Sonography and CT allow direct visualization of the normal or inflamed appendix.
The normal appendix can be identified in 67-100% of patients without appendicitis who undergo CT .
At sonography the normal appendix is less frequently visualized, with results varying between 0-82% , reflecting the operator dependency of sonography.
One of the most important imaging criteria in the evaluation of appendicitis is the outer diameter of the appendix.
Although an overlap of appendiceal diameters in normal and inflamed appendices has been reported, a threshold value of 6-7 mm is most commonly used . (Fig. 1).
A normal appendix has a maximum outer diameter of 6 mm, is surrounded by homogeneous non-inflamed fat, and often contains intraluminal gas  (Fig. 2).
An inflamed appendix has a diameter larger than 6 mm, and is usually surrounded by hyperechoic inflamed fat at sonography (Fig. 3a).
Other strongly supportive signs of inflammation include the presence of an appendicolith, cecal apical thickening
Another supportive sign for appendicitis is hypervascularity of the appendix wall on color Doppler sonography  (Fig. 3b).
At CT the inflamed appendix is surrounded by fat-stranding (Fig. 4).
Gastrointestinal nonsurgical mimics of Appendicitis
Mesenteric adenitis has been reported to be the second most common cause of right lower quadrant pain after appendicitis, accounting for 2-14% of the discharge diagnoses in patients with a clinical suspicion of appendicitis .
It is defined as a benign self-limiting inflammation of right-sided mesenteric lymph nodes without an identifiable underlying inflammatory process, occurring more often in children than in adults.
Sonography and CT show clustered adenopathy (Fig. 5).
Because adenopathy also frequently occurs with appendicitis, the normal appendix must be confidently visualized on imaging studies before assigning a diagnosis of mesenteric adenitis. Infectious enterocolitis may cause mild symptoms resembling a common viral gastroenteritis, but it may also clinically present with features indistinguishable from appendicitis .
This latter presentation may occur in bacterial ileocecitis, caused by Yersinia, Campylobacter, or Salmonella. Imaging studies show mural thickening of the terminal ileum and cecum without inflammation of the surrounding fat (Fig. 6), and moderate mesenteric adenopathy.
Epiploic appendages are small adipose protrusions from the serosal surface of the colon. An epiploic appendage may undergo torsion and secondary inflammation, causing focal abdominal pain that simulates appendicitis when located in the right lower quadrant.
Epiploic appendagitis is a self-limiting disease that has been reported in approximately 1% of patients clinically suspected of having appendicitis .
Sonography and CT depict an inflamed fatty mass adjacent to the colon (Fig. 7), containing a characteristic hyperattenuating ring of thickenend visceral peritoneal lining on CT .
Omental infarction has a pathophysiology and clinical presentation similar to that of epiploic appendagitis, with the infarcted fatty tissue being a right-sided segment of the omentum. Imaging shows a cakelike inflamed fatty mass (Fig. 8), larger than in epiploic appendagitis and lacking a hyperattenuating ring on CT.
In some cases it may be difficult to distinguish epiploic appendagitis from omental infarction (Fig. 9), however, this distinction has no clinical importance as both have a similar benign natural history .
Right-sided colonic diverticulitis
Right-sided colonic diverticulitis may clinically mimic appendicitis or cholecystitis, though the patient's history is generally more protracted. In contrast to sigmoid diverticula, right-sided colonic diverticula are usually true diverticula, that is, outpouchings of the colonic wall containing all layers of the wall.
This may possibly explain the essentially benign self- limiting character of right-sided diverticulitis .
Sonography and CT findings consist of inflammatory changes in the pericolic fat with segmental thickening of the colonic wall, at the level of an inflamed diverticulum (Fig. 10).
Crohn disease often causes long-standing symptoms, but up to one third of patients with ileocecal Crohn disease present with initial symptoms so acute that they are misdiagnosed as appendicitis .
In the acute active phase of ileocecal Crohn disease, imaging shows transmural bowel wall thickening, often predominantly of the submucosal layer, with frequent inflammatory changes of the surrounding fat (Fig. 11).
Uncomplicated Crohn disease can initially be treated with anti-inflammatory drugs.
Other nonsurgical mimics of appendicitis
Gynecologic conditions such as pelvic inflammatory disease or a hemorrhagic
functional ovarian cyst can cause acute pelvic pain that may simulate appendicitis.
In pelvic inflammatory disease the imaging findings vary according to the severity of the disease, and may be normal in early conditions.
In more advanced stages, findings may include enlargement of the internal genital organs with indistinct contours, and free pelvic fluid (Fig. 12).
In absence of a drainable tubo-ovarian abscess, treatment is medically with antibiotics.
An hemorrhagic ovarian cysts appears as a complicated cyst at sonography and a high- attenuation adnexal mass at unenhanced CT, and does not require any treatment.
Urolithiasis may present with right lower quadrant pain when obstruction is caused by a distal ureteral stone. Unenhanced CT (Fig. 13) is more accurate in detecting ureteral stones than sonography,
Ultrasound may show both hydronephrosis and hydroureter as signs of obstruction (Fig. 14).
Rectus sheath hematoma
A rectus sheath hematoma may be easy to diagnose in patients presenting with a
painful palpable mass under anticoagulant therapy, however, small nonpalpable hematomas
may clinically masquerade as appendicitis and also occur in patients without anticoagulantia
Sonography and CT show a hemorrhagic mass within the sheath of the rectus abdominis muscle (Fig. 15).
No treatment is required other than adjusting any anticoagulant therapy.
A broad spectrum of nonsurgical diseases may clinically present as appendicitis in
patients without appendicitis.
The radiologist should be aware of the sonographic and CT features of these alternative disorders, as a correct imaging diagnosis prevents an unwarranted operation and unnecessary hospital resource use.