it lies beneath the peritoneal covering of the caecum and may be fused to the caecum or the posterior abdominal wall. The lumen of the appendix is wide in the infant and obliterated after mid-adult life. Acute appendicitis, which is usually caused by obstruction of the lumen, is therefore rare in the extremes of life. The appendix is supplied by the appendicular artery which reaches it in the mesoappendix from the ileocolic artery. This is its sole supply, and if infection causes thrombosis of this artery, gangrene and perforation of the appendix results (compare with the gallbladder, which receives a rich collateral supply from the liver in the gallbladder bed and in which gangrene and perforation are rare). Lymph drainage is to the paracolic nodes along the ileocolic artery to the SMA group. Radiological features of the appendix Plain abdominal film Faecoliths or fluid levels of the appendix may be visible on plain films of the abdomen in the right iliac fossa in approximately 10% of individuals. Ultrasound The appendix is identified as a blind-ended tube arising from the posterior aspect of the caecum. Unlike nearby loops of ileum, it does not display peristalsis. Its position is variable, with the subcaecal appendix being least likely to be obscured by caecal gas. If in a retrocaecal position, visualization of the appendix is aided by compression of the caecum. The appendix can be found by finding the junction of the terminal ileum with the colon and then scanning carefully just below this level.
