NEUROIMAGING / NEUROSONOLOGY

Transcranial Doppler ultrasound

Created 24/10/2023, last revision 11/11/2023

Trancranial Doppler ultrasound (TCD/TCCD) is a non-invasive imaging technique helpful in evaluating flow in intracranial vessels  → general indications for the examination

Transcranial examination procedure

Methods

  • the probe utilizes ultrasound with a frequency of 2-2.5 MHz
    •  in pulse-wave (PW) mode, the device registers the frequency shift (∆f) created by the reflection of ultrasound from moving blood particles and converts it into a graphic record and audible sound using FFT transformation
  • transcranial Doppler sonography (TCD) allows measurement of flow (by obtaining the Doppler waveform) in intracranial arteries
    • velocities are calculated from the frequency shift of the reflected ultrasound beam,
    • TCD displays no anatomical structures; the examined vessel is determined by the insonation depth and probe position
      • correction of the insonation angle is not possible (insonation angle is considered to be 0 grade)
  • transcranial color-coded Doppler (TCCD)  
    • TCCD provides enhanced diagnostic capabilities compared to standard TCD
    • in addition to Doppler waveform, TCCD integrates B-mode imaging to visualize anatomical structures, as well as color, energy, or B-flow modes for vessel identification
    • sample volume can be thus reliably positioned within the desired arterial segment
    • angle-corrected flow velocities can be obtained

      • these velocities are up to 30% higher than uncorrected velocities obtained on TCD
  • limitation: an impenetrable temporal bone window (affecting ~ 5-20% of patients)
    • a large proportion of these patients can be investigated using contrast agents  (ECHOVIST, SONOVUE)
TCD showing MCA vasospasm
Transcranial color-coded Doppler (TCCD) with normal waveform in the MCA

Transcranial windows

TCD/TCCD is typically examined using a high-frequency transducer placed over specific cranial windows (regions where ultrasound imaging can be performed with minimal interference from bone):

  • temporal   Temporal window (horizontal plane)
    • horizontal plane: terminal ICA (TICA),  segment M1-3, A1-2, P1-2, AComA, PComA
      • anterior temporal artery (ATA) can be detected in P2 segment
    • anterior coronal plane:  better visualization of the terminal ICA   Anterior and posterior coronal plane
    • posterior coronal plane:  distal BA, PCA, and possibly SCA
  • transforaminal (posterior circulation) – V4, BA, potentially PICA, AICA, or SCA)  Transforaminal window
  • transorbital (carotid siphon and ophthalmic artery) – reduce power to 10-15%   Transorbital TCCD examination of the ophthalmic artery  Transorbital TCCD examination of the ophthalmic artery
  • complementary approaches:
    • submandibular (distal extracranial ICA segment) Submandibular approach to examine the distal segment of the ICA
    • frontal (A2)
    • occipital (dural sinuses, BA+VA)
Window quality assessment
  • in 5–20% of cases, cranial hyperostosis is the cause of poor insonation window, particularly in the temporal region
    • incidence can vary among populations; factors such as age and ethnicity can influence this rate
    • even in the absence of a color flow signal, a satisfactory Doppler waveform may sometimes be acquired   Bad temporal window, Doppler waveform can be obtained
    • consider the use of echo contrast agents to enhance image quality
  • the quality of the bone window is crucial for the sensitivity and specificity of transcranial sonography
The quality of the temporal bone window can be assessed in five degrees:
absent
  • no intracranial structures are visible in B-mode (even the brainstem is indistinguishable)
very bad
  • only the brainstem area can be evaluated in B-mode
  • no intracranial vessels can be visualized, and no flow waveform can be obtained
bad
  • at least some intracranial structures, in addition to the brainstem, can be imaged in B-mode
  • at least one main artery of the circle of Willis (usually MCA or PCA) can be visualized, and a sufficiently good-quality flow curve can be obtained from it
average
  • all intracranial structures are clearly visible in B-mode; areas of poor echogenicity may be present
  • most (at least 2) arteries of the circle of Willis can be imaged, and good-quality flow curves can be obtained from them
perfect
  • all intracranial structures are clearly visible in B-mode
  • all arteries of the circle of Willis can be visualized, and high-quality flow curves can be obtained

Examination

Correct evaluation of the Doppler waveform in the examined arteries is crucial;  it is essential to take into account the findings in the extracranial arteries and to consider additional factors (hypotension, significant changes in pCO2, arrhythmias, marked anemia, valvular defects, etc.)

  • assess the flow pattern (laminar x turbulent) Laminar and turbulent flow
  • evaluate Peak Systolic Velocity (PSV), End Diastolic Velocity (EDV), Resistance Index (RI), and Pulsatility Index (PI), as well as systolic acceleration (acceleration time)
    • compare flow on both sides (unless there is pathology there, too) and between individual arteries
  • normal findings
    • interhemispheric difference < 20-30%
    • PSV: MCA > ACA, ACM > PCA, BA > VA
    • cut-offs for normal PSV and EDV (see in the table)
    • RI < 0.8 and PI < 1.0
  • identify possible steal syndrome
    • subclavian-vertebral steal  Steal syndrome in the vertebral artery (retrogradef low) and basilar artery (bidirectional flow)
    • flow diversion due to arterial occlusion (such as increased ACA flow in case of MCA occlusion; PSV ACA>MCA)   Flow diversion due to the MCA occlusion
    • steal syndrome due to AVM or DAVF
  • search for embolic signals (HITS)  Detection of embolic signals (HITS) on TCCD (spontaneous and provoked during TCD/TCCD bubble test)
  • asssess cerebral vasomotor reactivity (CVR) in selected cases
  • note other waveform alterations
    • arrhythmias (e.g., atrial fibrillation)   Atrial fibrillation on TCCD
    • altered flow due to valvular defects or impaired ejection fraction

Normal finding

  • continuous, laminar flow, no aliasing
  • normal systolic acceleration (AT)
  • normal PSV/EDV and resistance indices (see table below)
    • PSV/EDV ± < 2SD
    • PI/RI ± < 1SD
  • no HITS, no turbulence or low-frequency murmurs
  • normal CVR
  • no arrhythmias
Average velocities in cm/s (with angle correction) +- 1 standard deviation (SD)
Artery
flow velocity and resistance index
 age
20 – 40 years
 40 – 60 years > 60 years
MCA
PSV (cm/s)
 91 -126 84 – 120 78 – 109
EDV (cm/s) 37 – 70 35 – 61 28 – 48
RI 0,48-0,59 0,47-0,58 0,5-0,65
ACA PSV (cm/s) 67-102 62-96 49-91
EDV (cm/s) 31-50 30-46 25-39
RI 0,5-0,59 0,48-0,57 0,51-0,6
PCA PSV (cm/s) 50-78 45-73 41-73
EDV (cm/s)
21-37
 21-35 17-31
RI 0,48-0,56 0,48-0,56 0,49-0,66
VA PSV (cm/s) 46-76 38-74 38-62
EDV (cm/s) 23-43
18-40
 14-29
RI
 0,44-0,57 0,44-0,57 0,48-0,63
BA PSV (cm/s) 48-80 49-82 37-65
EDV (cm/s) 26-43 19-40 13-25
RI
 0,46-0,56 0,46-0,56 0,5-0,62

Czech neurosonologic standard ČNS JEP

Assessment of arterial stenosis

  • a multiparametric evaluation is necessary; an isolated PSV obtained from the stenotic segment is not sufficient
    • obtain PSV, PI, and RI in the stenosis and in the pre-and poststenotic segments (if possible) Multiparametric evaluation of the stenosis - obtain PSV, PI and RI in the stenosis and in the pre- and poststenotic segments
    • assess PSV ratio and collateral circulation
  • prestenotic segment: a resistant (↑PI and RI) and decreased flow
  • stenotic segment:
    • turbulent flow with increased PSV Stenosis of P1 segment (PSV ratio 3)
      • PVS should be > 30% higher than contralateral (unless stenosis is bilateral)
      • PSV ratio (PSV in stenosis / pre- or poststenotic segment) (see table)  Stenosis in M1 segment of the MCA (PSV ratio 2-3)
    • aliasing phenomenon (manifestation of turbulences in color flow mode) Aliasing
  • poststenotic segment: decreased PSV, systolic acceleration, PI, and RI
PSV ratio
(stenosis / pre- or poststenotic segment)
 Stenosis (%)
mild stenosis 1.3-2 < 50%
moderate stenosis 2-3 50-70%
severe stenosis > 3 > 70%

STENOSIS IN M2 SEGMENT OF THE MCA


Stenotic turbulent flow at the origin of M2 segment


Flow in proximal M1 segment


Flow in distal M2 segment

STENOSIS IN THE M1 SEGMENT OF THE MIDDLE CEREBRAL ARTERY


MCA stenosis in color flow mode (CFM) characterized by aliasing


Poststenotic flow in M2 segment


Turbulent flows at the stenosis segment with PSV 185/76 cm/s. The PSV ratio (stenotic/prestenotic segment) is 3, corresponding to a stenosis near 70%


Flow in the proximal MCA (prestenotic segment)

M1 stenosis

Assessment of arterial occlusion

  • transcranial Doppler (TCD or TCCD) is an ideal noninvasive, real-time bedside tool for the evaluation of cerebral vessels, particularly in the setting of an acute stroke

    • strong correlation exists between TCCD and CTA
    • an abnormal TCD/TCCD study, defined as either no or asymmetrical MCA flow, is associated with poor outcome
  • direct signs of occlusion:
    • absence of the artery in color mode with good visualization of other arteries (or veins)
    • absent or altered flow (TIBI/COGIF criteria)  M1 occlusion on TCCD
  • indirect signs:
    • decreased PSV and increased PI and RI proximal to the occlusion
    • with M1 or multiple M2 branches occlusion, flow diversion can be detected ( PSVACA > PSVMCA)  ACA/MCA flow diversion
  • assessment of basilar artery occlusion (BAO), especially of its distal segment, may be challenging
    • due to numerous branches, the findings in the proximal segments may appear practically normal
    • the patency of the distal BA can be verified by inducing undulations in the PCA by tapping the extracranial VA (usually behind the ear)  Undulations detected in PCA during tapping of the extracranial vertebral artery demonstrate patency of the basilar artery.

TIBI (Thrombolysis in Brain Ischemia)

  • the TIBI classification was developed to grade residual flow
  • it correlates with initial stroke severity, clinical recovery, and mortality in acute stroke patients
    • no improvement in the residual flow correlates with the absence of early clinical recovery and increased mortality  (Demchuk, 2012)
TIBI 0
TIBI 0 – absent flow
– lack of regular pulsatile flow signals despite varying degrees of background noise
TIBI 1
TIBI 1 – minimal flow
– systolic spikes of variable velocity and duration
– EDV = 0
– possible reverberating flow
TIBI 2
TIBI 2 – blunted flow
– flattened systolic flow acceleration of variable duration compared to control side
– PI < 1,2
– positive EDV
TIBI 3
TIBI 3 – dampened flow
– normal systolic flow acceleration
– positive EDV
– decreased flow (MFV) by > 30% compared to the control (healthy) side
TIBI 4
TIBI 4 – stenotic flow
– MFV > 80cm/s + velocity difference (MFV) of > 30% compared to the control side (increased velocity)
– turbulence  + velocity difference (MFV) of > 30% compared to the control side (increased velocity)
TIBI 5 TIBI 5 – normal flow
– symmetrical flow or < 30% MFV difference compared to the control side

According to  [Demchuk, 2001]

 Recanalization assessment  [Clotbust, 2007]
Complete recanalization   TIBI 4-5
Parcial recanalization  increase of TIBI by ≥1 grade (but not to 4 or 5)
Reocclusion  decrease of TIBI by ≤1 grade
Post-IVT full recanalizatiin (TIBI 1 to TIBI 5)

COGIF

Differentiation of TIBI grades 1-3 (minimal flow, blunted flow, dampened flow) may be difficult. The flow patterns of these grades not only reflect partial M1 recanalization but also the hemodynamic situation in other segments. The COGIF scoring system was designed to more clearly follow the recanalization process; it is exclusively based on known hemodynamic changes of the Doppler spectrum. The time course of grades during the serial examination must be encoded.

COGIF GRADE (Consensus On Grading Intracranial Flow obstruction)
COGIF 1
COGIF 1
– no flow
  COGIF 2  COGIF 2
COGIF 2
– low flow
– EDV = 0
COGIF 3
COGIF 3
– low flow
– EDV > 0
COGIF 4b COGIF 4a
COGIF 4
4a – normal (symmetrical flow)
4b – focal flow acceleration (probable stenosis)
4c – high segmental flow (hypercirculation)
Recanalization assessment 
 Complete recanalization
   COGIF 4
 Partial recanalization
 improvement by ≥1 grade (but not to 4)
No change
 baseline COGIF grade persists
Worsening  decrease by ≥1 grade

[Nedelmann, 2009]

Transcranial findings in extracranial stenosis/occlusion

  • ↓PSV and EDV
  • ↓ PI, RI
  • decreased systolic acceleration   Imparied flow in the ipsilateral MCA due to carotid occlusion
  • development of collateral circulation
    • retrograde flow in the ophthalmic artery (OA)
    • retrograde flow in the ipsilateral ACA
    • well-marked AComA and/or PComP with accelerated and turbulent flow Collateral flow via PComA in a patient with the ICA occlusion
  • impaired cerebral vasomotor reactivity (CVR)
Carotid occlusion with altered flow in the ipislateral MCA

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Transcranial Doppler ultrasound
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