Multilevel Assessment of Atherosclerotic Extent Using a 40-Section Multidetector Scanner after Transient Ischemic Attack or Ischemic Stroke

Research Design

In the second part of our study, we used data collected in the first part of the study.⁶

Imaging Protocols

We performed contrast-enhanced MDCT by using a Brilliance 40 scanner (Philips Healthcare, Best, the Netherlands), with iomeprol (Iomeron 400; Bracco Diagnostics, Milan, Italy) injected into the right cubital vein with an 18-gauge catheter. The patient was placed in the supine, head-first position. A 2-step protocol was performed: first, electrocardiogram (ECG)-gated aortic and heart acquisitions were performed in the head-to-feet direction, encompassing the aortic and heart area from the top of the aortic arch to the diaphragm. The following parameters were used: 40 detectors, individual detector width of 0.625 mm, retrospective ECG gating, tube voltage of 120 kV, tube current of 300 mAs, pitch of 0.2, and half-rotation reconstruction. Iomeprol 70 mL and then saline solution 60 mL were injected at 4 mL/s. A bolus-tracking method was used with an attenuation threshold of 200 Hounsfield units in the ascending aorta. Reconstruction parameters for the axial sections were a 1.5-mm effective section thickness, 1-mm increments, a reconstruction filter Cardiac B, and an adapted field of view. Retrospective ECG-gated reconstruction was performed at 40% and 75% of the R-R interval. Then, 2 minutes later, a non–ECG-gated acquisition from the aortic arch to the vertex (approximately 50 cm) was performed with the following parameters: feet-to-head direction, section thickness of 1.2 mm, pitch of 1.2, tube voltage of 120 kV, amperage of 300 mAs per section, reconstruction filter B, and the bolus tracker set on the aortic arch with an attenuation threshold at 200 Hounsfield units. Iomeprol 50 mL and then saline solution 60 mL were injected at 4 mL/s, for a total injected contrast material volume of 120 mL. The patient underwent imaging with the arms over the head during the aortic and heart acquisitions and with the arms at the sides during the second acquisition. General guidelines for ECG-gated cardiac MDCT were followed regarding the qualifications of the personnel, radiation dose monitoring, and the safety rules for contrast agent and β-blocker administration.^7⇓–9 β-Blockers (esmolol hydrochloride; Brevibloc; Baxter, Deerfield, Illinois) (0.5–1.0 mg per kilogram of body weight) were administered intravenously, if necessary, when the heart rate was higher than 80 beats per minute. CT was performed even in cases of atrial fibrillation. The total radiation dose and the heart rate of each patient during the examination were recorded. The thyroid gland was irradiated once during the acquisition from the aortic arch to the vertex.

Imaging Evaluation

The diagnostic work-up was focused on the following vascular abnormalities by use of MDCT with CTA:

• Atherosclerotic lesion of the carotid arteries leading to ≥50% stenosis. The degree of carotid stenosis was measured with the North American Symptomatic Carotid Endarterectomy Trial (NASCET) criteria.¹⁰ A measure (measure 1) was taken of the diameter of the narrowest portion of the cervical ICA in the axial plane. This was compared with the maximal diameter (measure 2) of the cervical ICA distal to the carotid bulb at a location in which the imaging plane was orthogonal to the artery, the arterial walls were parallel, and where there was no arterial disease. The degree of cross-sectional stenosis was calculated in percent as: percent stenosis (1−[measure 1/measure 2]) × 100%, analogous to the method used in NASCET. In cases of poststenotic dilation of the ICA, we used as a denominator for the ratio calculation the diameter of the internal carotid well beyond the bulb, where the walls are parallel. Near-occlusion cases were defined as the presence of notable stenosis of the ICA bulb and distal ICA caliber reduction, compared with 1) expected size, 2) the controlateral ICA, and 3) the ipsilateral external carotid artery). In these cases, we did not do a ratio calculation and arbitrarily assigned as 95% stenosis as established in NASCET.^10⇓–12

• Atherosclerotic lesion of the vertebral arteries leading to ≥50% stenosis

• Aortic arch atheroma of > 4 mm

• Intracranial artery stenosis ≥50%

• Coronary artery stenosis ≥50%

Atherosclerotic extent was classified in 0, 1, 2, 3, and 4 atherosclerotic levels according to the number of arterial territories (aortic, coronary, cervical, intracranial) affected by at least 1 vascular abnormality as we described previously.

Statistical Analysis

Continuous variables were expressed as mean (standard deviation), and categoric variables were expressed as percentages. We compared continuous variables by using the t test or the Mann-Whitney test where appropriate, and categoric variables by using the Pearson χ² test or the Fisher exact test where appropriate. The associations between atherosclerotic extent (< 2 vs ≥2 atherosclerotic levels) and main vascular risk factors were measured by calculation of adjusted odds ratios and 95% confidence intervals by logistic regression analyses.

Multivariable models were adjusted for age, sex, diabetes, hypertension, dyslipidemia, and tobacco. The distribution of arterial disease combinations in patients with ≥2 atherosclerotic levels was compared by use of the Fisher exact test. A P value < .05 was considered statistically significant.

We performed statistical analysis by using STATA, version 11.0 (StataCorp, College Station, Texas) and R software, version 2.10.1 (http://www.r-project.org/).