Acute appendicitis is a medical emergency involving the inflammation of the vermiform appendix, a small pouch attached to the large intestine. Prompt and accurate diagnosis is required because a delay can lead to rupture, significantly increasing the risk of serious infection and complications. Physicians rely on the computed tomography (CT) scan as a highly reliable method to confirm the diagnosis and guide treatment.
Why CT Imaging is the Preferred Diagnostic Standard
Computed tomography has emerged as the preferred imaging modality for suspected appendicitis in adults due to its superior diagnostic accuracy. Modern CT scanners boast high sensitivity (94% to 98%) and high specificity (up to 97%) in identifying the condition. This level of accuracy is significantly greater than that of ultrasound, especially in adult patients where body habitus or bowel gas can obscure the appendix.
The CT scan provides cross-sectional images of the entire abdomen and pelvis. This comprehensive view allows the physician to visualize the appendix clearly and inspect surrounding structures. The ability to scan a wide area is crucial for ruling out other causes of pain that can mimic appendicitis, such as kidney stones or ovarian pathology.
Intravenous contrast material is administered to enhance the visibility of internal structures. The contrast agent highlights blood flow, making the inflamed wall of the appendix appear brighter than normal tissue. Intravenous contrast is generally accepted as beneficial for clearly outlining the appendix and detecting any potential complications.
Key Radiological Signs of Acute Appendicitis
The CT diagnosis of acute appendicitis is based on identifying specific visual findings within the right lower quadrant of the abdomen. The most direct sign is an enlarged appendix, typically measuring greater than 6 millimeters in diameter. This dilation occurs because the appendix is obstructed, causing fluid to build up inside the tubular structure.
Inflammation also causes the appendiceal wall to appear thicker than normal, often exceeding 2 to 3 millimeters. Following intravenous contrast injection, this thickened wall demonstrates increased enhancement, appearing brighter due to increased blood flow. In cross-section, this combination of a fluid-filled center and an enhancing wall creates the visual known as the “target sign.”
A highly specific and common finding is periappendiceal fat stranding, which refers to the hazy, ill-defined appearance of the fatty tissue surrounding the appendix. Normally, fat appears uniformly dark on a CT scan, but inflammation causes fluid to leak into the fat, giving it a characteristic dirty or streaky texture.
Another important marker is the presence of an appendicolith, a small, calcified fecal stone within the appendix. Because it is dense, the appendicolith appears bright white on the scan and is often the source of the obstruction. While the presence of an appendicolith strongly suggests appendicitis, the diagnosis can still be made without one.
Recognizing Complications and Differential Diagnoses
CT scans are invaluable not only for confirming acute appendicitis but also for assessing its severity by detecting complications. The most serious complication is perforation, where the inflamed wall breaks open, spilling infectious contents into the abdominal cavity. On the CT image, perforation may be indicated by a focal defect or gap in the enhancing wall of the appendix.
Signs of perforation include the presence of extraluminal air (free gas outside the intestinal tract) or an extraluminal appendicolith that has escaped the appendix. Following perforation, an abscess may form, which appears as a localized pocket of fluid and pus with a distinct, enhancing rim. A phlegmon, an ill-defined mass of severely inflamed tissue, shows up as severe fat stranding and an ill-defined fluid collection in the right lower quadrant.
The robust imaging capability of the CT scan is also crucial in distinguishing appendicitis from other conditions that cause similar right lower quadrant pain. These alternative diagnoses are often referred to as “mimics” of appendicitis. Conditions such as Crohn’s disease, cecal diverticulitis, or right ureteric obstruction (kidney stones) display unique features on the CT scan.
For example, a CT scan can differentiate appendicitis from diverticulitis of the cecum by showing inflammation centered on a colonic diverticulum instead of the appendix. In female patients, the CT scan helps rule out gynecological issues, such as ovarian cyst rupture or pelvic inflammatory disease, by clearly visualizing the reproductive organs.