Associations between Carotid Artery Wall Thickness and Cardiovascular Risk Factors Using Multidetector CT

Patient Population

An MDCTA dataset of 191 consecutive patients (131 men, 60 women; median age, 70 years; age range, 39–85 years) was retrospectively analyzed. Each patient gave written informed consent. Each MDCTA examination was performed when it was clinically indicated and was ordered by the patient's physician as part of routine clinical care, as described previously.^10,15,16

In our institution, the inclusion criteria for performing MDCTA are as follows: a prior clinical indication for CTA of the supra-aortic vessels (when possible also confirmed by US-ECD study¹⁵) as stated by the referring physician and established by the attending radiologist. In particular, the main reason for referral to MDCTA is the presence of a sonogram showing a pathologic stenosis and/or a plaque alteration, or when sonography cannot provide sufficient information about the degree of stenosis—for example, in the presence of large calcified plaques with acoustic shadowing, high carotid bifurcation, or a thick neck (edema, obese patients). “Plaque alteration” was the presence of a heterogeneous plaque, an irregular surface, intraplaque hemorrhage, and/or the presence of ulceration in the plaque. We defined “symptomatic” as a patient who had a TIA or stroke. We considered “TIA” as a brief (<24 hours) episode of neurologic dysfunction, such as dysarthria, dysphasia, hemiparesis, hemiparesthesia, or monocular blindness. If the episode of neurologic dysfunction exceeded 24 hours, it was classified as a “stroke.” We defined “asymptomatic” as descriptive of a patient who had no history of symptoms, either remote or present, at the time of the examination.

In our department, we studied the carotid arteries of asymptomatic patients with diabetes older than 50 years of age and of patients who were undergoing cardiac interventions for coronary artery disease, aortic interventions, and lower leg artery surgery.

Exclusion criteria for the study consisted of contraindications to iodinated contrast media, such as a known allergy to iodinated contrast material, or elevated renal function test results.

This retrospective review evaluated existing clinical data and records. No additional procedures were performed. The review was conducted in accordance with the guidelines of the research committee of our institution. Part of our patient cohort had been included in previous studies.^15,17–19

Vascular risk factors that are known before a stroke or TIA, coexisting comorbidities, and treatment before a stroke or TIA are systematically recorded at our institution. “Essential hypertension” was defined in those individuals who had systolic blood pressure >140 mm Hg and/or diastolic blood pressure >90 mm Hg or who were being treated with blood pressure−lowering drugs. “Diabetes” was indicated by abnormal fasting plasma glucose levels (>7.9 mmol/L) or the current use of insulin or an oral hypoglycemic agent. The use of insulin or oral diabetes medication was also considered diagnostic for diabetes. “Dyslipidemia” was defined as abnormal fasting plasma cholesterol (low-attenuation lipoprotein cholesterol) levels (fasting cholesterol, >5.0 mmol/L) or the current use of lipid-lowering agents. Cigarette smoking status was categorized as never, former (24 months), or current.

MDCTA Technique

We use a standardized technique as previously described.^15,17–19 All patients underwent MDCTA of the supra-aortic vessels by using a 4-multidetector-row CT system (Mx8000, Philips Healthcare [formerly Picker International], Andover, Massachusetts). Written consent to perform MDCTA was obtained after discussion of the risks associated with contrast-enhanced MDCTA and the potential benefits derived from this examination. Patients were placed in the supine position, with the head tilted back to prevent dental artifacts on the images. Patients were also instructed not to breathe and not to swallow. One hundred ten milliliters of a contrast medium (iopromide, Ultravist 370; Schering, Berlin, Germany) was injected into an antecubital vein, by using a power injector at a flow rate of 5–6 mL/s and an 18-ga intravenous catheter. CT technical parameters were as follows: matrix, 512 × 512; FOV, 11–19 cm; mAs, 180–220; kV, 120–140. Images were reconstructed with a section thickness of 0.6 mm. Angiographic acquisition included the circle of Willis; the lower landmark was the aortic arch. None of the patients included in the study had a medical history of cardiac output failure; any contraindications to iodinated contrast media, such as a known allergy; or elevated renal function test results.

Evaluation of Carotid Image Quality and Artifacts

In this study, evaluation of image quality and artifacts was performed by 2 radiologists with 5 and 10 years of experience in MDCTA angiography of the carotid arteries. The readers were blinded to the patient's symptoms. They were asked to evaluate the overall image quality on a 5-point scale. On this scale, 5 corresponded to excellent image quality; 4, to good image quality; 3, to adequate image quality; 2, to marginally acceptable image quality; and 1, to unacceptable image quality. Values 1 and 2 included those cases in which the carotid artery wall did not show a well-defined cleavage plane, with nearby structures such that it was not possible to clearly observe the hypoattenuated adipose tissue that allows the end of the carotid wall to be defined.

The readers were also asked to assess the impact of image artifacts on a 5-point scale. On this scale, 5 corresponded to the complete absence of imaging artifacts; 4, to mild artifacts not interfering with diagnostic decision making; 3, to moderate artifacts slightly interfering with diagnostic decision making; 2, to pronounced artifacts interfering with diagnostic decision making (though it was still possible to arrive at a diagnosis), and 1, to a situation in which artifacts completely hindered diagnostic decision making. Patients with image quality 1 and 2 and with image artifacts 1 and 2 were excluded from this study.

Evaluation of CAWT

For the MDCTA examination, both right and left carotid arteries were measured. CTA source axial images were considered. Two radiologists independently evaluated CAWT, blinded to each other's results. Window level, window width, and magnification were freely modifiable The CAWT was measured at the thickest point of the common carotid artery wall where there was no evidence of a plaque (Fig 1). This point was selected because if the wall is measured close to a stenosis, then it should be thicker. Three measurements for each carotid artery were performed, and the individual subject's mean CAWT values were then obtained as an average of values for each carotid artery. We measured the CAWT between the leading edge of the opacified vessel lumen and the external visible limit of the arterial wall, where it was surrounded by adjacent adipose tissue. Measurements between the 2 observers were averaged; in fact, in a previous study¹⁵ we observed, by using this technique, a concordance correlation coefficient that was extremely high (ρ = 0.923).

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Fig 1.

A, MDCTA axial image of a 64-year-old man. B, 3× magnification. White arrow in A and B indicates the CAWT (1.42 mm).

For analysis of data in symptomatic patients, we considered only the measurements in the carotid artery concordant with the symptoms (though both right and left carotid arteries were measured), and in the asymptomatic patients, we considered the highest CAWT value of the right and left carotid arteries.

Statistical Analysis

The normality of each continuous variable group was tested by using the Kolmogorov-Smirnov Z-test. Comparison of mean CAWT values in patients with and without stroke was performed by using a Student t test because the normality of the variable was accepted in both groups. We performed logistic regression analysis to examine the relationship between CAWT and the independent variables hypertension, dyslipidemia, diabetes mellitus, and smoking. To consider CAWT a dichotomous variable, as required in logistic regression analysis, we considered increased CAWT values to be those >1 mm and normal CAWT values to be those <1 mm, because a risk of stroke was demonstrated by Saba et al¹⁵ for CAWT values >1 mm. ROC curve analysis was also performed and the Az was determined for hypertension, dyslipidemia, diabetes mellitus, and smoking, but in this case considering the CAWT as a continuous variable. A P value < .05 indicated statistical significance. R software (www.r-project.org) was used for statistical analyses.