Acute appendicitis is a common cause of abdominal pain, often requiring prompt diagnosis to prevent serious complications. Ultrasound imaging offers a non-invasive, radiation-free method to assess the appendix, making it a preferred initial diagnostic tool, particularly for children and pregnant individuals. The goal is to visualize the vermiform appendix and determine its outer diameter and characteristics to assess for inflammation.
Visualization and Localization Techniques
The first step in measuring the appendix involves the challenging task of locating the structure, which is highly variable in position. Sonographers typically start the examination in the right lower quadrant, often over McBurney’s point, which is roughly one-third of the distance from the anterior superior iliac spine to the umbilicus. The patient’s reported area of maximum tenderness is also a useful starting point for the investigation.
The technique of graded compression is fundamental to successful visualization. This involves applying slow, firm pressure with the ultrasound transducer to displace overlying loops of gas-filled bowel. This compression pushes gas away, allowing sound waves to reach the appendix. A normal appendix appears as a blind-ending, tubular structure that is compressible and arises from the cecum.
High-resolution linear array transducers are used for this scan, though lower-frequency probes may be necessary for patients with a larger body habitus. The appendix is often located near the psoas muscle and the iliac vessels, which serve as helpful anatomical landmarks. Once a tubular structure is identified, it must be traced to its blind end to confirm it is the appendix and not a loop of small bowel.
Standard Measurement Criteria
Once the appendix is visualized, measurement is performed to determine if the structure is enlarged. The maximum outer diameter (MOD) is the most important measurement. This measurement is taken in the transverse view, perpendicular to the long axis of the structure.
The diameter is measured from the outer wall edge to the opposing outer wall edge. The measurement must include all layers of the appendiceal wall but exclude any adjacent periappendiceal fat or surrounding cecal wall. The transverse view may display the appendix with a “target sign” appearance, representing the concentric layers of the bowel wall.
In addition to the outer diameter, the thickness of the appendiceal wall can be measured as a secondary metric. A wall thickness of 3 millimeters or greater suggests progression toward appendicitis. The entire length of the appendix should be examined, as inflammation may be confined only to the tip.
Translating Measurements into Diagnosis
The measurements obtained on ultrasound are translated into diagnostic criteria for acute appendicitis. The accepted threshold for an inflamed appendix is a maximum outer diameter of 6 millimeters (mm) or greater. A measurement less than 6 mm is considered normal, though 6 to 8 mm may be indeterminate.
Non-compressibility is a key diagnostic feature confirming inflammation, especially when combined with an increased diameter. A normal appendix is typically compressible, often nearly to obliteration. An inflamed, swollen appendix resists the pressure applied by the transducer. This lack of compressibility helps distinguish true inflammation from distention caused by intraluminal contents like feces or fluid.
Secondary signs of inflammation further support the diagnosis. These include echogenic mesenteric fat surrounding the appendix, which appears brighter due to inflammatory infiltration. An appendicolith, a calcified fecal stone obstructing the lumen, may be visible, casting an acoustic shadow. The presence of free fluid in the periappendiceal region and a loss of the normal layered “target sign” appearance suggest a more advanced inflammatory process.
Reliability and Alternative Imaging
The accuracy of appendix ultrasound is highly dependent on the skill and experience of the sonographer. This operator-dependent variability is a primary limitation of the technique. The success rate for visualizing the appendix varies widely; in some settings, the appendix may not be seen in up to 45% of cases.
Several factors can impede successful visualization and accurate measurement. Excessive bowel gas is a major limiting factor, as the air scatters sound waves and obscures the view. Patient body habitus, particularly obesity, makes effective graded compression more difficult and limits the penetration of the ultrasound beam. If the appendix is located in an atypical position, such as behind the cecum (retrocecal), it is harder to find and measure.
When ultrasound is inconclusive, or if there is strong clinical suspicion despite non-diagnostic findings, physicians may turn to alternative imaging. Computed tomography (CT) offers higher accuracy but exposes the patient to ionizing radiation, a concern especially in children and young adults. Magnetic Resonance Imaging (MRI) is a non-ionizing option that provides excellent soft tissue contrast, serving as a viable alternative, particularly for pregnant women and children when ultrasound is technically limited.