Symposium Highlights Imaging Advances

By Kerri Wachter

NEW YORK -- With imaging playing a greater role in the planning and performance of vascular surgery procedures, advances in imaging technology were highlighted by several experts speaking at the Veith symposium on vascular medicine sponsored by the Cleveland Clinic.

In particular, the question was addressed as to whether virtual histology intravascular ultrasound may improve the success of stent placement.

Virtual histology intravascular ultrasound (VH IVUS) allows vascular surgeons to assess the severity of carotid artery disease, alter stent size and placement, and assess the completeness of treatment, said Dr. Donald B. Reid, of the University of Glasgow and a consultant vascular and endovascular surgeon at Wishaw Hospital (Scotland).

VH IVUS takes advantage of the fact that different constituents of plaque reflect ultrasound waves back at different frequencies. These data are processed using special software to produce a color-coded map of vessel disease with white indicating calcification, dark green indicating fibrous plaque, light green indicating fibrofatty plaque, and red indicating necrotic lipid core plaque (associated with plaque rupture).

VH IVUS has already been validated in the coronary artery but "is currently of great interest in the periphery, particularly in carotid stenting because it's likely to show us how plaque will behave at the moment of treatment," said Dr. Reid.

Will it resist stent expansion or will it risk embolization to the brain? However, VH IVUS still requires validation in the setting of stenting. The IVUS probe is inserted into the internal carotid artery via a sheath. Dr. Reid uses a cerebral protection device. IVUS allows measurements to better choose balloon and stent sizes.

Another issue discussed was whether currently recommended Doppler ultrasound thresholds may lead to the overuse of internal carotid interventions in asymptomatic patients.

While the Society of Radiologists in Ultrasound currently recommends using a peak systolic velocity (PSV) of greater than 230 cm/sec as the primary criterion for identifying stenosis of the internal carotid artery (ICA) greater than 70% but less than occlusion, such a threshold may still be too low and its use could lead to unnecessary carotid interventions in patients with an otherwise low risk of stroke, said Dr. Gregory L. Moneta, chief of the division of vascular surgery at Oregon Health and Science University, Portland, and staff surgeon at the Portland VA Medical Center.

The risk of stroke in asymptomatic patients "is too low to justify prophylactic carotid intervention based on the threshold level suggested by the Society of Radiologists in Ultrasound," said Dr. Moneta.

In 2003, a consensus conference recommended using a PSV of greater than 230 cm/sec as the primary criterion for stenosis of the internal carotid artery (ICA) greater than 70% but less than occlusion. This criterion is less than an angiographically-validated criterion of a PSV greater than 290 cm/sec for ICA stenosis greater than 60%.

To assess the potential clinical impact of using a PSV greater than 230 cm/sec to select patients for prophylactic carotid intervention, the researchers evaluated the risk of progression of ICA stenosis and risk of stroke in asymptomatic patients with a PSV between 230 cm/sec and 290 cm/sec.

In all, 562 patients were enrolled as part of a prospective trial of the progression of atherosclerosis, sponsored by the National Institutes of Health. Of these, 87 patients (61% males) had a PSV between 230 cm/sec and 290 cm/sec either at entry or subsequently. Roughly a quarter of patients had diabetes. Patients were evaluated every 6 months for an average of 27 months to identify new neurologic symptoms and duplex-determined progression of ICA stenosis.

Roughly half of patients (49%) progressed to a PSV greater than 290 cm/sec, with a mean time to progression of 20 months. Of these, roughly half (49%) underwent carotid intervention. Three patients had ipsilateral strokes, though two strokes occurred after PSV exceeded 290 cm/sec.

The incidence of stroke in patients with a PSV in the range of 230-290 cm/sec was 1%. During follow-up, 97% of patients were asymptomatic.

Progression of ICA stenosis was associated with diabetes and an elevated contralateral PSV. Progression was independent of initial PSV, diastolic velocities, hypertension, and hypercholesterolemia.

A third presentation addressed the issue of the future of coronary CT angiography.

Coronary CT angiography (CTA) is a rapidly evolving technique capable of volumetrically imaging the lumen, said Dr. Geoffrey D. Rubin, chief of cardio-vascular imaging, Stanford (Calif.) University.

In an informal meta-analysis using weighted averages, Dr. Rubin estimated that sensitivity of CTA for detecting significant lesions is 90% and negative predictive value for 64-slice CTA is 97%.

"The ability of CT to specifically filter out patients who do not have disease is a tremendously beneficial characteristic that hopefully can minimize the prevalence of negative arteriograms," said Dr. Rubin.

Dr. Rubin predicts that further technical improvements will enhance the reliability and robustness of coronary CTA, management decisions based on CTA will improve health outcomes, and coronary CTA will play a key role in the triage of patients with chest pain.

However, as long as stenting is the preferred treatment for CAD, angiography will remain the preferred test for assessing high-risk patients, he added.

Dr. Reid, Dr. Moneta, and Dr. Rubin disclosed that they have no potential conflicts of interest in these devices.