Transcranial Doppler (TCD) is a non-invasive ultrasound technology used to assess blood flow velocity in the major basal intracranial arteries on a real time, beat-to-beat basis. Blood flow velocity is calculated and used to make determinations about intracranial hemodynamics.
 

 TCD may be used to evaluate intracranial effects of extracranial lesions, including information on collateralizing channels and tandem stenosis.  In addition, TCD allows direct perioperative evaluation of middle cerebral artery blood flow velocity in carotid endarterectomy patients, and is typically used to guide shunt placement, and to monitor for re-occlusion and hyperperfusion syndrome.

 

The vessels in the circle of Willis lie at known depths in the brain.  The operator selectively evaluates specific vessels by controlling the placement of the ultrasonic sample volume and the transducer’s orientation.  Depth of insonation, flow direction, velocity, and audio pitch assist in vessel identification.  TCD signals have been extensively correlated with angiograms, and are accepted as reliable measurements of intracranial blood flow velocity.

TCD may be performed from three physical approaches, known as ultrasonic windows.  

 

The transtemporal window allows evaluation of the middle cerebral artery, internal carotid artery bifurcation, anterior cerebral artery, and posterior cerebral artery.  This window lies within the area of the zygomatic arch, and three variants (anterior, middle and posterior) may be identified.

 

The transorbital window allows insonation of the ophthalmic artery and the internal carotid artery siphon.  (When using this window, the ultrasonic power output of the instrument should be lowered to 10 or 20%.)

 

The sub-occipital window allows serial evaluation of major portions of the vertebral and basilar arteries.  In some patients, tortuous anatomy may limit the portions of the basilar artery that are available for sampling.

 

To perform the examination, the operator places a 2 MHz transducer on the appropiate ultrasonic window.  An ultrasound beam is sent out from the unit through the transducer directly into the vessel.  The depth of penetration (or length of the beam) is controlled by the operator.  The TCD instrument evaluates the difference between the signal sent (transmitted frequency) and the signal returned (reflected frequency).  Spectral analysis then calculates this difference, also known as the Doppler frequency shift.  Flow direction is also indicated, as flow coming towards the transducer is displayed above the baseline and flow going away from the transducer is displayed below the baseline.

 

By comparing established typical values with actual examination results, the interpreter can make determinations about vessel hemodynamics.  For example, stenosis and occlusions may cause elevated velocity levels.  Mean velocity is representative of, but does not directly measure, blood flow in the vessel.  Pulsatility index describes the shape of the waveform and the relationship between peak systole and end diastole.  It is believed to represent, primarily, an estimation of downstream vascular resistance.  Resistance Index provides another measure of downstream vascular resistance.

 

Additional diagnostic criteria, such as flow direction, change or absence of signal, differences in velocity values on left and right sides, waveform shape, and others, are also used in interpretation.