Comparison of carotid plaque tissue characteristics in patients with acute coronary syndrome or stable angina pectoris: assessment by iPlaque, transcutaneous carotid ultrasonography with integrated backscatter analysis

From 195 patients who had undergone percutaneous coronary intervention and carotid ultrasonography in our department from January 2010 to June 2014, patients with suitable carotid plaques for iPlaque analysis were enrolled in this study. The selection criteria for suitable iPlaque analysis were as follows: (1) maximal intima-media thickness (IMT) of the plaque ≥ 1.5 mm, (2) located in either the common or internal carotid artery, (3) located in the far wall, and (4) without severe stenosis (defined as > 75 % area stenosis). Of them, 26 patients were diagnosed as having ACS, and 169 were diagnosed with SAP. The ACS group included 8 patients with ST elevation myocardial infarction, 12 patients with non-ST elevation myocardial infarction, and 4 patients with unstable angina pectoris. To compare the tissue characteristics of the carotid artery with those in the 26 ACS patients, 38 age- and gender-matched subjects were randomly selected from the SAP patients. If the true difference of average IB values of carotid plaque in the ACS and SAP is 2.0 with standard deviation 2.5 according to a previous study [13], our sample size were enough to reject the null hypothesis that the population means of the ACS and SAP groups are equal with probability (power) 0.85. The Type I error probability associated with this test of this null hypothesis is 0.05.

ACS included acute myocardial infarction and unstable angina pectoris. The diagnosis of acute myocardial infarction was made if the patient had typical chest pain with characteristic electrocardiogram changes and elevation of at least one positive biomarker (creatinine kinase [CK], CK-MB, or troponin T). A diagnosis of unstable angina pectoris was made when there was angina with a progressive crescendo pattern, or angina that occurred at rest. SAP was defined as no episodes of angina at rest in a patient who underwent percutaneous coronary intervention more than six months previously. Hypertension was defined as a systolic blood pressure >140 mmHg and/or a diastolic pressure >90 mmHg, or current treatment with antihypertensive medication. Diabetes was defined as a fasting blood glucose ≥126 mg/dl, hemoglobin A1c ≥6.5 %, and/or the need for oral hypoglycemic agents or insulin. Hyperlipidemia was defined as plasma total cholesterol >220 mg/dl or the use of lipid-lowering therapy. Smoking was defined as current or previous smoking [23].

Carotid ultrasound examinations were performed using the Logiq 7 with a 10 MHz linear-array transducer (GE Healthcare, Milwaukee, WI, USA). All carotid scans were performed by a single well-trained operator, who had no information on the clinical characteristics of the patients. The position of the probe was adjusted so that the ultrasonic beam was vertical to the artery wall, and maximal IMT was measured manually in the common carotid artery, bulb and internal carotid artery according to the standard method recommended by Japanese Society of Ultrasound in Medicine [15]. Images of longitudinal sections of carotid plaque of maximal IMT were digitally stored with RAW data. The data were analyzed off-line by our dedicated software (iPlaque), as described later. The study was in accordance with the Declaration of Helsinki, and was approved by the Institutional Review Board of the University of Tokushima. Each subject gave informed written consent.

In the iPlaque analysis, ultrasound attenuation was adjusted by 2.0 dB/mm at first. Then, each IB value was normalized to the calibrated value by setting the IB value of blood near the plaque to −70 dB. The edge of the plaque of interest was manually traced, and then the program calculated the plaque area and the average IB value in the plaque. The area of each tissue component was categorized in accordance with published IB thresholds, and the amount of each component as a percentage of the whole plaque area was calculated. The IB thresholds used were: −30.14 dB≦IB = calcification (display in red); −46.18 dB≦IB < −30.14 dB = dense fibrosis (display in yellow); −61.23 dB≦IB < −46.18 dB = fibrosis (display in green); − IB < −61.23 dB = lipid pool (display in blue) [22].

To assess measurement reproducibility, 20 patients with ACS were selected randomly and we evaluated two different measurements on the 20 ACS patients. The correlation between the measured values of percentage blue area by the two observers (Observer I and Observer II) was regarded as the inter-observer correlation. The correlation between the first and second values determined by one observer (Observer I) was regarded as the intra-observer correlation. The coefficients of variation of interobserver and intraobserver was 7.3 % and 6.6 %, respectively. Also, we plotted the mean differences against the mean of 2 measurements according to the methods described by Bland - Altman.

Data are presented as mean ± SD. Data were tested for normality using the Kolmogorov-Smirnov test. Continuous variables were compared using unpaired Student’s t tests or Mann–Whitney test as appropriate, whereas categorical variables were compared using Chi-square test or Fisher’s exact test, as appropriate.　Receiver-operating characteristic (ROC) curves were generated to determine optimal cutoff values of continuous variables. Correlation coefficients were calculated by using the least-squares fit (forced through zero), and Bland - Altman analysis was used. Linear regression model assumptions of linearity, independence of observations, normality of error terms, homoscedasticity were all checked. All statistical analyses were carried out with Medcalc Software 12.7.5.0 (Ghent, Belgium). A p-value <0.05 was considered significant.