A head-to-head comparison between CT and virtual histology intravascular ultrasound (VH-IVUS) should increase confidence in the ability of CT to characterize coronary artery plaques and justify the use of a semiqualitative approach to plaque characterization for clinical reporting purposes.
That pronouncement was made in a commentary by Szilard Voros, MD, medical director of cardiovascular MRI and CT at the Fuqua Heart Center in Atlanta, Ga., in the April 2008 issue of JACC Cardiovascular Interventions.
Voros, commenting on a study by Gabija Pundziute, MD, and colleagues from the Netherlands, Lithuania and Belgium, said the work of these researchers is an important step in furthering the role of CT in clinical cardiology. “It seems that with regards to coronary plaque characterization, the CAT (scan) is indeed out of the bag.”
Researchers used CT (Aquilion, Toshiba Medical Systems) to obtain a coronary calcium (CAC) scan without contrast, followed by ECG-gated CT angiography. The data were reconstructed and transferred to a remote workstation for postprocessing (Vitrea 2, Vital Images or Advantage, GE Healthcare).
The investigators used sharp reconstruction algorithms to reduce partial volume effects when extensive coronary calcifications were present.
Two experts visually classified 168 plaques in 50 patients into one of three categories: noncalcified plaques (29 percent), mixed plaques (42 percent) and calcified plaques (29 percent).
For each IVUS exam, researchers used a 20-MHz, 2.9-F, phased-array IVUS catheter (Eagle Eye, Volcano Corp.). Data were analyzed off-line using dedicated Volcano software. Plaques were characterized based on spectral analysis of IVUS backscattered signals. Four tissues were differentiated:
- fibrotic tissue labeled in dark green
- fibro-fatty in light green
- necrotic core in red, and
- dense calcium in white
They observed a moderate but significant correlation between the Agatston score as measured by CT and the volume of dense calcium on VH-IVUS. Voros said that this correlation between the two modalities should “further increase our confidence in the value and validity of CAC scoring by MSCT.”
Pundziute and colleagues found that noncalcified plaques by CT contained more fibrous and fibrofatty tissue by VH-IVUS compared with calcified plaques, and calcified and mixed plaques contained more dense calcium compared with noncalcified plaques. These are important findings, said Voros, because they “validate a visual, qualitative, and clinically applicable approach to plaque characterization.”
Finally, researchers found that thin-cap fibroatheromas identified with VH-IVUS were most prevalent in mixed plaques as identified by CT: 32 percent of mixed plaques met criteria for thin-cap fibroatheromas by VH-IVUS compared with 13 percent of noncalcified and 8 percent of calcified plaques.
“It is important to realize,” wrote the researchers, “that the insufficient spatial resolution of MSCT precludes exact location of lipid accumulation in the plaque as well as visualization of the thin fibrous cap. Accordingly, direct identification of thin-cap fibroatheromas on MSCT is at present not feasible.”
Voros, however, suggests that “we pay particular attention to mixed plaques in designing larger-scale outcomes studies using MSCT technology.”
|Coronary Plaque Characteristics on VH-IVUS in 3 Types of Plaques on CT|
|VH-IVUS Characteristic||Noncalcified Plaques (48)||Mixed Plaques (71)||Calcified Plaques (49)||p Value|
|Lesion length (mm)||22.33||19.53||21.51||0.55|