Contrast-enhanced MR aids risk assessment of carotid plaque

U.K. researchers are using contrast-enhanced MR imaging to identify the stability of carotid plaques. The technique holds potential for determining risk in atheroma patients and tracking plaque progression in response to treatment.

Vulnerable atheromatous plaques can be characterized by their large necrotic lipid core, which is separated from the vessel wall by a thin fibrous cap. This cap can be weakened by macrophage infiltration and inflammation, making macrophage imaging a good marker of plaque stability.

Macrophage imaging can be achieved by using contrast-enhanced MR with ultrasmall iron oxide particles, said Dr. Jean-Marie U-King Im at a special session on experimental vascular imaging at the European Congress of Radiology.

"USPIO has mainly been used in human studies for lymph node imaging, but by pure coincidence, signal intensity changes were observed in human atheroma as well," U-King Im said. "Studies in rabbits have confirmed that these particles are taken up by macrophages."

Researchers from the University of Cambridge and Addenbrookes Hospital used the technique on 23 symptomatic patients awaiting carotid endarterectomy. Patients underwent T1-weighted fast spin-echo and T2*-weighted spiral MR before and 24 to 96 hours after infusion with the USPIO agent Sinerem (2.6 mg/hr). The team then compared areas of plaque signal loss on postinfusion images with histology results from plaque retrieved during surgery.

Histologic analysis showed USPIO accumulation in 20 out of the 23 cases, though two of the negative findings related to patients whose symptoms had occurred six months previously, U-King Im said. Double-reading of the contrast-enhanced MRI showed new areas of signal loss in 19 of the 23 cases.

"Two of the negative MR findings were from patients with very heavily calcified plaques. This can be a problem because on the preinfusion MR, calcification already appears as an area of low signal intensity," he said.

Taking histology as the gold standard, UPSIO-enhanced MR had sensitivity and specificity approaching 80% for plaque macrophage detection, U-King Im said. The positive predictive value came out as 90%, and the negative predictive value as 63%.

The team is proceeding with longitudinal studies in asymptomatic atheroma patients to assess the potential for USPIO-enhanced MRI in identifying and monitoring vulnerable plaques.