Carotid Ultrasound - Introduction

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Indications for Carotid US

• Evaluation of patients with hemispheric neurologic symptoms, including stroke, transient ischemic attack, and amaurosis fugax
• Evaluation of nonhemispheric or unexplained neurologic symptoms
• Evaluation of patients with a cervical bruit
• Evaluation of pulsatile neck masses
• Preoperative evaluation of patients scheduled for major cardiovascular surgical procedures
• Follow-up evaluation of patients with proven carotid disease
• Evaluation of postoperative post interventional (carotid endarterectomy, stenting, carotid-to subclavian artery bypass
  grafting)
• Intraoperative monitoring of vascular surgery
• Evaluation of patients with carotid reconstruction after extracorporeal membrane oxygenation bypass
• Screening of high risk patients; atherosclerosis elsewhere
• Evaluation of suspected subclavian steal syndrome
• Evaluation for suspected carotid artery dissection, arteriovenous fistula, or pseudoaneurysm
• Evaluation of patients with syncope, seizures, or dizziness
• History of head and neck radiation

Limitations of carotid US

• Vessels are small (0.3-1.5cm), good hand eye coordination required
• We only see the extracranial arteries
• The origin of the common carotid arteries difficult to image
• The vertebral arteries difficult to see DD hypoplasia or complete occlusion of the vertebral arteries
• Rely on indirect signs to quantify intracranial stenosis
• Sometimes difficult to differentiate the ECA from the ICA
• Calcifications make grading of stenosis difficult
• Trauma (cervical collar)
• Image quality

Difficult regions to image

• Cranial portions of the external and internal carotid arteries
• Origin of the left common carotid artery
• Origin of the vertebral arteries
• Cranial segments of the vertebral arteries
• Segments of the vertebral arteries that are posterior to
  the transverse processes

 

 

Branches of the external carotid artery

• Superior thyroid artery
• Ascending pharyngeal artery
• Lingual artery
• Facial artery
• Occipital artery
• Posterior auricular artery
• Maxillary artery
• Superficial temporal artery

 

 

Artery wall

Artery wall Intima Inner layer, consists of an endothelial lining of endothelial cells and an elastic membrane (tunica elastica) Media Middle layer, muscular and elastic components Adventitia Collagen tissue, contains vasa vasorum (in larger arteries)

 

 

Segments of the carotid artery

 

 

Angle of insonation

• The angle of the ultrasound beam relative to the tissue (vessel wall)
• The strongest echo are produced when the angles of incidence approaches the angle of reflection
• The best B Mode image is achieved if the structure is hit perpendicular (90°)
• The angle of insonation plays an important role when using color Doppler

 

 

Pulsed wave Doppler

• In carotid ultrasound we use mainly pulsed wave Doppler (PW-Doppler)
• With PW Doppler we are able to measure blood flow velocity within a sample volume
• The Doppler velocities we measure are very dependant on the angel between the blood flow direction and the transducer
• The more the angle moves towards 90° the higher the measurement error will be
• Therefore we use angel correction. We tell the scanner in which angle blood is actually flowing
• The principle of Doppler is used in color- and spectral Doppler
• Blood flow moving towards the transducer is depicted in red (color Doppler)
• Blood flow moving towards the transducer is depicted in blue (color Doppler)
• Blood flow moving toward the transducer is depicted above the baseline (spectral Doppler)
• Blood flow moving away from the transducer is depicted below the baseline (spectral Doppler)
• The shade of color (yellow-red, light blue - dark blue) allows us to semiquantitatively assess the velocity of blood flow

 

 

Aliasing

• Is a Doppler phenomenon where the intermittent sampling rate (PRF) is too low to record a certain velocity
• This results in an inability to measure the true velocity and flow direction
• It occurs in both color and PW Doppler (because both of theses techniques use „intermittent„ sampling)
• In carotid Doppler this results in a sudden shift from red to blue or blue to red, we often see a mosaic of different colors
• Aliasing is helpful in US because it allows us to quickly detect regions where blood flow velocity is high
• High blood flow velocity can be present in stenosis or tortuous vessels
• The velocity at which aliasing occurs can be set on the scanner (PRF, aliasing velocity, Nyquist limit)
• The value depicted on the color bar tells us at which velocity aliasing occurs. We call this the Nyquist limit

 

 

Artefacts

Artefacts Explanation Examples Acoustic enhancement Structure posterior to tissues that transmits US well is enhanced Posterior enhancement behind cysts, urinary or gallbladder Acoustic shadowing The US waves are completely absorbed or reflected (solid structures). Structures behind such tissue is not visible The vessel posterior so a calcified plaque can not be visualised. Prosthetic material, bone (rib shadow) Beam width artifact When the ultrasound beam is wider than the diameter of the reflector being scanned. Adjacent structure are included into the beam and displayed as false structures When scanning an anechoic structures (Cystic structures) they might appear solid Blooming artifact When the color Doppler signal extends beyond the true boundaries of the vessel, spreading into adjacent regions with no actual flow Over gaining of color Doppler (A structure in a vessel might be overlapped by color and thus missed (Carotid US) Mirror Image artifact Mirror image artefacts occur in the presence of strong reflectors. The wave is reflected several times and structures are displayed several times Pericardium, diaphragm, echogenic, reflection of liver lesion into the thorax Reverberation When the US beam reflects back and forth between Comet tail artefact in lung ultrasound, two strong parallel reflectors. Appears similar to mirror artefacts Comet Tail artefact in lung ultrasound Near field clutter Near field clutter occurs due to high amplitude oscillations of piezoelectric crystals. It involves the near field and may hinder identification of structures that are close to the transducer Echocardiography, display of false apical structures (which are close to the transducer in an apical view).

 

 

Transducer orientation

 

 

Tortuose vessels

Tortuose vessels Type I Tortuosity Tortuosity – a non-rectilinear stretch of an artery with an angulation >90° Type II Looping A 360 angulation of an artery on its transverse axis (‘‘coil’’ configuration) Type III Kinking The inflection of 2 or more segments of an artery with an internal angle of 90° or less.

 

 

Segmentation ICA

Segmentation ICA Proximal ICA From bifurcation to where the bulb ends Mid ICA From mid to 3cm from bifurcation Distal 3cm from bifurcation

Note: in clinical practice segmenting the ICA will be more of a
visual judgement. One can also use an approach where you
take the bulb as the lower boundary and the highest region
where you can still see the ICA as the upper boundary.
Halfway in between is the border between mid and distal ICA

 

 

Where to perform spectral Doppler

Mandatory Specific situations Distal CCA Peak systolic and diastolic velocities, ratio calculations Distal + mid ICA Proximal ICA Peak systolic and diastolic velocities, ratio calculations Mid CCA Prox ECA Maximal velocity. Spectral Doppler of ECA is important to determine which vessel is ECA and which is ICA Proximal CCA At a stenotic lesion Peak systolic and diastolic velocities - quantification of stenosis At tortuous ́segments To determine the haemodynamic significance and if a stenosis is also present within the tortious segment

 

 

Doppler characteristics - different vessels

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)

 

 

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

 

 

Ratios

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

 

 

External vs internal - parameters

Morphologic clues Doppler clues Position Color Doppler pulsatility Size Flow in side branches Shape Spectral Doppler shape Branches Manoeuvres (temporal tap)

Message: It is important to discern these vessels because a problem in the external carotid artery will not lead to a stroke, while a problem in the internal carotid artery can!

 

 

External vs internal - morphology

Morphologic clues Position Larger variation of the position in relationship to each other, The ICA is most commonly posterior and lateral to the ECA Size When imaging the carotid artery from anterior the ECA will more frequently be closer to the transducer than the ICA Shape The ICA has the bulb (is wider at its origin) Branches The ECA has side branches

 

 

External vs internal - Doppler

Doppler clues Side branches Use color Doppler to detect side branches Shape of the curves Diastolic velocities are higher in the ICA. You will find a rapid dip in the curve after systole in the ECA, and sometimes short retrograde flow. Often steeper deceleration in the ECA Temporal tap Tap on the temporal artery (anterior to the ear). You will find little „reflection“ waves in the spectrum Pulsatility Color Doppler shows more pulsatility in the ECA than the ICA (larger difference between max. systolic and diastolic velocity Tip: Move the sample volume more cranial into the ICA - the diastolic component is larger there