**2. Transcranial Doppler principles**

The *Doppler effect* is the shift in frequency emitted by a source moving in relation to an observer as perceived by the observer. The shift is to higher frequencies when

the distance between the source and the observer decreases and to lower frequencies when the distance increases. In TCD, the source is a red blood cell reflecting an echo, and the observer is the ultrasound probe.

TCD instruments are generally calibrated to measure blood flow velocity when blood is moving either directly toward the ultrasound probe (0°) or directly away from it (180°). This principle is important because if insonation is at an angle other than 0° or 180°, only a fraction of the true velocity is measured. Certain areas on and around the skull (*windows of insonation*) help the operator achieve insonation directly in line with blood flow and avoid signal attenuation from the skull and other tissue. **Figure 1** shows TCD acoustic windows. The arteries that are examined at those windows are as follows:


Temporal window is commonly accessed as it provides optimal visualization of MCA, which is frequently involved artery in stroke. Temporal window is an areas of the skull that is located just above the zygomatic arch, approximately 1 cm posterior to the midpoint of a line connecting the lateral canthus of the eye to the auditory

**Figure 1.** *TCD acoustic windows.*

#### *Clinical Application of Transcranial Doppler in Cerebrovascular Diseases DOI: http://dx.doi.org/10.5772/intechopen.111665*

meatus. The ultrasound probe is positioned on the temporal bone and angled to obtain a transcranial view of the MCA. This allows for detection of emboli, stenosis, and changes in blood flow velocity that may indicate cerebral ischemia or Vasospasm.

An understanding of cerebral hemodynamics, including relevant anatomy, physiology, and pathophysiology, is critical for the accurate acquisition and interpretation of intracranial Doppler data. To this end, it is important to understand the typical vascular distributions and the manifold factors that can affect cerebral blood flow. The cerebral vasculature has autoregulatory mechanisms that compensate for changes in cardiac output and blood viscosity, thereby maintaining relatively constant cerebral blood flow (and cerebral blood flow velocity as measured with TCD). Nevertheless, extreme changes in hemostasis will affect cerebral blood flow. For example, in larger arteries, atherosclerotic plaques cause arterial stenosis alters blood flow velocity. In smaller arteries, various precipitants (e.g., arterial carbon dioxide, intracranial pressure, and mean arterial pressure) alter the flow, and the upstream effects can be inferred from TCD.
