Planning Ahead With 3-D Imaging

Hank Russell

NEW YORK -- Dr. Eric E. Roselli and colleagues emphasized the importance of three-dimensional (3-D) imaging in designing fenestrated stent grafts for juxtarenal aneurysms.

"You have to plan ahead because the devices are custom designed to fit the patient's anatomy," Dr. Roselli, of the Cleveland Clinic Foundation, said during his presentation at the Veith symposium on vascular medicine sponsored by Montefiore Medical Center.

Variability in branch anatomy, pathology, and tortuosity of the thoracic, thoracoabdominal, and pelvic vasculature requires the use of 3-D image reconstruction techniques, such as center line of flow, to plan surgical and nonsurgical treatments, and to design and assess the devices used to treat these segments of the aorta, he said.

The problem in constructing the fenestrated grafts, Dr. Roselli said, is when people rely on conventional two-dimensional (2-D) imaging or standard axial imaging for the overall assessment of the vascular tree and its pathology.

"One measurement that you might achieve using 2-D imaging can be totally distorted because of the different angles of the aorta," he said. "Three-dimensional imaging allows you to get a real picture of the total anatomy of the aorta. Again, the importance of planning ahead and having accurate design allows for successful deployment of the device. If the device isn't designed accurately, and you try to deploy that device, you run the risk of covering a renal artery versus an aortic artery, which can have severe consequences."

Dr. Roy K. Greenberg, director of endovascular research at the Cleveland Clinic, became aware of 3-D imaging when he started working on fenestrated grafts. He said in his presentation that he has yet to find a better planning method than 3-D imaging, and it is time for the other surgeons to get up to speed. "There's a whole group of people we need to bring to light the concepts in terms of 3-D image manipulation for the planning of endovascular procedures," he said.

The two methods Dr. Greenberg uses are the gated CT scan and the nongated CT scan. The gated CT scan is used to visualize the aorta only at a specific point in the cardiac cycle and to rely on a relatively low heart rate to evaluate a small volume of aortic tissue. In his practice, Dr. Greenberg uses gated CT scans when looking at the ascending aorta or the aortic valve, or when trying to differentiate proximal and distal dissections. Nongated CT scans are used when evaluating more distal pathologies.

The nongated CT scan is a three-phase scan consisting of noncontrast, arterial, and 5-minute delayed phases. Diagnosis is done with a 3-mm slice thickness and a 3-mm reconstruction interval.

If a patient already has a stent graft placed, the native images are reconstructed using a high-resolution kernel or filter to apply edge-detection algorithms to look for the integrity of the graft. Meanwhile, the arterial phase of the scan is constructed with a low-resolution smoothing kernel or filter. Both the native and arterial phase reconstructions have a 1-mm slice thickness with a 0.8-mm reconstruction interval.

"We use subtraction methods to subtract objects that we're not necessarily interested in," Dr. Greenberg said. "This provides "eyes and fingers" on what would normally be exposed to the surgery, he said.

During his presentation, Dr. Greenberg chose some cardiology and vascular surgeons to work with radiologists so the surgeons could see how imaging during aortic procedures is done. "My hope is that both surgeons and radiologists--radiologists are more familiar with this than the surgeons are--will embrace these technologies," he said, "because without these imaging technologies, I think the success of complex interventions will be very much in danger."

"When you look at carotid arterial stenosis or lower-extremity vascular disease, you get so much more information from a three-dimensional imaging modality than you do from, for example, angiography," Dr. Greenberg added. "To my mind, standard diagnostic angiography is dead."