Fig. 18  10-year-old girl with advanced appendicitis. The appendix is fluid filled and enlarged with a diameter of 12 mm (arrow). Extensive adjacent inflammation is seen. There is also diffuse pelvic peritonitis and small amounts

of ascites (asterisk). At surgery perforated appendicitis was found. Right ovary (arrowhead). Courtesy of Dr. Rosemarie Forstner

3.3.2\ Value of Imaging

Ultrasound is the primary diagnostic imaging modality for suspected acute appendicitis; however, this is often non-diagnostic due to limitations in identifying the normal appendix, and variations in appendiceal location (Paulson et al. 2003). CT is highly sensitive and specific in the diagnosis of appendicitis (rates of 90–95% and 95–100%, respectively) is often performed when ultrasound is non-diagnostic. Due to its lack of ionizing radiation, MR is an alternative, highly useful imaging tool in the assessment of acute appendicitis (sensitivity and specificity rates of 96%) and is particularly useful as a first line investigation in pregnant women (sensitivity and specificity of 94% and 97%, respectively) and children (sensitivity and specificity of 96%) (Petkovska et al. 2016).

3.4\ Diverticulitis

Colonic diverticulosis is a very common condition in Western society, affecting 5–10% of the

population over 45 years, and 80% over 85 years of age (Ferzoco et al. 1998).

Diverticula are small sacculations of mucosa and submucosa through the muscularis of the colonic wall. They develop at the point where the nerve and blood vessel penetrate the muscularis between the teniae coli and mesentery (Horton et al. 2000). The most common location for diverticula is the sigmoid colon. Acute diverticulitis occurs when the neck of a diverticulum is occluded by food particles, stool, or inflammation, resulting in microperforation of the diverticulum with surrounding mild pericolic inflammation. This can lead to a localized abscess or, if adjacent organs are involved, a fistula. The inflammation is usually contained by peri-colonic fat and mesentery and without this free perforation and peritonitis can occur. The commonest clinical symptom is left-lower- quadrant pain and tenderness, which is often present for several days before admission. Low-grade fever and mild leukocytosis are common but their absence does not exclude diverticulitis.

Right-sided diverticulitis occurs in only 1.5% of patients in Western countries but is more common in Asian populations and tends to affect younger patients (Kang et al. 2004). Diverticulitis of small intestine or transverse colon is rare (Pereira et al. 2004).

3.4.1\ Imaging Findings

On CT, diverticulae appear as small, air-filled outpouchings of the colonic wall. On MRI airfilled diverticulae are hypointense against the high-signal-intensity peri-colonic fat. The most common imaging finding in diverticulitis is paracolic fat stranding, which is characteristically more severe than the focal colonic wall thickening (Fig. 19). The key to distinguishing diverticulitis from other inflammatory conditions affecting the colon is the presence of diverticulae in the involved segment (Pereira et al. 2004). Contrast-enhanced CT or fat-suppressed T1-weighted contrast-enhanced images provide the best assessment of thickening of the colonic wall and the peri-colonic fat stranding. Other common imaging findings include thickening of the lateral conal fascia and a small volume of ascites in the cul-de-sac. Accumulation of fluid in the root of the sigmoid mesentery is known as the comma sign.

Fig. 19  Sigmoid diverticulitis. Multiple air containing diverticula are found along the sigmoid colon. In this patient with acute pelvic pain, focal wall thickening, stenosis, and paracolic fat stranding (arrow) are signs of acute diverticulitis involving the distal sigmoid colon. R rectum

Complications of diverticulitis include diverticular phlegmon and abscess, colo-vesical fistula, and perforation. Phlegmon is a heterogeneous inflammatory mass found adjacent to the diverticulitis (Onur et al. 2017). An abscess occurs in up to 30% of cases and on CT appears as a hypodense fluid collection with a contrastenhancing rim and surrounding inflammatory changes. It may contain air or air–fluid levels (Horton et al. 2000). A colo-vesical fistula is suspected when air is seen in the bladder and there is thickening of the bladder wall adjacent to a diseased segment of bowel (Labs et al. 1988). Focal contained perforations can complicate diverticulitis; these appear as small extra-luminal deposits of air or extravasation of oral contrast material. Pneumoperitoneum is a rare finding in patients with diverticulitis (Horton et al. 2000).

3.4.2\ Differential Diagnosis

The most important differential diagnosis is colon carcinoma. The presence of pericolic lymph nodes suggests the diagnosis of colon cancer rather than diverticulitis (Chintapalli et al. 1999). A long segment on involved colon (>10 cm), engorgement of adjacent sigmoid mesenteric vasculature, and the presence of fluid in the root of the sigmoid mesentery favor the diagnosis of diverticulitis (Horton et al. 2000; Cobben et al. 2003). It is not always possible to distinguish diverticulitis from colon cancer and the two entities can coexist in 3–18% of patients (Cobben et al. 2003). The presence of pelvic abscess may raise the possibility of PID in the differential diagnosis, although the extent of inflammatory change in the bowel is usually diagnostic.

3.4.3\ Value of Imaging

The role of imaging in diverticulitis is to exclude complications and predict the necessity for emergent surgery. If an abscess is detected CT-guided percutaneous drainage may be performed. MR imaging can be useful in the diagnosis of right-sided diverticulitis in young or pregnant patients with suspected appendicitis.