Sometimes, a high-velocity jet with bruit can be found at midline depth, highly suggestive of flow interception through ACoA. In patients with hemodynamically significant ICA disease, TCD may demonstrate the following indirect signs indicating ACoA filling : (1) a decrease in mean flow velocity (MFV) and pulsatility in the ipsilateral middle cerebral artery (MCA) together with normal flow in the contralateral MCA (2) reversed direction of flow in the ipsilateral A 1 segment of the ACA (3) no significant change in MFV of the ipsilateral MCA and A 1 ACA by compression test of ipsilateral common carotid artery, but decreased MFV by compression test of contralateral common carotid artery, and (4) increased blood flow in contralateral A 1 ACA, presenting as MFV 1.2 times greater than the ipsilateral A 1 ACA. TCD can only determine flow findings consistent with anterior cross-filling via the ACoA rather than measure flow velocity in the ACoA itself (fig. Collateral Flow via the Circle of WillisĬollateral flow through the ACoA cannot be reliably distinguished from the neighboring A 1 and A 2 segments of the anterior cerebral artery (ACA) due to the smaller ACoA length and diameter compared to a large sample volume with which ultrasound intercepts this area. TCD can directly detect the following collateral channels. Furthermore, it can be used for follow-up evaluations of cerebral collaterals after vascular interventions such as thrombolysis, carotid endarterectomy or stenting. In chronic cerebrovascular steno-occlusive disease, TCD is nowadays routinely used in many centers for screening of ICA stenosis. The great advantage of TCD is its noninvasiveness and its ability for repeated examinations. TCD has been recommended by the American Association of Neurology for the evaluation of the collateral pathways in the condition of an internal carotid artery (ICA) occlusion. Indirect assessment of collaterals can be accomplished by transcranial Doppler (TCD) ultrasound. However, the applicability of catheter angiography is limited by its invasive nature and associated risks. Conventional angiography provides the most reliable information about the circle of Willis and leptomeningeal collaterals and is, therefore, considered the gold standard for collateral flow assessment.
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