Standards of the Polish Ultrasound Society - update. Examination of extracranial carotid and vertebral arteries.

The role of a standard is to describe examination techniques, recommended norms as well as principles necessary to obtain results and draw appropriate conclusions, rather than a description of pathologies. The paper presents a technique for carotid artery examination as well as currently recommended standards. According to San Francisco Consensus from 2003, a significant stenosis of 70% may be detected when systolic velocity in the internal carotid artery stenosis is >230 cm/s, and the diastolic velocity is >100 cm/s. The common carotid artery velocity is also taken into account. Hemodynamic evaluation is therefore the primary method for the assessment of the degree of internal carotid artery stenosis. It is important that the examination is performed at an insonation angle of 60° as measurements at higher angles result in an exponential increase in measurement error. Also, an extended version of standards involving measurements performed behind stenosis, which are used in some clinics, is included in the paper. The paper further presents guidelines for the description of the identified pathologies, which ensure that the findings prove unambiguous for clinicians, especially when stenosis eligible for surgical intervention is detected. Morphological measurement of stenosis (according to NASCET criteria) is only of supplementary character (confirming morphological grounds for the increase of velocity) and has no critical importance, especially due to high measurement divergence. Description of atherosclerotic plaques, especially hypoechoic ones, which are considered potentially unstable and may lead to a raised risk of stroke, is a very important element of examination. The paper is based on Standards of the Polish Ultrasound Society and updated based on the latest literature reports.

Introduction

Carotid artery evaluation is an essential part of Doppler ultrasound examination. Every year about 35 000 people in Poland suffer from stroke – 80% of these cases are ischemic, while nearly 1/4–1/3 are associated with extracranial carotid artery stenosis(. Therefore, the assessment of the degree of stenosis within these vessels is vital in the case of patients with the risk of stroke. Since nearly 80–90% of endarterectomy procedures are performed based on ultrasonography, Doppler ultrasound is of great importance for the whole process of patient management(. It involves an assessment of the accessible segments of the common carotid arteries (CCA), internal carotid arteries (ICA) as well as a basic assessment of the external carotid arteries (ECA) and vertebral arteries (VA).

Equipment

Examinations are performed using a device for vascular evaluation, i.e. fitted with a Doppler module. The module allows for both spectral flow recording and color flow imaging; it also features a mode for simultaneous 2D morphological analysis as well as color and spectral flow imaging – a so called triplex mode.

Special vascular probes with a footprint of max. 45 mm are used in the examination. The frequency of a probe should range between 5–7 and 8–12 MHz; higher frequencies do not allow for a sufficient wave penetration in thicker tissues or under difficult examination conditions. In such cases 3–5 MHz convex probe may be used, however, it should be noted that although its lower frequency does not allow for a detailed morphological imaging of atherosclerotic lesions, color imaging enables an assessment of vascular course and diameter as well as the velocity of blood flow.

The probes should be multi-frequency or broadband. The system should feature an option of bending the beam of ultrasound waves by at least 15° in order to obtain an image and record the flow spectrum. The number of transmission channels should be as high as possible. The device may feature a module for automated measurement of intima-media thickness (IMT) over a longer vascular segment. The dynamics of imaging should be as high as possible in order to obtain a better image of deeper vascular structures.

The following elements are also essential:

measurement software;

image-archiving system.

Preparation for the procedure

The procedure does not require special preparation.

Technique

The examination is performed with the patient in the supine position with the neck slightly extended and head turned away from the side being examined. In order to avoid dizziness or pain resulting from head extension when placed on a flat surface, it is recommended for some patients to have slightly raised head, neck and the upper abdomen – the top part of the examination table is elevated at 30° during examination.

First, the vessels are examined in a transverse plane, beginning in the supraclavicular region, and moving the probe up towards the angles of the mandible. This is done in order to determine the topography of blood vessels – their mutual relationship, as well as to identify the site of common carotid artery bifurcation and to perform an initial assessment of both the location and extent of atherosclerotic lesions.

The main part of the examination is performed in the longitudinal planes in the vessel axis in order to assess the length of lesions as well as to perform blood flow velocity measurements. Depending on the anatomical conditions, the examination is performed from the anterolateral and lateral (anteriorly and posteriorly from the sternoclavicular muscle) approach as well as posterior-lateral approach – by applying the probe towards the back from the muscle, with a strong neck rotation. In the case of calcified atherosclerotic lesions located in the lateral part of the aortic bulb, examination through the medial-anterior approach, i.e. with the probe placed directly at the trachea vincinity and with the ultrasound beam directed to the side, is preferable as it allows for a better visualization of a patent vessel lumen. In the case of patients with favorable anatomical conditions, examination through the jugular veins is recommended as it allows to obtain artifact-free images of the carotid artery walls. When applying the probe from the anterolateral and medial-anterior approach, it should be noted, that there is a possibility of carotid body stimulation caused by to strong compression – the pressure applied during the examination should be minimal. In the case of uncommon clinical situations such as dizziness or disability (patient on a wheelchair), the examination may be performed with a patient in sitting position, facing the device, with the head slightly extended in the direction opposite to the side being examined. The examination of carotid arteries should be performed according to the same scheme and always in the same imaging (and documenting) order for each vessel(.

Technical aspects of examination

Velocity measurements are always performed in the long axis of the vessel, more precisely in the axis of the stream of blood flowing within a vessel, while in the case of stenosis – in the flow channel axis. The width of the gate should be 1/3 to 1/2 of vessel lumen (so as not to include the vessel wall if possible). In normal cases, measurements are performed in the central part of vessel cross-section(.

Standard measurement sites:

CCA – the mid-CCA, at least 2 cm below sinus (higher blood flow velocity in the lower CCA);

ICA – in a normal vessel – about 1 cm above the bulb, in a region with a constant diameter (i.e. above the turbulences associated with blood flow in the wider part of the vessel;

ECA – about 1 cm above the origin of the vessel and above the origin of the superior thyroid artery;

vertebral arteries (if possible) – in the V1 segment, prior to their entry into the transverse spinal processes, at V2, at the site of appropriate visualization of the vessel between the processes and suitable for a correction of velocity measurement angle.

Measurement method

The angle of access to a vessel depends on probe physical position in relation to the vessel and, if necessary, on the electronic deflection of the ultrasound beam. Combining these two parameters usually allows to obtain the angle of access to a vessel – below 60°. Measurements at higher angles are clinically worthless as angles of more than 60° result in an exponential increase in velocity measurement error(. It is obvious that, in the absence of stenosis, the flow channel is parallel to the walls of the vessels and their straight course renders the correction of the angle uncomplicated.

Measurements at vessel curvatures should be avoided, and in the case of winding vessels, a relatively straight vessel segment should be selected. If this is impossible, it is necessary to record velocities at a relatively wide measurement gate in the central part of the flow channel (blood flow velocity in the lateral part of a curved blood vessel is physiologically 10–15% higher compared to the inner part of the curve).

In the presence of atherosclerotic lesions, the stenotic blood flow channel may not be parallel to its walls, therefore correction should be performed along the channel with the highest flow velocity, best visualized by color imaging and with the apparatus set for higher velocity recording.

In the case of arrhythmia, measurements are performed after second/third contraction after a pause, when contractions are more regular. In complicated cases, average velocity of several contractions should be used.

Measurements

The examination of each patient should involve the measurements of blood flow velocity in:

the middle CCA;

ICA – about 1 cm above the bulb;

ECA – about 1–2 cm above the origin;

VA – at V1 and/or V2.

Normal values for flow velocity are shown in tab. 1.

Tab. 1

Normal range of velocities

	PSV	EDV
CCA	0,8–1,2 m/s	0,1–0,3 m/s
ICA	0,8–1,2 m/s	do 0,3 m/s
ECA	0,8–1,2 m/s	do 0,25 m/s
VA	<0,6 m/s	0,05–0,2 m/s

It should be noted that there is a decrease in blood flow velocities in the carotid vessels with advancing age – the upper values of velocity norms in the elderly are usually associated with pathology.

Records/examination characteristics

Each description of carotid and vertebral artery ultrasound examination should include the following data:

patient's full name, age;

date of examination;

the name of device and the type of probe, including its frequency.

Photographic documentation (from a videoprinter, computer printer or recorded on electronic media) with patient's full name should be included in the characteristics.

The style of description should depend on whether stenosis qualifying for surgical intervention (hemodynamically significant) or only minor lesions, which do not qualify for surgical intervention, were observed. Such a division will facilitate both the decision regarding further management as well as communication with the clinician.

Examination characteristics in the case of hemodynamically significant stenosis

The first paragraph should include stenosis description and parameters. For instance: “Hemodynamically significant stenosis in the left ICA of >70%. The peak systolic velocity in the stenosis (PSV) of 240 cm/s, enddiastolic velocity (EDV) of 105 cm/s. Systolic velocity in the CCA: 55 cm/s”.

In the case of hemodynamically significant ICA stenoses (exceeding 50%), the description should additionally include flow velocities in the upper part of the vessel (significantly decreased at stenoses of more than 80%) and (at ICA stenoses) in the middle-upper part of the CCA – necessary for ICA/CCA index calculation.

In the case of high grade stenoses (>70%), the characteristics should include flow velocities in the remaining carotid vessels as these may serve as collateral circulation, and thus reduce the blood flow through stenotic vessels, which leads to a false evaluation of the degree of stenosis.

The description should include atypical location of CCA bifurcation – low/high; necessary for planned surgical procedure.

In the case of minor and medium stenosis (up to 50%), the description should include the peak flow velocities (PSV and EDV) registered in the area of stenosis. Stenoses of up to 30% do not influence spectrum or blood flow velocities within vessels.

The description should preferably include the range of stenosis, most preferably with accuracy of ±5–10%, e.g. 40–50% stenosis, 70–80% stenosis, etc.

The range should not differentiate further management of stenoses of 50 and 70%, i.e. there are no ranges 45–55% or 65–75% – the operator is required to determine the clinical category to which the patient belongs.

In the event of considerable divergence between hemodynamic and morphological assessments of the degree of vascular stenosis, both values should be included in the results.

Morphological stenosis description should include the precise location of stenosis in relation to the internal carotid bulb – the length of lesions and the type of plaques (soft, fibrous, calcified) should be provided as this may affect the choice of treatment.

Morphology of the identified plaques should be described in detail for future comparison of the evolved lesions. Atherosclerotic plaque characteristics should include: thickness and, in the case of major lesions, length, echogenicity (with particular emphasis on hypoechoic plaques, i.e. potentially unstable lesions), surface (smooth/rough, missing plaque fragments and their size), location (concentric or eccentric – in the lateral part of the bulb).

The characteristics should include major (of more than 50%) differences in the diameters of homonymous vessels.

The characteristics should include blood flow velocities in the internal carotid arteries, even if no stenoses or other pathologies were found.

With reference to vertebral arteries, their diameters should be reported in the case of hypoplasia, i.e. below 2 mm (as found in 20% of patients). It should be noted that physiological differences in flow velocity may account for up to 30%, and the registered spectra should be symmetrical and similar in shape (comparable PI).

In the case of normal carotid and vertebral vessels, it is not necessary to include blood flow velocity in other vessels apart from flow velocity in the internal carotid arteries as images of these vessels with their blood flow velocities are included in the photographic documentation.

In the case of pathological flow in vertebral arteries, the following details should be reported: the direction of blood flow in the vessel, changes in velocities and flow resistance by comparison with the opposite side as well as conclusions drawn from the data obtained. In the case of subclavian steal syndrome, the examination should additionally include an evaluation of subclavian arteries.

Additional comments

Examination characteristics, depending on the data obtained, should end with diagnostic conclusions, including the degree of stenosis expressed as a percentage as well as suggestions on complementary examinations (CT, MR, angiography). In the case of significant degree of stenosis (>70%), conclusions should include (regardless of the referring specialist) a referral for vascular surgeon consultation.

Records

The following data should be provided even if no lesions were found:

images of the common carotid sinus with the internal carotid bulb;

a record of the spectrum obtained in the internal carotid artery in the region of laminar flow as well as velocity measurement;

spectrum from the vertebral artery.

Additionally, the following should be performed in the case of identified lesions:

plaque image (low-echoic plaque, ulceration, etc.) with measurements;

systolic and diastolic velocity measurement in the ICA stenosis;

systolic and diastolic velocity measurement in distally from stenosis;

measurements of the common carotid (CCA) systolic velocity;

measurement of systolic velocity in the external carotid artery stenosis, if present.

Evaluation of carotid artery lesions

Evaluation of the intima-media complex

Examination of the intima-media thickness (IMT) is performed in the upper CCA segments, about 10 mm below its bifurcation, in a longitudinal plane, with strong (specular) reflection from the surface of the internal membrane, and involves several measurements over longer wall segments with a result as a mean value (fig. 1). High quality equipment and probes are required for the examination. In the enlarged images, total IMT thickness, i.e. hypoechogenic middle part of the vessel together with echogenic inner membrane, is measured. The thickness of intima-media increases with advancing age. Normal values are 0.6 in females and 0.7 in males. Thickness of 0.9 mm (F) and more (M) is considered pathological(. The measurement of IMT in the internal carotid artery may be problematic due to difficulties in obtaining a perpendicular approach angle to the vascular wall.

Fig. 1

The measurement of IMT as an averaged measurement from the outline over a longer segment

Many authors believe that a single measurement in the thickest point is sufficient. Therefore, in this type of measurement, the characteristics should include focal/point measurement.

Atherosclerotic plaque assessment

Atherosclerotic plaque is defined as vessel wall thickening (IM thickness) of more than 1.5 mm or lesion protrusion into vascular vessel wall at a depth of more than 50% of the vessel thickness(.

Atherosclerotic plaque measurements include:

thickness – for comparison in subsequent examinations;

length – mainly in order to determine the extent of incision in the case of planned surgical procedure;

echogenicity – the risk of neurological events increases with decreasing plaque echogenicity (fig. 2);

Fig. 2

Low-echoic atherosclerotic plaque with an increased risk of rapture

surface roughness/ missing plaque fragments – as a potential source of embolic material;

location – concentric/eccentric lesion;

lesions in relation to CCA bifurcation.

Massive calcification of the plaque may hinder or even prevent the assessment of the degree of vascular stenosis. In such circumstances, CT angiography or MR angiography should be performed.

Morphological evaluation of the degree of stenosis

Morphological evaluation of the degree of carotid stenosis based on morphological measurements is used as a supplementary examination. Therefore, it should not be used as a primary element of stenosis degree assessment. Morphological stenosis is a reduction in vascular diameter.

It should be defined according to NASCET criteria (measurement of stenosis diameter and normal lumen diameter in the cephalad region) using the following formula:

where:

A – flow channel diameter at the stenosis

B – cephalad vascular segment diameter

The measurement of stenosis according to ECST method (C – A/C, where: C – vascular diameter at the level of stenosis, A – the diameter of the flow channel in the stenosis) is practically not used.

Measurement of cross-sectional area is of historical value.

Stenosis – hemodynamic assessment

Hemodynamic assessment is the primary method of determining the degree of carotid artery stenosis. Examination of these vessels does not in fact involve morphological assessment of arteries, but is a functional and dynamic method evaluating the degree of cerebral circulation impairment based on both direct and indirect measurements of the amount and velocity of cephalad blood flow. Measurements are performed at an angle of attack i.e. the angle of 60° between the axis of ultrasound beam and the axis of the blood flow in a vessel, which in the case of stenosis may not be parallel to the long axis of the vessel. A universal ALAP principle (“as low as possible”, i.e. measurement at the smallest angle) should be applied in all blood flow Doppler measurements.

Three groups of patients may be distinguished in terms of clinical diagnostics:

with a stenosis of <50%, who do not require surgical intervention;

with a stenosis of 50 up to 70%, when surgical intervention is necessary only in some cases;

with a stenosis of >70%, when (in the absence of contraindications) surgical procedure or stent placement is a method of choice.

Blood flow velocities in these patient groups, measured within or in the vicinity of stenosis, should be within the ranges listed in tab. 2.

Tab. 2

Patient categories based on the size of stenosis

I	PSV < 1,5 m/s	EDV < 0,4/0,5 m/s
II	1,5 < PSV < 2,3 m/s	0,5 < EDV < 1,0 m/s
III	PSV > 2,3 m/s	EDV > 1,0 m/s

Tab. 3 shows currently and widely recommended standards regarding the identification of the hemodynamic size of stenosis, based on which patients are qualified for surgical procedures.

Tab. 3

Recommended standards for hemodynamic assessment of the size of internal carotid artery stenosis in accordance with the Society of Radiologists in Ultrasound (Consensus Conference on Carotid Ultrasound, San Francisco, 22–23rd October 2002)

	ICA PSV	Plaque	ICA/CCA PSV	ICA EDV
Normal	<125 cm/s	No	<2,0	<40 cm/s
<50%	<125 cm/s	<50% diameter reduction	<2,0	<40 cm/s
50–69%	125–230 cm/s	50% diameter reduction	2,0–4,0	40–100 cm/s
70%	>230 cm/s	50% diameter reduction	>4,0	>100 cm/s
Critical stenosis	Very low or undetectable	Visible	Various	Various
Occlusion	Undetectable	Visible, vascular lumen in not visible	-	-

The same criteria should be used for the assessment in post-endarterectomy patients. It should be noted that carotid artery restenosis is diagnosed when residual stenosis is >50%.

The same hemodynamic criteria are used for the evaluation of stent patency. Currently used stents are tapered, i.e. their upper part is slightly narrower than the lower part, therefore morphological assessment of stent is not fully reliable (flow turbulence in the vicinity of the upper part of a vascular prosthesis may also occur)(.

EDV measurement of >1.0 m/s is the most accurate, single parameter for the determination of ICA stenosis of more of 70%. In the case of stenosis of more than 70%, stenosis velocity/ CAA velocity ratio should be >4.0. In the case of stenosis of >75%, there is a substantial reduction in blood flow in the upper portion of the ICA – the quotient of the velocity measured at stenosis to the measurement performed at least 2–3 cm above should be >5.0. In the case of stenosis of >80%, extended acceleration time of >0,2 s is observed(.

Tab. 4 shows other standards for carotid artery stenosis characteristics, which also assess the flow above stenosis(.

Tab. 4

Other standards used in carotid stenosis description, including stenotic blood flow

Degree of stenosis (%)	ICA PSV m/s	ICA EDV m/s	Turbulence	ICA/CCA	ICA/ICA high
0–30	<1,25	<0,4	-	<2	<1,3
30–40	<1,25	<0,4	+	<2	<1,3
40–50	<2,0	<0,5	+	<2	<1,3
50–60	<2,0	0,5–0,7	+ +	2–4	1,3–2,5
60–70	<2,5	<1,0	+ +	<4,5	2,5–5,0
70–90	>2,5	>1,0	+ +	>4,5	>5,0
>90	>2,5	>1,3	+ + +	>4,5	>8,0
>95	<2,5	<1,0	+ + +	any	any

Fig. 3 shows 70-percent internal carotid artery stenosis due to hypoechoic atherosclerotic plaque.

Fig. 3

Hemodynamic stenosis – about 70%. A. Hypoechoic plaque resulting in 70-percent stenosis. B. Stenosis resulting in an increased systolic velocity up to 288 cm/s, and diastolic velocity up to 103 cm/s

Fig. 4 shows 80–90-percent internal carotid artery stenosis.

Fig. 4

Hemodynamic stenosis – about 80–90%. A. Atherosclerotic plaques cause morphologically visible high grade stenosis. B. A systolic velocity of 602,5 cm/s and diastolic velocity of 270 cm/s were obtained in spectral Doppler

Carotid artery occlusion

Vascular occlusion is diagnosed based on the lack of blood flow in the vessel. In the early phase after lumen closure, the vessel is wide, easily visualized, filled with atherosclerotic plaques characterized by different echogenicity as well as with hypoechoic thrombi. Several months later the lesions become fibrous, their echogenicity increases, while the vascular diameter decreases, thus hindering vessel visualization. In the case of vertebral arteries, frequently occurring deviations of vascular course and diameter combined with difficult examination conditions in patients with cervical spine degenerative lesions hinder their evaluation. Making definitive statements should be avoided in such circumstances – when vascular visualization is impossible, the vessels should not be described as obstructed but information concerning the conditions preventing the proper assessment should be included. Vertebral artery visualization lacking flow spectral recording corresponds to occlusion.

Summary

Doppler ultrasonography of carotid arteries is a precise method enabling evaluation of carotid stenosis. There are a number of factors affecting its results – vascular stenosis, disrupted blood flow registered in these vessels (upstream and downstream), other anomalies as well as collateral circulation within carotid vessels. Only combined evaluation of all these elements allows for a diagnosis.