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Introduction
Ischemic stroke is a leading cause of death and disability worldwide, with approximately 20% to 30% attributed to carotid atherosclerotic plaques [1,2]. Current guidelines for large artery stroke assessment typically require carotid stenosis ≥50% [3], thus excluding non-stenotic plaques (stenosis <50%) and leaving these patients without specific risk prevention. Nevertheless, non-stenotic carotid plaques play a recognized role in cryptogenic stroke [4]. Consequently, there is growing interest in using non-invasive imaging to identify high-risk features of non-stenotic plaques, with the goal of improving risk stratification and management in patients with mild carotid stenosis [5].
Ultrasound (US) is the first-line imaging modality for carotid plaque evaluation due to its safety, high spatial resolution, efficiency, and absence of ionizing radiation [6]. Advances in US techniques have improved the detection of vulnerable plaques [7-9]. High-frame rate vector flow imaging (VFI) is a novel US-based method that quantifies multidirectional flow velocities and visualizes multidimensional blood flow and vortex patterns in real time [10]. By characterizing biomechanical conditions around the plaque—which contribute to its development—high-frame rate VFI can provide insights into plaque vulnerability [11]. In the authors’ recent work, VFI-derived biomechanical parameters emerged as promising non-invasive indicators correlated with the risk of carotid plaque in mild stenosis [12].
Shear wave elastography (SWE) is another innovative US technique that enables quantitative, non-invasive assessment of tissue stiffness, which reflects plaque characteristics such as the presence of a thin fibrous cap and lipid necrotic core [13-16]. Previous research and the present study have also demonstrated a potential clinical benefit of using SWE imaging to assess carotid plaque vulnerability [17-19]. However, the combined quantitative evaluation of carotid plaques using high-frame rate VFI and SWE for risk prediction in mild stenosis remains unexplored.
To better understand the causal role of non-stenotic carotid plaques in ischemic stroke, this retrospective study was performed to quantitatively evaluate peri-plaque biomechanical parameters and plaque elasticity using B-mode US, high-frame rate VFI, and SWE in patients with mild stenosis and a single plaque at the carotid bifurcation (CB). To minimize bias, inclusion criteria required that any ipsilateral ischemic lesion be confined to the territory supplied by the carotid artery bearing the solitary CB plaque. The authors then compared these US-derived parameters between patients with recent ipsilateral ischemic stroke and those without. These findings may offer novel diagnostic insights for identifying patients with mild carotid stenosis at high risk of stroke.
Materials and Methods
Compliance with Ethical Standards
The study protocol was approved by the relevant institutional ethics committee of Shanghai General Hospital ([2022]028), and informed consent was obtained from all participants.
Study Population and Patient Selection
This retrospective case-control study initially included 616 patients (aged 37-86 years) diagnosed with carotid plaques who underwent B-mode US, high-frame rate VFI, and SWE examinations between September 2023 and April 2024 (Fig. 1).
The inclusion criteria were as follows: (1) the presence of a single plaque located at the CB and (2) mild carotid stenosis (<50%) on B-mode US, as defined by the North American Symptomatic Carotid Endarterectomy Trial criteria [20]. The exclusion criteria comprised: (1) other thromboembolic sources, such as cardiac or intracranial embolism; (2) transient ischemic attack without definitive imaging evidence of an anterior circulation lesion ipsilateral to the CB plaque; (3) poor image quality caused by issues such as severe plaque calcification (Gray-Weale–Nicolaides Type V [21]); (4) multiple plaques, moderate to severe carotid stenosis, or plaques located outside the CB; (5) a history of neck irradiation or prior carotid surgery/stenting; and (6) the absence of head computed tomography (CT) or magnetic resonance imaging (MRI) examinations, or other incomplete clinical data.
Patients were classified as symptomatic or asymptomatic based on head CT or MRI findings. The symptomatic group comprised individuals who experienced their first anterior circulation ischemic stroke ipsilateral to the carotid plaque within the previous 30 days [22,23]. The asymptomatic group included patients without a history of stroke or intracranial lesion.
Clinical Variables and US Parameters
The following variables were collected: (1) demographic data, including age, sex, and body mass index (BMI); (2) laboratory values, including total cholesterol (TC), triglycerides, low-density lipoprotein (LDL), high-density lipoprotein, serum creatinine, and uric acid; (3) medical history, including hypertension, diabetes mellitus, and hyperlipidemia; (4) medication history, including statins, antihypertensives, and hypoglycemics; (5) personal history, including smoking and alcohol consumption; (6) B-mode US parameters, including plaque echogenicity, plaque length, and plaque thickness; (7) high-frame rate VFI parameters, namely wall shear stress (WSS), oscillatory shear index (OSI), and turbulence (Tur) index; and (8) plaque elasticity on SWE, specifically the Young modulus (YM).
US Data Acquisition
All standard B-mode US, high-frame rate VFI, and SWE examinations were performed using a Mindray Resona R9 system (Mindray Bio-Medical Electronic Co., Ltd., Shenzhen, China), as previously described [12,24]. A linear-array transducer with a 3-11 MHz frequency range was employed for all scans. Examinations were conducted by radiologists with more than 5 years of carotid US experience; in the event of disagreement, a senior radiologist with over 10 years of experience made the consensus decision.
B-Mode US Analyses
The carotid arteries were examined bilaterally in both longitudinal and transverse planes, and B-mode US parameters—including plaque echogenicity, plaque length, and plaque thickness—were documented [12,17,24]. Color Doppler flow imaging (CDFI) was then performed in the longitudinal plane along the plaque surface, followed by Doppler spectral analysis. Peak systolic velocity, end-diastolic velocity, and resistance index were recorded for further analysis.
High-Frame Rate VFI Protocol
For VFI video recordings, the following settings were used: arrow life cycle of 25 ms; frequency of 5.0 MHz; depth range of 2-4 cm; and arrow density of 10%. The frame rate was set to 498 Hz with a pulse repetition frequency of 4.0 kHz. Each plaque was recorded in the longitudinal plane for 36 seconds. A representative image illustrating blood flow velocity vectors, streamlines, and vortices over one carotid plaque is shown in Fig. 2.
For each plaque, the optimal frame at peak systole was extracted from the VFI video for analysis. Measurement points (blue dots) were placed on the upstream and downstream plaque surfaces to derive WSS and OSI values (Fig. 2). Then, measurement boxes (green 2 mm×2 mm squares) were positioned on those same surfaces to calculate the Tur index in both regions (Fig. 2).
SWE Examination
SWE was performed on the plaque’s longitudinal section as previously described [17]. A sampling box was positioned to encompass the entire plaque, with real-time quality control. Builtin software was used to quantify the YM values, a process that involved manually delineating several regions of interest: the entire plaque margin (delimited by lines) (Fig. 3A, B) and the downstream and upstream plaque areas (denoted by 2 mm-diameter circular region of interests) (Fig. 3C). Each measurement was repeated three times consecutively, and the mean value was recorded. The images were saved for analysis.
Assessment of Inter- and Intra-observer Agreement
Inter-observer agreement for VFI and SWE parameters was evaluated in a blinded fashion by two independent radiologists using images from 30 randomly selected patients. To assess intra-observer agreement, one radiologist re-analyzed the data from 30 randomly selected patients after a 1-month interval, without access to the initial results.
Statistical Analyses
All analyses were performed using SPSS Statistics for Windows, version 26.0 (IBM Corp., Armonk, NY, USA). Categorical variables were presented as frequency (%) and analyzed using the Pearson chi-square test. Normally distributed continuous variables were expressed as mean±standard deviation and compared using one-way analysis of variance, whereas non-normally distributed data were reported as median (interquartile range [IQR]) and evaluated with the non-parametric rank-sum test. Multivariate logistic regression models were constructed to examine associations between VFI parameters, SWE parameters, B-mode US parameters, and the occurrence of ischemic stroke, with odds ratios and 95% confidence intervals calculated. Receiver operating characteristic (ROC) curves were generated to assess each model’s diagnostic performance, and optimal cutoff values were determined using the maximum Youden index (sensitivity+specificity-1). The Spearman rank correlation coefficient was used to explore relationships between VFI parameters and SWE values. Intra- and inter-observer reliability were quantified by the Kendall W coefficient of concordance. A two-sided P-value of less than 0.05 was considered to indicate statistical significance.
Results
Clinical Characteristics
Of the 616 patients initially identified, 494 were excluded for various reasons (Fig. 1), leaving 122 patients for analysis. Among these 122 patients, 82 (67.2%) were male. The median age was 67 years (IQR, 61.75 to 71 years), and the median BMI was 23.81 kg/m2 (IQR, 21.73 to 26.20 kg/m2). Clinically, 75 patients (61.5%) had hypertension, 51 (41.8%) had diabetes mellitus, and 67 (54.9%) had hyperlipidemia. As shown in Table 1, the symptomatic group (n=60) had significantly higher TC and LDL levels than the asymptomatic group (n=62) (P=0.011 and P=0.001, respectively), while other clinical characteristics did not differ significantly between groups (all P>0.05).
Comparisons of US-Based Parameters between Symptomatic and Asymptomatic Groups
As shown in Table 2, plaque length and thickness were greater in the symptomatic group (P<0.05), whereas plaque echogenicity and all CDFI parameters did not differ significantly between groups (P>0.05).
Regarding VFI-derived biomechanical forces at the plaque periphery (Table 2), the symptomatic group exhibited significantly higher maximum and mean WSS values on both upstream and downstream plaque surfaces (P<0.05). The Tur index for the downstream surface was also significantly elevated in the symptomatic group compared to the asymptomatic group (P<0.05). However, OSI and the Tur index on the upstream surface, as well as downstream OSI, did not differ significantly between groups (all P>0.05). Notably, mean YM for the entire plaque and for its downstream region were significantly lower in the symptomatic group (P=0.015 and P=0.035, respectively). In contrast, mean upstream YM did not differ significantly between the two groups (P>0.05) (Table 2).
Comparisons of VFI and SWE Parameters within Symptomatic and Asymptomatic Groups
Table 3 presents a within-group comparison of VFI parameters between the plaque’s upstream and downstream surfaces in both symptomatic and asymptomatic groups. In both groups, upstream WSS values were significantly greater than downstream values (P<0.001), whereas upstream OSI and Tur index values were significantly lower than their downstream counterparts (P<0.001).
SWE parameters between the plaque’s upstream and downstream areas were also compared within both asymptomatic and symptomatic groups (Table 3). In both groups, the mean YM in the downstream area was significantly lower than in the upstream area (P<0.001).
Correlations between VFI and SWE Parameters in Symptomatic and Asymptomatic Groups
As shown in Supplementary Tables 1 and 2, no significant correlations were observed between VFI parameters on the plaque’s upstream surface and SWE values in the upstream area, nor between VFI parameters on the downstream surface and downstream SWE values, in either the symptomatic or asymptomatic group (all correlation coefficients r ranged from -0.3 to 0.3; all P>0.05).
Associations of US-Based Parameters in Mild Stenosis with Ischemic Stroke
Multivariable logistic regression models were constructed to explore predictors of ischemic stroke (Table 4). In model 1, mean WSS and Tur index on the downstream plaque surface, along with maximum WSS on the upstream surface, were significantly associated with stroke occurrence. After adjustment for the SWE-derived mean YM and B-mode US parameters (plaque length and thickness), the downstream Tur index (cutoff, 1.4%; area under the curve [AUC], 0.607; sensitivity, 0.774; specificity, 0.450) and the whole-plaque mean YM (cutoff, 16.52 kPa; AUC, 0.627; sensitivity, 0.839; specificity, 0.400) remained significantly correlated with ischemic stroke in patients with mild stenosis (models 2 and 3) (Table 4).
Regarding predictive performance Supplementary Table 3 shows that model 3 achieved the highest area under the ROC curve (AUC, 0.765), with a sensitivity of 0.871 and a specificity of 0.533. This model outperformed the downstream Tur index alone (AUC, 0.607), the plaque mean YM alone (AUC, 0.627), model 1 (AUC, 0.737), and model 2 (AUC, 0.752) (Supplementary Table 3), indicating that the combined use of high-frame rate VFI and SWE examinations improves stroke risk prediction.
Consistency Analyses of High-Frame Rate VFI and SWE Examinations
Assessment of intra- and inter-observer agreement for these parameters demonstrated exceptionally high concordance (Supplementary Table 4). For intra-observer analysis, the Kendall W ranged from 0.826 to 0.953 (all P<0.05) for VFI parameters and from 0.917 to 0.969 (all P<0.05) for SWE measurements. Inter-observer Kendall W coefficients ranged from 0.786 to 0.917 (all P<0.05) for VFI and from 0.906 to 0.973 (all P<0.05) for SWE.
Discussion
This study emphasizes that peri-plaque biomechanical conditions and carotid plaque stiffness are correlated with ipsilateral ischemic stroke in patients with mild stenosis, a finding that may inform the development of diagnostic algorithms for assessing plaque vulnerability and stratifying stroke risk. The primary findings show that higher Tur index values on the downstream plaque surface and lower mean YM values within the plaque are significantly associated with ischemic stroke in this population. Furthermore, the combined application of high-frame rate VFI and SWE examinations yielded significantly better predictive performance for stroke risk than either modality alone.
This study’s entry criteria required either brain ischemic lesions ipsilateral to the carotid artery or the absence of such lesions, confirmed by MRI/CT, for the inclusion of symptomatic and asymptomatic patients with mild stenosis. This approach enabled meaningful analyses while minimizing bias in comparisons between the two groups. By restricting enrollment to patients with a single plaque at the CB and excluding other embolic sources, the results provided a more precise understanding of the relationships between US parameters of mildly stenotic carotid plaques and ipsilateral ischemic stroke, extending the authors’ previous research [12,24].
In this study, the symptomatic group exhibited higher serum TC and LDL levels, along with significantly greater plaque length and thickness, suggesting more pronounced lipid metabolism disturbances and inflammatory activity in their carotid arteries—findings consistent with prior studies [25-27]. However, no significant differences were observed between groups in hypertension or diabetes prevalence, medication use, or smoking and alcohol history, with fairly widespread use of antidiabetics, statins, and antihypertensives in both groups. While hypertension is a well-established risk factor for ischemic stroke in patients with moderate to severe stenosis [28], the present study focused exclusively on patients with mild stenosis. Additionally, plaque echogenicity on B-mode US did not differ significantly between groups, which may be attributable to the exclusion of severely calcified plaques in this analysis.
Atherosclerotic carotid plaques predominantly develop in the CB region, exhibiting complex flow patterns [29,30]. High-frame rate VFI enables real-time visualization of unstable flow and yields robust biomechanical parameters [31]. In this study, VFI was used to quantify WSS, OSI, and Tur index on both upstream and downstream plaque surfaces at the CB in symptomatic and asymptomatic patients. WSS represents the shear force exerted by blood flow on the plaque surface, whereas OSI quantifies the temporal oscillation in fluid WSS during the cardiac cycle. Symptomatic patients exhibited significantly higher WSS on both surfaces compared to asymptomatic individuals, suggesting that increased WSS may be associated with carotid plaque rupture even in mild stenosis; this finding was consistent with previous research, including the authors’ work [12,24,32,33]. However, OSI did not differ significantly between groups for either surface, possibly because mild stenosis does not generate the severe recirculation vortices seen in advanced disease [32]. The Tur index quantifies flow complexity [34,35], and a significantly higher downstream Tur index was observed in symptomatic patients. This finding indicated more intense turbulence around carotid plaques and thus greater plaque instability, as previously reported [34,35].
Additionally, within both symptomatic and asymptomatic groups, WSS values on the downstream plaque surface were lower than those upstream, while OSI and Tur index values were higher downstream compared to upstream. This pattern likely reflects the downstream surface’s exposure to multiple recirculation vortices and a more oscillatory hemodynamic environment, driven by high blood flow velocity in that region [32].
YM, a plaque elasticity value that reflects plaque stiffness, is determined by plaque composition and is correlated with vulnerability [16,36]. In the present study, symptomatic patients exhibited lower YM values for both the entire plaque and its downstream region, suggesting that reduced elasticity in these areas may serve as a key imaging biomarker in mild carotid stenosis, in line with prior findings [37]. However, YM was not significantly correlated with VFI-derived parameters (WSS, OSI, and Tur index) in the symptomatic or asymptomatic group. The elastic response of plaques to hemodynamic forces at the CB is multifactorial [38]; radial and circumferential strains result from axial and lateral displacements of the plaque during vascular pulsation [39]. Accordingly, two-dimensional VFI measurements on longitudinal sections may overlook key spatial deformation information, and three-dimensional elastography could provide more comprehensive indicators.
The multivariable logistic regression models demonstrated that the downstream Tur index and the plaque’s mean YM were independently correlated with ischemic stroke in patients with mild stenosis. The present study is the first to integrate high-frame rate VFI and SWE examinations for the US-based prediction of carotid-related ischemic stroke. Models combining biomechanical parameters with plaque elasticity outperformed those based on a single modality in predictive accuracy.
As atherosclerosis is a systemic disease, combining high-frame rate VFI with SWE for the visualization of carotid plaques may yield novel insights into plaque instability throughout the arterial system. Furthermore, this study demonstrates that both high-frame rate VFI and SWE are reproducible, reliable imaging techniques, highlighting the value of US-derived biomarkers in ischemic stroke risk assessment. In contrast, Doppler-based parameters did not differ significantly between groups, consistent with previous findings that dramatic changes in surface flow velocity occur only in severe stenosis [40]. Because data on mild carotid stenosis are scarce, the present research series adds important evidence for identifying patients with mild stenosis at high risk of stroke—particularly through multimodal US approaches. Importantly, these evaluations are clinically feasible, as all of these non-invasive imaging modalities (B-mode US, high-frame rate VFI, and SWE) can be performed on a single US system.
This study had certain limitations. First, it had a retrospective case-control design and a relatively low sample size. Future multicenter prospective studies with larger cohorts are needed to confirm these results and evaluate their clinical utility. Second, high-frame rate VFI protocols for carotid assessments are not yet standardized; consensus guidelines on acquisition parameters and analysis methods should be established to reduce operator dependence. Third, the utility of invasive US biomarkers for stroke risk prediction in patients with moderate to severe carotid stenosis remains to be explored. Finally, the study excluded plaques with severe calcification to optimize VFI accuracy; investigating these rigid, hemodynamically complex lesions will require the development of novel imaging techniques.
These findings confirm that a higher Tur index on the downstream plaque surface and reduced plaque elasticity are associated with ipsilateral ischemic stroke in patients with mild stenosis. Combined high-frame rate VFI and SWE examinations can enhance stroke risk prediction and hold promise for improving risk stratification and patient management.
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