Is B-mode ultrasound alone a sufficient screening tool for carotid stenosis? A pilot study

In 2000, cerebrovascular accidents (CVAs) accounted for 7% of all deaths in Canada [1]. The incidence of CVAs has increased more than 100% in low to middle income countries in the last four decades, reaching an epidemic level [2].

The main surgical intervention to prevent CVAs is carotid endarterectomy (CEA), which treats stenosis at the level of the carotid bifurcation. CEA decreases the risk of future CVA in both symptomatic and asymptomatic patients [3]-[5].

Carotid stenosis is usually diagnosed with ultrasound. Elective carotid ultrasound is performed solely by ultrasound technologists and imaging specialists (e.g. radiologists and vascular surgeons). Carotid ultrasound is generally performed on symptomatic patients who present with a possible CVA or a transient ischemic attack. Carotid ultrasound can also be used to screen asymptomatic patients [6].

The yield of screening asymptomatic patients depends on the patient group that is screened. In the general population over 65 years of age, the prevalence of clinically important carotid stenosis is 1% [7]. In a study of patients who were referred to a vascular surgeon for non-carotid disease, carotid ultrasound found significant stenoses in 21% of patients [8]. If screening was restricted to patients with a carotid bruit, at least half of significant carotid stenoses would be missed [9]. Risk factors (e.g. hypertension, smoking, cardiac disease, dyslipidemia, or a family history of stroke) place patients at an increased risk of cerebrovascular disease. However, significant stenosis may be found in as little as 1% of such populations [7]. Screening of patients at risk with elective ultrasound may therefore not be cost-effective.

In many ultrasound labs, elective carotid ultrasound involves the evaluation of the common carotid artery (CCA), the CCA bifurcation, the internal carotid artery (ICA), the external carotid artery (ECA), the vertebral artery, and the subclavian artery. It is performed using both B-mode and Doppler ultrasound, a combination termed ‘duplex’ ultrasound. This extensive evaluation is time-consuming, requiring 30 min or more to complete. Most carotid duplex exams are normal or show only mild disease.

When screening for carotid stenosis, only those areas treated by CEA may need to be evaluated. When B-mode shows no evidence of stenosis, evaluation by Doppler may be unnecessary because velocity measurements are normal when the stenosis is less than 50%. Stenoses less than 50% are not clinically significant and no further investigation would be needed. The use of only B-mode at the level of the carotid bifurcation would make the exam much less time-consuming and possibly tip the cost-benefit ratio in favor of more screening. It also opens up the possibility that clinicians who perform point-of-care ultrasound (POCUS) could add carotid screening to their practice. Only those patients with a screening B-mode ultrasound suggesting a stenosis over 50% would need to undergo an elective duplex scan for the purposes of measuring velocities.

The primary objective of this study was to determine the frequency with which a carotid stenosis of greater than 50% is found using spectral Doppler when no such stenosis was visible using B-mode.

A total of 200 patients were sampled. There were 100 patients from group a and 100 patients from group b. This resulted in 400 carotid ultrasounds: 200 on the left and 200 on the right. The patient's baseline characteristics can be found in Table 1.

The results from the CEA group (group a) are listed in Table 2. The interpretations by our study investigator as compared with the elective ultrasound results yielded a sensitivity of 99.3%, a specificity of 40%, a negative predictive value of 94.7%, and a positive predictive value of 85%.

The results from the stroke clinic group (group b) are listed in Table 3. The interpretations by our study investigator as compared with the elective ultrasound results yielded a sensitivity of 100%, a specificity of 65%, a negative predictive value of 100%, and a positive predictive value of 36.3%. Inter-rater reliability was 65% for both groups combined. The kappa statistic was 0.61 (95% CI: 0.38, 0.83) for the left carotid and 0.33 (95% CI: 0.13, 0.53) for the right carotid artery.

When the data for the CEA group (group a) were unblinded and compared to the sonographer's report, it was noted that there were two reports which indicated a greater than 50% stenosis based on velocity criteria, but no drawing was made on the diagram. In both cases, the radiologist report indicated a stenosis of less than 50%. These two cases were removed from the study. One ultrasound report documented a greater than 50% stenosis but only minimal stenosis was drawn by the technologist. The radiologist report also concluded that the stenosis was greater than 50%. This ultrasound scan was kept in the study and accounts for the one false negative found in Table 2.

When the data for the stroke clinic group (group b) were unblinded and compared to the sonographer's report, it was noted that there were seven ultrasound reports in the stroke clinic group which indicated greater than 50% stenosis in at least one vessel based on velocity criteria, yet no drawing was made on the diagram. These results were compared with the radiologist's interpretation. In six of these cases, the radiologist interpretation concluded that there was less than 50% stenosis. The radiologist interpreted the remaining case as showing greater than 50% stenosis. Given the absence of a drawing by the technologist, these seven reports were removed from the study. An eighth report was also removed because both the technologist and the radiologist commented that the study was of poor quality and the results were inconclusive.

Of the 200 patients in the study, two patients were found in both groups. These patients were initially seen in the stroke clinic and referred for a CEA based on their carotid ultrasound results. These scans were unknowingly reviewed twice and thus were left in the study in both groups.

The objective of this study was to determine the frequency with which carotid stenoses of greater than 50% are found based on velocity criteria when the B-mode ultrasound suggested that the scan was normal or indicative of a stenosis of less than 50%.

In the CEA group, there were 153 ultrasounds which showed a stenosis of greater than 50% based on velocity criteria. In only one of these cases did the technologist's drawing appear to show a stenosis of less than 50%. The finding of a less than 50% stenosis on B-mode when the measured velocities indicate a greater than 50% stenosis is therefore a rare event. This suggests that the use of B-mode alone may detect the vast majority of clinically important carotid stenoses.

In the stroke clinic group, the reinterpretation of the B-mode portion of the scan yielded a sensitivity and negative predictive value of 100% when compared to velocity criteria for stenoses above or below 50%. The specificity was 65%. The high sensitivity and negative predictive value supports the theory that a B-mode-only scan could be used to screen for significant carotid stenosis. The modest specificity would be acceptable for a screening test.

This pilot study is the first known study of its kind. The results support the theory that it may be possible to use carotid B-mode ultrasound without Doppler to reliably determine if there is stenosis above or below 50%. If further research corroborates and further develops this theory, more ubiquitous screening may be possible. An ultrasound performed with B-mode alone would be simpler and faster to perform. In the authors' experience, a B-mode scan requires less than 5 min to complete when scanning the left and right carotid systems. This would open up the possibility that clinicians who perform POCUS could add carotid stenosis screening to their practice. In fact, the ability of internists trained to perform carotid ultrasound has been studied and yielded positive results. Ray et al. have shown that internists can achieve acceptable sensitivity and high specificity for carotid plaque detection, resulting in high positive and negative predictive values [10].

This pilot study supports the theory that it may be possible to use carotid B-mode ultrasound without Doppler to reliably determine if there is stenosis above or below 50%. Further research is required before carotid ultrasound using B-mode alone can be recommended.

SS and TB made substantial contributions to the design, analysis and interpretation of data, the drafting and revision of the manuscript. SS conceived of the study. TB acquired the data. Both SS and TB gave their final approval and agreed to be accountable for all aspects of the work.