摘要：Frequency, wavelength, resolution, and depth are interlinked. These seem to be complex physics stuff to grasp, but it is worth und

Frequency, wavelength, resolution, and depth are interlinked. These seem to be complex physics stuff to grasp, but it is worth understanding them because it can help you to get the best images from your ultrasound machine.This website will simply it all for you so don't worry.

The first step is to understand what the terms mean. Each of the terms will be described to you, starting with "frequency".

频率、波长、分辨率和深度

频率、波长、分辨率和深度是相互关联的。这些似乎是复杂的物理知识，但值得了解，因为它可以帮助您从超声波机器中获得最佳图像。这个网站将为您提供一切，所以不用担心。

第一步是理解这些术语的含义。每个术语都将从“频率”开始向您描述。

As you may recall, ultrasound waves are created by a vibrating piezo electric crystal. The vibration creates alternate high pressure and low pressure areas which travel forwards.

Frequency is the number of high pressure / low pressure cycles in a period of one second.

频率

你可能还记得，超声波是由振动的压电晶体产生的。振动产生交替的高压和低压区域，这些区域向前移动。

频率是指一秒钟内高压/低压循环的次数。

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In the image below, the high frequency wave has an higher number of cycles of high /low pressure areas in a period of one second. Similarly, the low frequency wave has a lower number of cycles of high / low areas of pressure per second.

The unit of frequency is Hertz, which is abbreviated to Hz. Hz refers to the "number of cycles per second". In the image below, the low frequency has 2 high /low pressure cycles per second. In other words, the low frequency wave below has a frequency of 2 Hz. Similarly, the high frequency wave below has 4 high /low pressure cycles per second. In other words, the high frequency wave below has a frequency of 4 Hz. These small numbers have been used only to explain the concept of frequency to you. In reality, ultrasound operates at many million cycles per second. (e.g. about 2 million Hz to 20 million Hz (2-20 MHz)

Wavelength is the distance between identical points in adjacent cycles of a waveform. For an example, you can measure wavelength by measuring the distance between two adjacent highest pressure points in the wave.

Or similarly, you could have measured the distance between two adjacent lowest pressure points to get the wavelength.

In either case, the wavelength is the same for a given wave.

Wavelength and frequency are importantly interrelated. When the frequency is increased the wavelength becomes shorter. Similarly, when the frequency is decreased, the wavelength becomes longer. This has some important implications when you use ultrasound in clinical practice and this will be explained to you later.

First let us discover why high frequency waves have a shorter wavelength. In the following image, the upper wave has a higher frequency. You will recall that high frequency means higher cycles per second (one cycle = one high pressure area followed by one low pressure area). In this example, the high frequency wave has four cycles per second (the high pressure areas are shown with a purple dot). Similarly, the lower wave has a lower frequency and therefore has less cycles per second.

This means that the high frequency wave has more cycles that are "squeezed" into the one second time frame. The low frequency wave has less cycles "squeezed" into the same one second time frame. Thus the cycles in a low frequency wave are more more wide apart. As discussed before, wavelength can be measured between two adjacent high pressure areas of a wave. You can now see that low frequency waves have a longer wavelength. High frequency waves have a shorter wavelength.

In more "serious" physics terms, one would describe this as "wavelength is inversely proportional to frequency". This can be expressed as the equation shown in the yellow square below:

We have discussed frequency and wavelength. These can affect "resolution". So let us understand what resolution means.

Resolution is the ability to see two things as two things.

If the resolution is good, the picture will be clear and the two objects will look like two objects.

If the resolution is poor, the picture will be blurred and the two objects will look like one.

Our aim is to get the best possible resolution from our ultrasound machine. Higher the resolution, higher will be the quality of the image we see.

For slightly complex physics reasons, shorter wavelengths give a better resolution.

But we also know that wavelength and frequency are related to each other. To get a shorter wavelength we should use a high frequency.

So to get a good resolution, we should use a high frequency because a high frequency has a short wavelength.

i.e. high frequency = short wavelength = good resolution

If the ultrasound waves do not penetrate the body to a sufficient depth, you may miss seeing the structure that you want to see.

The depth of penetration is related to the frequency of the ultrasound wave. Higher frequencies have a shorter depth of penetration. Lower frequencies have a longer depth of penetration.

So in clinical practice, you might think that using the lowest frequency is the best because it gives a longer depth of penetration. However it isn't as straightforward as the next section will explain.

来源：hoogoow