NEUROIMAGING

Neurosonology

Created 24/11/2021, last revision 29/04/2023

Introduction

modern imaging method, which comprises
- duplex/triplex examination of the extracranial arteries
- transcranial color-coded duplex sonography (TCCD)

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 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)

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.)
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

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

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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:
- 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

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)