6. Spectral Doppler

6.1 When should spectral Doppler be applied in carotid ultrasound?

Spectral Doppler is part of every standard carotid scan. In carotid ultrasound we use pulsed wave Doppler. In contrast to continuous Doppler (CW-Doppler), which is mainly used in echocardiography, spectral Doppler allows the measurements of flow velocities within in a sample volume. Since flow velocities are elevated in stenosis, and because these flow velocities increase proportionally with the degree of stenosis spectral Doppler allows us to quantify stenotic lesions.

Applications of Spectral Doppler

Differentiation ECA from ICA

Measure flow velocities

To determine if a stenosis is heamodynamically significant

Quantification of stenosis

Determine the direction of flow

See if distal (intracranial) flow obstruction is present

6.2 What is the sample volume in spectral Doppler?

The sample volume is the region where the velocities are measured. This region can be adjusted with the help of the trackball and is depicted as an interrupted line (Doppler line). The width of the sample volume can also be adjusted. The wider the sample volume is the more spectral broadening you will have. An additional line denotes the Doppler angle correction.

Image components of spectral Doppler. The sample volume is defined by the interrupted Doppler line.

6.3 How should I optimize the spectral Doppler tracing for carotid ultrasound

• Optimize the velocity scale and the base line (shift)
• The curve should be above the baseline (if not invert scale)
• Use a time scale which allows you to observe at least 3 beats at once
• Position your sample volume in the middle of the artery
• The sample volume should be between 15 and  30mm
• Optimize the angle correction (45-60°)
• If you lose the signal - go back to the B mode/color image and try again
• Use automatic curve tracing only if the signal quality is good and tracking is optimal

Information provided on the ultrasound screen.

6.4 At which locations should spectral Doppler be performed in carotid ultrasound?

Spectral Doppler measurements should be performed in the common carotid artery, the proximal internal and external carotid artery. It is advisable too also obtain signals from the proximal common carotid artery and the distal + mid carotid artery. It is mandatory to also measure the flow velocity at the site of a suspected stenosis (look for the maximal velocity) and distal to the stenosis.

Locations at which a spectral Doppler recording should be performed. Red denotes positions which are required even in a minimal protocol. Yellow: positions for an extended protocol. In addition, spectral Doppler should always be performed at and distal to a suspected stenotic lesion.

6.5 What are the Doppler characteristics of the arteries of the neck?

Spectral Doppler allows us to identify vessels. Not only is it possible to distinguish arteries from veins but we can also see different flow patterns in different arteries. In general we distinguish high- low and mixed resistance arteries. Arteries that supply the brain are usually low resistance while arteries that supply muscles show a high resistance pattern

Artery

Type

Characteristic

Common carotid artery

Mixed resistance

Intermediate systolic / diastolic ratio

Internal carotid artery

Low resistance

Lowest systolic to diastolic ratio

External carotid artery

High resistance

Highest systolic / diastolic ratio, rapid dip in the curve after systole

Vertebral artery

Low resistance

Similar to ICA (low systolic / diastolic ratio)

Subclavian artery

Very high resistance

Very little diastolic flow, high systolic / diastolic ration, diastolic flow reversal (triphasic  flow)

Typical spectral Doppler flow patterns seen during carotid ultrasound (common carotid artery CCA; Internal carotid artery, ICA; external carotid artery, ECA)

Typical flow in the mid common carotid artery (mixed resistance vessel), normal peak systolic velocity (89 cm/s) and an end-diastolic velocity of 16cm/s

Typical flow in the proximal internal carotid artery (ICA). Note that the end- diastolic velocity is rather high (11cm/s) compared to the maximal systolic velocity (33cm/s). The ratio is 2.8. This is typical for a low resistance artery.

6.6 What are the normal flow velocities in the carotid artery?

It is important to measure both end-systolic and diastolic velocities. There is a fairly high variation of the maximal velocities in the arteries of normal individuals. The velocities depend on age, sex, heart rate, stroke volume, resistance and the width of the artery. The following table shows the upper limits of normal that have been proposed in the literature.

Normal values

Common carotid artery

Peak systolic velocity

< 125cm/s (30-40cm/s)

Large variations in velocity, higher velocities found in paediatric population (up to 180cm/s)

End diastolic velocity

40cm/s?

No data available but usually less than 40cm/s

Internal carotid artery

Peak systolic

< 125cm/s

Usually below 100cm/s

Enddiastolic

< 40cm/s

Low diastolic velocities are indicative of a distal obstruction

Ratio

ICA / CCA ratio

< 2

Women tend to have slightly higher ratios than men, the ratio increases with age, usually the ratio is between 0.4 - 1.4

6.7 Which ratios / calculations are used to quantify the degree of carotid artery stenosis?

At the site of a stenosis (<50%) we see elevated systolic and diastolic velocities. The absolute velocities allow us to quantify the degree of stenosis. In addition we can also compare velocities and calculate ratios. The following ratios are used in carotid artery ultrasound:

Ratios

ICA/CCA ratio

Diagnose and quantify stenosis

S/D ratio

Helps to distinguish high from low resistance vessels, helps to diagnose and quantify stenosis

Resistance index

Is calculated as: (peak systolic velocity – end diastolic velocity)/peak systolic velocity. A value of of 1 indicates systolic blood flow but no diastolic blood flow (high resistance vessel)

End diastolic ratio ICA/CCA

Helps to quantify the degree of stenosis