target="_blank" rel="nofollow" href="#fb3_img_img_6490c939-e834-5df3-98e5-854c43951ec0.jpg" alt=""/> Possible compression test if the information provided is expected to have implications—e.g., for:
– Clarifying the indication for surgery in multiple-vessel disease (e.g., internal carotid artery stenosis with contralateral occlusion)
– Assessment of the afferent components of the various collateral pathways
– Assessment of the hemodynamic consequences of a potential vascular occlusion (e.g., progressive asymptomatic internal carotid artery stenosis)
– Testing the quality of the collateralization
– Detecting any collateral pathways that cannot be spontaneously imaged (e.g., posterior communicating branch) by inducing hyperperfusion
– Assessment of the risk of intraoperative clamping (see below) before or during carotid thromboendarterectomy or proximal embolic protection. Evidence for sufficient residual perfusion includes:
– In cases of ≤ 80% stenosis of the internal carotid artery: mean residual flow velocity in the middle cerebral artery > 30–40% of the resting value
– In cases of ≤ 80% stenosis of the internal carotid artery: mean residual flow velocity ≥ 30 cm/s
– Correlates with the risk of hemodynamic watershed infarction—corresponds to the remaining CO2-induced dilation capacity in the intracerebral vessels
– Methods:
– Breath-holding index
– Doppler CO2 testing
– Apnea–hyperventilation testing
– Acetazolamide (Diamox) testing
Fig. 1.2–6 Apnea test. Top: the power Doppler profile of the middle cerebral artery bilaterally, with massively reduced amplitude on the right, reduced pulsatility and prolonged acceleration time as signs of poor intracerebral collateralization in subtotal internal carotid artery stenosis. Bottom: inverse steal phenomenon in the apnea test as a sign of an absent autoregulation reserve and steal in a vascular area that is still capable of reacting.
The apnea–hyperventilation test is the fastest exploratory test in routine work: resting Vmax is measured, followed by respiration for at least 30 seconds once a second or once per pulse and measurement of Vmax, and finally an apnea phase induced at mid-respiration for a maximum duration, during which Vmax is measured again. Standard: increase (apnea) and decrease (hyperventilation) by at least 15% in comparison with the resting values (Widder 1999).
Anterior cerebral artery
There are as yet no validated Doppler or duplex ultrasound criteria for grading stenoses of the anterior cerebral artery. The critical velocity from which a > 50% stenosis must be assumed to be present is 155 cm/s (Baumgartner et al. 1999). The criteria mentioned in connection with the middle cerebral artery can be used as an approximation. It is sometimes difficult to differentiate organically fixed stenoses from functional stenoses in collateral function: circumscribed flow accelerations argue more for localized stenoses, while longer flow accelerations—particularly in combination with other signs of collateralization (such as retrograde perfusion in the contralateral anterior cerebral artery, compression tests) suggest relative stenoses with collateral function.
Posterior cerebral artery
There are as yet no validated Doppler or duplex ultrasound criteria for grading stenoses of the posterior cerebral artery. The critical velocity from which a > 50% stenosis must be assumed to be present is 145 cm/s (Baumgartner et al. 1999). The criteria mentioned in connection with the middle cerebral artery can be used as an approximation. Particular sites of predilection for arteriosclerotic stenoses are the start of the P2 segment, the posterior arch, and more rarely the P1 outflow region. In the P1 segment, relative stenoses due to hyperperfusion in collateral function of the posterior cerebral artery via the posterior communicating branch or stenotic signals from the hyperperfused posterior communicating branch must be taken into consideration (caution: risk of possible confusion); color-coded imaging can be helpful for differentiation here.
Vertebral artery
See under extracranial occlusion processes.
Basilar artery
The head of the basilar artery is a site of predilection for arteriosclerotic lesions. There are as yet no validated Doppler or duplex ultrasound criteria for grading stenoses of the basilar artery. The critical velocity from which a > 50% stenosis must be assumed to be present is 140 cm/s (Baumgartner et al. 1999), but suspicion should already be raised at flow velocities of 100–120 cm/s. The criteria mentioned in connection with the middle cerebral artery can also be used as an approximation.
Basilar artery hypoplasia must be assumed when there is extracranial evidence of bilateral vertebral artery hypoplasia, particularly if the total diameter of the two vertebral arteries is less than 5 mm. Occlusion of the basilar artery (basilar thrombosis) must be assumed with:
Basilar occlusion cannot be definitively excluded only by evaluating the findings from the extracranial vertebral artery (particularly with older, collateralized occlusions that have developed gradually).
Other applications for transcranial Doppler and duplex ultrasonography
Evidence of spontaneous cerebral emboli/HITS analysis
High-intensity transient signals (HITS) with a relevant signal intensity and temporal latency in their occurrence in two sample volumes (multigating procedure) in the main trunk of the middle cerebral artery represent high-intensity signal peaks
