Nonenhanced volumetric CT demonstrates strength for endoleak detection

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Volume-rendered images from nonenhanced CT study at 12-month follow-up from stent placement. The volume essentially remained the same (174.6 vs. 174.2 cm3). Image and caption courtesy of the Radiological Society of North America.

Serial volumetric analysis of aortic aneurysm with nonenhanced CT serves as an adequate screening test for endoleak, causing volumetric increase of more than 2 percent from the volume seen at the previous examination, according to a retrospective analysis published this month in Radiology.

A team from the departments of radiology and vascular surgery at the University of Wisconsin-Madison examined the results of a volumetric nonenhanced CT algorithm for the detection of endoleak in the repair of endovascular abdominal aortic aneurysms (AAA).

“An unwanted complication after endovascular AAA repair is the persistence of blood flow within the excluded aneurysm sac (outside the lumen of the endoluminal graft)—that is, as endoleak,” wrote the authors.

According to the researchers, CT has demonstrated that it is an efficient and accurate modality with which to assess aneurysm morphology, define the position and integrity of the endostent, and identify endoleaks.

Patients requiring repeated follow-up examinations potentially have impaired renal function and would benefit greatly from nonenhanced volumetric analysis as compared with screening contrast-enhanced CT angiography for endoleaks.

In addition, radiation exposure and cost can be greatly reduced compared with those associated with the traditional combined nonenhanced CT and contrast-enhanced CT angiography protocol.

Patients who underwent at least two CT scans for the purpose of comparing aneurysm volume measurements were eligible for inclusion in the study; a total of 70 patients met these criteria, with 230 scans available for evaluation. In 153 of the 230 observations, the protocol initially consisted of both nonenhanced CT and contrast-enhanced CT angiographic scans.

All CT scans were obtained with a 16-slice (LightSpeed 16; GE Healthcare) or 64-slice (LightSpeed VCT; GE) scanner. The scanning protocol consisted of three steps: contrast–enhanced CT angiography before endovascular stent placement; contrast-enhanced CT angiography zero to three months after repair to depict immediate complications; and nonenhanced CT at three, six and 12 months after repair.

At each follow-up visit, aortic volume analysis was performed the same day using 3D image reformations (Volume Viewer 2 Advantage, GE).

“Volumes were calculated in a standardized fashion from the level of the renal arteries to the common iliac bifurcation,” the authors reported.

The researchers found that with their proposed nonenhanced CT algorithm, the absence of endoleaks causing more than 2 percent volumetric increase could be identified without intravenous contrast material.

“Endoleaks associated with minimal aortic volume increase of less than 2 percent did not require any intervention,” the authors noted.

The technique also substantially reduced radiation exposure.

“With our protocol of nonenhanced CT for volumetric analysis and limited contrast-enhanced CT angiography, radiation exposure is reduced by approximately 69 percent–82 percent in an average-sized patient compared with radiation exposure in the traditional protocol of combined nonenhanced CT and contrast-enhanced CT angiography with delayed-phase postcontrast CT protocol,” they wrote.

The researchers cautioned that they do not recommend using nonenhanced CT alone in the initial three months after repair because volumetric changes may not yet be large enough to be detected, and the prevalence of endoleaks is highest in the immediate postoperative period.

“This protocol has the important benefits of reduced scanning time and radiation exposure and potentially reduced cost and risk of nephropathy from contrast material,” they concluded.