- 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.
- 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.
- 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).
- 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.
- 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).
- 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.
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.