NEUROIMAGING
Neurosonology
Created 24/11/2021, last revision 29/04/2023
Introduction
- the combination of both methods allows a comprehensive non-invasive diagnosis of cerebrovascular disease:
- detection of occlusions, stenoses, and vasospasms
- assessment of collateral circulation and vasomotor reactivity (VMR)
- monitoring of recanalization (→ TIBI)
- 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 considered experimental methods)
- detection of occlusions, stenoses, and vasospasms
- the examination is non-invasive, cheap, without side effects
Indications for neurosonology
- patients with stroke symptoms in the carotid or vertebrobasilar (VB) 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 a 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 right-to-left shunt (RLS) in suspected paradoxical embolism (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
- 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 mimick 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.)
- aliasing (visible in color Doppler, not power Doppler!) appears as red to blue hues (may mimick 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
- 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
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Pulse wave Doppler
- spectral Doppler permits the graphical display of velocities over time
- pulsed Doppler enables recording of Doppler shifts in a 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” the blood flow during the exam
- higher velocities = high-pitched sounds, lower velocities= low-pitched sounds
- during the examination, assess:
- 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
Examined vessels
Usual arteries to be examined:
- extracranial:
- 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)