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 within the Doppler spectrum of blood components—i.e., spontaneous cerebral emboli. There is a 15-fold increase in the risk of stroke when there is evidence of HITS—e.g., in patients with 60% asymptomatic internal carotid artery stenosis—in comparison with negative HITS findings (Spence et al. 2005).
Testing for persistent patent foramen ovale (PFO)
An ultrasound contrast medium that will not enter the capillaries is injected into a large antecubital vein or the common femoral artery; after approximately 8 seconds, a Valsalva maneuver is carried out for approximately 4 seconds, possibly supported by compression in the abdominal area, with further ultrasound imaging for 5–10 seconds. When there is an intermittent cardiac right–left shunt due to PFO, there is a mean contrast appearance time of 9 ± 6 s (< 15 s) in comparison with 24 ± 9 s (> 15 s) with transpulmonary passage. A relevant shunt is present if more than 10 emboli appear at rest and/or more than 25 emboli appear after the Valsalva maneuver within the time stated. Larger persistent foramina lead to showers of emboli that can no longer be detected individually. Lower emboli rates are probably not relevant. The advantage of TCD in comparison with transesophageal echocardiography is that the procedure is not invasive, the patient can still cooperate (with the Valsalva), evidence of noncardiac shunts is also possible (explaining occasional differences), and the sensitivity is comparable for relevant shunts.
Intracranial pressure monitoring
Diastolic flow/pulsatility correlates with intracerebral pressure/outflow resistance; the parameters along the course are highly sensitive and measurement of absolute values is of course not possible (with the exception of diastolic zero flow, in which case intracranial pressure corresponds to the diastolic pressure, with phasic flow when there is a further rise in intracranial pressure).
Diagnosis of cerebral death
Transcranial Doppler ultrasound is an approved procedure used to shorten the waiting time before cerebral death is diagnosed. Prerequisites include not only availability of an examiner with the relevant expertise, but also confirmation that ultrasonography can be carried out in the patient and appropriate adjustment of the device settings (high gain, maximum transmission power, high reception speed, low wall filter, large measurement volume ≥ 15 mm). Typical Doppler findings in cerebral perfusion standstill:
Prerequisites: