Ultrasound proves just as useful as CT for detecting free air

Japanese researchers have established by studying nearly 500 patients that ultrasound is just as useful as CT in diagnosing intraperitoneal free air in patients suffering from abdominal pain or acute injury.

After blunt trauma, it is important to diagnose gastrointestinal perforation, especially in the peritoneal cavity. In the U.S., CT is used almost exclusively for the workup of acute abdominal pain, but Dr. Yoshihiro Moriwaki from the Critical Care and Emergency Center at Yokohama City University and colleagues argue for ultrasound.

Ultrasound is simple, rapid, inexpensive, and dynamic, and it can be used as often as necessary without harm to the patient, Moriwaki said. Ultrasound can also be used in remote locations such as emergency scenes and ambulances and with unstable patients unable to travel to a radiographic examination area.

In their study, Moriwaki and colleagues examined 484 patients using ultrasound, abdominal radiography, and abdominal CT (Arch Surg 2009;144[2]:137-141). Ultrasound detected intraperitoneal free air (IPFA) in 85.2% (46) of the blunt trauma patients (54). In the remaining eight patients, three did not show IPFA even on CT, and two did not show IPFA in front of the liver surface but rather in the intermesenteric space. In the other three patients, IPFA could not be visualized by ultrasound despite the presence of sufficient amounts for its detection.

Ultrasound demonstrated 85.7% sensitivity and 99.6% specificity in patients with blunt abdominal trauma. In patients with severe acute abdominal pain, sensitivity was 85% and specificity was 100%, according to the researchers.

When it comes to ultrasound, however, the most important issue is interpretation. At the Critical Care and Emergency Center in Japan, ultrasound is used often so the physicians have honed their technique and increased their familiarity with the device. As such, they are skilled at detecting IPFA.

Ultrasound diagnosis of IPFA is operator-dependent because many of the techniques for detection are dynamic and require patience, according to Dr. Christine S. Cocanour from the department of surgery at the University of California, Davis Medical Center in Sacramento.

"In busy hospitals in the U.S., where technicians perform nearly all ultrasounds, ultrasound will probably not replace CT or plain radiographs for the diagnosis of free air unless we truly decide that we need to decrease medical costs," she said in an accompanying editorial.

Moriwaki does not, however, advocate ultrasound replacing CT.

"I believe that, essentially and practically, we should not compare the usefulness of CT alone with ultrasound alone for diagnosing gastrointestinal perforation, but should compare CT alone with CT + ultrasound," Moriwaki told Diagnostic Imaging.

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