摘要：Before proceeding to explain the doppler effect, let us quickly revise what frequency means. Frequency is the number of oscillatio

Before proceeding to explain the doppler effect, let us quickly revise what frequency means. Frequency is the number of oscillations of the sound wave that occurs per one second. In the example below, the high frequency has four oscillations per second and the low frequency has two oscillations per second.

Now let me try and explain the Doppler effect. As you will recall, the ultrasound machine measures the distance to things by transmitting ultrasound waves from the probe and seeing how long the wave takes to return back to the probe.

The transmitted wave has a certain frequency. The wave that returns to the probe also has a certain frequency. When the wave is bounced back from a stationary object such as a nerve, both the transmitted and the returned waves have the same frequency.

However, if we repeat the same thing with an object moving towards the probe, something interesting happens. Imagine that the red disc below is an red blood cell moving towards the probe ( I know it is a rather large red blood cell !).

Again we send out an ultrasound wave .

This ultrasound wave reaches the moving red blood cell and bounces back. However, this time if you measured the frequency of the returned wave, it will not be the same as the frequency of the transmitted wave. The wave that bounces off an object moving towards the probe will have a higher frequency than the frequency of the wave transmitted from the probe. This is becuase the moving object "squashes" the waves as it moves towards the probe (see diagram below) . This is an example of the doppler effect. When a wave is sent to an object that is moving towards the transmitting probe, the doppler effect makes the frequency of the returning wave to be higher than the frequency of the wave sent out. The faster the object moves towards the transmitting probe, the higher will be the difference.

Ultrasound imaging devices can use the doppler effect to help us in many ways, including helping us to identify blood vessels.When you scan structures without using the doppler effect, the machine simply sees how long the waves take to return back to the probe and constructs an image.

However, this image does not clearly show which of the circles is a blood vessel and which is a nerve. Fortunately, blood vessels have one big difference from nerves. Blood vessels are full of rapidly moving red blood cells. When using an ultrasound machine with the ability to look for the doppler effect, the machine constructs an image in the usual way by seeing how long waves take to return back to the probe. But in addition, it also analyses the frequency of the returned waves. Whenever the returned wave has a frequency different to the frequency of the transmitted wave, the machine knows that the place where those waves bounced back from have moving objects. To help you to see these areas of moving cells, the ultrasound machine "adds colour" to areas showing the "doppler effect". In the image below, this helps you to differentiate nerve from blood vessels. The nerve has no moving cells, so there is no doppler effect and therefore no colour is added by the machine. The blood vessel has rapidly moving cells which cause a doppler effect, and where this occurs, the machine adds the colour red to help you identify it. This makes it easy for you to identify the blood vessel.

Let me try and explain how the doppler effect happens. When an ultrasound wave meets something moving towards it, the wave gets 'compressed' by the object.

The compression of the wave into a smaller length means that the oscillations of high and low pressure areas of the wave become more concentrated. As the wave gets compressed, it has more oscillations (high pressure / low pressure areas) per second than before ( i.e. the frequency has increased). This explains how the frequency of waves reflected from objects moving towards the probe have an higher frequency than the frequency of the wave sent out.

The Doppler effect also occurs for objects moving AWAY from the transmitting probe. Again there is a difference between the frequency of the transmitted waves and the frequency of the returning waves. However this time , the returning waves have a LOWER frequency than the frequency of the waves transmitted. The faster the object moves away, the greater will be the frequency difference. The reason for this decrease in frequency is the opposite of the explanation given before. In this case, the object moving away 'stretches' the wave. The stretching reduces the number of oscillations per one second.

To summarise, the doppler effect causes the frequency of waves reflected from a moving object to be different from the frequency of the wave sent out of the probe.

If the object is moving towards the probe, the reflected frequency is increased.

If the object is moving away, the reflected frequency is decreased.

Ultrasound machines tell us doppler effect information using colour. It uses different colours to show the direct and speed of flow. This helps you to identify vessels.Ultrasound machines tell us doppler effect information using colour. It uses different colours to show the direct and speed of flow. This helps you to identify vessels.

来源：每日科技观