Figure 3
From: Aneurysmal subarachnoid haemorrhage from a neuroimaging perspective
Technical aspects. Transcranial Doppler ultrasonography (TCD) probe emits low-frequency (2 MHz), pulsed waves that cross the skull at specific points called acoustic windows. The emitted waves are reflected back from the moving red blood cells at an altered frequency fe. The Doppler effect (fe) is the difference in frequency between the transmitted wave f0 and the received wave fe: fd = fe – f0. The red blood cells' velocity can then be calculated according to the following mathematic equation: V = c?×?fd / 2?×?f0?×?cosθ. θ is the angle formed by the ultrasound beam and the blood flow. Ideally this angle should approach 0° so that the ultrasound beam is parallel to the blood flow (cos0° = 1). The higher this angle, the lower the velocity read by the machine. If the ultrasound beam is perpendicular to the blood flow, no velocity will be detected (cos90° = 0). This concept is fundamental to the performance and interpretation of TCD results. The velocities can be underestimated, never overestimated.