Case 7: Introduction to appendicitis.
This patient has appendicitis. The appendix is enlarged. It does not fill with oral contrast. It is surrounded by mesentery with increased attenuation (mesenteric stranding). The appendiceal mucosa enhances more than the surrounding bowel. The cecum is thickened at the appendiceal orifice.
Appendicitis is one of the most common emergency room diagnosis. The criteria for an abnormal appendix are as follows: distended greater than 6 mm from outer wall to outer wall; non filling with air or oral contrast; hyper-enhancing mucosa; and stranding in the surrounding (i.e. have increased attenuation when compared to the rest of the intra-abdominal fat). Often, the cecum is thickened in the region where the appendix originates. All of these findings are seen in this case.
The first step in evaluating for appendicitis is to find the appendix. The appendix is a blind ending tubular structure that arises from the cecum opposite the ileocecal valve from the ascending colon. Normally, the ascending colon is superior to the ileocelcal valve and the cecum is inferior. The appendix then originates from the medial surface of the cecum, inferior to the ileo-cecal valve. A common strategy for identifying appendix is to find the ileocecal valve, scroll down 3-5 images, and there you will magically find the appendix.
However this does not always work: many people have a mobile right lower quadrant and the structures can move. A technique that works consistently is to "run" the large bowel from the rectum to the cecum. (Surgeons "run" the bowel with their hands. The CT scanner is cleaner and less malodorous.) Start at the anus, scroll through the rectum, up through the sigmoid colon and descending colon. Follow the lumen of the bowel up and down as you need to. Don't skip segments. Identify the splenic flexure, traverse rightwards along the transverse colon, then follow the ascending colon to the ileocecal valve. The valve is easy to find because a loop of terminal ileum will merge with the colon. Follow the lumen of the colon past the ileocecal valve into the cecum. The origin of the appendix is located on the same side of the cecum as the ileocecal valve. If the cecum has flipped this area may be superior to the valve, not inferior. However, its origin will be opposite the ileocecal valve from the ascending colon. Once the appendix leaves the cecum it can travel in any direction. Coronal images often help with visualization, but I am rarely unable to find the appendix on the axial images. Using this method I can almost always find the appendix quickly. I also know that I have evaluated the entire bowel for other things like colitis or diverticulitis.
Go back to the cine and trace the bowel from anus to cecum. You can easily find the ileocecal valve. The ascending colon is above this level. The cecum is below. The appendix originates in the expected location.
Incidentially, this patient has a horseshoe kidney, an abdominal aortic aneurysm with intraluminal thrombus, and a left common iliac artery aneurysm with intraluminal thrombus.
Take home points
A nice recent review article is found in the American Journal of Roentgenology:
Pinto Leite et al. CT Evaluation of Appendicitis and Its Complications: Imaging Techniques and Key Diagnostic Findings. AJR 2005; 185:406-417.