Extracranial

When the probe (i.e. a transducer) - frequency 5-10 MHz, is applied to the skin surface, a proportion of the ultrasonic waves emitted are reflected back from structures of varying acoustic impedence and are detected by the same probe. These reflected waves are reconverted into electrical energy and displayed as a two-dimensional image (p-mode).

When the probe is directed at moving structures, such as red blood cells within a blood vessel lumen, frequency shift of the reflected waves occurs (the Doppler effect) proportional to the velocity of flowing blood. Doppler ultrasound uses continuous wave (CW) or pulsed wave (PW). The former measures frequency shift anywhere along the path of the probe. Pulsed ultrasound records frequency shift at a specific depth.

Duplex scanning combines P-mode with doppler, simultaneously providing images from the vessels from which the velocity is recorded.

Colour Coded Duplex (CCD) uses colour coding to superimpose flow velocities on a two dimensional ultrasound image.

Normal vessels exhibit laminar flow and the probe detects a constant velocity.

Detector

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f ,

L 3

With stenosis the probe detects a wide spectrum of Probe velocity

P-mode (real time) scanning images the arterial wall rather than the passage of red blood cells - producing a 'map' of the lumen.

Applications: assessment of extracranial carotid and vertebral arteries.

Intracranial - transcranial Doppler ultrasound

By selecting lower frequencies (2 MHz), ultrasound is able to penetrate the thinner parts of the skull bone. Combining this with a pulsed system gives reliable measurements and flow velocity in the anterior, middle and posterior cerebral arteries and in the basilar artery. Applications:

Assessment of intracranial haemodynamics in extracranial occlusive/stenotic vascular disease. Detection of vasospasm in subarachnoid haemorrhage.

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