Introduction

  • modern imaging method, which comprises
    • duplex/triplex examination of the extracranial arteries  Nerosonology - extracranial utrasound modalities
    • transcranial color-coded duplex sonography (TCCD)   Transcranial color-coded duplex sonography (TCCD)   Triplex mode (B-mode + CDI + spectral doppler) depicting stenosis of P1 portion of PCA
  • the combination of both methods allows extensive non-invasive diagnosis of cerebrovascular diseases:
    • detection of occlusions, stenoses, and vasospasms
    • assessment of collateral circulation and vasomotor reactivity (VMR)
    • monitoring of recanalization
    • monitoring of embolization (detection of PL shunt, monitoring during CAS or CEA)
    • diagnosis of cerebral circulatory arrest (brain death) → see here
    • therapeutic use – sonothrombolysis, sonolysis (currently experimental methods)
  • the examination is non-invasive, cheap, with no side effects

Indications for neurosonology

  • examination of patients with stroke symptoms in the carotid or vertebrobasilar (VB) territory, including amaurosis fugax
  • monitoring of recanalization therapy
  • examination of patients with chronic cerebrovascular disease, especially those with previously documented extra- and/or intracranial artery disease, assessment of collateral circulation and vasomotor reactivity
  • examination of patients with a focal neurological deficit of unclear origin in which a possible vascular etiology is considered
  • examination of persons with trauma in the cervical region or with local pathological findings in the neck, which may relate to the vascular system
  • examination of patients with pathological physical examination (cervical murmurs, asymmetry of arterial blood pressure on upper extremities, etc.)
  • follow-up of patients after CEA/CAS
  • examination before major surgical procedures (especially cardiac and vascular surgery in extracorporeal circulation)
  • examination of patients with cardiovascular disease and multiple vascular risk factors
  • examination of patients with vascular malformations, especially AV shunts (detection of supplying vessels, monitoring after therapeutic intervention) → DAVF
  • examination in the DDx of headaches, especially migraine
  • detection of the right-left shunt (RLS) in suspected episodes of paradoxical embolization (PFO, pulmonary AV shunts)

B-mode

  • B-mode is a two-dimensional ultrasound image display composed of bright dots representing the ultrasound echoes
  • time (echo distance) and amplitude (echogenicity) of the returned echo signal are processed   A and B-mode
  • B-mode allows the visualization and assessment of anatomical structures in the insonated area
  • the vascular wall is shown as a double line with a hypoechoic center
  • an important parameter is the ultrasound resolution
    • lateral resolution is worse than axial resolution and depends on the size and shape of the probe
    • lateral resolution deteriorates with increasing distance from the probe (unlike axial resolution)

Color Doppler Imaging (CDI)  / Color Flow Mode (CFM)

  • CDI/CFM allows the visualization of flow direction and velocity within the color box, defined by the operator
  • the Doppler shifts of returning ultrasound waves within the color box are color-coded based on average velocity and direction  Kinking in the Color Flow Mode (CFM)  Carotid stenosis on CDI
    • flow is usually depicted as red when traveling toward the probe and blue when traveling away from the probe (specific user settings)
    • shades of each color denote different velocities (lighter shade = higher velocity)
  • the color is superimposed over the B-mode image and allows easier identification of the course of the vessels and examination of the flow channel
  • under physiological circumstances, the aliasing does not occur in arteries (unless the Nyquist limit is reached)
    • aliasing (visible in colored Doppler, not power Doppler!) is encountered as red to blue hues ( appears as flow in the opposite direction), not separated by a black region of no flow. The artifact disappears if the velocity scale is increased above the peak flow velocity   Ultrasound artifacts - aliasing
    • aliasing helps detect areas of increased flow (e.g., stenosis, arteriovenous fistula, etc.)
  • advantages:
    • displays flow velocity for both small and large vessels at different depths
    • helps show flow information at deeper depths
    • improves separation of nearby arteries and veins

Power Doppler Imaging (PDI)

  • PDI encodes the energy (power) of moving particles  Atherosclerotic plaques in power doppler (PDM)
  • at the optimal gain setting, the lumen of the artery is homogeneously filled with color
  • independent of insonation angle, no signal aliasing, more sensitive to slow flow
  • no directional information, lower frame rate, artifacts from surrounding slow-moving tissue
    • in new machines, directional information is shown on Directional PDI Maps

B-flow mode

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Pulse wave Doppler

  • spectral Doppler permits the graphic display of velocities over time
    • pulsed Doppler enables the recording of Doppler shifts in the user-defined area (the sample volume/gate) within the B-mode image
    • curves provide information about absolute velocities and the direction of blood flow
  • Doppler signals are also converted to audio signals, which enable the investigator to “hear” blood flow during the investigation
    • higher velocities = high-pitched sounds, lower velocities= low-pitched sounds
  • during the examination, assess:
    • peak systolic velocity (PSV)
    • end-diastolic velocity (EDV)
    • resistance index (RI) and pulsatility index (PI)   Resistance and pulsatility index
      • RI = (PSV-EDV) / PSV
      • PI = (PSV-EDV) / Vmean
      • Vmean = 1/3 (PSV-EDV) + EDV – automatically calculated by the device as a time-weighted average of systolic and diastolic flow velocities
  • under physiological conditions, the flow is laminar, continuous, without turbulences and low-frequency murmurs
  • measure the flow in multiple segments (see below)
  • angle correction is necessary – the angle between the blood flow direction in the artery and the ultrasound beam should be ≤ 60 degrees  The angle between the direction of blood flow in the artery and the ultrasound beam should be ≤ 60 degrees
Doppler waveform
Pulse wave doppler

Examined vessels

Usual arteries to be examined:

  • extracranial:
    • common carotid artery (CCA)
    • internal carotid artery (ICA)
    • external carotid artery (ECA)
      • superficial temporal artery (one of two terminal branches of the external carotid artery) in suspected temporal arteritis   Temporal arteritis - enlarged walls in superficial temporal artery (STA)  Temporal arteritis 
    • vertebral artery (VA)
    • subclavian artery (SA)
  •  TCCD:
    • terminal ICA (carotid siphon)
    • middle cerebral artery (MCA)
    • anterior cerebral artery (ACA)
    • posterior cerebral artery (PCA)
    • vertebral artery (VA)
    • basilar artery (BA)
    • ophthalmic artery (OA)
    • anterior communicating artery (ACom/ACoA)
    • posterior communicating artery (PCom/PCoA)
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