Fig. 17.1
Variability of measurements of PSV by three ICAVL-accredited technologists from an in vitro flow model
In a clinical study, the interobserver variability of measurements of PSV from the internal carotid artery (ICA) by three experienced ICAVL-accredited vascular laboratory technologists was quite large [2]. For individual patients, variability of PSV measurements ranged from −25 to +35% of the average value. When we compared the three technologists, no systematic overestimation or underestimation of the ICA PSV was found. Although individual PSV measurements varied significantly, nonetheless we noted that the severity of carotid stenosis can be reproducibly categorized into ranges (i.e., <50%, 50–70%, >70%).
Variability of Diagnostic Velocity Criteria
Numerous studies have evaluated the ultrasound criteria for diagnosing the severity of carotid stenosis. There is variability between laboratories related to the method for defining the percentage stenosis, different machines, and differences in technique. The following is not meant to be a comprehensive review but rather a summary of a few key articles that address the variability in diagnostic criteria. They all defined percentage angiographic diameter stenosis using the North American Symptomatic Carotid Endarterectomy Trial (NASCET) criteria.
A consensus group organized by the Society of Radiologists in Ultrasound made the following recommendations1 for the ultrasound diagnosis of carotid artery disease [3]. They determined that a PSV of ≥125–230 cm/s was consistent with ≥50–69% stenosis and that a PSV >230 cm/s was consistent with the diagnosis of ≥70–99% stenosis. Other parameters were considered to be helpful in certain situations where PSV may be inaccurate (e.g., the presence of contralateral high-grade stenosis or occlusion, discrepancy between visual assessment of the carotid plaque and the ICA PSV, low cardiac output, or hyperdynamic cardiac state). An end-diastolic velocity (EDV) of ≥40 cm/s and an ICA/CCA PSV ratio of ≥2 were consistent with 50–69% stenosis, and an EDV of ≥100 cm/s and a ratio of ≥4 were consistent with 70–99% stenosis.
AbuRahma et al. [4] evaluated the accuracy of the consensus criteria and noted that an angiographic stenosis of 50–69% was detected with a sensitivity of 93%, specificity of 68%, and overall accuracy of 85% for a PSV of 125–230 cm/s. For detecting a ≥70% stenosis, a PSV of >230 cm/s had a sensitivity of 99%, specificity of 86%, and overall accuracy of 95%. For detecting any grade of stenosis, ICA PSV was significantly better than end-diastolic velocity (EDV) or ICA/CCA ratio. Adding the EDV values or the ratios to the PSV values did not improve accuracy. The accuracy of detecting a 50–69% stenosis was improved by using an ICA PSV of 140–230 cm/s rather than 125–230 cm/s.
Braun et al. also noted that when the consensus criteria were used in their patient population, the accuracy was similar to the consensus panel report. A cutoff of 230 cm/s was very accurate; however, a PSV ≥240 cm/s was slightly more accurate in their laboratory [5]. They agreed that other parameters (EDV, ICA/CCA PSV ratio, and ICA/CCA EDV ratio) only needed to be used in borderline situations.
Shaalan et al. concluded that a >50% stenosis was optimally detected with s PSV ≥155 cm/s, and an ICA/CCA ratio of ≥2.0. For ≥80% stenosis, a PSV of ≥370 cm/s, an EDV of ≥140 cm/s, and an ICA/CCA ratio of ≥6 were equally reliable in the diagnosis [5, 6].
In a meta-analysis [7], an angiographic stenosis of ≥50% was identified with a peak systolic velocity ≥130 cm/s [sensitivity of 98% (95% confidence intervals [CI] of 97–100%) and specificity of 88% (95% CI, 76–100%)]. For the diagnosis of angiographic stenosis of ≥70%, a peak systolic velocity ≥200 cm/s has a sensitivity of 90% (95% CI, 84–94%) and a specificity of 94% (95% CI, 88–97%).
These diagnostic criteria provide an excellent guide to individual laboratories; however, in general it is best practice to have an ongoing quality assurance program to verify the accuracy of the velocity criteria in your patient population and with your machines and ultrasound techniques.
Clinical Situations Affecting PSV Measurements
Contralateral Stenosis
Flow velocity can be increased because of a contralateral carotid stenosis ; hence, velocity criteria may overestimate the severity of stenosis. This is particularly problematic when the contralateral artery is severely stenosed or occluded. Heijenbrok-Kal et al. normally use a PSV threshold of 230 cm/s for the diagnosis of 70–99% [8]. However, they found that accuracy was improved by using separate PSV velocities for ipsilateral and contralateral carotid arteries. Specifically, for diagnosing a 70–99% stenosis, the optimal PSV threshold for the ipsilateral artery was 280 cm/s and for the contralateral artery, 370 cm/s. Severe contralateral carotid stenosis is a common cause of a false-positive test, i.e., reporting an ipsilateral carotid stenosis as severe when it is in fact less severe.
Error After Previous Endarterectomy, Patch, or Stent
Patients who have previously had a carotid endarterectomy with or without patch angioplasty or a carotid stent may develop a recurrent stenosis. Current velocity criteria overestimate the severity of these stenoses after carotid endarterectomy with patch closure [9] or after carotid stenting [10, 11]. Higher velocity criteria need to be adopted.
Very Severe Carotid Stenosis Versus Complete Occlusion
Peak systolic velocity increases as the severity of the stenosis increases. However, when the stenosis severity is very severe (95–99%), the velocity begins to fall. Preocclusive lesions are associated with only “trickle flow” and the velocity may be <40 cm/s [12].
A total occlusion is diagnosed when there is no detectable patent lumen on grayscale US and no flow on spectral, power, and color Doppler US [3]. Note that it can be very difficult to be certain if an artery is occluded or very severely stenosed with trickle flow even if all ultrasound modalities are used. Since the distinction can have major therapeutic implications, if the results of the ultrasound are uncertain, it is best to report that the artery is either very severely stenosed (trickle flow) or completely occluded.
Gender
PSV velocity in women averages 10% higher than in men [13]. They showed that the optimal PSV for ≥60% stenosis was 160 cm/s for men and 180 cm/s for women and for ≥70% stenosis was 185 cm/s for men and 202 cm/s for women. Although gender is not often considered in interpreting a carotid Doppler examination, this study indicates that gender potentially has important implications for patient care.
Cardiac Output
PSV may be lower if the cardiac output is low or higher in hyperdynamic situations.
Failure to Define Percentage Severity of Stenosis
There is disagreement on the definition of a percentage stenosis. Grading of the severity of a carotid stenosis is complex for several reasons. Although most express the severity of a stenosis as a percentage diameter reduction, some still consider the percentage area stenosis as most appropriate because the change in cross-sectional area in fact determines the increase in velocity.
In defining a stenosis based on diameter reduction, the numerator is the diameter at the site of the maximum carotid stenosis. The denominator has been defined differently in the two major randomized carotid surgery studies. The North American Symptomatic Carotid Endarterectomy Trial (NASCET) definition uses the diameter of the normal distal internal carotid artery as the denominator, whereas the European Carotid Surgery Trial (ECST) uses the estimated diameter of the bulb. Table 17.1 shows a comparison of percentage diameter stenosis calculated by the NASCET and ECST definitions.
Table 17.1
Comparison of NASCET and ECST measurements of percentage diameter stenosis
NASCET % | ECST % |
|---|---|
30 | 65 |
40 | 70 |
50 | 75 |
60 | 80 |
70 | 85 |
80 | 91 |
90 | 97 |
In an audit of vascular laboratories in the United Kingdom, Walker and Naylor noted that 26% used the NASCET measurement method, 31% used the ECST method, and 43% did not know [14]. To overcome confusion caused by two grading systems, the NASCET definition is suggested.
Angle of Insonation Incorrect
One drawback of Doppler-based blood velocity measurements is that the operator must manually specify the angle between the Doppler ultrasound beam and the artery. Using Doppler ultrasound, velocity is calculated from the following: (1) the frequency shift (i.e., the difference between the frequency transmitted by the ultrasound machine and the returned frequency which may be higher or lower if it is reflected by moving red blood cells), (2) the speed of the ultrasound in tissue, and (3) the angle between the ultrasound beam and the path of the red blood cells.
The technologist subjectively aligns the cursor of the sample volume in the center of the artery and parallel to the walls of the artery and at an angle of 60°. Although angles between 45° and 60° are considered satisfactory, the most consistent results are obtained with an angle of 60°—see below. In measuring the velocity, the largest source of error results from errors in accurately aligning the cursor of the sample volume.
Technologist Incorrectly Aligns Cursor
It is quite easy to make a small error in aligning the cursor to be parallel to the artery walls and hence make an error in the measurement of the Doppler angle. Figure 17.2 shows the errors in the measurement of velocity based on errors in the beam/artery angle. Clearly, even quite small errors in angle measurement can result in significant errors in velocity measurement and consequently on evaluation of the severity of a stenosis. Figure 17.3 shows the PSV measurements that would be recorded if the probe/vessel angle is in error. Note that relatively small errors in the angle result in significant PSV measurements and errors in the interpretation of the severity of stenosis.
Fig. 17.2
If the probe/vessel angle is assumed to be correctly positioned at 60°, this figure shows the percentage error in the measurement of the PSV if there is actually an error in setting the probe/vessel angle. If the angle is in fact 29°, the velocity will be 29% less than expected
Fig. 17.3
PSV measurements may be in error if the probe/vessel angle is incorrectly evaluated. This figure shows the magnitude of the error for errors from +15° to −20° and for different velocity measurements. These errors may result in different interpretations of the severity of a stenosis
Error from Different Angles 45–60
Beach et al. noted that angle-corrected Doppler velocity measurements were higher when higher Doppler examination angles were used [15]. Tola and Yurdakul compared the velocity measurement obtained at 60° and 45° insonation angles [16]. Even though the measurements were corrected for the probe/vessel angle, measurements at 45° were about 24% lower than at 60°.
Transducers use multiple elements to form the beam and consequently there are multiple angles to the flow axis. The angle of the middle part of the beam is used to calculate velocity. However, because of the multiple angles, there are a range of velocities (some lower and some higher than the calculated velocity). This geometric spectral broadening may result in an overestimation of the velocity.
To minimize this error, most recommend using a consistent angle of insonation, usually 60°. Although a smaller angle, for example, 45°, will result in smaller errors in velocity estimation, most use 60°. Most critically, it’s important to use a consistent angle and validate the results in each vascular laboratory.
Actual Flow Vector Is Not Parallel to Walls
In calculating velocity, the angle is that between the ultrasound beam and the axis of flow. However, this cannot be determined in a clinical situation, and consequently the sample volume is positioned so that it is parallel to the side walls of the artery on the assumption that the flow direction is parallel to the walls. However, it is important to note that the flow may not be parallel to the walls in certain circumstances, e.g., in a tortuous, kinked, or coiled artery or beyond an asymmetrical stenosis. In these cases, it may be impossible to measure an accurate PSV.
Beam and Sample Volume Positioned Close to the Ends of the Transducer
The most consistent measurements of PSV will be obtained with the beam and sample volume position ed in the center of the transducer rather than at the ends of the transducer. Multiple elements from the transducer are used to produce the beam, and the angle of insonation is calculated from the center of the beam. When the beam is positioned near the end of the transducer, fewer elements may be used to produce the beam than when the beam is positioned in the center of the image. With more active elements, the angle of some of the elements will be significantly greater than the mean angle used to calculate the PSV, and the measured velocity will be higher than obtained when fewer elements are used near the edge of the transducer. Using a consistent beam positioned along the transducer minimizes these errors.
Sample Volume Is Positioned Incorrectly
Peak Velocity Not at Stenosis
Positioning the Doppler sample volume in the throat of a stenosis is important in order to obtain accurate and reproducible measurements of PSV that can quantify the severity of the stenosis. We showed that the flow field is complex at and beyond a stenosis [17]. The peak frequency is maximum within the throat of the stenosis and returns to the pre-stenotic value within five tube diameters distal to the stenosis. Hence, reproducible peak velocity measurements are obtained only if the Doppler sample volume is positioned at or very near the throat of the stenosis. Figure 17.4 shows that the PSV is maximum in the throat of the stenosis and quickly falls distal to the stenosis. Figure 17.5 illustrates these observations in a clinical example.
