How It Works
Blood flow velocity is recorded by emitting a high-pitched sound wave from the ultrasound probe, which then bounces off of various materials to be measured by the same probe. A specific frequency is used (usually a multiple of 2 MHz), and the speed of the blood in relation to the probe causes a phase shift, wherein the frequency is increased or decreased. This frequency change directly correlates with the speed of the blood, which is then recorded electronically for later analysis. Normally a range of depths and angles must be measured to ascertain the correct velocities, as recording from an angle to the blood vessel yields an artificially low velocity.
Because the bones of the skull block the transmission of ultrasound, regions with thinner walls – insonation windows – must be used for analyzing. For this reason, recording is performed in the temporal region above the cheekbone/zygomatic arch, through the eyes, below the jaw, and from the back of the head. Patient age, gender, race and other factors affect bone thickness, making some examinations more difficult or even impossible. Most can still be performed to obtain acceptable responses, sometimes requiring using alternate sites from which to view the vessels.
Read more about this topic: Transcranial Doppler
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