Instruments
- Transducer frequency from 7.5 – 12 MHz. The higher frequencies give you better images but lower frequencies may need to be used to look at proximal common carotid, distal internal carotid, and vertebral arteries.
- The 5 MHz frequency should be used whenever possible when generating spectral waveforms through pulsed Doppler exam on ICA, as most published studies use this frequency.
Positioning and Exam Techniques
- Patient’s neck and head is slightly extended and turned to the contralateral side about 45 degrees off midline.
- Transverse (cross-sectional) images are obtained first to characterize the plaques, if any. Longitudinal is sub-optimal for visualizing eccentric nature of the plaque. However, transverse view is never used to obtain flow information. Measuring lumen diameter and degree of stenosis through transverse view is notoriously inaccurate and should not be used.
- Longitudinal views are used for most of the exam and to obtain color Doppler and spectral waveform analyses. Doppler angle of 60 degrees should be used to obtain flow velocity.
Plaque Characteristics and Morphology
Plaque morphology comes in various descriptions, but does not affect the velocity criteria. Examples of plaque description includes homogenous, heterogenous, calcified, and various echogenicities (hyperechoic, hypoechoic, anechoic). If the presence of plaque is noted on B mode but velocity is not increased, then it is classified as <50% stenosis, rather than just normal carotid without disease.
Waveform Analysis
The common carotid artery (CCA) normally has a sharp systolic rise, clear window, and relatively high diastolic velocity. The CCA waveforms resemble combination of ICA and CCA, with an end systolic “dip” that does become negative.
Velocity Criteria
When taking ICA to CCA ratio, CCA velocity should be taken from 2 cm proximal to the bifurcation. The peak systolic velocity (PSV) is usually elevated first. In severe stenosis, the end diastolic velocity (EDV) becomes elevated, thus it is a more sensitive marker of stenosis. The PSV ratio between ICA and CCA (ICA/CCA) is also an important diagnostic criteria as it normalizes the entire carotid system, rather than relying on ICA velocity alone.
It assumes:
- Doppler angle of 60 degrees
- CCA velocity taken from mid CCA, or 2 cm proximal to the bulb (the most straight part of CCA)
- ICA velocity taken from first 3 cm of ICA
| Stenosis | PSV | EDV | ICA/CCA Ratio | Plaque |
| Normal | <125 cm/s | <40 cm/s | <2.0 | No plaque visible |
| <50% stenosis | <125 cm/s | <40 cm/s | <2.0 | Some plaque visible |
| 50-69% stenosis | 125-230 cm/s | 40-100 cm/s | 2.0-4.0 | Visible plaque |
| >70% stenosis | >230 cm/s | >100 | >4.0 | Visible plaque |
| Near occlusion | Variable | Variable | Variable | Visible plaque |
| 100% | No flow | No flow | - | Visible plaque |
Post Carotid Stent Criteria
There is no exact consensus. However, several published data generally suggests the criteria for native disease above overestimates incidence of stent re-stenosis. The following criteria can be roughly applied to predict >70% stenosis:
- In-stent PVS > 300 cm/s
- In-stent EDV > 90 cm/s
- and in-stent PSV to CCA PVS ratio > 4
The University of Maryland criteria based on their experience is as follows:
| Stenosis (%) | Peak Systolic Velocity (cm/s) |
| <20 | <150 |
| 20-49 | 150-219 |
| 50-70 | 220-239 |
| >70 | >300 |
| >80 | >340 |
Intraoperative Duplex
- For carotid endarterectomy, ICA PSV > 150 cm/s is abnormal. Residual plaque shelf in CCA should be less than 2 mm. A velocity of >150 and ratio of 2-2.5 will require careful inspection. Velocity of >300 cm/s and ratio > 3.5 indicate severe stenosis and immediate repair should be undertaken.
- For carotid stenting, PSV < 150 cm/s is also used. IVUS can also be used to make sure residual stenosis is < 20%.