Relationship between Coronary Artery Calcium Score by Multidetector Computed Tomography and Plaque Components by Virtual Histology Intravascular Ultrasound

Patient population

This study was a retrospective, single-center study. From January 2007 to November 2009, a total of 172 CAD patients with 250 coronary lesions who underwent MDCT and VH-IVUS at Chonnam National University Hospital were enrolled in this study.

Laboratory analysis

The blood samples were centrifuged, and serum was collected and stored at -70℃ until the assay was performed. Absolute creatine kinase-myocardial band levels were determined by radioimmunoassay (Dade Behring Inc., Miami, FL, USA). Cardiac-specific troponin I levels were measured by a paramagnetic particle, chemiluminescent immunoenzymatic assay (Beckman, Coulter Inc., Fullerton, CA, USA). The serum levels of total cholesterol, triglyceride, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol were measured by standard enzymatic methods. High-sensitivity C-reactive protein was analyzed turbidimetrically with sheep antibodies against human C-reactive protein; this has been validated against the Dade-Behring method (12).

Coronary angiographic analysis

Coronary angiogram was analyzed with validated QCA system (Phillips H5000 or Allura DCI program, Philips Medical Systems, Eindhoven, the Netherlands) (13). With the outer diameter of the contrast-filled catheter as the calibration standard, the minimal lumen diameter and reference diameter were measured in diastolic frames from orthogonal projections.

Cardiac MDCT analysis

In the absence of contraindications (contraindication to intravenous contrast agents [contrast allergy] or elevated serum creatinine [> 1.3 mg/dL] or atrial fibrillation or frequent ventricular ectopy [> 10 extra systoles per minute], or a heart rate > 90 beats per minute), all patients with a resting heart rate of > 70 bpm received 50-100 mg of metoprolol orally 1-2 hr before scan. Blood pressure and pulse were rechecked immediately prior to performing the prodedure. CT examinations were performed with a two-phase, contrast-enhanced, electrocardiogram-gated, MDCT scanner (Sensation Cardiac 64, Siemens, Forcheim, Germany) set at a 0.75-mm section thickness with a gantry rotation time of 330 msec and a kernel value of B25f. The scan delay was determined using the bolus tracking technique: when a threshold of 120 Hounsfield Units (HU) was reached in the ascending aorta at the level of origin of the coronary arteries, a delay of 5 seconds was applied before the scan was initiated.

CACS were determined on a workstation (CaScore, Siemens, Germany) with software for coronary artery calcification according to the Agatston method as previously described (14). We classified the CACS as follows: Group I (calcium score = 0), Group II (calcium score = 1-100), Group III (calcium score = 101-400), Group IV (calcium score > 400) (Fig. 1A).

Fig. 1
The examples of coronary artery calcium detected by cardiac multidetector computed tomography (MDCT) (A) and plaque components assessed by virtual histology-intravascular ultrasound (VH-IVUS) (B).

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IVUS imaging and analysis

All pre-intervention grey-scale and VH-IVUS examinations were performed after intracoronary administration of 300 µg nitroglycerin. A 20-MHz, 2.9F IVUS imaging catheter (Eagle Eye, Volcano Corp, Rancho Cordova, CA, USA) was advanced > 10 mm beyond the lesion; and automated pullback was performed to a point > 10 mm proximal to the lesion at a speed of 0.5 mm/sec.

Grey-scale IVUS and VH-IVUS data were analyzed by 2 independent observers. The levels of reproducibility for external elastic membrane, lumen, and plaque plus media cross-sectional areas using the Spearman rank-order correlation coefficients were 0.95, 0.97, and 0.97, respectively. Similarly, for plaque components by VH-IVUS, reproducibility for the fibrous, fibro-fatty, dense calcium, and necrotic core volume measurements using the Spearman rank-order correlation coefficients were 0.95, 0.92, 0.93, and 0.93, respectively.

Quantitative volumetric grey-scale and VH-IVUS analyses were performed across the entire lesion segment, and cross-sectional analysis was performed at the minimum lumen area sites and at the largest necrotic core sites. Conventional quantitative volumetric grey-scale IVUS analysis was performed according to the American College of Cardiology Clinical Expert Consensus Document on Standards for Acquisition, Measurement and Reporting of Intravascular Ultrasound Studies (15). Measurements were made by every 1-mm interval for the region of interest, which was defined as the segment between distal to proximal reference sites that were the most normal looking within 5 mm proximal and distal to the lesion. Volumetric data were generated by the software using Simpson's method. External elastic membrane and lumen cross-sectional areas were measured. Plaque plus media cross-sectional area was calculated as external elastic membrane minus lumen cross-sectional area; and plaque burden was calculated as plaque plus media divided by external elastic membrane cross-sectional area. Total atheroma volume was calculated by summation of atheroma area from each measured image as: TAV = Σ (external elastic membrane area-lumen area). The percent atheroma volume was determined using the formula: PAV = 100 × (Σ [external elastic membrane area-lumen area]/Σ [external elastic membrane area]). VH-IVUS analysis classified the color-coded tissue into four major components: green (fibrous), yellow-green (fibro-fatty), white (dense calcium), and red (necrotic core) (16). VH-IVUS analysis was reported in absolute amounts and as a percentage of plaque area or volume (Fig. 1B). Thin-cap fibroatheroma was defined as a necrotic core ≥ 10% of plaque area in at least 3 consecutive frames without overlying fibrous tissue in the presence of ≥ 40% least 3 plaque burden.

Statistical analysis

The statistical package for social sciences (SPSS) for Windows, version 15.0 (Chicago, IL, USA) was used for all analyses. Continuous variables were presented as the mean value ± SD; comparisons were conducted by ANOVA. Discrete variables were presented as percentages and frequencies; comparisons were conducted by chi-square test, where appropriate. Pearson's correlation was used to evaluate the correlation between VH-IVUS parameters and CACS. A P value < 0.05 was considered statistically significant.

Ethics statement

The protocol was approved by the institutional review board of Chonnam National University Hospital (2010-06-112). Informed consent form was exempted because this study was a retrospective enrollment by review of hospital records.

Baseline characteristics

The baseline characteristics are summarized in Table 1. Group IV was oldest, and hypertension was observed most frequently in Group IV. The neutrophil count and hemoglobin level were lowest in Group IV. The high-density lipoprotein-cholesterol level was lowest in Group IV.

Table 1
Baseline characteristics

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Coronary angiographic findings

Coronary angiographic findings are summarized in Table 2. There were no significant differences in the target vessel, lesion location, and QCA data among four groups.

Table 2
Coronary angiographic findings

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Grey-scale and VH-IVUS findings

Grey-scale IVUS findings are summarized in Table 3. At the proximal reference, plaque burden was greatest in Group IV. At the distal reference, lumen cross-sectional area was smallest and plaque burden was greatest in Group IV. At the minimum lumen site, lumen cross-sectional area was smallest and plaque burden was tended to be greatest, and IVUS lesion was longest in Group IV. At the largest necrotic core site, plaque plus media cross-sectional area was greatest in Group IV. The calcium arc was greatest and calcium length was longest in Group IV and most of the calcium was observed at superficial location in Group IV. By volumetric analysis, external elastic membrane volume and total atheroma volume and percent atheroma volume were greatest in Group IV.

Table 3
Grey-scale intravascular ultrasound findings

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VH-IVUS findings are summarized in Table 4. At the proximal reference, absolute dense calcium and necrotic core areas were greatest in Group IV. At the distal reference, absolute dense calcium and necrotic core areas were greater in Group III and IV than Group I and II. At the minimum lumen area site, absolute dense calcium area was greatest in Group IV (Fig. 2A). At the largest necrotic core site, absolute dense calcium and necrotic core areas were greatest in Group IV. At the proximal reference, relative dense calcium and necrotic core areas were greatest in Group IV. At the minimum lumen area site, relative dense calcium area was greatest, and in contrast relative fibrous area was smallest in Group IV (Fig. 2B). At the largest necrotic core site, relative dense calcium area was greatest, and in contrast relative fibrous area was smallest in Group IV. By volumetric analysis, absolute fibrous and fibro-fatty and dense calcium and necrotic core volumes were greatest in Group IV (Fig. 3A). Relative dense calcium and necrotic core volumes were greatest, and in contrast relative fibrous volume was smallest in Group IV (Fig. 3B). Also thin-cap fibroatheromas were observed more frequently in Group IV (10% vs 12% vs 8% vs 33%, P = 0.046, respectively).

Fig. 2
Absolute (A) and relative (B) plaque components at the minimum lumen area sites. Absolute dense calcium and relative dense calcium (%) areas were greatest in Group IV; in contrast, relative fibrous (%) area was smallest in Group IV.

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Fig. 3
Absolute (A) and relative (B) plaque components by volumetric analysis. Absolute and relative dense calcium (%) and necrotic core (%) volumes were greatest in Group IV; in contrast, relative fibrous (%) volume was smallest in Group IV.

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Table 4
Virtual histology intravascular ultrasound findings

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Correlations between VH-IVUS parameters and CACS

The absolute plaque and dense calcium and necrotic core volumes and the relative dense calcium volume correlated positively with CACS (r = 0.470, P < 0.001, r = 0.318, P < 0.001, r = 0.313, P < 0.001, respectively), in contrast the relative fibrous volume correlated negatively with CACS (r = -0.328, P < 0.001).