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Appendicitis Imaging

Plain abdominal radiographs have been used as the first imaging technique in suspected appendicitis. However, its diagnostic accuracy is limited. More than 50% of them are normal and when positive, radiological signs usually indicate perforation (Johnson and Coughlin 1989). This fact makes abdominal plain radiography useless in the early diagnosis of appendicitis and adds unnecessary cost and radiation exposure. Appendicolith is the only specific sign (10% of cases) of appendicitis. When this finding is associated with abdominal pain, the chance of having appendicitis is 90%, and at least 50% of them have perforated or gangrenous appendicitis (Holgersen and Stanley-Brown 1971) (Fig. 1.56a).

US is a widely available and inexpensive modality with the potential for highly accurate imaging in the patient suspected of having acute appendicitis. The utility of US lies primarily in children with equivocal clinical findings, both to establish the diagnosis of appendicitis and to aid in diagnosis of other abdominal and pelvic conditions that may mimic appendicitis, particularly gynecological diseases. In experienced hands, the inflamed appendix can be demonstrated in 90% of patients with appendicitis (Puylaert 2001). High-resolution transducers (5 and 15 MHz), familiarity with the RLQ anatomy, experience, and patience are necessary. The graded compression US technique described by Puylaert (Puylaert 1986) is helpful in positioning the appendix into the most focused area of the ultrasonographic beam and to displace and compress bowel loops, but is not always required in children. The peritoneal cavity is screened for bowel pathology with five or six vertically orientated, overlapping lanes using a broad-based probe. Puylaert (2003) refers to this as "moving the lawn." The examination begins with identification of the ascending colon, which appears as a non-peristaltic structure containing gas and fluid. The transducer is then moved inferiorly to identify the terminal ileum, which is easily compressible and displays active peristalsis. The cecal tip where the appendix arises is approximately 1-2 cm below the terminal ileum (Sivit 2001).

The normal appendix can be detected on US in more than 80% of asymptomatic patients (Rioux 1992; Wiersma et al. 2005). It may be identified as a tubular, mobile and blind-ended structure, measuring 6 mm or less in the anteroposterior diameter (Jeffrey et al. 1988; Vignault et al. 1990). As with any intestinal bowel loop, five concentric layers that are alternately hyperechoic and hypoechoic can be identified corresponding from inner to outer: hyperechoic mucosal surface, hypoechoic mucosa, hyperechoic submucosa (due to vessels, connective tissue and fat content), hypoechoic muscular, and hyperechoic serosa sometimes in smooth continuity with the hyperechoic periappendicular fat (Kimmey et al. 1989) (Fig. 1.57). The most prominent layer of the normal appendix in children is the hypoechoic mucosa due to the follicular lymphoid tissue of the mucosal lamina propria (Spear et al. 1992).

Several authors have reported a marked overlap of diameters of normal and acutely inflamed appendices measured on US in children. In 1997, HaHN et al. found diameters of 6 mm or more in 79 (82%) of 96 cases of histologically proven normal appendices with lymphatic hyperplasia. They therefore concluded that high diagnostic accuracy in children can be achieved only by considering several US criteria simultaneously, including appendiceal compressibility, location of the point of tenderness, presence of hyperechoic periappendiceal inflamed fatty tissue, appendiceal shape, appendicoliths, air in the appendiceal lumen, and blood flow in the appendiceal wall detected on color Doppler US (Rettenbacher et al. 2001). Furthermore, the analysis of morphologic appendiceal changes may add enough information to avoid the limitation of the nonspecific numerical (6-mm diameter) criteria in

Fig. 1.56a-e. Obstructive appendicitis. a Plain abdominal radiograph showing an appendicolith (arrow) indenting the cecal luminogram (C). b,c Longitudinal and axial US show typical obstructive appendicitis produced by an appendicolith (A) causing distal appendiceal dilatation (arrows).The axial US scan shows howthe purulent luminal content expands the five layers ofthe appendiceal wall producing a thin concentric rings pattern (arrows). d Diagram showing the thin rings pattern. e Histologic specimen shows a dilated appendix

Fig. 1.56a-e. Obstructive appendicitis. a Plain abdominal radiograph showing an appendicolith (arrow) indenting the cecal luminogram (C). b,c Longitudinal and axial US show typical obstructive appendicitis produced by an appendicolith (A) causing distal appendiceal dilatation (arrows).The axial US scan shows howthe purulent luminal content expands the five layers ofthe appendiceal wall producing a thin concentric rings pattern (arrows). d Diagram showing the thin rings pattern. e Histologic specimen shows a dilated appendix

the accurate diagnosis of appendicitis. Based on our own experience (dei Pozo et al. 1992) and appen-diceal anatomic considerations (Spear et al. 1992; Puylaert 1990; Borushok et al. 1990), distinctive axial US patterns have been organized (three/two/ one ring) to help in appendiceal identification and in evaluating the involvement stage. Each of the three rings closely correlates to one of the hyperechoic anatomical layers, from inner to outer: mucosal surface, submucosa, and serosa. Like a countdown, the progressive loss of those rings leads to perforation (3 rings, 2 rings, 1 ring, perforation) (Fig. 1.58).

The centrifugal loss of the hyperechoic appendiceal layers (rings) and, outside the organ, the periap-pendicular fat prominence, the irregularity of the appendiceal contour, and the presence of collections or echogenic ascites, are linked to increasing inflammation and necrosis.

In US examination, the inflamed appendix enlarges (greater than 6 mm in AP diameter), becoming a noncompressible structure (Puylaert 1986; Jeffrey et al. 1988). Initially, the transmural involvement stresses the concentric layered appearance of the normal appendix, turning from a rela-

Fig. 1.57a-e. US images of the normal appendix. a Round transverse section of the proximal part of a normal appendix (arrow) behind the terminal ileum (I). "Target" appearance with patent concentric layers. b Longitudinal section of a normal appendix (arrows) showing five layers that are alternately hyperechoic and hypoechoic from inner to outer: hyperechoic mucosal surface, hypoechoic mucosa, hyperechoic submucosa, hypoechoic muscular, and hyperechoic serosa. The hypoechogenicity of the mucosa corresponds to the follicular lymphoid tissue of the mucosal lamina propria, the most prominent layer of the normal appendix in children. c Normal appendix with the lumen collapsed surrounded by ascites. The fatty mesoappendix (M) appears as a tiny hyperechoic structure close to the appendix. Compared c with d, e Diagram and histologic specimen show the hyperechoic appendiceal layers as three concentric rings, e Histologic specimen showing lymphoid hyperplasia

Fig. 1.57a-e. US images of the normal appendix. a Round transverse section of the proximal part of a normal appendix (arrow) behind the terminal ileum (I). "Target" appearance with patent concentric layers. b Longitudinal section of a normal appendix (arrows) showing five layers that are alternately hyperechoic and hypoechoic from inner to outer: hyperechoic mucosal surface, hypoechoic mucosa, hyperechoic submucosa, hypoechoic muscular, and hyperechoic serosa. The hypoechogenicity of the mucosa corresponds to the follicular lymphoid tissue of the mucosal lamina propria, the most prominent layer of the normal appendix in children. c Normal appendix with the lumen collapsed surrounded by ascites. The fatty mesoappendix (M) appears as a tiny hyperechoic structure close to the appendix. Compared c with d, e Diagram and histologic specimen show the hyperechoic appendiceal layers as three concentric rings, e Histologic specimen showing lymphoid hyperplasia

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