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). 

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