Sonographic Features of ACD
Several key sonographic features have been described in ACD:
1. A thick hypoechoic wall with a central hyperechoic center (target phenomena), which is seen in up to 40% of cases. This structure is known as the Parulekar pseudokidney (Figure 3). Although it was one of the initial sonographic signs described for the diagnosis of ACD, later studies have found it to be nonspecific. Thus, the primary emphasis is placed on a thickened hypoechoic wall.
2. Diverticula, which are seen in up to 50% of cases (Figures 2, 3, and 7).
3. Changes in pericolic fat. This sign is usually seen as a rigid hyperechoic zone surrounding the colon, representing omental or pericolic fat that is encasing the inflammation (Figures 3, 5, and 7).
4. Enlarged fluid-filled loops of bowel.
5. Air-containing diverticula manifesting as hyperechoic areas within the lumen where there is acoustic shadowing as a result of air residue (Figure 2).
6. An abscess presenting primarily as a cystic mass with hyperechoic debris.
7. Local pain and tenderness on compression.
Sonographic Approach to Evaluating the Abdomen
In scanning the gastrointestinal tract, the graded compression procedure is used. The examination is performed with a curvilinear 3.5–5.0-MHz probe, which is used in the majority of cases. However a high-frequency linear 5–12-MHz probe may also be used, primarily in the pediatric patient, the thin patient, and the elderly patient who has decreased muscula is most helpful in the evaluation of superficial disease affecting the left colon as well as the sigmoid colon. In addition, an endocavitary (transrectal or tranvaginal) probe may be used when indicated for the evaluation of difficult-to-access areas such as the sigmoid colon, keeping in mind that this approach is more invasive, requires additional examination time, and may be a source of discomfort, particularly to the elderly patient. Before commencing the systemic evaluation of the abdomen, it is critical to focus on the patient’s most painful area. It is precisely this ability to communicate with the patient and to perform a focused evaluation in real time that distinguishes sonography from all other diagnostic modalities and renders it an invaluable extension of the physical examination.
Subsequently, it is recommended to perform a systemic evaluation of the abdomen by commencing in the right upper quadrant with the ascending colon, with its characteristic haustra in its constant anatomic location.
From there, the right lower quadrant is evaluated, reaching the blind-ending loop of bowel, the cecum. The terminal ileum and the appendix are then evaluated, followed by the transverse and descending portions of the colon. It is recommended to follow the sigmoid colon into the pelvis and to attempt visualizing the rectum with the bladder as an acoustic window. Optimal visualization may be attained with a half-full bladder. The small bowel is then scanned and recognized by its valvulae conniventes, while paying attention to the perienteric soft tissue and fat. The normal colon is seldom recognized on sonography, and its wall thickness is less than 3 mm (Figure 1). As such, whenever the colonic wall measures greater than 5 mm, underlying disease must be suspected (Figures 2–6). Although bowel gas and peristalsis may hinder a proper sonographic evaluation of the normal gastrointestinal tract, with underlying disease there tends to be a thickened wall, a narrowed lumen, and decreased peristalsis, all of which facilitate the sonographic evaluation. The key to differentiating the sigmoid colon from the small bowel lies in identifying its stable location, visualizing the colonic lumen that lacks vulvulae conniventes, and ascertaining the absence of peristalsis that is normally pathognomonic for the small bowel.