Ultrasound, which avoids the use of ionizing radiation, is often the preferred initial imaging choice for evaluating patients, particularly children and pregnant women, who have symptoms suggesting appendicitis. The goal is to visualize the appendix and determine its maximum outer diameter, assessing for signs of inflammatory enlargement. This diagnostic approach helps to differentiate acute appendicitis from other causes of abdominal pain, guiding the decision for surgical or medical management. The effectiveness of the ultrasound relies heavily on the sonographer’s ability to first locate the appendix and then accurately size it.
Preparation and Visualization Techniques
The process begins with careful patient positioning, typically with the patient lying supine, which can be modified to the left lateral decubitus position if the appendix is difficult to locate. A high-frequency linear array transducer, usually operating between 7 and 12 MHz, is selected to provide high-resolution images of the superficial structures in the right lower quadrant. For patients with a larger body size, a lower-frequency convex probe may be necessary to achieve adequate tissue penetration.
The defining technique for finding the appendix is graded compression, which involves applying steady, increasing pressure with the transducer to displace overlying bowel gas and fluid. This pressure pushes the appendix closer to the probe face, making it visible as a blind-ending tubular structure originating from the cecum. The appendix is usually found anterior to the iliac vessels and the psoas muscle, which serve as reliable anatomical landmarks. If the appendix is not immediately visualized in the area of maximal tenderness, the sonographer will systematically trace the ascending colon down to the cecum.
The Standard Measurement Technique
Once the appendix is clearly visualized, the standardized measurement technique is applied to determine its size. The maximum outer diameter (MOD) is the primary measurement and must be taken in the transverse plane, measuring from the outer wall to the outer wall of the structure. This measurement should be carefully performed to exclude any surrounding periappendiceal fat or fluid collection that could artificially inflate the reading.
Electronic calipers are used to precisely mark the widest distance across the appendix. A crucial step before accepting the measurement is confirming that the appendix is non-compressible; an inflamed appendix will remain rigid when pressure is applied, unlike a normal bowel loop, which collapses. The appendix must also be observed for a lack of peristalsis, or wave-like contractions, which is another sign of inflammation.
A secondary measurement sometimes taken is the thickness of the appendiceal wall. The entire length of the appendix should be scanned, as inflammation can be confined only to the tip, which could be missed if only a short segment is measured. Ensuring that the tip is identified confirms the structure is indeed the appendix and not another loop of small bowel.
Interpreting the Measurements
The resulting maximum outer diameter measurement is directly used to determine the likelihood of acute appendicitis. The commonly accepted threshold for an inflamed appendix is 6 millimeters (mm) or greater. An appendix measuring less than 6 mm is considered normal, especially if it is compressible and shows no secondary signs of inflammation.
Measurements falling into a borderline range, typically between 6 mm and 8 mm, require careful consideration of other findings to make a definitive diagnosis. In the transverse view, an inflamed appendix often presents with a characteristic “target sign,” showing alternating concentric hypoechoic (dark) and hyperechoic (bright) rings corresponding to the layers of the bowel wall. This target appearance confirms that the structure being measured is the appendix.
The appendix’s wall thickness can also contribute to the diagnosis, with a single wall thickness of 3 mm or more being another indicator of inflammation. Some studies suggest a three-category interpretation scheme: a diameter less than or equal to 6 mm is highly unlikely to be appendicitis, greater than 8 mm is highly suspicious, and the range between 6 and 8 mm is equivocal, necessitating a search for additional diagnostic features.
Beyond Measurement: Other Diagnostic Indicators
When the appendix cannot be visualized, or the size measurement is borderline, other sonographic signs become important for diagnosis. The presence of periappendiceal fluid is a strong indicator of appendicitis. Another sign is the increased echogenicity, or brightness, of the pericecal fat surrounding the appendix, which suggests inflammatory infiltration.
Color Doppler studies check for hypervascularity (increased blood flow) in the appendiceal wall, a common feature of acute inflammation. An appendicolith, a calcified stone within the appendix lumen that casts an acoustic shadow, is another finding that strongly supports the diagnosis, though it can occasionally be seen in a normal appendix. Non-visualization of the appendix, especially without secondary signs of inflammation, often carries a high negative predictive value, suggesting appendicitis is unlikely.
The effectiveness of ultrasound is highly dependent on the sonographer’s skill and can be limited by factors like excessive bowel gas or a patient’s body habitus. If the ultrasound remains non-diagnostic or equivocal, alternative imaging methods like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) may be used to confirm or rule out the diagnosis.