Diagnostic Imaging: CT: Technique and Accuracy
Visualization of the appendix on CT is dependent on the size of the appendix, the presence of periappendiceal fat, adequate bowel opacification, and CT technique. In general, CT examinations for appendicitis in children should be performed using oral contrast and rectal contrast if necessary. Thin (3-5 mm) images are obtained through the expected region of the appendix (third lumbar vertebral level to the pubic symphysis). Although intravenous contrast may not be necessary in adults, it should be employed in pediatric patients, who tend to have a paucity of retroperitoneal fat, to better delineate normal and abnormal anatomy in the abdomen and pelvis (26,27). In younger or thin children, optical magnification of the right lower quadrant may be helpful to better delineate and accurately measure the appendix. If there is no abnormality identified in the pelvis, or an inflammatory process extends in a cranial direction to the upper pelvis, imaging should be extended to include the abdomen (19).
In larger patients, such as obese or older adolescent children and adults, CT images may be obtained without administration of intravenous or enteric contrast in order to avoid the possibility of obscuring an appendicolith. If an abnormally dilated appendix (with or without an appendicolith) with surrounding inflammatory changes in the retroperitoneal fat is identified, there is no need for further imaging (27).
The appendix is seen on CT examination in 50% of healthy children, in 100% of those with non-perforated appendicitis. and in 30% of children with perforated appendicitis (28). With perforation, the appendix may be fragmented or destroyed and difficult to visualize. Non-visualization of the appendix on CT scans is not a reliable sign of appendicitis; this may also occur in normal children.
Based on the data of several recent studies, CT performed for clinical suspicion of appendicitis with atypical symptoms has a sensitivity of 87 - 96%, specificity of 89 - 97%, positive predictive value of 95%, negative predictive value of 95%, and accuracy of 94% (27,29).
If CT of the appendix is performed for all patients with clinically suspected appendicitis, including typical and atypical presentation, it has a sensitivity of 100%, specificity of 95%, positive predictive value of 97%, negative predictive value of 100%, and overall accuracy of 98% for the diagnosis of appendicitis (30).
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Diagnostic Imaging: CT: Findings in Appendicitis
The primary CT signs which are considered pathognomonic for appendicitis include:
Friedland et al found that primary signs were the basis for diagnosis of appendicitis in 80% in their select group of 20 children with surgically proven appendicitis (28). Similarly, these primary signs were the basis of the diagnosis of appendicitis in 95% of the adult patients studied by Malone et al (27).
With perforation, the position of an appendicolith may change as it is free to move within the peritoneal cavity. In the group of children studied by Friedland et al, 70% had perforated appendicitis. Fifty percent of the appendicoliths in this series were found outside of the appendiceal lumen. These children also had a higher incidence of bowel wall thickening (11%) and small bowel obstruction (30%). This is again due to the greater proportion of patients in this study with appendiceal perforation and accompanying inflammatory changes affecting the adjacent bowel (28).
Secondary CT signs which are suggestive of, but not diagnostic for, appendicitis include:
- pericecal inflammatory process with obscuration of the fat plane anterior to the psoas muscle,
- periappendiceal fatty infiltration,
- inflammatory changes or abscess in the right lower quadrant, with or without visualization of an abnormal appendix or appendicolith,
- pelvic abscess,
- thickening of the cecum (arrow-head sign) and terminal ileal wall (seen in 50% of children and 5% of adults with appendicitis),
- small bowel obstruction (seen in 30% of children and 10% of adults with appendicitis),
- and free fluid in the pelvis.
If perforation occurs, an abscess may be identified, most commonly seen in the right lower quadrant (periappendiceal), but also possible between loops of adjacent bowel (interloop abscess), in the pelvic cul-de-sac, within the mesentery, or in a perihepatic or subdiaphragmatic location. Generalized peritonitis may also develop following appendiceal perforation. Hepatic abscesses which occur secondary to septic mesenteric thrombophlebitis may be detected as hypodense lesions in the liver; lymphomatous or leukemic lesions may have a similar CT appearance.
A false positive CT diagnosis of appendicitis may be made in the presence of small bowel obstruction, a gangrenous infarcted cecum, mucocele of the appendix, or neoplastic or inflammatory processes involving the cecum or terminal ileum.
CT assessment for appendicitis may hampered by the presence of normal fluid-filled bowel loops which may be confused with an abnormal appendix or with radiodense pits and medications in the distal ileum which may be mistaken for an appendicolith. False negative CT examinations may occur in slender patients and children in whom there is a paucity of periappendiceal and retroperitoneal fat.
Review all cases presentations in CT Findings of Appendicitis
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