Transcranial Duplex

Average Cerebral Artery Velocities

In transcranial doppler (TCD), the flow velocity is reported as mean doppler flow velocity, or time-averaged peak velocity. The mean velocity is similar to equation as mean arterial pressure (MAP):

Mean velocity = EDV + (PSV-EDV)/3

Based on sizes of blood vessels,  blood flow (velocity) should be MCA > ACA >PCA. For exam purposes, the ballpark velocities fore each cerebral artery are as follows:

  • Middle cerebral artery (MCA): mid 50s to 60s cm/s
  • Anterior cerebral artery (ACA): High 40s to low 50s cm/s
  • Posterior cerebral artery (PCA): 40s
  • Basilar artery (BA): High 30s to low 40s
  • Vertebral artery (VA): High 30s to low 40s

Another point to know. It is more important to compare velocities of both sides in the same patient, as hemodynamic parameters are consistent in the same subject and will provide more useful data than comparing to historical values.

Normal TCD Flow Patterns:

In transtemporal view (most common)

  • MCA: Toward the probe.
  • ACA: Away from the probe.
  • PCA: P1 segment toward the probe, P2 away.
  • MCA > ACA > PCA relationship should remain true.
  • ACoA and PCoA are not usually seen unless flow is increased to serve as collateral pathway.

In transocciptal/suboccipital view, patient is asked to tile chin down toward chest. Probe is placed on base of skull looking into foramen magnum.

  • Vertebral artery flows away from probe. The BA should be in line with the dominant verebral artery in the same direction.

Transorbital view is uncommon

  • Evaluates ophthalmic artery, which flows toward the probe. Can also see the ICA siphon, which can be away, bidirectional, or towards depending on location.
  • Ophthalmic artery waveforms should be high resistance pattern similar to a normal ECA. Mean velocity is around 21 cm/s.
  • Carotid siphon velocity is around 40s cm/s.

Interpreting TCD

  • In general, you need to carefully assess the flow velocity and direction, as well as presence of ACoA and PCoA flow. An ipsilateral decrease in MCA flow with reverse and increased ACA flow, +/- presence of ACoA flow would indicate ipsilateral ICA stenosis or occlusion with contralateral compensation via ACoA.
  • If MCA velocity > 80 cm/s, there is a clinically significant stenosis (30% stenosis). For 50% stenosis, velocity is usually above 100 cm/s, with a velocity ratio > 2.0.

Intraoperative TCD

  • During carotid endarterectomy, MCA velocity decrease by 60% is associated with poor collateral flow. Shunt is therefore warranted.

Vasospasm

In the setting of subarachnoid hemorrhage, the cerebral arteries can vasospasm, causing ischemia. It is unlikely to be the focus of RPVI, but below are some parameters:

  • MCA mean velocity > 120 cm/s, MCA/ICA ratio 3-6: moderate proximal MCA vasospasm.
  • MCA > 200 cm/s , MCA/ICA ratio > 6, severe proximal MCA spasm.
  • ACA velocity 130 cm/s, PCA 110 cm/s, vertetral artery 80 cm/s are associated with spasm.
  • Basilar artery > 85 cm/s, BA/VA ratio > 3.0, severe spasm.

The MCA to distal ipsilateral ICA mean velocity ratio is called Lindegaard ratio.

Basically, just remember that if the mean velocity is elevated with some sort of ratio in the setting of subarachnoid hemorrhage or intracranial aneurysm, the answer is vasospasm. The lower the velocity, the lower the risk of developing brain ischemia. Treatment is first medical, then neuroendovascular intervention if medical treatment is inadequate.

Hyperperfusion Syndrome

Hyperperfusion syndrome is relatively rare but known clinical presentation after carotid endarterectomy. In this case, the TCD mean velocities of one or several cerebral vessels may be elevated to as much as 3x the normal. This makes sense because hyperperfusion is the brain getting too much blood.

Now, carefully read through the vasospasm paragraph again and compare hyperperfusion to vasospasm. Cerebral velocities are increased in both situations. However, when the brain is vasospastic, the arteries constrict and elevate the blood flow velocities. The brain tissues are in fact ischemic. But in hyperperfusion, the velocities are elevated simply because there is more blood flow going through the vessels.

If you encounter a question with elevated TCD velocities on the test, carefully read the clinical scenario and pick the best answer. If the patient is status post carotid surgery, hyperperfusion is likely the right answer.

Cerebral Vasomotor Reactivity (VMR)

Although there is probably a very low chance of this being tested on RPVI, we will review this briefly. VMR looks at the range of percent change of MCA velocity in response to Inhaled CO2. Specifically, during hypercapnia state, the MCA blood flow, represented as mean velocity Vm, increases. During hypocapnia, MCA Vm decreases. The range of change compared to baseline is expressed below:

VMR = (Hypercapnia Vm x 100/ Baseline Vm) – (Hypocapnia Vm x 100/Baseline Vm)

Normal VMR is 86%± 10%. VMR < 38% indicates abnormal state. It is associated with prior infarcts, chronic ischemic symptoms, and hypostatic TIAs.

Circle of Willis

  • The circle of Willis is incomplete in about 50% of the population.
  • Absence of posterior communicating artery (PCoA) on one side (9%), both sides (9%), and absence of one proper PCA, also known as fetal origin PCA (9%) are among the most common variants.
  • The absence of anterior communicating artery (ACoA) is  the most rare at 1%.
  • In computer modeling, the ACoA is a more important collateral pathway than PCoA.