Normal norms of carotid vessel wall volume in Taiwanese adults as measured using three-dimensional ultrasound
Authors:
Chih-Chen Liao, 1 , Pei-Ya Chen, 2 , Shinn-Kuang Lin, 3 ,
1 Stroke Center and Department of Neurology, Taipei Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Taiwan 2 Stroke Center and Department of Neurology, Taipei Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Taiwan; School of Medicine, Tzu Chi University, Hualien, Taiwan 3 Stroke Center and Department of Neurology, Taipei Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation, Taiwan; School of Medicine, Tzu Chi University, Hualien, Taiwan
Corresponding Author:
Shinn-Kuang Lin
keywords: carotid ultrasound, intima–media thickness, normal norms, three‐dimensional ultrasound, vessel wall volume
Abstract
Purpose:The three-dimensional (3D) measurement of vessel wall volume (VWV) and plaque volume is sensitive for predicting cardiovascular risk. We established the normal norms of carotid VWV.
Methods: We retrospectively enrolled 352 patients with normal findings of the carotid ultrasound studies. Two-dimensional carotid intima-media thickness (IMT) was measured online. Grayscale 3D images of both sides of the carotid arteries were analyzed offline for measurement of IMT (QIMT) and VWV.
Results: The median age of the enrollees was 59 years. The median carotid IMT, QIMT, and VWV was 0.61 mm, 0.72 mm, and 90 mm3, respectively. No differences in IMT and VWV were observed between men and women or between the right and left side. We stratified participants into four groups, namely young adults (≤50 years), middle-aged adults (51–65 years), older adults (66–75 years), and senior adults (≥75 years). All the values of measured variables increased with advancing age. The median VWV of each group was 84, 90, 100, and 112 mm3, respectively. The increment percentage from young to senior adults was similar in terms of IMT and VWV. Nevertheless, the difference in the value of VWV (28 mm3) was much larger than that in IMT (0.18 mm). All three measured variables exhibited a positive linear correlation with age.
Conclusion: Both IMT and VWV have positive linear correlations with age. The application of QIMT measurements was limited by its inconsistent accuracy. VWV not only has a strong correlation with IMT but also enables observation of dynamic vessel wall changes, which is valuable for clinical
observational studies.
Keywords: carotid ultrasound, intima–media thickness, normal norms, three‐dimensional ultrasound,
vessel wall volume
INTRODUCTION
The carotid arteries are one of the most easily screened arteries of the vascular system. Carotid atherosclerosis is associated with cardiovascular and cerebrovascular events(1,2), and the degree of carotid atherosclerosis has been used to estimate and classify individuals’ cardiovascular risk(3). The prevalence of increased carotid intima–media thickness (IMT) and carotid plaque is increasing in the general population worldwide(4). Carotid
IMT is the combined thickness of the intimal (20%) and medial (80%) layers of the carotid artery(5). Meta-analysis studies have demonstrated that carotid IMT only minimally improves predictive power beyond traditional risk factors(6). Carotid plaque is a more powerful predictor of cardiovascular risk than carotid IMT alone. However, focal thickening of the IMT typically coexists with protuberant plaque, and the two conditions are indistinguishable from each other. Therefore, measuring the thickness of the vessel wall, which comprises the intima–media and protuberant plaque, is more reasonable
than measuring the IMT or plaque alone(7). Carotid ultrasound is a noninvasive and reproducible study and is generally regarded as the standard imaging technique for primary evaluation of carotid atherosclerosis and plaque burden(5,8,9). With the continual advancements in ultrasound equipment and relevant software, the IMT, vessel wall, and plaque of the carotid artery can now be detected on a three-dimensional (3D) plane. Smaller
values of measured IMT can be converted to higher vessel wall volume (VWV) values, which enhances the feasibility of monitoring the sequential changes of carotid plaque in a shorter duration and with fewer subjects during clinical research(3,10). Furthermore, 3D assessment of plaque volume is progressively more sensitive in predicting cardiovascular risk than mere assessment of plaque presence(6). We developed a standardized method of 3D ultrasound measurement of carotid VWV with a reasonable interobserver differences and high reliability(7). In this study, we established the normal norms of carotid VWV in Taiwanese adults and their association with age and carotid IMT.
METHODS
A 3D transducer was available as part of the ultrasound equipment in our institute, and rapid 3D carotid ultrasound scanning of bilateral carotid arteries is included in the regular carotid ultrasonography program. Through a retrospective review, patients who were referred to the sonographic laboratory from the outpatient clinic for carotid duplex sonography and had normal findings of bilateral carotid arteries on a B-mode scan with available 3D images were enrolled in this study. Ethical approval for this study was provided by the Institutional Review Board of Taipei Tzu Chi Hospital, New Taipei City, on August 7, 2020 (no. 09-X-063). Informed written consent was waived because the study constituted a retrospective data analysis. The detailed procedures of the two-dimensional (2D) and 3D carotid ultrasound were described in a previous report(7). In brief, we used an Affiniti 70 ultrasound system (Philips Healthcare, Bothell, WA, USA) to measure the 2D carotid IMT at a mean distance of 1 cm from the far wall of the distal common carotid artery (CCA) bilaterally with the built-in autotrace function of the scanning equipment. Grayscale 3D images of the carotid artery from the distal CCA to the internal carotid artery on both sides were acquired using a single sweep VL15-3 transducer. The 3D images were analyzed offline on a computer by using the vascular plaque quantification function of the Philips QLAB software. The standardized method of measuring the carotid VWV was employed, with measurements taken from a 1-cm segment of the distal CCA that was 1 cm proximal to the carotid bifurcation (Figure 1A) with a preset IMT of 0.4 mm. The intima–media area was defined as the area between the outer and inner vessel walls. Plaque area was defined as the area between the lumen boundary and inner vessel wall, and the wall area was defined as the sum of the intima–media area and plaque area (Figure 2B). Measured data from all frames were exported as an Excel file. From the exported Excel file, we manually computed the mean intima–media thickness (QIMT) and VWV of the measured 1-cm segment at the junction of the distal CCA and CCA bifurcation. The 3D image measurements were taken by a trained neurology resident. Continuous variables were computed as the median (first to third quartile). The Mann–Whitney U and Kruskal–Wallis tests were used to evaluate differences in continuous variables. P < 0.05 was considered statistically significant. The correlation between the measured variables and age was analyzed through linear regression. All statistical analyses were performed using the MedCalc software package (version 18, Mariakerke, Belgium).
RESULTS
A total of 352 patients were enrolled in this study, including 147 men and 205 women. A total of 704 carotid arteries (including right and left sides) were measured. The median age was 59 (51–68) years, with no difference noted between men and women. The median 2D carotid IMT of the 704 carotid arteries in the 352 patients, which was automatically traced using the ultrasound equipment, was 0.61 (0.52–0.70) mm (Table 1). The median QIMT and VWV, which were manually measured using QLAB software, were 0.72 (0.63–0.85) mm and 90 (86–98) mm3, respectively. No differences in IMT and VWV were observed between men and women or between the right and left side of the carotid arteries. A higher QIMT was observed in women and on the right side than in men and on the left side. The median value of the manually measured QIMT (0.74 mm [0.63–0.85 mm]) was higher than that of the automatically traced IMT (0.61 mm [0.52– 0.70 mm]; P < 0.0001; Figure 2).
To identify the effect of age on the carotid vessel wall, we classified the enrollees into four age groups, namely young adults (≤50 years), middle-aged adults (51–65 years), older adults (66–75 years), and senior adults (age: ≥75 years). The number of participants in the senior adults group was only 23, which is possibly attributable to the lower proportion of senior adults with normal carotid
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