Craniocervical vessels Doppler ultrasonography

Doppler Ultrasound of Craniocervical Vessels

Doppler Ultrasonography is a non-invasive diagnostic technique used to evaluate blood flow within blood vessels. When applied to the craniocervical vessels, it assesses circulation in the major arteries supplying the brain, located both in the neck (extracranial) and within the skull (intracranial).

The method is based on the Doppler effect: Ultrasound waves emitted by a transducer (probe) travel into the body and reflect off moving red blood cells within the vessels. If the blood cells are moving towards the transducer, the frequency of the reflected ultrasound waves increases; if they are moving away, the frequency decreases. The magnitude of this frequency shift is proportional to the velocity of the blood flow. The Doppler equation relates this frequency shift to the flow velocity, considering the angle between the ultrasound beam and the direction of blood flow (angle of insonation).

Modern ultrasound systems often combine Doppler techniques with B-mode imaging (which creates a grayscale anatomical image of the vessel walls). This is known as Duplex Ultrasound (B-mode + Doppler) or Triplex Ultrasound (B-mode + Color Doppler + Spectral Doppler), allowing visualization of vessel structure (plaque, stenosis) alongside precise measurement of blood flow velocities and patterns within specific areas.

Key parameters assessed include:

• Blood Flow Velocity: Peak Systolic Velocity (PSV), End-Diastolic Velocity (EDV).
• Waveform Analysis: Shape of the spectral Doppler waveform, indicating resistance in the downstream vascular bed (e.g., low resistance in internal carotid artery vs. high resistance in external carotid artery).
• Pulsatility Index (PI) and Resistive Index (RI): Calculated indices reflecting downstream vascular resistance.
• Presence and Direction of Flow: Confirming vessel patency and identifying abnormal flow patterns (e.g., reversed flow indicating collateral circulation).
• Detection of Stenosis: Identifying narrowed segments by visualizing plaque and observing focal increases in flow velocity and post-stenotic turbulence.

While blood viscosity (influenced by factors like hematocrit and fibrinogen) can affect flow dynamics, Doppler ultrasound primarily measures velocity, which is then used to infer information about vessel patency, stenosis severity, and downstream resistance.

Indications for Doppler Ultrasonography

Doppler ultrasound of the craniocervical vessels is a cornerstone in neurovascular diagnostics. Common indications include:

• Screening for and quantifying carotid and vertebral artery stenosis (narrowing), often caused by atherosclerosis (plaque buildup), as a major risk factor for Ischemic stroke, cerebral ischemia.
• Evaluation of patients with Transient Ischemic Attack (TIA) or stroke to identify potential embolic sources or flow limitations.
• Assessment of Vertebrobasilar insufficiency (VBI) with vertigo symptom, often involving assessment of vertebral artery flow.
• Monitoring known stenosis progression or evaluating results after intervention (e.g., carotid endarterectomy or stenting).
• Evaluation of pulsatile tinnitus or suspected subclavian steal syndrome.
• Assessment of suspected carotid or vertebral artery dissection.
• Transcranial Doppler (TCD) specific indications:
  • Detection of cerebral vasospasm after subarachnoid hemorrhage.
  • Evaluation of intracranial stenosis.
  • Assessment of collateral circulation pathways.
  • Detection of right-to-left shunts (e.g., Patent Foramen Ovale - PFO) using bubble studies.
  • Monitoring during surgical procedures (e.g., carotid endarterectomy).
  • As an ancillary test in the evaluation of Increased intracranial pressure and hydrocephalus (by assessing flow patterns and pulsatility).
  • As part of brain death evaluation (demonstrating cessation of cerebral blood flow).
• Evaluation following Traumatic brain injury (TBI) or Cervicocranial syndrome and whiplash neck injury, particularly if vascular injury (e.g., dissection, vasospasm) is suspected.
• Investigation of certain types of Headache, migraine, particularly if vascular mechanisms like vasospasm are considered (though less common).
• While not primary tools, sometimes used adjunctively in evaluating complex cases of Ischemic brain disease or Encephalopathy to rule out major vascular contributions.
• Somatoform autonomic dysfunction (Generally low yield unless specific vascular symptoms are present).

The procedure uses specialized ultrasound equipment with probes (transducers) operating at frequencies typically ranging from 2 MHz (for TCD) to 10 MHz (for superficial neck vessels). Data interpretation involves analyzing absolute velocity values, comparing side-to-side symmetry, evaluating waveform morphology, and calculating indices.

 

Technique: Extracranial and Transcranial (TCD)

Extracranial (Neck Vessels):

The patient lies supine, typically with the head slightly turned away from the side being examined. Using a linear transducer (usually 5-10 MHz) with coupling gel applied to the skin, the examiner systematically visualizes and assesses the common carotid artery (CCA), the carotid bifurcation, the internal carotid artery (ICA), the external carotid artery (ECA), and the vertebral arteries (VA) as they course through the neck. B-mode imaging identifies plaque morphology and location, while Color and Spectral Doppler measure flow velocities and identify areas of stenosis or abnormal flow patterns.

Intracranial (Transcranial Doppler - TCD):

TCD uses lower frequency transducers (typically 2 MHz) to penetrate the skull bones through specific "acoustic windows" where the bone is thinnest. Gel is applied to the skin over these windows. Common windows include:

• Transtemporal Window: Located above the zygomatic arch. Allows access to the Middle Cerebral Artery (MCA), Anterior Cerebral Artery (ACA), Posterior Cerebral Artery (PCA), and terminal ICA. Vessel identification relies on depth of sample volume, direction of flow relative to the probe, waveform characteristics, and response to compression maneuvers (e.g., CCA compression).
• Transorbital Window: Ultrasound beam directed through the closed eyelid (with caution and reduced power). Allows access to the Ophthalmic Artery (OA) and the carotid siphon (part of the ICA within the skull).
• Transforaminal (Suboccipital) Window: Located below the occiput at the base of the skull, aiming through the foramen magnum. Allows access to the intracranial segments of the Vertebral Arteries (VA) and the Basilar Artery (BA). Requires neck flexion, which should be done cautiously, especially if raised ICP is suspected.

Normal flow velocities vary by age and specific artery. For example, Mean Flow Velocity (MFV) in the MCA of adults is typically in the range of 30-80 cm/s, generally higher in children and decreasing with age.

 

Interpretation and Clinical Significance

Interpreting Doppler ultrasound results involves integrating B-mode findings (plaque, vessel diameter) with Doppler parameters (velocities, waveforms, indices).

• Stenosis Grading: Primarily based on increased flow velocities (PSV, EDV) at the site of narrowing, often using established criteria (e.g., NASCET or ECST criteria correlations for carotid stenosis). Post-stenotic turbulence is also a key sign.
• Vessel Occlusion: Absence of detectable flow signal within a visualized vessel segment.
• Vasospasm: Characterized by significantly increased flow velocities (especially MFV in TCD) often developing days after subarachnoid hemorrhage.
• Increased Intracranial Pressure (ICP): TCD may show indirect signs like increased Pulsatility Index (PI), reduced diastolic flow, or eventually reverberating/oscillating flow patterns or cessation of flow in severe cases.
• Brain Death: TCD can contribute to the diagnosis by demonstrating characteristic flow patterns indicating cessation of cerebral circulation (e.g., oscillating flow, systolic spikes only, or complete absence of flow). Typically, this follows a pattern of increasing resistance, reduced diastolic flow, then reverberating flow, and finally signal loss.
• Collateral Flow: Detecting reversed flow in certain vessels (e.g., ophthalmic artery, anterior communicating artery) can indicate significant stenosis or occlusion in major supply arteries.

Asymmetry in flow velocities between corresponding arteries on opposite sides can be significant, potentially indicating unilateral stenosis or altered perfusion, although some degree of asymmetry can be normal. Markedly elevated velocities (e.g., >100-120 cm/s in MCA) warrant investigation for vasospasm, severe stenosis, or compensatory hyperperfusion.

The main contraindication is the presence of open wounds or dressings over the required probe placement sites. Operator skill and experience are crucial for accurate acquisition and interpretation. Factors like patient cooperation, skull thickness (for TCD), and presence of anatomical variations can affect the examination.

References

1. Grant EG, Benson CB, Moneta GL, et al. Carotid artery stenosis: gray-scale and Doppler US diagnosis--Society of Radiologists in Ultrasound Consensus Conference. Radiology. 2003 Nov;229(2):340-6. DOI: 10.1148/radiol.2292030516. PMID: 14500858. (Consensus guidelines for carotid ultrasound)
2. Alexandrov AV, Sloan MA, Tegeler CH, et al. Organization and performance of transcranial Doppler laboratory. Report of the Practice Standards Subcommittee of the American Academy of Neurology. Neurology. 2004 Jan 13;62(1):13-7. DOI: 10.1212/01.wnl.0000103007.26659.19. PMID: 14718690. (TCD practice standards)
3. Baumgartner RW, Mathis J, Sturzenegger M, Mattle HP. Transcranial color-coded duplex sonography in the evaluation of collateral flow patterns. Stroke. 1997 Jan;28(1):104-9. DOI: 10.1161/01.str.28.1.104. PMID: 8996500. (Application of color TCD)
4. Naqvi TZ, Lee MS. Carotid ultrasonography. JACC Cardiovasc Imaging. 2014 Jun;7(6):630-40. DOI: 10.1016/j.jcmg.2013.10.021. PMID: 24925308. (Review focusing on cardiovascular aspects)
5. Purkayastha S, Sorond F. Transcranial Doppler ultrasound: technique and application. Semin Neurol. 2012 Nov;32(5):411-20. DOI: 10.1055/s-0033-1334476. Epub 2013 Feb 19. PMID: 23440664. (Review of TCD technique and applications)