Contrast-Enhanced Ultrasound Aids in Endoleak Detection


SCOTTSDALE, ARIZ. — Duplex ultrasound with contrast enhancement delivered by continuous infusion shows promise in the detection of endoleaks following endovascular aneurysm repair, Dr. Ruth L. Bush said at an international congress on endovascular interventions sponsored by the Arizona Heart Institute.

“Our early results strongly suggest that contrast enhancement with a continuous infusion technique could be used as a primary diagnostic imaging modality for follow-up endograft surveillance,” possibly replacing CT as the standard for graft surveillance, said Dr. Bush of the Baylor College of Medicine, Houston.

Following endovascular aneurysm repair (EVAR), patients must be assessed periodically for sac size, stent-graft integrity, and endoleaks. CT is a fine imaging modality for this surveillance, but at the cost of substantial doses of ionizing radiation and exposure to iodinated contrast media, which can be nephrotoxic. Furthermore, CT scanning is expensive. According to some reports, more than 65% of postoperative costs following EVAR are related to CT scanning.

Unfortunately, color duplex ultrasound has been shown to have a lower sensitivity and a lower positive predictive value than CT has in this surveillance. Delivery of contrast enhancement in a bolus improves echogenicity and can even detect slow endoleaks that are not visible in CT. But the disadvantage of this technique is that bolus injection of the contrast medium allows for only a short scanning time—less than 10 minutes—so multiple injections are usually necessary. The contrast is provided by microbubbles, which must be small enough to pass through the pulmonary capillaries. The various gas microbubble contrast media are generally considered safe and to have low toxicity. However, it has proven difficult to maintain these microbubbles in the systemic circulation.

Some of these problems are avoided by continuous infusion of the contrast medium. Dr. Bush uses a syringe filled with contrast medium and normal saline and infuses the mixture at the rate of 4 cc/min to extend the scanning time to 20 minutes or more.

In a preliminary study in 20 patients, contrast-enhanced ultrasound found one type 1 endoleak and nine type 2 endoleaks. In those same patients, color duplex ultrasound found one type 1 endoleak and only four type 2 endoleaks, and CT found one type 1 endoleak and six type 2 endoleaks.

The patients' body type affected scanning time. The investigators noted a direct relationship between scanning time and body mass index.

And it's important to control scanning parameters carefully. For one thing, the syringe holding the contrast medium needs to be agitated continually to avoid breakdown of the microbubbles.

And it's necessary to optimize the harmonic imaging on the ultrasound machines, decrease the mechanical index and the compression, and adjust the focal zone to be below the aorta.

“All of this was done in an attempt to maintain the integrity of the microbubbles,” Dr. Bush said. “If you have a mechanical index turned up too high or the compression and the focal zone adjusted [imperfectly], the microbubbles would shatter and you won't get a good result.”

The learning curve for this technique is about 10–15 patients, she said.

Delivery of contrast enhancement in a bolus can even detect slow endoleaks that are not visible in CT. DR. BUSH

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