ULTRASOUND OF THE CAROTIDS - NORMAL

Note: Hover cursor over images for highlighted anatomy.

Normal carotid bifurcation. Common carotid artery (CCA). Internal carotid artery (ICA). External carotid artery (ECA) Normal Common Carotid artery (CCA). Note the smooth echogenic intimal surface.

 

The CCA is readily visible. Locate it in transverse and rotate into longitudinal. The CCA will have a doppler trace that is representative of both upstream and down stream influences. Any cardiac arhythmia or significant left heart valvular problems may be relected in the wave form (eg via a audible and visible 'flutter'). Similarly, the CCA waveform is a combination of both ICA and ECA waveforms.

 

Assess the bifurcation in transverse. Rotate on the ECA origin to sample it longitudinally. The ECA waveform is high resistance and may have retrograde flow in diastole. A 'temporal-tap' (TT) was employed here to confirm it was the ECA.

 

The ICA will have low resistance flow, with constant forward flow during diastole. The ICA origin incoporates the bulb which may create a degree of turbulent flow. This should not be mistaken for spectral broadening secondary to pathology. Normal Carotid bifurcation with the ICA bulb and branch off the ECA.

 

Distal ICA scan plane. You may only be able to see a few cm of the ICA if there is a high bifurcation. Unless the vessel is tortuous, you should see a low resistance waveform with a clean spectral window beneath the trace

 

Transverse bifurcation scan plane. These transverse images show the difference in ICA-bulb vs ECA at the bifurcation and then approximately 1cm further distal.

 

From the mid-distal CCA slide and angle posteriorly to visualise the cervical transverse processes and the vertebral artery. The vertebral arteries can be variable in diameter. They should always demonstrate antegrade flow (toward the brain) and be low resistance similar to the ICA.
Arrows indicate normal flow direction in the extra cerebrovascular circulation. Arrows indicate the flow direction in a right sided subclavian steal syndrome.

 

 

CAROTID ULTRASOUND PROTOCOL

ROLE OF ULTRASOUND

  • To examine the extra-cranial cerebrovascular supply for signs of arterial abnormalities that may be responsible for cerebral or vascular symptoms.
  • Also for preoperative screening of patients with known cardio-vascular risk factors
For a table showing criteria for ICA stenosis classification.


LIMITATIONS

Patient's with short thick necks or with high bifurcations pose technical difficulties however manipulation of settings and probe choice will result in an adequate examination in 99% of cases.

EQUIPMENT SELECTION AND TECHNIQUE

  • Use a linear, mid frequency range probe (5-8MHZ). Be prepared to change probes (or frequency output of probes) to adequately assess deeper or tortuous structures.
  • Use of a 3-6MHz curvilinear probe is useful for distal ICA in patients with high bifurcations, very thick necks and vertebral areties in arthritic necks.

SCANNING TECHNIQUE

  • Explain the examination to patient, and obtain adequate and relevant history.
  • Patient positioned supine on the bed, with head slightly extended over pillow.
  • Scan with patient's head turned slightly away from the side being examined.
  • Begin the examination by assessing vessels in B-Mode, optimising factors such as frequency, depth, gain, TGC and focal zone.

ANGLE:

  • Always angle correct to the flow NOT the vessel wall. They arent always the same and it may not be in the centre of the vessel.
  • Less than 60 degrees ( beyond 60degrees, error is exponentially increased)
  • Ideally an angle of 0 degrees provides least error and greatest doppler shift. This is rarely acheivable but as we approach 0 degrees, our human inter-observer error error is diminishing.
  • With modern equipment, accurate angle correction is acheivable.

CCA

  • Begin proximally in transverse and follow distally to the bifurcation.
  • Assess the course (i.e. if tortuous) and the presence of any intimal thickening or plaque.
  • Repeat in longitudinal plane.
  • Use colour to assess patency of vessel and the direction of flow.
  • Use 'Heel/Toe' technique to optimize insonation of vessel, apply colour box and Doppler sample gate with appropriate steering and angle correction.
  • Measure the Peak Systolic (PSV) and end diastolic velocities (EDV).

BIFURCATION

  • Assess in transverse and longitudinal for pathology.
  • Identify the origins of the ICA and ECA arteries. The ECA has small branches (usually the thyroglossal artery). The ECA also usually has a smaller diameter, arises laterally and has a higher resistance waveform (ie lower diastolic flow than a normal ICA).
  • 'Temporal Tapping' may also be used to confirm that you are examining the ECA. This involves gently tapping the temporal artery (approximately 1-2cm anterior to the top of the ear) whilst sampling the ECA with doppler. You will see reverberations in the trace corresponding to your tapping.
  • A normal ICA will have no branches and usually a lower resistance waveform.
  • Measure the Peak Systolic (PSV) and end diastolic velocities (EDV) of the ECA.

Be aware of the possibility of a 'Carotid bulb tumour' which whilst relatively rare, is a clinically significant finding. This will occur at the bifurcation, outside the vessels, possibly exerting extrinsic compression on the carotid artery.

INTERNAL CAROTID ARTERY (ICA)

  • Follow the vessel intially in B-mode and then using colour doppler. Look for stenoses highlighted by aliasing in the colour doppler.
  • Take Doppler samples in the proximal and distal segments and anywhere else that pathology or an altered waveform is detected.
  • Ensure you angle correctly to the direction of the flow indicated by the colour doppler prior to calculating velocity. IMPORTANTLY, this angle may not correspond to the course of the vessel.

VERTEBRAL ARTERIES

  • Return to a longitudinal plane of the CCA and angle the beam postero-laterally to visualise the vertebral artery.
  • Ensure suitable PRF and gain for these smaller, deeper vessels. The flow should be low resistance flow ( presence of forward diastolic flow).
  • Confirm the flow is antegrade i.e. towards the head - (normal) or retrograde (suggesting subclavian steal syndrome).

SUBCLAVIAN ARTERIES

  • If there is the suggestion of retrograde vertebral artery flow, examine the subclavian artery for a tight stenosis or occlusion that could result in subclavian steal syndrome.
  • The pathology will usually be located between the CCA origin and vertebral origin.
  • Similarly, if there is low systolic, high diastolic flow in the common carotid artery this may be related to CCA origin or subclavian pathology.


ADJACENT STRUCTURES

Always keep in mind the surrounding anatomy in the neck that may be of clinical significance. For example enlarged lymph nodes or thyroid pathology. Brief documentation may be made and formal follow up studies can be performed if clinically indicated.

BASIC IMAGING

A Carotid ultrasound series should include the following images;

  • Common Carotid artery in B mode - long, trans
  • Common Carotid artery with colour & spectral doppler
  • External Carotid artery origin B mode
  • External Carotid artery origin with colour & spectral doppler
  • Internal Carotid artery origin B mode
  • Internal Carotid artery origin with colour & spectral doppler
  • Vertebral artery spectral doppler showing patency and flow direction
  • Document:
    • The normal anatomy.
    • Any plaque found (in 2 planes).
    • All peak systolic and end diastolic velocities

 


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