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].
Limitations
There are several limitations to this study. The drawing of the sonographer was reinterpreted by one of the study investigators to determine the degree of carotid stenosis using B-mode alone. Due to the subjective nature of the drawing, what the sonographer interpreted as <50% stenosis on the diagram may not have been interpreted as such by our investigators. Conversely, what the sonographer interpreted as ≥50% may not have been interpreted as such by our investigators, as was the case for the one false negative in Table 3. Further research should determine the optimal residual diameter or area of residual lumen as seen on B-mode which would allow one to objectively rule out clinically important stenosis.
When developing the study hypothesis, the authors considered using the B-mode images to interpret the degree of stenosis. Unfortunately, the degree of stenosis is determined with elective ultrasound using velocity criteria alone, as measured by the technologist. The B-mode images are not generally used for determining the degree of stenosis. The result is that the images are of insufficient quality to allow for reinterpretation of stenosis. The ideal methodology would have been to prospectively perform the ultrasound with a focus on determining the degree of stenosis using only B-mode, without relying on the velocity criteria. The retrospective nature of this study precluded doing so. Future research should include a prospective study comparing an assessment of stenosis using B-mode only to elective ultrasound using velocity criteria.
Our interpretations were dependent on the sonographer's drawing of stenosis on the carotid diagram. In some cases, the drawings may not have reflected the image seen on the screen. There were two cases in group a and seven cases in group b where the technologist failed to draw the stenosis. These cases were withdrawn from the study. This may have affected test characteristics.
Based on the kappa statistic, agreement was substantial for the left carotid artery but only fair on the right. This may be related to the limited experience in the technique of the author performing all of the measurements as well as the subjectiveness of reinterpreting the technologists' drawings.
Although stroke clinic patients were included in the study (group b) to add cases of mild or no stenosis, the rate of significant stenosis was higher than it would have been in an unselected population. This may have resulted in work-up bias. Future research should be performed in populations with a lower rate of stenoses over 50%.
Carotid ultrasound represents only one aspect of the management of carotid stenosis. The results of this study and the theoretical benefit of a B-mode-only carotid ultrasound should be considered in the larger context of the target population most likely to benefit from early detection and evolving research in the medical and surgical management of carotid stenosis.
