Vascular Sonography Methods: Ultrasound Evaluation of the Intracranial Cerebrovascular System

Vascular Sonography Methods Unit 7: Ultrasound Evaluation of the Intracranial Cerebrovascular System

Required Readings

  • Kupinski, A. M. (2023). The Vascular System 3rd edition. Philadelphia, PA: Wolters Kluwer.
       - Chapter 10: Intracranial Cerebrovascular Examinations

  • Size, G., Lozanski, L., Russo, T. (2024). Inside Ultrasound Vascular Reference Guide, 2nd ed. Pearce, AZ. Inside Ultrasound, Inc.
       - Anatomy: pp. 1-9 and 116-117
       - Cerebrovascular Events: pp. 81-84
       - Additional, but not required: Intracranial Cerebrovascular Testing: pp. 116-131 (Size, G.)

Additional Resources

  • Supportive content is available in the Canvas Course.

  • Vocabulary can be found in a separate vocabulary document.

Anatomy Overview

  • Circle of Willis (refer to Kupinski, Size, and handouts for illustration and detailed anatomy).

Medical History/Risk Factors/Physical Considerations

  • Refer to Kupinski Table 10-1 for transcranial Doppler (TCD) considerations.

TCD Equipment

  • Utilizes a dedicated non-imaging machine, employing Power M-mode and Pulsed Wave (PW) Doppler.

Intracranial Vessel Identification

  • Identification based on five primary criteria:
      1. Approach (cranial window) - zero angle of insonation is assumed.
      2. Sample volume depth - critical for accurate measurement.
      3. Direction of blood flow - relative to the ultrasound transducer.
      4. Spatial relationship of one artery to another.
      5. Flow velocity - categorized as MCA > ACA > PCA = BA = VA.

Vessel Identification Criteria

  • Refer to additional documents provided in Canvas. Students are advised to create flashcards for memorization.

Anatomical Approaches to Access Cerebral Vasculature

  • Common Approaches:
      - Transtemporal
      - Transorbital
      - Suboccipital (also known as foramen magnum or transforaminal)
      - Submandibular

  • Atlas Loop - A fifth window sometimes used (Right and Left of midline) often grouped with suboccipital in many vascular labs.

TCD Diagnosis

  • Primary diagnostic tools:
      - Spectral waveforms
        - Velocity (Time-Averaged Maximum Mean Velocity - TAMMV)
        - Changes in waveform characteristics
        - Direction of flow

Intracranial Stenosis and Occlusion

  • Atherosclerosis is the leading cause of intracranial stenosis.

  • Most common intracranial sites for stenosis include:
      - MCA (Middle Cerebral Artery)
      - Carotid siphon
      - Terminal ICA bifurcation (TICA)

  • Consequences of stenosis may include:
      - Formation of microemboli
      - Significant stenosis or occlusion.

Diagnosis Methodology

  • Stenosis diagnosis relies on identifying:
      - Focal velocity increases
      - Velocity differences between sides
      - Downstream hemodynamic effects, assessing for collateral pathways.

Collateral Pathways

  • In cases of significant intracranial or extracranial stenosis, the brain compensates by increasing collateral flow through autoregulation.

  • Key Collateral Pathways:
      - Circle of Willis: Most prevalent collateral pathway.
        - Anterior communicating artery (ACoA) commonly connects the two hemispheres.
        - Posterior communicating artery (PCoA) facilitates anterior and posterior circulation integration, especially with bilateral ICA disease.
      - ICA/ECA network: The second most frequent collateral pathway. Arteries communicate to ensure alternative flow routes.
        - ECA to ICA connections characterized by reverse flow in the ophthalmic artery observed via the transorbital window.

Examples of ECA Collaterals

  • Important collaterals that may be relevant to board examinations include:
      - Superficial temporal artery (ECA branch) connects to the supraorbital artery (ICA branch).
      - Middle meningeal artery (ECA) connects to the ophthalmic artery (ICA).
      - Facial or angular artery (ECA) connects to nasal artery (ICA).

Monitoring Techniques in TCD:

For Vasospasm Detection

  • TCD can detect elevated blood velocities suggesting potential vasospasm, particularly vital after subarachnoid hemorrhage (SAH).

  • Vasospasm onset typically occurs 5-10 days post-initial event; patients are monitored in Neuro ICU during this period.

Management Strategies

  • Depend on enhancing cerebral blood flow (CBF) through methods including:
      - Induced hypertension and hypervolemia.
      - Hemodilution to reduce blood viscosity.
      - Specialized interventions such as balloon angioplasty and focal application of verapamil (vasodilator).

  • Loss of autoregulation can lead to dangerously increased intracranial pressures (ICP).

TCD Monitoring Techniques

  • TCD for Emboli Monitoring and Bubble Studies:
      - Microemboli detection is achieved by observing bilateral MCA flow with secure transducers.
      - Microbubbles, especially in agitated saline, exhibit a distinct Doppler signature called HITS (high-intensity transient signals) or MES (microemboli signals).

Perioperative Monitoring

  • TCD serves as a monitoring tool during surgeries to:
      - Detect microemboli.
      - Ensure optimal brain perfusion by evaluating collateral routes and flow volumes.
      - Adjust surgical techniques based on real-time emboli monitoring to mitigate the risk of stroke.

Identifying Emboli in Transit

  • TCD can detect emboli indicative of cardiac origin or originating from other vessels. Continuous patient monitoring for at least 30 minutes post-intervention is recommended.

Assessing Cerebral Circulatory Arrest

  • TCD can be integral in diagnosing cerebral circulatory arrest, especially in conjunction with EEG and angiography.

  • Per American Academy of Neurology guidelines, confirmation requires two TCD studies alongside other testing modalities (e.g., EEG).

Waveform Progression in TCD Measurement

  • As cerebral edema develops, TCD measurements indicate increasing Pulsatility Index (PI):
      - Early stage PI > 1.2 along with clinical presentation suggests high resistance.
      - No diastolic flow, with only systolic flow remaining indicates no brain perfusion.

Monitoring During Thrombolytic Treatment

  • TCD is utilized during acute cerebrovascular accidents (CVAs) to assess the effectiveness of thrombolytic agents on recanalization, employing the TIBI score system.

TCD for Vasomotor Reactivity Assessment

  • TCD enables evaluation of vasomotor reactivity (VMR), assessing the brain’s capacity to modify flow in response to CO2 levels.

  • It evaluates maximum vasodilation among high-risk stroke populations through both VMR and CO2 testing methodologies.

Pitfalls in TCD Application

  • Accuracy of TCD is highly dependent on operator skill, necessitating substantial training.

  • Key limitations include the angle of insonation influencing velocity calculations and issues with patient cooperation.

TCD Imaging (TCDi)

Equipment and Approaches

  • Utilizes duplex ultrasound with a phased array sector transducer, employing the same accesses as TCD.

  • For vessel identification with TCDi, refer to directional landmarks such as sphenoid wing and anterior clinoid process.

Advantages and Pitfalls of TCDi

  • Advantages: Enhanced accuracy in identifying vessels and potentially less extensive training requirements.

  • Pitfalls: Larger transducer profile and limited utility for procedural monitoring, requiring patient cooperation to achieve successful results.

Diagnostic Criteria for TCD/TCDi

Parameters for Assessment

  • Velocity: Time-Averaged Mean Maximum Velocity (TAMMV). Note that this definition differs from that used in hemodialysis contexts.

  • Critical diagnostic ratios include:
      - Gosling’s Pulsatility Index (PI): calculated as extPI=racextPSV−extEDVextTAMMVext{PI} = rac{ ext{PSV} - ext{EDV}}{ ext{TAMMV}}, with values interpreted as follows:
        - High resistance: PI > 1.2
        - Normal resistance: PI < 0.6     - Focal increase in PI indicative of stenosis or narrowing.   - Lindegaard Ratio (LR): extLR=racextMCATAMMVextsubmandibularICATAMMVext{LR} = rac{ ext{MCA TAMMV}}{ ext{submandibular ICA TAMMV}} used for assessing anterior circulation vasospasm     - Elevated velocities indicate hyperemia with LR < 3.0; vasospasm if > 3.0; severe vasospasm if > 6.0.
      - Sviri Ratio (SR): for assessing posterior circulation vasospasm measured as: extSR=racextBATAMMVextaverageofbilateralextracranialvertebralarteryTAMMVext{SR} = rac{ ext{BA TAMMV}}{ ext{average of bilateral extracranial vertebral artery TAMMV}}, with a cutoff of >3.0 indicating severe vasospasm.

Special Considerations in Vascular Sonography

  • Key factors influencing cerebral blood flow and velocities include:
      - Age: Flow velocities decrease with age.
      - CO2 Levels: Increased CO2 leads to vessel dilation, enhancing CBF.
      - Cardiac Output: CBF is autoregulated in response to cardiac output until a threshold is surpassed.
      - Fever: Elevated body temperatures boost CBF.
      - Hematocrit Levels: Severe anemia increases CBF, which is relevant when interpreting TCD velocities.

Neuro ICU Protocols

  • Awareness is critical concerning conditions influencing ICP; nursing protocols must be respected such as not altering bed levels without clearance when EVDs are present.

  • Surgical interventions that may be present include burr holes and craniectomies, along with various peripheral and central line setups.

Sickle Cell Disease (SCD) and TCD

  • TCD is vital for evaluating children and adults with SCD, emphasizing crucial diagnostic criteria for pediatric patients.

  • SCD specific criteria indicate that median velocity for MCA and/or distal ICA should be > 200 cm/s to warrant diagnosis for increased stroke risk.

Conclusion

The management of cerebrovascular events is complex and requires an in-depth understanding of both anatomy and the technical aspects of TCD. Sonographers must remain current in protocols and diagnostic methodologies to effectively assess and manage patient outcomes.