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PERIPHERAL VASCULAR ULTRASOUND
narrowing, but there are potential pitfalls. Spurious flow voids can be created due to a poor angle of insonation or inappropriate PRF or filter settings, which may lead to an overestimate of the degree of narrowing. If the color gain is set too high, it is possible for the color to appear to ‘bleed’ out of the vessel lumen, and this can lead to an underes- timate of the degree of narrowing (Fig. 7.6).
Spectral Doppler
As the quantity of atheroma in the vessel increases, it becomes more difficult to estimate the degree of narrowing from the image, especially in the pres- ence of calcified or anechoic atheroma. However, velocity criteria are used to grade the degree of stenosis once the vessel becomes narrowed by a 50% reduction in diameter. Over the years, several criteria have been produced for grading carotid artery disease, many of which have been published, and this has revealed many discrepancies.
The various criteria have been produced by comparing Doppler measurements with those of angiography, which has its own limitations, as the gold standard.
Most criteria for grading carotid stenoses are based on the peak systolic velocity (PSV) and end diastolic velocity (EDV) in the ICA, and the ratio of the PSV in the ICA to that in the CCA. Unlike the grading of stenosis in other parts of the arterial system, where there is often a proximal segment of normal vessel which can be used to calculate a velocity ratio, the geometry of the carotid bulb makes the situation less straightforward. The ratio of the PSV in the ICA to that in the CCA will partly depend on the relative dimensions of the CCA and ICA, and this is further complicated by the variable geometry of the carotid bulb. Many crite- ria use absolute velocities in grading the narrowing. Using a combination of absolute velocity measure- ments and velocity ratios potentially reduces the pitfalls of using velocity criteria alone. For example, an increase in PSV can arise due to hypertension, age-related changes in vessel wall compliance or increased flow to supply a collateral pathway. However, an increase would be seen in both the CCA and ICA, and the absence of a significant velocity increase would reassure the sonographer
that there was no significant evidence of a stenosis. Conversely, an abnormal velocity ratio in the pres- ence of low velocities, possibly due to low cardiac output, may help to identify a stenosis.
Table 8.2 gives some examples of criteria for grading carotid stenosis that have been developed by investigators at different centers over the years. These centers have divided the degree of narrow- ing into different bands. Following the ECST and NASCET trials, it is especially useful to distinguish between stenoses of 70% and 70% in order to select the group of patients who would benefit from surgery, given the appropriate symptoms. Bluth et al (1988) produced their criteria using an early duplex scanner with a stand-off Doppler ele- ment, as opposed to a linear array transducer. These older systems were less prone to intrinsic spectral broadening and therefore produced differ- ent results from many modern linear array systems, which tend to overestimate peak velocities. Intrinsic spectral broadening (see Ch. 6) may lead to errors in velocity measurements that are depend- ent on the angle of insonation. Some centers choose to overcome angle-dependent variations in velocity recordings by using a fixed angle of 60°.
Most of the sets of criteria listed in Table 8.2 have been correlated against angiography using the NASCET method of reporting angiographic find- ings. Therefore a 70% stenosis as defined by these criteria would relate to a 80% diameter reduction as measured by the ECST method. Staikov et al (2002) compared ultrasound criteria for detecting stenosis using both the NASCET and ECST methods of measuring angiograms and showed that the velocity criteria produced for a 70% stenosis by the NASCET method are sim- ilar to the criteria produced for a 80% stenosis using the ECST method. The criteria suggested by Sidhu & Allan (1997) are based on results reported by Moneta et al (1993, 1995). Table 8.2 includes criteria published following the Carotid Artery Stenosis Consensus conference (Grant et al 2003). The consensus makes many recommendations, including the use of plaque estimate (% diameter reduction) with B-mode and color Doppler ultra- sound, along with ICA PSV criteria as primary parameters. ICA/CCA PSV ratio and ICA EDV are recommended as additional parameters. Nicolaides et al (1996) have described another velocity ratio,