For appendix CT scan, bring a tape measure

Imagers know that you can skip the contrast in heavier patients undergoing CT scans for suspected appendicitis, but just how heavy is heavy enough? 

Researchers at McMaster University in Ontario, Canada, have developed one indicator: body girth. In a scientific paper presented Tuesday, they reported that in patients with an abdominal girth of 105 cm or more, it is safe to skip the contrast and still get a good image of the appendix.

In CT scans, abdominal fat helps visualize the appendix. Without the fat, contrast is needed to bring out the organ.

Investigators retrospectively evaluated unenhanced helical CT scans of 108 patients to determine whether a normal appendix could be identified by both a radiology resident and a staff abdominal radiologist. They developed a computer program to estimate abdominal girth at the level of the iliac crests, said Dr. Kelly Ainsworth, the presenter and a fourth-year radiology resident.

The appendix was seen in 74 patients and not seen in 34. A statistical analysis determined that abdominal girth averaged 19.42 cm larger in those patients in whom the appendix was visible (p <0.001) than in those in which it was not visible. An ROC analysis demonstrated that for those whose abdominal girth is greater than 105 cm (p <0.001), an unenhanced CT scan adequately identifies the appendix.

Besides reducing the risk attendant on contrast injection, an unenhanced scan also reduces the time needed to administer and wait for the contrast. In slim patients, ultrasound or contrast CT is necessary, Ainsworth said.

Ainsworth was asked about other potential measures, such as body mass index. She said the facility is continuing its research into other indicators, including visceral fat.