We can also display this information visually which is what the US machine would do for us. The faster the moving object, the greater the perceived change in frequency. Once the ambulance moves away from us the wavelengths now are further and further away from each other and we perceive a lower frequency. If its coming towards us the frequency appears higher since the wavelengths are compressed together. If the vehicle remains stationary we perceive one unique frequency but if we hear and ambulance coming towards us and then moving away from us we hear a difference in frequency due to the movement of the vehicle. Let imagine we have an ambulance with a siren emitting the same frequency. If the red blood cell is moving away from the probe, its wavelength is higher and has a lower frequency. The reflected sound wave then returns towards the probe with a different frequency to that which was emitted. This vessel contains moving red blood cells that interacts and reflect this sound wave. The ultrasound probe emits a frequency that travels to a vessel. Lets dig a bit deeper for better understanding. The Doppler effect is the measured change in frequency that corresponds to a change in velocity which in turn is related to a change in flow. TCD uses the Doppler effect so its understanding is paramount in its clinical application.
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