CT Measurements of AAAs May Hide Inaccuracy

By Hank Russell

NEW YORK -- Computed tomography seems to be the imaging modality of choice for many vascular surgeons, but Dr. George H. Meier III presented some important concerns regarding its accuracy at the Veith symposium on vascular medicine sponsored by the Cleveland Clinic.

The most important issues are the angle of the aorta as it comes down from the chest, and the use of conventional axial CTs, said Dr. Meier of the surgery department at the Eastern Virginia Medical School, and chief of vascular surgery, the Vascular and Transplant Specialists, both in Norfolk, Va.

"If there are twists and turns or some configurations to it, then anything we can do to an image of a CT can sometimes be erroneous because of the angle, and that's where the difference comes from," he said.

These errors occur because "the aorta doesn't follow the natural axis of everyone's body," he said. "As the aneurysm forms, [the aorta] becomes more tortuous."

In a CT scan, the machine orients itself to the long axis of the body rather than the long axis of the artery. In addition, the CT scanner is designed to image the straight, healthy arteries.

"CT scans have been [considered to be] very objective," Dr. Meier said. "The problem is, [the imaging is] blind to the clinical situation."

In addition, CT scans overestimate the diameter size. "That's the good news, which is also the bad news," he said. "Because of the twists and turns, you'll [be able to] pick it out, but you'll assume it's bigger than it is."

Dr. Meier noted that CT is mostly used by vascular surgeons in the United States, whereas their UK counterparts rely on ultrasound. "CT is easily available and doesn't require anyone to think," he said.

"With ultrasound, the technologist becomes more critical. You've got to have more education; you've got to have someone smart on the other end of the transducer. With a CT scanner, you just have somebody pushing the buttons and setting it up."

He added that with ultrasound, the technologist is able to make a wide range of adjustments for an accurate reading. "That's what a lot of people hate about ultrasound: A good technologist can make or break you," he said.

The other benefits of ultrasound Dr. Meier listed include the cost--ultrasound machines cost $200,000, whereas a CT scanner costs $1.5 million--and there is no radiation concern. According to Dr. Meier, 5-6 CT scans will expose a patient to the maximum radiation dosage allowed by OSHA. The only problem with ultrasound, Dr. Meier said, is that it provides a limited view of the surrounding organs, especially if there is gas inside the patient's bowels. "It's a little more finicky," he said. "With CT, you can do imaging anytime, anywhere, but at a cost."

There is a way to enhance the accuracy of CT, he pointed out. "The diameter of the aorta [in a CT scan] is the same as in the ultrasound scan if you use flow-directed stacking reconstructions," he said, "but it requires an additional process in order to get these true diameters."

The best way to determine the flow is the use of multislice CT scans. "What that does is it provides us with a continuous data set so that we can reformat it," Dr. Meier said. "That's the advantage of these multislices: You get a lot more data sets that you can manipulate."

To make the CT reading as accurate as an ultrasound reading, adjust the diameter for the flow during postprocessing, determine the flow's angle and direction, stack the reconstructed images, and then look perpendicular to the flow.