NEUROIMAGING

Neurosonology

Created 24/11/2021, last revision 06/11/2023

Definition and capabilities of neurosonology

neurosonology refers to the use of ultrasonography to evaluate the cerebral circulation and nervous system (such as peripheral nerves and brain structures)
it enables noninvasive assessment of blood flow in intracranial and extracranial vessels and is an essential part of the diagnosis and monitoring of cerebrovascular diseases
it includes:
- duplex/triplex examination of the extracranial arteries
- transcranial sonography (TCD/TCCD)

the combination of both methods allows a comprehensive, noninvasive diagnosis of cerebrovascular disease:
- detection of occlusions, stenoses, and vasospasms
  - including the evaluation of the impact of extracranial stenoses/occlusions on intracranial circulation (reduction of velocities and indices)
- assessment of collateral circulation
- assessment of cerebral vasomotor reactivity (CVR or CVMR)
- detection of AV malformations
  - assessment of the feeding arteries (“feeders” – characterized by increased flow velocities and decreased pulsatility index and resistance index
- monitoring of recanalization in acute stroke (→ TIBI)
- periprocedural monitoring during CAS or CEA
  - periprocedural detection of embolizations (HITS)
  - indication for shunt placement during CEA when Vmean drops to < 30 cm/s or 50% of baseline values after artery occlusion
- detection of R-L shunts (bubble test) and spontaneous microembolizations
- diagnosis of cerebral circulatory arrest (brain death) → see here
- therapeutic applications – sonothrombolysis, sonolysis (currently considered experimental methods)
- diagnosis of intracranial hypertension

Indications for neurosonology

patients with stroke symptoms in the carotid or vertebrobasilar (posterior) territory, including amaurosis fugax
monitoring of recanalization therapy (→ TIBI)
patients with chronic cerebrovascular disease, especially those with previously documented extra- and/or intracranial artery disease, assessment of collateral circulation and vasomotor reactivity
patients with a focal neurological deficit of unknown origin in whom a possible vascular etiology is considered
patients with trauma to the cervical spine or with local pathological findings in the neck that may be related to the vascular system
patients with a pathological physical examination (cervical murmurs, blood pressure asymmetry on upper extremities, etc.)
follow-up of patients after CEA/CAS
patients scheduled for major surgery, especially cardiac and vascular surgery in extracorporeal circulation (ECC)
patients with cardiovascular disease and multiple vascular risk factors
patients with vascular malformations, especially AV shunts (detection of feeding vessels, post-procedure monitoring) → DAVF
DDx of headache, especially migraine
detection of the right-to-left shunt (RLS) in suspected paradoxical embolism (PFO, pulmonary AV shunts)

Ultrasound modalities

→ physical principles of ultrasound

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
B-mode allows the visualization and assessment of anatomical structures in the insonated area
the vascular wall is displayed 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 operator-defined color box
the Doppler shifts of the returning ultrasound waves within the color box are color-coded based on average velocity and direction
- flow is typically depicted as red when moving toward the probe and blue when moving 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 vessel course and examination of the flow channel
under physiological conditions, aliasing does not occur in arteries (unless the Nyquist limit is reached)
- aliasing (visible in color Doppler, not power Doppler!) appears as red to blue hues (may mimic 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
- aliasing helps to detect areas of increased flow (e.g., stenosis, AV fistula, etc.)
advantages:
- shows flow velocity for both small and large vessels at different depths
- helps show flow information at greater depths
- improves separation of adjacent arteries and veins

Power Doppler Imaging (PDI)

PDI encodes the energy (power) of moving particles
at 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 newer machines, directional information is shown on Directional PDI Maps

B-flow mode

B-flow is a unique flow mode (on GE systems) that directly images blood reflectors and tissue simultaneously, providing an accurate morphologic representation of intraluminal blood flow throughout the entire field of view
compared to other flow modes, B-Flow has the following advantages:
- displays the true vessel diameter
- has a high spatial resolution to show fine vessel details (including plaque morphology) and flow in larger vessels
- is angle independent
B-flow may be helpful to
- assess high-grade stenosis
- visualize flow around a soft (hypoechoic) atherosclerotic plaque

Pulse wave Doppler

spectral Doppler permits the graphical display of velocities over time
- pulsed Doppler enables the recording of Doppler shifts in a user-defined area (sample volume/gate) within the B-mode image
- curves provide information about absolute velocities and the direction of flow
Doppler signals are also converted to audio signals, which enable the investigator to “hear” the blood flow during the exam
- higher velocities = high-pitched sounds, lower velocities= low-pitched sounds

during the examination, assess the following:
- peak-systolic velocity (PSV)
- end-diastolic velocity (EDV)
- Resistance Index (RI) and Pulsatility Index (PI)
  - RI = (PSV-EDV) / PSV
  - PI = (PSV-EDV) / V_mean
  - 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 turbulence and low-frequency murmurs
measure flow in multiple segments (see below)
angle correction is necessary – the angle between the direction of blood flow in the artery and the ultrasound beam should be ≤ 60 degrees (usually 50-60°)

Doppler waveform

several factors influence the character of the flow:
- heart ejection fraction (EF)
- flow volume and pressure
- resistance and geometry of the vascular system
- vessel wall elasticity
- blood viscosity
the Doppler waveform refers to the morphology of pulsatile blood flow on spectral Doppler ultrasound
- monophasic – forward systolic flow that continues into diastole, lacking reverse diastolic flow (e.g., in the ICA, MCA, etc.)
- triphasic (typically in the subclavian artery)
  - systolic forward flow
  - early diastolic reverse flow
  - late diastolic forward flow (slower than in systole)
typical items assessed within the Doppler waveform:
- acceleration time (systolic acceleration)
  - time required for blood flow to reach peak velocity from the onset of systole (measured in milliseconds)
  - a prolonged acceleration time may indicate proximal arterial stenosis/occlusion, stenosis of the aortic valve, or impaired cardiac contractile force
- peak systolic velocity (PSV) – the highest flow velocity during systole
- dicrotic notch
  - a small, downward deflection observed after the peak systolic velocity. This notch corresponds to aortic valve closure and the beginning of diastole in the cardiac cycle
  - a prominent dicrotic notch may be indicative of increased peripheral resistance, commonly observed in conditions such as hypertension
  - conversely, a blunted or absent dicrotic notch may indicate low systemic vascular resistance and may be seen in conditions like sepsis
- end-diastolic velocity (EDV)
  - EDV is the velocity at the end of diastole; it is influenced by peripheral resistance (including distal stenosis)
- mean velocity (V_mean) is calculated
- resistance index and pulsatility index can be calculated from the above-stated values
  - resistance index (RI) is calculated as (PSV – EDV) / PSV
  - pulsatility index (PI) is calculated as (PSV – EDV) / mean velocity
vessels are classified according to the peripheral resistance
- high-resistance limb type (typically the ECA)
- low-resistance parenchymal type (typically the ICA, VA)
- the CCA waveform has an intermediate profile under physiological conditions because it depends on peripheral resistance in both intracranial (brain) and extracranial regions (skin, muscles)

Normal Doppler waveform in extracranial vessels

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
- vertebral artery (VA)
- subclavian artery (SA)
TCCD:
- terminal ICA (TICA)
- 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/AComA)
- posterior communicating artery (PCom/PComA)